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Shear Failure of Column-Beam Joint
bars also are effective in confining the joint concrete and resisting horizontal
shear forces. Providing closed-loop ties in the joint requires some extra effort.
Indian Standard IS:13920-1993 recommends continuing the transverse loops
around the column bars through the joint region. In practice, this is achieved by
preparing the cage of the reinforcement (both longitudinal bars and stirrups) of
all beams at a floor level to be prepared on top of the beam formwork of that
level and lowered into the cage. However, this may not always be possible
particularly when the beams are long and the entire reinforcement cage becomes
heavy. The gripping of beam bars in the joint region is improved first by using
columns of reasonably large cross-sectional size. The Indian Standard
IS:13920-1993 requires building columns in seismic zones III, IV and V to be at
least300mm wide in each direction of the cross-section when they support
beams that are longer than 5m or when these columns are taller than 4m
between floors (or beams). The American Concrete Institute recommends a
column width of at least 20 times the diameter of largest longitudinal bar used in
adjoining beam. In exterior joints where beams terminate at columns,
longitudinal beam bars need to be anchored into the column to ensure proper
gripping of bar in joint. The length of anchorage for a bar of grade Fe415
(characteristic tensile strength of 415MPa) is about 50times its diameter. This
length is measured from the face of the column to the end of the bar anchored in
the column. In columns of small widths and when beam bars are of large
diameter a portion of beam top bar is embedded in the column that is cast up to
the soffit of the beam, and a part of it overhangs. It is difficult to hold such an
overhanging beam top bar in position while casting the column up to the soffit
of the beam. Moreover, the vertical distance beyond the 90ºbend in beam bars is
not very effective in providing anchorage. On the other hand, if column width is
large, beam bars may not extend below soffit of the beam. Thus, it is preferable
to have columns with sufficient width. Such an approach is used in many codes
[e.g., ACI318, 2005]. In interior joints, the beam bars (both top and bottom)
need to go through the joint without any cut in the joint region. Also, these bars
must be placed within the column bars and with no bends.](https://image.slidesharecdn.com/earthquakeresistantbuildingswithcoverpreface-250604111419-27b36dd3/75/Earthquake-Resistant-Buildings-with-cover-preface-pdf-29-2048.jpg)
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Earthquake Resistant Buildings with cover & preface.pdf
- 1. DESIGN & CONSTRUCTION PRACTICES ADDITIONAL DIRECTORATE GENERAL TECHNICALEXAMINATION IHQ OF MoD (ARMY) QMG’S BRANCH EARTHQUAKE RESISTANT BUILDINGS 01/2018-19
- 3. Earthquake Resistant Buildings- Design and Construction Practices Note All rights of translation and Re-publication Reserved. This booklet is strictly for private circulation.
- 5. Tele: 35343/35340(A) AddlDte Gen Tech Examination 23019948/23018182(Civ) Quartermaster General Branch Integrated HQ of MoD (Army) Kashmir House, Rajaji Marg DHQ PO New Delhi – 110 011. No.98150/Adv/ /Q/ADGTE Jul 2018 MGOL, Southern Command, Pune MGOL, Eastern Command, Kolkata MGOL, Western Command, Chandimandir MGOL, Central Command, Lucknow MGOL, Northern Command, Jammu MGOL, South Western Command, Jaipur Chief Engineer, Southern Command, Pune - 908541 Chief Engineer, Eastern Command, Kolkata - 908542 Chief Engineer, Western Command, Chandimandir - 908543 Chief Engineer, Central Command, Lucknow - 908544 Chief Engineer, Northern Command, C/o 56 APO - 908545 Chief Engineer, South Western Command, Jaipur - 908546 EARTHQUAKE RESISTANT BUILDINGS : DESIGN & CONSTRUCTION PRATICES A tech paper on Earthquake Resistant Building : Design & Construction Practices is fwd herewith alongwith soft copy in CD for your info and and further distribution pl. (SK Mishra) Col Dir TE Encls: As above for ADGTE Copy to: QMG Sectt E-in-C Sectt E-in-C Br (DGW) STE, Pune - 900449 STE, Chennai - 900432 STE, Kolkata - 900285 STE, Chandigarh - 160009 STE, Lucknow - 900450 STE, Jammu Cantt - 900277 STE, Jaipur - 908746 STE MAP New Delhi - 110 011
- 6. (i) PREFACE: EARTHQUAKE RESISTANTBUILDING IN INDIA: PREPARING FOR SAFETY 1. Recent earthquakes have created much curiosity about personal safety among the people. Since its major impact is seen on the buildings hence it is important that they meet seismic codes requirements to ensure safety of the people living in them. As many countries have already started implementing seismic codes, India too prepares for seismic proof buildings. According to bureau of Indian standards on earthquake engineering, several codes have been produced in construction of quake resistant structures and regarding tests & measurements therewith. 2. The earthquake that shattered neighbouring Nepal in 2015 was a display of how small human efforts can look when the nature strikes. But then, the battered nation got up and started rebuilding from scratch. This was a proof of how humans go on despite the many hindrances. The same is true of the more recent tragedies in Japan and Ecuador. 3. India, too, has a geophysical position that makes it earthquake resistant. This is why when Nepal witnessed the massive destruction of life and property, we felt only tremors. However, we must keep in mind that the entire Northeast, Uttarakhand, Gujarat and Himachal Pradesh are some of the highly earthquake- prone areas in India. Besides, many other zones like Delhi-National Capital Region also fall under high-risk category. In view of that, the country has to be ready in case of a disaster and more so in constructing earthquake-resistant structures. And this is why the government has put in place several measures.
- 7. (ii) 4. The standardsmaintained for mitigating the hazards of Earthquake are mentioned beneath. Criteria for Earthquake Resistant Design of Structures (IS 1893:1984) 5. This standard mainly deals into earthquake resistant design of buildings and gives a map showing seismic zones based on the seismic intensity. The provisions made are applicable to all structures such as elevated structures etc. General provisions and Buildings (IS 1893 (Part 1):2002) 6. Standard contains design criteria, including design spectrum, main attributes of buildings, seismic zoning & coefficients of area that are general in nature and applicable to all structures. These provisions of this standard ensure that no structure suffer damage from the sudden movement in the earth's crust. Industrial Structures Including Stack Like Structures (IS 1893(Part 4):2005) 7. Mainly dealing with quake proof industrial design, this standard ensures that the structures possess minimum strength to withstand minor earth quake which has been seen occurring frequently in many parts of the country. Earthquake Resistant Design and Construction of Buildings (IS 4326:1993) 8. From general principles on earthquake design to guidance in selection of construction materials, providing seismic strengthening of concrete buildings. The provisions laid are applicable for high risk zones 3, 4 and 5.
- 8. (iii) Improving Earthquake Resistanceof Earthen Buildings (IS 13827:1993) 9. This standard is for earthen structures in Seismic zones 3, 4 & 5. As per the guidelines, structure design should be light with simple rectangular plan and of single. Here qualitative tests have been suggested for the suitability of soil. Improving Earthquake Resistance of Low Strength Masonry Buildings (IS 13828:1993) 10. The guidelines focus on special features of structure design and construction in order to improve earthquake resistance of low-strength masonry buildings. The provisions made are applicable in all 2-5 seismic zones. The various provisions of IS 4326:1993 pertaining to general principles and special construction features for low-strength masonry buildings dealt with in this standard. Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces (IS 13920:1993) 11. This covers all design requirements, including detailing of monolithic reinforced concrete buildings so as to provide them with good ductility and adequate toughness to resist severe seismic shocks without collapse. Seismic Evaluation, Repair and Strengthening of Masonry Buildings (IS 13935:2009) 12. It includes selection of construction materials and techniques for repair and seismic strengthening of buildings damaged from earthquakes. It also covers the provisions of IS 4326 and IS 13828 that deals with seismic damageability assessment and retrofit of existing masonry buildings to upgrade seismic resistance of the structures. Criteria for Safety and Design of Structures Subject to Underground Blasts (ISS 6922:1973) 13. This standard specifically deals with the safety of structures and all buildings during underground blasts constructed in materials like concrete, brickwork as well as stone masonry. Criteria for Blast Resistant Design of Structures (IS 4991:1968) 14. This covers criteria for structure design for blast effects due to explosions above ground. However, blast effects from nuclear explosions are excluded in this.
- 9. (iv) Recommendations for SeismicInstrumentation for River Valley Projects (IS 4967:1968) 15. This mainly includes recommendations pertaining to instrumentation for investigation of seismicity and permanent installation of instruments in the appurtenant structures and in surrounding areas. 16. These standards are an endeavor to provide a guideline in design and repair of buildings under high risk zones. An RCC frame structure is an assembly of beams, columns and slabs interconnected so that the load gets transferred to the slabs then on to beams then columns and then to the lowers beams in such a way it reaches the soil. Failure of a builder to abide by seismic codes for construction may put the builder to various civil litigation disputes. This paper compiles the last advances and presents the theory. That is, purposes, principles, definition, contents, methodology and guidelines to make the architectural and the structural designs compatible.
- 10. 1 EARTHQUAKE RESISTANT BUILDINGS:DESIGN AND CONSTRUCTION PRACTICES Earthquake Occurrence 1. The Earth was formed by a large collection of material masses. Large amount of heat was generated by this fusion, and slowly as the Earth cooled, the heavier and denser materials sank to the center and the lighter ones rose to the top. The Earth consists of the Inner Core (radius ~1290km), the Outer Core (thickness ~2200km), the Mantle (thickness ~2900km) and the Crust (thickness ~5 to 40km). The Inner Core is solid and consists of heavy metals (e.g., nickel and iron), while the Crust consists of light materials (e.g., basalts and granites). The Outer Core is liquid in form and the Mantle has the ability to flow. At the Core, the temperature is estimated to be ~2500°C, the pressure ~4 million atmosphere sand density ~13.5 gm/cc; this is in contrast to ~25°C, 1 atmosphere and 1.5 gm/ccon the surface of the Earth. Convection currents develop in the viscous Mantle, because of prevailing high temperature and pressure gradients between the Crust and the Core. The energy for the above circulations is derived from the heat produced from the incessant decay of radioactive elements in the rocks throughout the Earth’s interior. These convection currents result in a circulation of the earth’s mass; hot molten lava comes out and the cold rock mass goes into the Earth. Many such local circulations are taking place at different regions underneath the Earth’s surface, leading to different portions of the Earth undergoing different directions of movements along the surface. 2. The convective flows of Mantle material cause the Crust and some portion of the Mantle, to slide on the hot molten outer core. This sliding of Earth’s mass takes place in pieces called Tectonic Plates. These plates move in different directions and at different speeds from those of the neighboring ones. The relative movement of these plate boundaries varies across the Earth; on an average, it is of the order of a couple to tens of centimeters per year. The sudden slip at the fault causes the earthquake…a violent shaking of the Earth during which large elastic strain energy released spreads out in the form of seismic waves that travel through the body and along the surface of the Earth. And, after the earthquake is over, the process of strain build-up at this modified interface between the tectonic plates starts all over again called the Elastic Rebound. Most earthquakes in the world occur along the boundaries of the tectonic plates as described above and are called Inter-plate Earthquakes(e.g.,
- 11. 2 1897 Assam(India) earthquake).A number of earthquakes also occur within the plate itself but away from the plate boundaries (e.g., 1993 Latur (India) earthquake); these are called Intra-plate Earthquakes. Here, a tectonic plate breaks in between. In both types of earthquakes, the slip generated at the fault during earthquakes is along both vertical and horizontal directions with one of them dominating sometimes. Large strain energy released during an earthquake travels as seismic waves in all directions through the Earth’s layers, reflecting and refracting at each interface. The instrument that measures earthquake shaking is called a seismograph. The point on the fault where slip starts is the Focus or Hypocenter, and the point vertically above this on the surface of the Earth is the Epicenter. Magnitude is a quantitative measure of the actual size of the earthquake. The most commonly used magnitude scale is the Richter Scale. Intensity is a qualitative measure of the actual shaking at a location during an earthquake and is assigned as Roman Capital Numerals. Magnitude of an earthquake is a measure of its size and on the other hand, intensity is an indicator of the severity of shaking generated at a given location. Seismic Zones in India 3. The varying geology at different locations in the country implies that the likelihood of damaging earthquakes taking place at different locations is different. Thus, a seismic zone map is required to identify these regions. Based on the levels of intensities sustained during damaging past earthquakes, the 1970 version of the zone map subdivided India into five zones – I, II, III, IV and V. The maximum Modified Mercalli (MM) intensity of seismic shaking expected in these zones are V or less, VI, VII, VIII, and IX and higher, respectively. Parts of Himalayan boundary in the north and north-east, and the Kachchh area in the west were classified as zone V. The seismic zone maps are revised from time to time as more understanding is gained on the geology, the seismo-tectonics and the seismic activity in the country. The Indian Standards provided the first seismic zone map in 1962, which was later revised in 1967 and again in 1970. The map has been revised again in 2002 and it now has only four seismic zones – II, III, IV and V. The areas falling in seismic zone I in the 1970 version of the map are merged with those of seismic zone II. Also, the seismic zone map in the peninsular region has been modified. Chennai now comes in seismic zone III as against in zone II in the 1970 version of the map. This 2002 seismic zone map is not the final word on the seismic hazard of the country, and hence there can be no sense of complacency in this regard. The national Seismic Zone Map presents a large-scale view of the seismic zones in the country. Local variations in soil type and geology cannot be represented at that scale. Therefore, for important projects, such as a major dam or a nuclear
- 12. 3 power plant, theseismic hazard is evaluated specifically for that site. Also, for the purposes of urban planning, metropolitan areas are micro-zoned. Seismic micro-zonation accounts for local variations in geology, local soil profile, etc. Seismic Effects on Structures 4. Earthquake causes shaking of the ground. So a building resting on it will experience motion at its base. In the building, since the walls or columns are flexible, the motion of the roof is different from that of the ground. This tendency of the superstructure to continue to remain in the previous position is known as inertia. Consider a building whose roof is supported on columns and taking the analogy of yourself on the bus: when the bus suddenly starts, you are thrown backwards as if someone has applied a force on the upper body, when the ground moves, even the building is thrown backwards, and the roof experiences a force, called inertia force. Clearly, more mass means higher inertia force. Therefore, lighter buildings sustain the earthquake shaking better. The inertia force experienced by the roof is transferred to the ground via the columns, causing forces in columns. These forces generated in the columns can also be understood in another way. During earthquake shaking, the columns undergo relative movement between their ends. But, given a free option, columns would like to come back to the straight vertical position, i.e., columns resist deformations. In the straight vertical position, the columns carry no horizontal earthquake force through them. But, when forced to bend, they develop internal forces. The larger is the relative horizontal displacement between the top and bottom of the column, the larger this internal force in columns. Also, the stiffer the columns are (i.e., bigger is the column size), larger is this force. For this reason, these internal forces in the columns are called stiffness forces. Earthquake causes shaking of the ground in all three directions – along the two horizontal directions (X and Y), and the vertical direction (Z). Also, during the earthquake, the ground shakes randomly back and forth (- and +) along each of these X, Y and Z directions. All structures are primarily designed to carry the gravity loads, the vertical acceleration during ground shaking either adds to or subtracts from the acceleration due to gravity. Since factors of safety are used in the design of structures to resist the gravity loads, usually most structures tend to be adequate against vertical shaking. However, horizontal shaking along X and Y directions (both + and – directions of each) remains a concern. Structures designed for gravity loads, in general, may not be able to safely sustain the effects of horizontal earthquake shaking. Hence, it is necessary to ensure adequacy of the structures against horizontal earthquake effects.
- 13. 4 Partial Collapse ofStone masonry walls during Uttarkashi earthquake Collapse of reinforced concrete columns (and building) during 2001 Bhuj earthquake 5. Under horizontal shaking of the ground, horizontal inertia forces are generated at level of the mass of the structure (usually situated at the floor levels). These lateral inertia forces are transferred by the floor slab to the walls or columns, to the foundations, and finally to the soil system underneath. So, each of these structural elements (floor slabs, walls, columns, and foundations) and the connections between them must be designed to safely transfer these inertia forces through them. Walls or columns are the most critical elements in transferring the inertia forces. But, in traditional construction, floor slabs and beams receive more care and attention during design and construction, than walls and columns. Walls are relatively thin and often made of brittle material like masonry. They are poor in carrying horizontal earthquake inertia forces along the direction of their thickness. Failures of masonry walls have been observed in many earthquakes in the past. Similarly, poorly designed and constructed reinforced concrete columns can be disastrous. The failure of the ground storey columns resulted in numerous building collapses during the2001 Bhuj (India) earthquake. Affect of Architectural Features on Buildings during Earthquakes 6. The behaviour of a building during earthquakes depends critically on its overall shape, size and geometry, in addition to how the earthquake forces are carried to the ground. Hence, at the planning stage itself, architects and structural engineers must work together to ensure that the unfavourable features are avoided and a good building configuration is chosen. A desire to create an aesthetic and functionally efficient structure drives architects to conceive wonderful and imaginative structures. Sometimes the shape of the building catches the eye of the visitor, sometimes the structural system appeals, and in other occasions both shape and structural system work together to make the structure a marvel. However, each of these choices of shapes and structure has significant bearing on the performance of the building during strong
- 14. 5 earthquakes. The widerange of structural damages observed during past earthquakes across the world is very educative in identifying structural configurations that are desirable versus those which must be avoided. 7. Size and Layout of Buildings. In tall buildings with large height-to- base size ratio, the horizontal movement of the floors during ground shaking is large. In short but very long buildings, the damaging effects during earthquake shaking are many. And, in buildings with large plan area like warehouses, the horizontal seismic forces can be excessive to be carried by columns and walls. In general, buildings with simple geometry in plan have performed well during strong earthquakes. Buildings with re-entrant corners, like those U, V, Hand + shaped in plan, have sustained significant damage. Many times, the bad effects of these interior corners in the plan of buildings are avoided by making the buildings in two parts. For example, an L-shaped plan can be broken up into two rectangular plan shapes using a separation joint at the junction. Often, the plan is simple, but the columns/walls are not equally distributed in plan. Buildings with such features tend to twist during earthquake shaking. The earthquake forces developed at different floor levels in a building need to be brought down along the height to the ground by the shortest path; any deviation or discontinuity in this load transfer path results in poor performance of the building. Buildings with vertical setbacks (like the hotel buildings with a few storeys wider than the rest) cause a sudden jump in earthquake forces at the level of discontinuity. Buildings that have fewer columns or walls in a particular storey or with unusually tall storey, tend to damage or collapse which is initiated in that storey. Many buildings with an open ground storey intended for parking collapsed or were severely damaged in Gujarat during the 2001 Bhuj earthquake. Buildings on slopy ground have unequal height columns along the slope, which causes ill effects like twisting and damage in shorter columns. Buildings with columns that hang or float on beams at an intermediate storey and do not go all the way to the foundation, have discontinuities in the load transfer path. Some buildings have reinforced concrete walls to carry the earthquake loads to the foundation. Buildings, in which these walls do not go all the way to the ground but stop at an upper level, are liable to get severely damaged during earthquakes. When two buildings are too close to each other, they may pound on each other during strong shaking. With increase in building height, this collision can be a greater problem. When building heights do not match, the roof of the shorter building may pound at the mid-height of the column of the taller one; this can be very dangerous.
- 15. 6 8. Looking ahead,of course, one will continue to make buildings interesting rather than monotonous. However, this need not be done at the cost of poor behaviour and earthquake safety of buildings. Architectural features that are detrimental to earthquake response of buildings should be avoided. If not, they must be minimised. When irregular features are included in buildings, a considerably higher level of engineering effort is required in the structural design and yet the building may not be as good as one with simple architectural features. Decisions made at the planning stage on building configuration are more important, or are known to have made greater difference, than accurate determination of code specified design forces. 9. Twisting of Buildings. Just like when you sit on a rope swing - a wooden cradle tied with coir ropes to the sturdy branch of an old tree. Consider the rope swing that is tied identically with two equal ropes. It swings equally, when you sit in the middle of the cradle. Buildings too are like these rope swings; just that they are inverted swings. The vertical walls and columns are like the ropes, and the floor is like the cradle. Buildings vibrate back and forth during earthquakes. Buildings with more than one storey are like rope swings with more than one cradle. If you sit at one end of the cradle, it twists(i.e., moves more on the side you are sitting). This also happens sometimes when more of your friends bunch together and sit on one side of the swing. Likewise, if the mass on the floor of a building is more on one side (for instance, one side of a building may have a storage or a library), then that side of the building moves more underground movement. This building moves such that its floors displace horizontally as well as rotate. Let the two ropes with which the cradle is tied to the branch of the tree be different in length. Such a swing also twists even if you sit in the middle. Similarly, in buildings with unequal structural members (i.e., frames and/or walls) also the floors twist about a vertical axis and displace horizontally. Likewise, buildings, which have walls only on two sides (or one side) and flexible frames along the other, twist when shaken at the ground level. Twist in buildings, called torsion by engineers, makes different portions at the same floor level to move horizontally by different amounts. This induces more damage in the frames and walls on the side that moves more. Many buildings have been severely affected by this excessive torsional behaviour during past earthquakes. It is best to minimize (if not completely avoid) this twist by ensuring that buildings have symmetry in plan (i.e., uniformly distributed mass and uniformly placed lateral load resisting systems). If this twist cannot be avoided, special calculations need to be done to account for this additional shear forces in the design of buildings; the Indian seismic code (IS 1893, 2002) has provisions for such calculations. But, for sure, buildings with twist will perform poorly during strong earthquake shaking.
- 16. 7 Earthquake Resistant Buildings 10.The engineers do not attempt to make earthquake proof buildings that will not get damaged even during the rare but strong earthquake; such buildings will be too robust and also too expensive. Instead, the engineering intention is to make buildings earthquake resistant; such buildings resist the effects of ground shaking, although they may get damaged severely but would not collapse during the strong earthquake. Thus, safety of people and contents is assured in earthquake-resistant buildings, and thereby a disaster is avoided. This is a major objective of seismic design codes throughout the world. The earthquake design philosophy may be summarised as follows: (a) Under minor but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged; however, building parts that do not carry load may sustain repairable damage. (b) Under moderate but occasional shaking, the main members may sustain repairable damage, while the other parts of the building may be damaged such that they may even have to be replaced after the earthquake. (c) Under strong but rare shaking, the main members may sustain severe (even irreparable) damage, but the building should not collapse. 11. Thus, after minor shaking, the building will be fully operational within a short time and the repair costs will be small. And, after moderate shaking, the building will be operational once the repair and strengthening of the damaged main members is completed. But, after a strong earthquake, the building may become dysfunctional for further use, but will stand so that people can be evacuated and property recovered. The consequences of damage have to be kept in view in the design philosophy. For example, important buildings, like hospitals and fire stations, play a critical role in post-earthquake activities and must remain functional immediately after the earthquake. These structures must sustain very little damage and should be designed for a higher level of earthquake protection. Collapse of dams during earthquakes can cause flooding in the downstream reaches, which itself can be a secondary disaster. Therefore, dams (and similarly, nuclear power plants) should be designed for still higher level of earthquake motion. 12. Damage to buildings due to earthquake is unavoidable. Design of buildings to resist earthquakes involves controlling the damage to acceptable levels at a reasonable cost. Contrary to the common thinking that any crack in
- 17. 8 Diagonal cracks incolumns and brittle failure of column concrete jeopardize vertical load carrying capacity of buildings – unacceptable damage the building after an earthquake means the building is unsafe for habitation, engineers designing earthquake-resistant buildings recognise that some damage is unavoidable. Different types of damage (mainly visualized through cracks; especially so in concrete and masonry buildings) occur in buildings during earthquakes. Some of these cracks are acceptable (in terms of both their size and location), while others are not. For instance, in a reinforced concrete frame building with masonry filler walls between columns, the cracks between vertical columns and masonry filler walls are acceptable, but diagonal cracks running through the columns are not. In general, qualified technical professionals are knowledgeable of the causes and severity of damage in earthquake-resistant buildings. Earthquake-resistant design is therefore concerned about ensuring that the damages in buildings during earthquakes are of the acceptable variety, and also that they occur at the right places and in right amounts. Likewise, to save the building from collapsing, you need to allow some pre-determined parts to undergo the acceptable type and level of damage. 13. So, the task now is to identify acceptable forms of damage and desirable building behaviour during earthquakes. Earthquake-resistant buildings, particularly their main elements, need to be built with ductility in them. Such buildings have the ability to sway back-and-forth during an earthquake, and to withstand earthquake effects with some damage, but without collapse. Ductility is one of the most important factors affecting the building performance. Thus, earthquake-resistant design strives to predetermine the locations where damage takes place and then to provide good detailing at these locations to ensure ductile behaviour of the building.
- 18. 9 Indian Seismic Codes 14.Seismic codes help to improve the behaviour of structures so that they may withstand the earthquake effects without significant loss of life and property. Countries around the world have procedures outlined in seismic codes to help design engineers in the planning, designing, detailing and constructing of structures. An earthquake-resistant building has four virtues in it, namely: (a) Good Structural Configuration: Its size, shape and structural system carrying loads are such that they ensure a direct and smooth flow of inertia forces to the ground. (b) Lateral Strength: The maximum lateral (horizontal)force that it can resist is such that the damage induced in it does not result in collapse. (c) Adequate Stiffness: Its lateral load resisting system is such that the earthquake-induced deformations in it do not damage its contents under low-to moderate shaking. (d) Good Ductility: Its capacity to undergo large deformations under severe earthquake shaking even after yielding, is improved by favourabledesign and detailing strategies. 15. Seismic codes are unique to a particular region or country. They take into account the local seismology, accepted level of seismic risk, building typologies, and materials and methods used in construction. Further, they are indicative of the level of progress a country has made in the field of earthquake engineering. The first formal seismic code in India, namely IS 1893, was published in 1962. Today, the Bureau of Indian Standards (BIS) has the following seismic codes: (a) IS 1893 (Part I), 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision). (b) IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (2nd Revision). (c) IS 13827, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Buildings IS 13828, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Low Strength Masonry Buildings.
- 19. 10 (d) IS 13920,1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces. 16. Countries with a history of earthquakes have well developed earthquake codes. Thus, countries like Japan, New Zealand and the United States of America, have detailed seismic code provisions. Development of building codes in India started rather early. Today, India has a fairly good range of seismic codes covering a variety of structures, ranging from mud or low strength masonry houses to modern buildings. However, the key to ensuring earthquake safety lies in having a robust mechanism that enforces an implements these design code provisions in actual constructions. Behaviour of Brick Masonry Structures During Earthquakes 17. Masonry buildings are brittle structures and one of the most vulnerable of the entire building stock under strong earthquake shaking. The large number of human fatalities in such constructions during the past earthquakes in India corroborates this. Thus, it is very important to improve the seismic behaviour of masonry buildings. A number of earthquake-resistant features can be introduced to achieve this objective. Ground vibrations during earthquakes cause inertia forces at locations of mass in the building. These forces travel through the roof and walls to the foundation. The main emphasis is on ensuring that these forces reach the ground without causing major damage or collapse. Of the three components of a masonry building (roof, wall and foundation), the walls are most vulnerable to damage caused by horizontal forces due to earthquake. A wall topples down easily if pushed horizontally at the top in a direction perpendicular to its plane (termed weak direction), but offers much greater resistance if pushed along its length (termed strong direction). 18. The ground shakes simultaneously in the vertical and two horizontal directions during earthquakes. However, the horizontal vibrations are the most damaging to normal masonry buildings. Horizontal inertia force developed at the roof transfers to the walls acting either in the weak or in the strong direction. If all the walls are not tied together like a box, the walls loaded in their weak direction tend to topple. To ensure good seismic performance, all walls must be joined properly to the adjacent walls. In this way, walls loaded in their weak direction can take advantage of the good lateral resistance offered by walls loaded in their strong direction. Further, walls also need to be tied to the roof and foundation to preserve their overall integrity. Masonry walls are slender because of their small thickness compared to their height and length. A simple way of making these walls behave well during earthquake shaking is by making them act together as a box along with the roof at the top and with the foundation at the bottom. A number of construction aspects are required to ensure this box
- 20. 11 action. Firstly, connectionsbetween the walls should be good. This can be achieved by ensuring good interlocking of the masonry courses at the junctions and employing horizontal bands at various levels, particularly at the lintel level. Secondly, the sizes of door and window openings need to be kept small. The smaller the openings, the larger is the resistance offered by the wall. Thirdly, the tendency of a wall to topple when pushed in the weak direction can be reduced by limiting its length-to-thickness and height- to-thickness ratios. Design codes specify limits for these ratios. A wall that is too tall or too long in comparison to its thickness, is particularly vulnerable to shaking in its weak direction. 19. Earthquake performance of a masonry wall is very sensitive to the properties of its constituents, namely masonry units and mortar. The properties of these materials vary across India due to variation in raw materials and construction methods. A variety of masonry units are used in the country, e.g., clay bricks (burnt and unburnt), concrete blocks (solid and hollow), stone blocks. Burnt clay bricks are most commonly used. These bricks are inherently porous, and so they absorb water. Excessive porosity is detrimental to good masonry behaviour because the bricks suck away water from the adjoining mortar, which results in poor bond between brick and mortar, and in difficulty in positioning masonry units. For this reason, bricks with low porosity are to be used, and they must be soaked in water before use to minimize the amount of water drawn away from the mortar. Various mortars are used, e.g., mud, cement-sand, or cement-sand-lime. Of these, mud mortar is the weakest; it crushes easily when dry, flows outward and has very low earthquake resistance. Cement-sand mortar with lime is the most suitable. This mortar mix provides excellent workability for laying bricks, stretches without crumbling at low earthquake shaking, and bonds well with bricks. The earthquake response of masonry walls depends on the relative strengths of brick and mortar. Bricks must be stronger than mortar. Excessive thickness of mortar is not desirable. A 10mm thick mortar layer is generally satisfactory from practical and aesthetic considerations. Indian Standards prescribe the preferred types and grades of bricks and mortars to be used in buildings in each seismic zone. 20. Indian Standards suggest a number of earthquake-resistant measures to develop good box-type action in masonry buildings and improve their seismic performance. For instance, it is suggested that a building having horizontal projections when seen from the top, e.g., like a building with plan shapes L, T, E and Y, be separated into (almost) simple rectangular blocks in plan, each of which has simple and good earthquake behaviour. During earthquakes, separated blocks can oscillate independently and even hammer each other if they are too close. Thus, adequate gap is necessary between these different blocks of the building. The Indian
- 21. 12 Standards suggest minimumseismic separations between blocks of buildings. However, it may not be necessary to provide such separations between blocks, if horizontal projections in buildings are small, say upto ~15-20% of the length of building in that direction. Inclined staircase slabs in masonry buildings offer another concern. An integrally connected staircase slab acts like a cross-brace between floors and transfers large horizontal forces at the roof and lower levels. These are areas of potential damage in masonry buildings, if not accounted for in staircase design and construction. To overcome this, sometimes, staircases are completely separated and built on a separate reinforced concrete structure. Adequate gap is provided between the staircase tower and the masonry building to ensure that they do not pound each other during strong earthquake shaking. Requirement of Horizontal Bands in Masonry Structures 21. Horizontal bands are the most important earthquake-resistant feature in masonry buildings. The bands are provided to hold a masonry building as a single unit by tying all the walls together and are similar to a closed belt provided around cardboard boxes. There are four types of bands in a typical masonry building, namely gable band, roof band, lintel band and plinth band, named after their location in the building. The lintel band is the most important of all and needs to be provided in almost all buildings. The gable band is employed only in buildings with pitched or sloped roofs. In buildings with flat reinforced concrete or reinforced brick roofs, the roof band is not required, because the roof slab also plays the role of a band. However, in buildings with flat timber or CGI sheet roof, roof band needs to be provided. In buildings with pitched or sloped roof, the roof band is very important. Plinth bands are primarily used when there is concern about uneven settlement of foundation soil. The lintel band ties the walls together and creates a support for walls loaded along weak direction from walls loaded in strong direction. This band also reduces the unsupported height of the walls and thereby improves their stability in the weak direction. During the 1993 Latur earthquake (Central India), the intensity of shaking in Killari village was IX on MSK scale. Most masonry houses sustained partial or complete collapse. On the other hand, there was one masonry building in the village, which had a lintel band and it sustained the shaking very well with hardly any damage. During earthquake shaking, the lintel band undergoes bending and pulling actions. To resist these actions, the construction of lintel band requires special attention. Bands can be made of wood (including bamboo splits) or of reinforced concrete (RC); the RC bands are the best. The straight lengths of the band must be properly connected at the wall corners. This will allow the band to support walls loaded in their weak direction by walls loaded in their strong direction. Small lengths of wood spacers (in wooden bands) or steel links (in RC bands) are used to make the straight lengths of wood runners or steel bars act together. In wooden bands,
- 22. 13 A building withlintel band in Killari village: no damage Building with no lintel band : Collapse of roof walls Killari village proper nailing of straight lengths with spacers is important. Likewise, in RC bands, adequate anchoring of steel links with steel bars is necessary. The Indian Standards IS:4326-1993 and IS:13828(1993) provide sizes and details of the bands. When wooden bands are used, the cross-section of runners is to be at least 75mm×38mm and of spacers at least50mm×30mm. When RC bands are used, the minimum thickness is 75mm, and at least two bars of 8mmdiameter are required, tied across with steel links of at least 6mm diameter at a spacing of 150 mm centers. ‘ 1993 Latur Earthquake Vertical Reinforcements in Masonry Structures 22. Horizontal bands are provided in masonry buildings to improve their earthquake performance. These bands include plinth band, lintel band and roof band. Even if horizontal bands are provided, masonry buildings are weakened by the openings in their walls. During earthquake shaking, the masonry walls get grouped into three sub-units, namely spandrel masonry (above lintel band), wall pier masonry (between lintel and sill bands) and sill masonry (below sill band). These masonry sub-units rock back and forth, developing contact only at the opposite diagonals. The rocking of a masonry pier can crush the masonry at the corners. Rocking is possible when masonry piers are slender, and when weight of the structure above is small. Otherwise, the piers are more likely to develop diagonal (X-type) shear cracking; this is the most common failure type in masonry buildings. In un-reinforced masonry buildings, the cross-section area of the masonry wall reduces at the opening. During strong earthquake shaking, the building may slide just under the roof, below the lintel band or at the sill level. Sometimes, the building may also slide at the plinth level. The exact location of sliding depends on numerous factors including building weight, the earthquake-induced inertia force, the area of openings, and type of doorframes used.
- 23. 14 23. Embedding verticalreinforcement bars in the edges of the wall piers and anchoring them in the foundation at the bottom and in the roof band at the top, forces the slender masonry piers to undergo bending instead of rocking. In wider wall piers, the vertical bars enhance their capability to resist horizontal earthquake forces and delay the X-cracking. Adequate cross-sectional area of these vertical bars prevents the bar from yielding in tension. Further, the vertical bars also help protect the wall from sliding as well as from collapsing in the weak direction. Sliding failure mentioned above is rare, even in unconfined masonry buildings. However, the most common damage, observed after an earthquake, is diagonal X-cracking of wall piers, and also inclined cracks at the corners of door and window openings. When a wall with an opening deforms during earthquake shaking, the shape of the opening distorts and becomes more like a rhombus - two opposite corners move away and the other two come closer. Under this type of deformation, the corners that come closer develop cracks. The cracks are bigger when the opening sizes are larger. Steel bars provided in the wall masonry all around the openings restrict these cracks at the corners. In summary, lintel and sill bands above and below openings, and vertical reinforcement adjacent to vertical edges, provide protection against this type of damage. Behaviour of Reinforced Concrete Structures During Earthquakes 24. In recent times, reinforced concrete buildings have become common in India, particularly in towns and cities. Reinforced concrete (or simply RC) consists of two primary materials, namely concrete with reinforcing steel bars. Concrete is made of sand, crushed stone (called aggregates) and cement, all mixed with pre-determined amount of water. Concrete can be molded into any desired shape, and steel bars can be bent into many shapes. Thus, structures of complex shapes are possible with RC. A typical RC building is made of horizontal members (beams and slabs) and vertical members (columns and walls) and supported by foundations that rest on ground. The system comprising of RC columns and connecting beams is called a RC Frame. The RC frame participates in resisting the earthquake forces. Earthquake shaking generates inertia forces in the building, which are proportional to the building mass. Since most of the building mass is present at floor levels, earthquake- induced inertia forces primarily develop at the floor levels. These forces travel downwards - through slab and beams to columns and walls, and then to the foundations from where they are dispersed to the ground. As inertia forces accumulate downwards from the top of the building, the columns and walls at lower storeys experience higher earthquake-induced forces and are therefore designed to be stronger than those in storeys above.
- 24. 15 Total horizontal earthquakeforce in a building increases downwards along its height 25. Floor slabs are horizontal plate-like elements, which facilitate functional use of buildings. Usually, beams and slabs at one storey level are cast together. In residential multi-storey buildings, thickness of slabs is only about 110- 150mm. When beams bend in the vertical direction during earthquakes, these thin slabs bend along with them. And, when beams move with columns in the horizontal direction, the slab usually forces the beams to move together with it. In most buildings, the geometric distortion of the slab is negligible in the horizontal plane; this behaviour is known as the rigid diaphragm action. Structural engineers must consider this during design. After columns and floors in a RC building are cast and the concrete hardens, vertical spaces between columns and floors are usually filled-in with masonry walls to demarcate a floor area into functional spaces (rooms). Normally, these masonry walls, also called infill walls, are not connected to surrounding RC columns and beams. When columns receive horizontal forces at floor levels, they try to move in the horizontal direction, but masonry walls tend to resist this movement. Due to their heavy weight and thickness, these walls attract rather large horizontal forces. However, since masonry is a brittle material, these walls develop cracks once their ability to carry horizontal load is exceeded. Thus, infill walls act like sacrificial fuses in buildings; they develop cracks under severe ground shaking but help share the load of the beams and columns until cracking. Earthquake performance of infill walls is enhanced by mortars of good strength, making proper masonry courses, and proper packing of gaps between RC frame and masonry infill walls. However, an infill wall that is unduly tall or long in comparison to its thickness can fall out-of-plane (i.e., along its thin direction), which can be life threatening. Also, placing infills irregularly in the building causes ill effects like short-column effect and torsion.
- 25. 16 26. Gravity loading(due to self-weight and contents) on buildings causes RC frames to bend resulting in stretching and shortening at various locations. Tension is generated at surfaces that stretch and compression at those that shorten (Figure 4b). Under gravity loads, tension in the beams is at the bottom surface of the beam in the central location and is at the top surface at the ends. On the other hand, earthquake loading causes tension on beam and column faces at locations different from those under gravity loading; the relative levels of this tension (in technical terms, bending moment) generated in members are shown in Figure 4d. The level of bending moment due to earthquake loading depends on severity of shaking and can exceed that due to gravity loading. Thus, under strong earthquake shaking, the beam ends can develop tension on either of the top and bottom faces. Since concrete cannot carry this tension, steel bars are required on both faces of beams to resist reversals of bending moment. Similarly, steel bars are required on all faces of columns too. For a building to remain safe during earthquake shaking, columns (which receive forces from beams) should be stronger than beams, and foundations(which receive forces from columns) should be stronger than columns. Further, connections between beams & columns and columns & foundations should not fail so that beams can safely transfer forces to columns and columns to foundations. When this strategy is adopted in design, damage is likely to occur first in beams. When beams are detailed properly to have large ductility, the building as a whole can deform by large amounts despite progressive damage caused due to consequent yielding of beams. In contrast, if columns are made weaker, they suffer severe local damage, at the top and bottom of a particular storey. This localized damage can lead to collapse of a building, although columns at storeys above remain almost undamaged. The Bureau of Indian Standards, New Delhi, published the following Indian standards pertaining to design of RC frame buildings: (a) Indian Seismic Code (IS 1893 (Part 1), 2002) – for calculating earthquake forces. (b) Indian Concrete Code (IS 456, 2000) – for design of RC members. (c) Ductile Detailing Code for RC Structures (IS 13920, 1993) – for detailing requirements in seismic regions. 27. Design Strategy for Beams. In RC buildings, the vertical and horizontal members (i.e., the columns and beams) are built integrally with each other. Thus, under the action of loads, they act together as a frame transferring forces from one to another. Beams in RC buildings have two sets of steel reinforcement, namely, long straight bars (called longitudinal bars) placed along its length and closed loops of small diameter steel bars (called stirrups) placed
- 26. 17 vertically at regularintervals along its full length. Beams sustain two basic types of failures: (a) Flexural (or Bending) Failure: As the beam sags under increased loading, it can fail in two possible ways. If relatively more steel is present on the tension face, concrete crushes in compression; this is a brittle failure and is therefore undesirable. If relatively less steel is present on the tension face, the steel yields first (it keeps elongating but does not snap, as steel has ability to stretch large amounts before it snaps; and redistribution occurs in the beam until eventually the concrete crushes in compression; this is a ductile failure and hence is desirable. Thus, more steel on tension face is not necessarily desirable. The ductile failure is characterized with many vertical cracks starting from the stretched beam face and going towards its mid-depth. (b) Shear Failure: A beam may also fail due to shearing action. A shear crack is inclined at 45° to the horizontal; it develops at mid-depth near the support and grows towards the top and bottom faces (Figure 2b). Closed loop stirrups are provided to avoid such shearing action. Shear damage occurs when the area of these stirrups is insufficient. Shear failure is brittle, and therefore, shear failure must be avoided in the design of RC beams. 28. Designing a beam involves the selection of its material properties (i.e, grades of steel bars and concrete) and shape and size; these are usually selected as a part of an overall design strategy of the whole building. And, the amount and distribution of steel to be provided in the beam must be determined by performing design calculations as per IS:456-2000 and IS13920-1993. 29. Design Strategy for Columns. Columns, the vertical members in RC buildings, contain two types of steel reinforcement, long straight bars (called longitudinal bars) placed vertically along the length and closed loops of smaller diameter steel bars (called transverse ties) placed horizontally at regular intervals along its full length. Columns can sustain two types of damage, namely axial-flexural (or combined compression bending) failure and shear failure. Shear damage is brittle and must be avoided in columns by providing transverse ties at close spacing. Designing a column involves selection of materials to be used (i.e, grades of concrete and steel bars), choosing shape and size of the cross-section, and calculating amount and distribution of steel reinforcement. The first two aspects are part of the overall design strategy of the whole building. The Indian Ductile Detailing Code IS:13920-1993 requires columns to be at least 300mm wide. A column width of up to 200mm is allowed if unsupported length is less than 4m and beam length is less than 5m. Columns that are required to resist earthquake forces must be designed to prevent shear
- 27. 18 Shear Failure ofColumn failure by a skillful selection of reinforcement. Closely spaced horizontal closed ties help in three ways: (a) They carry the horizontal shear forces induced by earthquakes, and thereby resist diagonal shear cracks. (b) They hold together the vertical bars and prevent them from excessively bending outwards (in technical terms, this bending phenomenon is called buckling). (c) They contain the concrete in the column within the closed loops. The ends of the ties must be bent as 135° hooks. Such hook ends prevent opening of loops and consequently bulging of concrete and buckling of vertical bars. 30. The Indian Standard IS13920-1993 prescribes following details for earthquake-resistant columns: (a) Closely spaced ties must be provided at the two ends of the column over a length not less than larger dimension of the column, one-sixth the column height or 450mm. (b) Over the distance specified in item (a) above and below a beam- column junction, the vertical spacing of ties in columns should not exceed D/4 for where D is the smallest dimension of the column (e.g., in a rectangular column, D is the length of the small side). This spacing need not be less than 75mm nor more than 100mm. At other locations, ties are spaced as per calculations but not more than D/2. (c) The length of tie beyond the 135° bends must be at least 10 times diameter of steel bar used to make the closed tie; this extension beyond the bend should not be less than 75mm.
- 28. 19 31. Construction drawingswith clear details of closed ties are helpful in the effective implementation at construction site. In columns where the spacing between the corner bars exceeds 300mm, the Indian Standard prescribes additional links with 180° hook ends for ties to be effective in holding the concrete in its place and to prevent the buckling of vertical bars. These links need to go around both vertical bars and horizontal closed ties; special care is required to implement this properly at site. In the construction of RC buildings, due to the limitations in available length of bars and due to constraints in construction, there are numerous occasions when column bars have to be joined. A simple way of achieving this is by overlapping the two bars over at least a minimum specified length, called lap length. The lap length depends on types of reinforcement and concrete. For ordinary situations, it is about 50 times bar diameter. Further, IS:13920-1993 prescribes that the lap length be provided only in the middle half of column and not near its top or bottom ends. Also, only half the vertical bars in the column are to be lapped at a time in any storey. Further, when laps are provided, ties must be provided along the length of the lap at a spacing not more than 150mm. 32. Strategy for Beam and Columns Joints. In RC buildings, portions of columns that are common to beams at their intersections are called beam- column joints. Since their constituent materials have limited strengths, the joints have limited force carrying capacity. When forces larger than these are applied during earthquakes, joints are severely damaged. Repairing damaged joints is difficult, and so damage must be avoided. Thus, beam-column joints must be designed to resist earthquake effects. Under earthquake shaking, the beams adjoining a joint are subjected to moments in the same (clockwise or counter- clockwise) direction. Under these moments, the top bars in the beam-column joint are pulled in one direction and the bottom ones in the opposite direction. These forces are balanced by bond stress developed between concrete and steel in the joint region. If the column is not wide enough or if the strength of concrete in the joint is low, there is insufficient grip of concrete on the steel bars. In such circumstances, the bar slips inside the joint region, and beams loose their capacity to carry load. Further, under the action of the above pull- push forces at top and bottom ends, joints undergo geometric distortion; one diagonal length of the joint elongates and the other compresses. If the column cross-sectional size is insufficient, the concrete in the joint develops diagonal cracks. 33. Diagonal cracking and crushing of concrete in joint region should be prevented to ensure good earthquake performance of RC frame buildings. Using large column sizes is the most effective way of achieving this. In addition, closely spaced closed-loop steel ties are required around column bars to hold together concrete in joint region and to resist shear forces. Intermediate column
- 29. 20 Shear Failure ofColumn-Beam Joint bars also are effective in confining the joint concrete and resisting horizontal shear forces. Providing closed-loop ties in the joint requires some extra effort. Indian Standard IS:13920-1993 recommends continuing the transverse loops around the column bars through the joint region. In practice, this is achieved by preparing the cage of the reinforcement (both longitudinal bars and stirrups) of all beams at a floor level to be prepared on top of the beam formwork of that level and lowered into the cage. However, this may not always be possible particularly when the beams are long and the entire reinforcement cage becomes heavy. The gripping of beam bars in the joint region is improved first by using columns of reasonably large cross-sectional size. The Indian Standard IS:13920-1993 requires building columns in seismic zones III, IV and V to be at least300mm wide in each direction of the cross-section when they support beams that are longer than 5m or when these columns are taller than 4m between floors (or beams). The American Concrete Institute recommends a column width of at least 20 times the diameter of largest longitudinal bar used in adjoining beam. In exterior joints where beams terminate at columns, longitudinal beam bars need to be anchored into the column to ensure proper gripping of bar in joint. The length of anchorage for a bar of grade Fe415 (characteristic tensile strength of 415MPa) is about 50times its diameter. This length is measured from the face of the column to the end of the bar anchored in the column. In columns of small widths and when beam bars are of large diameter a portion of beam top bar is embedded in the column that is cast up to the soffit of the beam, and a part of it overhangs. It is difficult to hold such an overhanging beam top bar in position while casting the column up to the soffit of the beam. Moreover, the vertical distance beyond the 90ºbend in beam bars is not very effective in providing anchorage. On the other hand, if column width is large, beam bars may not extend below soffit of the beam. Thus, it is preferable to have columns with sufficient width. Such an approach is used in many codes [e.g., ACI318, 2005]. In interior joints, the beam bars (both top and bottom) need to go through the joint without any cut in the joint region. Also, these bars must be placed within the column bars and with no bends.
- 30. 21 Ground Floor StiltFailure of Framed Structures 34. Reinforced concrete (RC) frame buildings are becoming increasingly common in urban India. Many such buildings constructed in recent times have a special feature – the ground storey is left open for the purpose of parking, i.e., columns in the ground storey do not have any partition walls (of either masonry or RC) between them. Such buildings are often called open ground storey buildings or buildings on stilts. 35. An open ground storey building, having only columns in the ground storey and both partition walls and columns in the upper storeys, have two distinct characteristics, namely: (a) It is relatively flexible in the ground storey, i.e., the relative horizontal displacement it undergoes in the ground storey is much larger than what each of the storeys above it does. This flexible ground storey is also called soft storey. (b) It is relatively weak in ground storey, i.e., the total horizontal earthquake forces it can carry in the ground storey is significantly smaller than what each of the storeys above it can carry. Thus, the open ground storey may also be a weak storey.
- 31. 22 36. Often, openground storey buildings are called soft storey buildings, even though their ground storey may be soft and weak. Generally, the soft or weak storey usually exists at the ground storey level, but it could be at any other storey level too. Open ground storey buildings have consistently shown poor performance during past earthquakes across the world (for example during 1999 Turkey, 1999 Taiwan and 2003 Algeria earthquakes); a significant number of them have collapsed. A large number of buildings with open ground storey have been built in India in recent years. For instance, the city of Ahmedabad alone has about 25,000 five-storey buildings and about 1,500 eleven-storey buildings; majority of them have open ground storeys. Further, a huge number of similarly designed and constructed buildings exist in the various towns and cities situated in moderate to severe seismic zones (namely III, IV and V) of the country. The collapse of more than a hundred RC frame buildings with open ground storeys at Ahmedabad (~225km away from epicenter) during the 2001 Bhuj earthquake has emphasised that such buildings are extremely vulnerable under earthquake shaking. The presence of walls in upper storeys makes them much stiffer than the open ground storey. Thus, the upper storeys move almost together as a single block, and most of the horizontal displacement of the building occurs in the soft ground storey itself. In common language, this type of buildings can be explained as a building on chopsticks. Thus, such buildings swing back-and- forth like inverted pendulums during earthquake shaking, and the columns in the open ground storey are severely stressed. If the columns are weak (do not have the required strength to resist these high stresses) or if they do not have adequate ductility, they may be severely damaged which may even lead to collapse of the building. 37. Open ground storey buildings are inherently poor systems with sudden drop in stiffness and strength in the ground storey. In the current practice, stiff masonry walls are neglected and only bare frames are considered in design calculations. Thus, the inverted pendulum effect is not captured in design. After the collapses of RC buildings in 2001 Bhuj earthquake, the Indian Seismic Code IS:1893 (Part 1) - 2002 has included special design provisions related to soft storey buildings. Firstly, it specifies when a building should be considered as a soft and a weak storey building. Secondly, it specifies higher design forces for the soft storey as compared to the rest of the structure. The Code suggests that the forces in the columns, beams and shear walls (if any) under the action of seismic loads specified in the code, may be obtained by considering the bare frame building (without any infills). However, beams and columns in the open ground storey are required to be designed for 2.5 times the forces obtained from this bare frame analysis. For all new RC frame buildings, the best option is to avoid such sudden and large decrease in stiffness and/or strength in any storey; it would be ideal to build walls (either masonry or RC walls) in the ground storey also. Designers can avoid dangerous effects of flexible and weak ground
- 32. 23 Consequences of openground storeys in RC frame buildings 2001 Bhuj Earthquake – one with Stilt Collapsed storeys by ensuring that too many walls are not discontinued in the ground storey, i.e., the drop in stiffness and strength in the ground storey level is not abrupt due to the absence of infill walls. The existing open ground storey buildings need to be strengthened suitably so as to prevent them from collapsing during strong earthquake shaking. The owners should seek the services of qualified structural engineers who are able to suggest appropriate solutions to increase seismic safety of these buildings. Short Column Effect 38. During past earthquakes, reinforced concrete (RC) frame buildings that have columns of different heights within one storey, suffered more damage in the shorter columns as compared to taller columns in the same storey. Two examples of buildings with short columns are buildings on a sloping ground and buildings with a mezzanine floor. Poor behaviour of short columns is due to the fact that in an earthquake, a tall column and a short column of same cross- section move horizontally by same amount. However, the short column is stiffer as compared to the tall column, and it attracts larger earthquake force. Stiffness of a column means resistance to deformation – the larger is the stiffness, larger is the force required to deform it. If a short column is not adequately designed for such a large force, it can suffer significant damage during an earthquake. This behaviour is called Short Column Effect. The damage in these short columns is often in the form of X-shaped cracking – this type of damage of columns is due to shear failure.
- 33. 24 39. Many situationswith short column effect arise in buildings. When a building is rested on sloped ground, during earthquake shaking all columns move horizontally by the same amount along with the floor slab at a particular level (this is called rigid floor diaphragm action). If short and tall columns exist within the same storey level, then the short columns attract several times larger earthquake force and suffer more damage as compared to taller ones. The short column effect also occurs in columns that support mezzanine floors or loft slabs that are added in between two regular floors. There is another special situation in buildings when short-column effect occurs. Consider a wall (masonry or RC) of partial height built to fit a window over the remaining height. The adjacent columns behave as short columns due to presence of these walls. In many cases, other columns in the same storey are of regular height, as there are no walls adjoining them. When the floor slab moves horizontally during an earthquake, the upper ends of these columns undergo the same displacement. However, the stiff walls restrict horizontal movement of the lower portion of a short column, and it deforms by the full amount over the short height adjacent to the window opening. On the other hand, regular columns deform over the full height. Since the effective height over which a short column can freely bend is small, it offers more resistance to horizontal motion and thereby attracts a larger force as compared to the regular column. As a result, short column sustains more damage. 40. In new buildings, short column effect should be avoided to the extent possible during architectural design stage itself. When it is not possible to avoid short columns, this effect must be addressed in structural design. The Indian Standard IS:13920-1993 for ductile detailing of RC structures requires special confining reinforcement to be provided over the full height of columns that are likely to sustain short column effect. The special confining reinforcement (i.e. closely spaced closed ties) must extend beyond the short column into the columns vertically above and below by a certain distance. In existing buildings with short columns, different retrofit solutions can be employed to avoid damage in future earthquakes. Where walls of partial height are present, the simplest solution is to close the openings by building a wall of full height – this will eliminate the short column effect. If that is not possible, short columns need to be strengthened using one of the well-established retrofit techniques. The retrofit solution should be designed by a qualified structural engineer with requisite background.
- 34. 25 Effective height ofcolumn over which it can bend is restricted by adjacent walls. Short-Column effect is most severe when opening height is small. Buildings with Shear Walls in Seismic Regions 41. Reinforced concrete (RC) buildings often have vertical plate-like RC walls called Shear Walls in addition to slabs, beams and columns. These walls generally start at foundation level and are continuous throughout the building height. Their thickness can be as low as 150mm, or as high as 400mm in high rise buildings. Shear walls are usually provided along both length and width of buildings. Shear walls are like vertically-oriented wide beams that carry earthquake loads downwards to the foundation. Properly designed and detailed buildings with shear walls have shown very good performance in past earthquakes. The overwhelming success of buildings with shear walls in resisting strong earthquakes is summarised in the quote by noted consulting engineer Mark Fintel “We cannot afford to build concrete buildings meant to resist severe earthquakes without shear walls.”
- 35. 26 42. Shear wallsin high seismic regions require special detailing. However, in past earthquakes, even buildings with sufficient amount of walls that were not specially detailed for seismic performance (but had enough well-distributed reinforcement) were saved from collapse. Shear wall buildings are a popular choice in many earthquake prone countries, like Chile, New Zealand and USA. Shear walls are easy to construct, because reinforcement detailing of walls is relatively straight-forward and therefore easily implemented at site. Shear walls are efficient, both in terms of construction cost and effectiveness in minimizing earthquake damage in structural and non-structural elements (like glass windows and building contents). Most RC buildings with shear walls also have columns; these columns primarily carry gravity loads (i.e., those due to self-weight and contents of building). Shear walls provide large strength and stiffness to buildings in the direction of their orientation, which significantly reduces lateral sway of the building and thereby reduces damage to structure and its contents. Since shear walls carry large horizontal earthquake forces, the overturning effects on them are large. Thus, design of their foundations requires special attention. Shear walls should be provided along preferably both length and width. However, if they are provided along only one direction, a proper grid of beams and columns in the vertical plane (called a moment- resistant frame) must be provided along the other direction to resist strong earthquake effects. Door or window openings can be provided in shear walls, but their size must be small to ensure least interruption to force flow through walls. Moreover, openings should be symmetrically located. Special design checks are required to ensure that the net cross-sectional area of a wall at an opening is sufficient to carry the horizontal earthquake force. Shear walls in buildings must be symmetrically located in plan to reduce ill-effects of twist in buildings. They could be placed symmetrically along one or both directions in plan. Shear walls are more effective when located along exterior perimeter of the building – such a layout increases resistance of the building to twisting. 43. Just like reinforced concrete (RC) beams and columns, RC shear walls also perform much better if designed to be ductile. Overall geometric proportions of the wall, types and amount of reinforcement, and connection with remaining elements in the building help in improving the ductility of walls. The Indian Standard Ductile Detailing Code for RC members (IS:13920-1993) provides special design guidelines for ductile detailing of shear walls. Shear walls are oblong in cross-section, i.e., one dimension of the cross-section is much larger than the other. While rectangular cross-section is common, L- and U-shaped sections are also used. Thin-walled hollow RC shafts around the elevator core of buildings also act as shear walls and should be taken advantage of to resist earthquake forces. Steel reinforcing bars are to be provided in walls in regularly spaced vertical and horizontal grids. The vertical
- 36. 27 and horizontal reinforcementin the wall can be placed in one or two parallel layers called curtains. Horizontal reinforcement needs to be anchored at the ends of walls. The minimum area of reinforcing steel to be provided is 0.0025 times the cross-sectional area, along each of the horizontal and vertical directions. This vertical reinforcement should be distributed uniformly across the wall cross-section. Under the large overturning effects caused by horizontal earthquake forces, edges of shear walls experience high compressive and tensile stresses. To ensure that shear walls behave in a ductile way, concrete in the wall end regions must be reinforced in a special manner to sustain these load reversals without loosing strength. End regions of a wall with increased confinement are called boundary elements. This special confining transverse reinforcement in boundary elements is similar to that provided in columns of RC frames. Sometimes, the thickness of the shear wall in these boundary elements is also increased. RC walls with boundary elements have substantially higher bending strength and horizontal shear force carrying capacity and are therefore less susceptible to earthquake damage than walls without boundary elements. Protecting Non-Structural Elements of a Building During Earthquakes 44. Structural Elements (SEs) in a building have a primary role of resisting the effects of earthquakes ground shaking, and of protecting life and property of building occupants. But, buildings contain many other items, such as contents, appendages and services & utilities, which are attached to and/or supported by SEs and affected by earthquake ground shaking; these items are called Non- Structural Elements (NSEs). 45. NSEs can be classified into three groups, namely: (a) Contents of buildings. Items required for functionally enabling the use of spaces, such as (i) furniture and other items, e.g., storage shelves, (ii) facilities and equipment, e.g., refrigerators, multi-level material stacks, false ceilings, and (iii) door and window panels and frames, or ply board or aluminum partitions. (b) Appendages to buildings. Items projecting out from buildings, either horizontally or vertically, such as chimneys, exterior glass or stone cladding (pasted on the building surface as façades), parapets, small water tanks rested on top of buildings, sunshades, advertisement hoardings and communication antennas atop buildings.
- 37. 28 Chemical spill inLab Library Book shelves Water Pipelines Electrical Control Panels (c) Services and utilities. Items required for facilitating essential activities in the buildings, such as water mains, electricity cables, air- conditioning ducts, rainwater drain pipes, and elevators. 46. During strong earthquake shaking, NSEs can slide or topple, or move or swing, if they are not secured well to SEs of the building. These actions can cause loss to functionality of NSEs and potential secondary disasters, e.g., spill of chemicals leading to laboratory fires. Loss of functionality of an NSE can be small or substantive depending on its importance, the function it serves, and its cost. For instance, if book shelves of a library are not properly secured, they can distort or topple; the former may only dislodge books, but the latter can cause threat to life. If gas pipelines are pulled apart or electric control panels are toppled, then both direct and indirect losses can be significant. With increasing sophistication in building systems, seismic performance of NSEs is becoming more important. In many earthquakes, economic losses due to damages in NSEs have been very substantial.
- 38. 29 47. NSEs candemonstrate either acceleration-sensitive or displacement- sensitive behaviour during earthquakes. (a) Acceleration-sensitive. NSEs may topple or slide, if not anchored adequately to SEs (e.g., a diesel generator unit on a floor, and expensive contents of museums). Thus, the SEs and the anchors by which NSEs are secured to SEs should be designed to resist the induced forces corresponding to the accelerations developed in these NSEs. (b) Displacement-sensitive. NSEs may bend, compress or stretch by large amounts during earthquake shaking (e.g., glass facades, water and gas pipes running between floors of a building, and electric lines running from a street pole to a building). Also, NSEs are significantly affected by the flexibility of SEs and their deformations. The connection of NSEs with SEs should be designed to accommodate relative displacements generated between support points on SEs with adequate slack. 48. Some NSEs are both displacement and acceleration sensitive, and they have to be designed for both forces and relative displacements. For example, false ceilings suspended from floor slabs above, may not only pull out vertically from slabs, but also swing laterally and knock on walls. Three strategies are adopted for design of NSEs in a building and their connections with SEs, namely: (a) Non-Engineered Strategy. Generic NSEs (e.g., glass bottles on shelves, and crockery) cannot be individually secured, but can be protected with simple strategies (e.g., hold-back strings). (b) Prescriptive Strategy. Factory-made, reasonably large NSEs (e.g., cupboards, refrigerators, laboratory equipment and large panel glass windows) often have manufacturer prescribed protection or anchorage details provided at the time of purchase. (c)Engineered Design Strategy. Large, specialised, massive NSEs (e.g., cooling plant of central air-conditioning systems, billboards) and those whose failure can be critical (e.g., fire hydrant pipes running along building height) require formal design calculations for protecting them.
- 39. 30 49. NSEs locatedin upper levels of buildings and their connections to SEs must be designed for shaking expected at those floor levels; this floor shaking can be different and even of higher intensity than the shaking at the ground level. Hence, NSEs that project vertically or horizontally from buildings at the upper elevations needs special attention. Some countries (e.g., USA) have provisions for engineered design of NSEs and their connections with SEs. Foundations of Earthquake-Resistant Buildings 50. The site of a building should be free from any collateral damage due to earthquake-related effects. Ideal sites are: (a) Away from a potential fault rupture zone. (b) Above the level of inundation under tsunami waves generated in the adjoining ocean by earthquakes. (c) Beyond the forest or wooded areas with potential fire hazard arising from. (d) Free from detrimental earthquake actions in the ground, like liquefaction, settlement and lateral spreading. 51. Even if the site is devoid of the above, steep slopes or vertical cuts in natural hills (otherwise safe under other loads acting on them) can slide during earthquakes. Vulnerable soil embankments can slide or spread laterally due to liquefaction. Other earthquake hazards at hill slope sites include rolling stones and debris. When the ground shakes underneath buildings with elongated plan or long span structures (e.g., suspension bridges), the motion at different supports may not be synchronous. Differential shaking of such structures at their supports induces additional effects and should be accounted for in their design. 52. Even if local soil stratum underneath a proposed structure is stable, ground shaking may be modified when earthquake waves propagate through the soil overlying rock layers; this phenomenon is referred to as Site Effect. Even when shaking at the base rock is moderate, the motion at a site may be amplified by soil above rock, and this needs to be accounted for in design. Site effect was noticed first in the 1819 Kutch earthquake in India. It was very prominent in 1985 earthquake that affected Mexico City; ground response was amplified by up to 7-8 times at building sites located on lake bed (which was akin to a bowl of jelly) in contrast to those located on hard rock in Mexico City. Peak Ground Acceleration (PGA) is a measure of severity of shaking of ground. During the 1985 earthquake, PGA at soft soil site (SCT) was significantly larger
- 40. 31 than at rockysite (UNAM). Amplification of ground motion depends on soil properties (e.g., shear modulus, damping, soil layers and their properties, saturated versus dry soil, and loose versus dense soil), and ground motion characteristics. In general, stiff soils have lower amplification, while soft soils higher. Seismic design codes provide design spectra for underlying soil strata of different soil types. 53. No structure can perform well, if it does not have a good foundation supported on strata that is stable during earthquakes. All principles applicable in foundation design of structures subjected to gravity loads, are applicable in foundation design of earthquake-resistant structures also. Concepts of foundation engineering, like Bearing Capacity and Settlement Criteria, are relevant to earthquake-resistant buildings also. Thorough geotechnical investigations at the site are a must for most design projects. In addition to traditional Standard and Cone Penetration Tests, other in-situ tests (e.g., Shear Wave Velocity Test and Pressure-meter Test) may be performed. Depending on geotechnical conditions, structural configuration and loads, a suitable type of foundation must be chosen. If soil type is hard, isolated footings may suffice under individual columns. But, these foundations must be tied to each other with beams at top of footings or within the footing depth to resist relative movement between column bases. On the other hand, if soil underneath is soft, other foundation types may become necessary, e.g., raft or pile foundations. If the site is susceptible to liquefaction, either ground improvement must be undertaken or the foundation must be carefully designed, such that it can carry the load even after the vulnerable soil layers have liquefied. For instance, in case of pile and well foundations, layers susceptible to liquefaction should be neglected in estimating stiffness and strength of the soil system. In case of lateral spreading, investigations beyond the property boundaries (lines) of the building under consideration may become necessary, especially in when plots are small. Also, lateral thrust offered by liquefied soil layers must be included in estimating force demands on foundations. 54. It is difficult to inspect and repair foundations after a severe earthquake. Further, damage to foundation can be detrimental to the stability of the structure. Hence, in seismic design, column damage in columns is preferred over foundation damage during strong shaking. This is achieved by adopting Capacity Design Concept; the foundation system needs to be designed for loads higher than the ultimate flexural capacity of column or of structural walls.
- 41. 32 Conclusion: Quality andEarthquake Safety 55. Quality is critical for ensuring safety of buildings during earthquakes. Appropriate measures are required to control quality in all activities related to development of earthquake-resistant buildings; if not, the weakest link will fail. While quality control is important also for buildings meant to resist effects other than those meant to resist earthquake shaking, there is a difference. Buildings meant to resist only gravity loads are designed to resist loads much higher(say about 2 to 3 times more) than the gravity loads that may arise during lifetime of the building. And hence, no damage occurs in buildings with minor structural deficiencies in individual members, because of availability of adequate margin in design. Thus, some error can be tolerated in design or workmanship without serious consequences or getting noticed. On the other hand, buildings meant to resist earthquake effects are designed for lateral earthquake loads much smaller (up to about 10 times smaller) than what may be experienced during severe shaking, if the building were to sustain no damage during severe earthquake shaking. This is because earthquakes occur rarely. Hence, ordinary buildings are expected to undergo damage during strong shaking. Every structural element is expected to respond in a certain way and is tested to its limit when strong shaking is experienced. Thus, deficiencies in structural elements can result in premature, unwanted or unwarranted failures. Because there is no margin, effects of poor quality are clearly noticed; the negative consequences of poor quality are most visible during severe shaking. Therefore, quality is far more important in buildings exposed to earthquake effects than in those exposed only to other load effects (e.g., gravity loads). 56. Quality control means adopting and ensuring formal procedures and processes that are based on scientific principles and professionally agreed norms. The need to ensure quality arises at every step of the building development process. These steps include: (a) Conceptualizing structural configuration. Architects and Structural Engineers need to work together to adopt a good configuration. (b) Designing the structure. Structural Engineers need to take utmost care while performing required calculations as per sound structural safety concepts and relevant design standards. (c) Preparing structural drawings. Structural Engineers and Draughts men need to comprehensively and accurately present structural design intent in well detailed drawings.
- 42. 33 (d) Selecting constructionmaterials. Contractors need to take utmost care in selecting the intended construction materials and adopting construction procedures as per standard specifications. (e) Converting structural drawings at site. Competent Site Engineers need to faithfully follow structural drawings to ensure that the design intent is actually realized in the building working with Certified Artisans, as per good construction practices laid down in standards and specifications. (f) Undertaking post-construction activities. Maintenance Engineers need to embed long-term maintenance steps (like preventing leaks), thereby avoiding structural damage) in post-construction handling of structures, and preventing damage to buildings (especially to critical structural members). 57. Quality Assurance, rigorous, independent monitoring and correction need to be undertaken by competent third-party professionals or professional agencies (other than those involved in the Quality Control effort) to ensure that the design intent is actually realized in buildings. This is referred to as Quality Assurance and is required in each of the activities mentioned above. Executives and Users have the responsibility of ensuring that their buildings are functional, safe and durable, in addition to being economical and aesthetic. Quality must be ensured by all stakeholders involved in the building delivery process, including architects, structural engineers, draughtsman, contractors, site engineers, artisans (e.g., bar benders, carpenters and masons), and maintenance engineers. Each activity needs to adhere to a pre-specified procedure laid down in design codes and standards. There is no single activity that is more important than the others, which alone determines the quality of the building being built. For instance, just designing the building for a higher seismic lateral force to compensate for poor quality in construction will not ensure a safe building. Even if one of the key stakeholders fails to deliver quality, overall earthquake safety of building may be jeopardised. Users need to seek professional services that comply with: (1) proper understanding and estimation of earthquake hazard at the site, (2) rigorous design, compliance with prevalent standards, specifications and bye-laws, (3) independent design review (peer review), (4) procurement of intended quality materials, (5) careful construction of the building, (6) independent construction audit, and (7) approved occupancy and use of buildings. Any shortfall in understanding or implementing (to the fullest) any of these aspects leads to compromising safety of life and property in the building. Services of competent professional architects and engineers are essential to incorporate the above aspects in buildings; these professionals need to have past experience of having successfully provided such services.
- 43. 34 Executives are facedwith many challenges in earthquake-resistant design and construction. These include: (a) Identifying competent architects and design engineers. There are many standards and specifications for earthquake-resistant design and construction of buildings, which architects and design engineers need to be conversant with. The mandatory curricula in architecture and engineering colleges often do not ensure that the required background is provided to graduates. Thus, it is unlikely that all architects and engineers practicing today understand earthquake behavior of structures, and the design techniques required to incorporate earthquake-resistance in them. So, building owners face a challenge related to selecting competent professionals to undertake earthquake resistant design of their buildings. (b) Complying with Building Codes and Zonal Controls. Local Zonal CEs require architects and design engineers to ensure safety of buildings through faithful compliance with various building codes and municipal bye-laws. This cannot happen only on the basis of voluntary effort by executives – it is the responsibility of zonal authorities to enforce compliance. But, a severe shortage of suitable adequately trained personnel in zonal offices can be a bottleneck for ensuring compliance on part of local executives. Alternate strategies are required to build a robust system for Enforcement of Earthquake Safety, e.g., independent peer review by consulting engineers of good standing. (c) Undertaking Hazard Estimation Studies. Seismic hazard assessment must consider many uncertainties. For ordinary buildings, it is best to adopt seismic design codes of the country. But, for projects of importance, site-specific studies are required, for which executives will require services of competent earthquake geologists, seismologists, earthquake geotechnical engineers and seismic structural engineers. 58. Faithfully converting construction drawings of buildings into actual structures is critical for ensuring earthquake safety of buildings. Competent contractors must be appointed by Executives to implement formal construction strategies and construct earthquake-resistant buildings. Quality control needs to be exercised at all stages of construction by Contractors. But, independent agencies need to test quality of all construction materials before accepting them. Similarly, competent engineers employed for site-supervision need to examine that work being is done as intended. These engineers employed for site inspection need to have requisite competence. Therefore, Competence-Based Licensing of Construction Engineers and Certification of Artisans are essential.
- 44. 35 59. Professional Ethics.Finally, Earthquake-resistant design and construction is possible only with high ethical standards employed by all personnel involved. A project can be successfully executed only by avoiding all three types of errors - Error of Intention, Error of Concept and Error of Execution. Error of intention is really an issue of ethics, while errors of concept and execution are of competence. For instance, a professional accepting an assignment beyond one’s competence is indulging in unethical practice. Similarly, if a professional realizes that one is unable to follow correct procedures and still proceeds with the project, it is an unethical practice. And finally, an engineer not following code provisions to reduce structural cost, indulges in unethical practice. In civil constructions, User’s take performance of a structure for granted. For instance, one drives over abridge unconsciously, assuming it is safe. Hence, it is critically important to ensure and enforce highest levels of ethical standards in the practice of engineering. It is not possible to legislate virtues. But, the situation can be alleviated to some extent by putting in place systems and procedures, e.g., competence-based registration of Contractor’s, wherein license to practice is given only after establishing that the agency has atleast a minimum set of skills required to practice design and construction, which may be revoked incase of a malpractice, and a robust regulatory system, with a rigorous enforcement protocol and implementation mechanism that allows for swift penalties and punishments to erring individuals. Such systems have been effective in many organisations and must be established in organisation like ours.