CE 72.32 (January 2016 Semester) Lecture 4 - Selection of Structural Systems

Dr. Naveed Anwar
Executive Director, AIT Consulting
Affiliated Faculty, Structural Engineering
Director, ACECOMS
Design of Tall Buildings
Hybrid Learning System

Dr. Naveed Anwar
Executive Director, AIT Consulting
Affiliated Faculty, Structural Engineering
Director, ACECOMS
Lecture 4: Selection of Structural
Systems
Design of Tall Buildings

Design of Tall Building: Hybrid Learning System, Dr. Naveed Anwar 3
• In nature, the structures of organisms differ according to their size. For example,
the structure of a large animal such as an elephant is radically different from
that of a dog or a mosquito.
• However, in spite of these obvious differences, until about the middle of the
seventeenth century, scientists believed that it was possible to build larger
structures simply by duplicating the form and proportion of a smaller one.
• The prevailing opinion was that if the ratios between structural elements in the
larger structure were made identical to the ratios in the smaller structure, the
two structures would behave in a similar manner.
Historical Prospective

• In 1638, Galileo was the first scientist to refute this
principle by citing examples from animate and
inanimate structures, thus formulating the idea of an
ultimate size for structures.
• He clearly recognized the effect of self-weight on the
efficiency of structures.
• These principles have since been extended, and
engineers have come to recognize that different
scales require different types of structures.
Historical Prospective
Galileo Galilei (1564 – 1642)

• The principles of efficient tall building structural design, known for some time,
are quite simple:
4 Commandments of Tall Buildings Design
1. Resist overturning forces due to lateral loads by
using vertical elements placed as far apart as
possible from the geometric center of the
building
2. Channel gravity loads to those vertical
elements resisting overturning forces
3. Link these vertical elements together with
shear-resisting structural elements that
experience a minimum of shear lag effects such
that the entire perimeter of the building resists
the overturning moments
4. Resist lateral forces with members axially
loaded in compression rather than those
loaded in tension due to overturning

Building Structural Systems

Design of Tall Building: Hybrid Learning System, Dr. Naveed Anwar
Beams, Columns, Two-way Slabs, Flat Slabs, Pile Caps
Shear Walls, Deep Beams, Isolated Footings, Combined Footings
Sub-structure and Member Design
Frame and Shear Walls
Lateral Load Resisting System Floor Slab System
Gravity Load Resisting System
Floor Diaphragm
The Building Structural System - Physical
7

• Gravity Load Resisting System (GLRS)
– The structural system (beams, slab, girders, columns, etc.) that act primarily
to support the gravity or vertical loads
• Lateral Load Resisting System (LLRS)
– The structural system (columns, shear walls, bracing, etc.) that primarily
acts to resist the lateral loads
• Floor Diaphragm (FD)
– The structural system that transfers lateral loads to the lateral load resisting
system and provides in-plane floor stiffness
The Building Structural System - Conceptual

Systems Based on Material
9
Systems Based on
Material
Concrete
Structural
Systems
Framed Systems
and Slabs
Concrete Walls
and Slabs
Ferrocement
Structures
Steel Structural
Systems
Steel Framing
and Cladding
Steel Trusses
and GI Sheets
Steel Beams and
Concrete Floors
Composite
Materials
Steel and
Concrete
Fiber Glass and
Plastic
Composite
Timber
Structures
Timber Framing
Plywood and
Laminated
Wood Panels
Brick and Stone
Masonry
Systems
Clay Bricks
Concrete Blocks

Systems Based on Method of Construction
10
Systems Based on
Method of
Construction
Cast- In Place, On-
site Construction
Pre-Cast, Pre-Made
Elements
Precast Concrete
Elements
Shop Fabricated Steel
Elements
Pre-Fabricated House
Complete Pre-Made
Structures or Parts
Mixed Construction
Some Pre-Cast, Some
On-Site

Reinforced Concrete Building Elements
11

Reinforced Concrete Building Elements
Reference: J. G. Macgregor. Reinforced Concrete: Mechanics and Design, 3rd ed.

Gravity Load Resisting Systems
The Components Needed to Complete the Load-Transfer Path
for Vertical Loads

• Slabs supported on long rigid supports
– Supported on stiff beams or walls
– One-way and two-way slabs
– Main consideration is flexural reinforcement
• Slab-system supported on small rigid supports
– Supported on columns directly
– Flat slab floor systems
– Main consideration is shear transfer, moment distribution in various parts
and lateral load resistance
• Slabs supported on soil
– Slabs on grade: light, uniformly distributed loads
– Footings, mat, etc. Heavy concentrated loads
Vertical Load Resisting Systems

• Buildings
– Flat slabs, one-way slabs, two-way slabs
– Isolated footings, combined footings, rafts
– Pre-cast slab panels, hollow core slabs
• Bridges and Highways
– Deck slab on girders
– Box girder slabs
– Slab on grade: pavements, approach slabs
• Water Retaining Structures
– Retaining wall systems
– Tank roof and floor slabs
Main Slab Types: Usage

• Plate
– Any member, or part of a member having a thickness which is much less
than its other dimensions
• Slab
– A plate resting on supports, generally horizontal and transferring vertical
loads directly
• Slab System
– A combination of various components, such as, slabs, beams, drop panels,
stiffeners, joists, girders and ribs, all acting together to transfer the loads to
supports
Basic Definition

• Slabs supported on long rigid supports
– Supported on stiff beams or walls
– One-way and two-way slabs
• Slab supported on small rigid supports
– Supported on columns
– Flat slab floor systems
• Slabs supported on soil
– Slabs on grade
– Footings, mat, etc.
Main Slab Types: Behavior

Some Sample Floor Slab Types
18
Slab Only
Hollow Core Slab
Pre-Cast Slab Panels
Beam and Slab
Beam-Slab
Girder-Beam Slab
Joist Slab
Girder-Joist Slab
Pre-Cast Slab-Beam
System
Flat Plate
Flat Plate + Capital
Flat Slab
Flat Slab + Capital
Flat Slab + Drop Panel
Flat Slab + Drop Panel
+ Capital
Waffle Slab
Band Slab
Inverted Band Slab
Metal Deck/ Wood
Deck
Composite Metal Deck
Hollow Block Slab
Composite Girder-Slab
Composite Truss Slab
Alpha Truss System
Wooden Beam, Rafter
Plank System
Sandwich Panels

2D Panel Types
19
2D Panel Types
Solid
Panels
Ribbed
Panels
Hollow
Core
Panels
Sandwich
Panels
Alpha
Truss
Panels
Arched
Panels
Folded
Panels

• Small “beams” or “ribs” are added to
increase bending stiffness and moment
capacity
• Ribs may be one-way or two-way
• Reduces weight for same solid thickness
• Reduces concrete cost
• Reduces minimum steel requirement
• Reduces insulation compared to solid
panel
Ribbed Panels

• Polystyrene, sandwiched between
layers of concrete
• Increases insulation compared to solid
wall
Precast Sandwich Panels

• Hollow cores inside wall or slab section
• Cores made by:
– Extrusion
– PVC/ GI Pipes
– Inflatable Tubes
• Increases insulation compared to solid wall
Hollow Core Slabs and Panels

• Used as permanent formwork for CIP
slabs
• Combines the benefits of precast and
CIP slab and walls
• Designed to carry dead load and some
construction live load
• May contain primary reinforcement
Alpha Truss Panels

Common Types of Floor Systems
(a) Two-way flat plate, (b) Two-way waffle,
(b) (c) Two-way flat slab with drops, (d) One-way beam and slab

Common Types of Floor Systems
(e) Skip joist wide module, (f) Two-way beam and slab,
(g) One-way joist slab, and (h) One-way flat slab

Difference Between Flat Plate and Flat Slab
26
Flat Plate
Flat Slab

Waffle System
Skip Joist System
(a) Building plan and
(b) Section A
One-way Joist System
(a) Building plan and
(b) Section A

Band beam system: (a) floor plan and (b) section

Haunch girder-framing system
Tapered haunch girder
Hammerhead haunch girder

Lateral Load Resisting Systems
The Components Needed to Complete the Load-Transfer Path
for Lateral Loads

• Purpose
– “To transfer lateral loads applied at any location in the structure down to
the foundation level”
• Single System
– Moment Resisting Frames
– Braced Frames
– Shear Walls
– Tubular Systems
• Dual System
– Shear Wall - Frames
– Tube + Frame + Shear Wall
Lateral Load Bearing Systems

• Primary Lateral Loads
– Load generated by wind pressure
– Load generated due to seismic excitation
• Other Lateral Loads
– Load generated due to horizontal component of gravity loads in inclined
systems and in un-symmetrical structures
– Load due to lateral soil pressure, liquid and material retention
Lateral Loads

• Bearing wall system
– Light frames with shear panels
– Load bearing shear walls
• Fully Braced System (FBS)
– Shear Walls (SW)
– Diagonal Bracing (DB)
Sample Lateral Load Resistance Systems

• Moment Resisting Frames (MRF)
– Special Moment-Resisting Frames (SMRF)
– Concrete Intermediate Moment-Resisting Frame (IMRF)
– Ordinary Moment-Resisting Frame (OMRF)
• Dual Systems (DS)
– Shear Walls + Frames (SWF)
– Ordinary Braced Frame (OBF)
– Special Braced Frame (SBF)

Moment Resisting Frame
35
• The load is transferred by shear in
columns that produces moment in
columns and in beams
• The Beam-Column connection is
crucial for the system to work
• The moments and shear from lateral
loads must be added to those from
gravity loads

• The lateral loads is primarily resisted by
the shear in the walls, which in turn
produces bending moment
• The openings in wall become areas of high
stress concentration which need to be
handled carefully
• Partial loads is resisted by the frames
• Traditionally, 75/25 distribution have been
used
Shear Wall and Frame

Shear Wall – Frame Coupled
37
• The walls are part of the frame and
act together with the frame members
• The lateral loads is primarily resisted
by the shear in the walls, which in
turn produces bending moment.
• Partial loads is resisted by the frame
members in moment and shear

Braced Frame
38
• The lateral loads is primarily resisted by
the Axial Force in the braces, columns and
beams in the braced zone.
• The frame away from the braced zone
does not have significant moments
• Bracing does not have to be provided in
every bay, but should be provided in every
story

Tubular Structure
39
• The system is formed by using closely
spaced columns and deep spandrel beams
• The lateral loads is primarily resisted by the
entire building acting as a big cantilever
with a tubular/box cross-section
• There is a “shear lag” problem between
opposite faces of the tube due to
inefficiency of column beam connection
• The height to width ratio should be more
than 5

Braced Tube Systems
40
• Diagonal braces are added to the basic
tubular structure
• This modification of the tubular system
reduces shear lag between opposite
faces

Various Schemes and Options for
Lateral Load Resisting Systems
• Frame Systems
• Wall Systems
• Tubular Systems
• Miscellaneous Systems

Frame Systems – Rigid Frame

• A rigid-frame high-rise structure typically comprises of parallel or orthogonally
arranged bents consisting of columns and girders with moment-resistant joints.
• Its unobstructed arrangement, clear of structural walls, allows freedom
internally for the layout and externally for the fenestration.
• Rigid frames are considered economical for buildings of up to about 25 stories,
above which their drift resistance is costly to control.
• If, however, a rigid frame is combined with shear walls, the resulting structure is
very much stiffer so that its height potential may extend up to 50 stories or
more.
Rigid Frame

• The horizontal stiffness of a rigid frame is governed mainly by the bending
resistance of the girders, the columns, and their connections, and in a tall
frame, also by the axial rigidity of the columns.
• The accumulated horizontal shear above any story of a rigid frame is resisted by
shear in the columns of that story.
• The shear causes the story-height columns to bend in double curvature with
points of contra-flexure at approximately mid-story-height.
• The moments applied to a joint from the columns above and below a particular
level are resisted by the attached girders, which also bend in double curvature,
with points of contra-flexure at approximately mid-span.
Rigid Frame

Rigid Frame
Rigid Frame: Forces and Deformations

Rigid Frame
Shear Wall–Frame Interaction

Rigid Frame
Bending deformation of rigid frame:
(a) Moment resisted by axial loads in columns, (b) Cantilever bending of shear wall

Shear Racking Component
Shear Deflection Analogy:
The lateral deflections of a story-high rigid frame due to beam and column rotations may be
considered analogous to the shear deflections of a
story-high segment of a shear wall

Shear Racking Component
Story mechanism:
Strong-column-weak-beam requirement aims at preventing story mechanism

Frame Systems –
Frame with Haunch Girders

• Typical office buildings usually have a lease depth of about 40 ft. (12.19 m)
from the core to the building exterior without interior columns.
• To span a distance of 40 ft., a girder depth of about 2 ft. – 6 in. (0.76 m) is
required unless the girder is post tensioned.
• Because the beam depth has quite an impact on the floor-to-floor height, and
is often limited due to additional cost for the increased height, it is a common
practice to seek shallow framing systems.
• A variable-depth haunch girder is one such solution.
Rigid Frame with Haunch Girders

• Using this system, no increase in floor-to-floor height is expected because the
bottom elevation of girder at midsection is flush with the shallow floor system
transverse to the haunch girder.
• Ample beamless space exists for passage of mechanical ducts.
• Examples of haunch girder buildings are shown in next slides.

Typical floor framing plan: Haunch girder scheme

Haunch Girder Elevation and Reinforcement

Haunch Girder Section

The Huntington. (Architects, Talbot Wilson & Associates; structural engineers,
Walter P. Moore and Associates; contractor, W. S. Bellows Construction Corp.)

A 28-story haunch girder building, Houston, Texas (b) photograph.
(Structural engineers, Walter P. Moore and Associates.)

A 28-story haunch girder building, Houston, Texas. (a) Typical floor plan

Frame Systems –
Flat Slab – Frame System

• The term flat slab-frame signifies that the flat slab behaves as a beam,
responding to lateral loads by developing bending moments and shear forces.
• The floor framing typically consists of a two-way system such as a flat plate,
flat slab, or waffle slab.
• A flat slab has column capitals, drop panels, or both.
• The intent of providing these is to increase the shear and moment resistance
of the system at the columns where the shears and moment are greatest.
Flat Slab - Frame System

• A drop panel is considered as part of a slab and its design is part of the slab
design, whereas, a column capital is deemed part of a column and its design is
considered along with the column design.
• A waffle slab consists of orthogonal rows of joists commonly formed by using
square domes.
• The domes are omitted around the columns to increase the moment and shear
capacity of the slab.
• Any of the three systems may be used in buildings assigned to Seismic Design
Category (SDC) A or B as an integral part of a lateral-resisting system.

Response of flat slab-frames to lateral loads:
Displacement compatibility between slab and walls.
One of the simplest framing techniques for a concrete building – a two-way floor slab
framing directly into columns without beams.

Flat Slab – Frame System
Typical floor systems for flat slab-frames: (a) flat plate, (b) flat slab with drop panels,
and (c) two-way waffle system.

Wall Systems –
Coupled Shear Walls

• A system of interconnected shear walls exhibits a stiffness that far exceeds the
summation of the individual wall stiffness.
• This is because the interconnecting slab or beam restrains the cantilever
bending of individual walls by forcing the system to work as a composite unit.
• The walls behave as if they are connected through a continuous shear-resisting
medium.
Coupled Shear Walls

• The system is economical for buildings in the 40-story range.
• Since planar shear walls carry loads only in their plane, walls in two orthogonal
directions are generally required to resist lateral loads in two directions.
• Placement of walls around elevators, stairs, and utility shafts is common
because they do not interfere with interior architectural layout.
• However, resistance to torsional loads must be considered in determining their
location.
Coupled Shear Walls

Coupled Shear Walls

Coupled Shear Walls
Representation of coupled shear wall by continuum model:
(a) Wall with openings, (b) Analytical model for close-form solution

Wall Systems –
Core-supported Structures

• Shear walls placed around building services such as elevators and stair cores
can be considered as a spatial system capable of transmitting lateral loads in
both directions.
• The advantage is that, being spatial structures, they are able to resist shear
forces and bending moments in two directions and also torsion particularly so
when link beams are provided between the openings.
• The shape of the core is typically dictated by the elevator and stair
requirements and can vary from a single rectangular core to multiple cores.
• Floor framing around the core typically consists of systems such as cast-in-
place mild steel reinforced or post tensioned concrete.

Examples of core-supported buildings: (a) cast-in-place shear walls with precast surround,
(b) shear walls with post tensioned flat plate, and (c) shear walls with one-way joist system

Concrete core with steel surround

Shear Walls with Perimeter Frames

Shear Walls with Interior Frames

Shear Walls with Outrigger Girders

Full depth interior shear walls acting as giant K-brace. (a) Plan and (b) schematic section

Wall Systems –
Spinal Wall Systems

• In this relatively new system, well suited for ultra tall residential towers, shear
walls are placed along both sides of corridors.
• These walls, often referred to as “spine” walls, run through the length of the
floors to resist lateral loads acting parallel to the corridors.
• To resist loads in a perpendicular direction, cross walls are placed in an
orthogonal direction to the spine walls. Interaction between the cross walls
occurs through the interconnecting floor system and/or link beams. To
improve the torsional resistance, additional shear walls are placed around the
elevator and stair cores.
Spinal Wall Systems

• An outstanding example is the Burj Dubai, the tallest building at this time, in
Dubai, UAE.
• The building is “Y”-shaped in plan. Each wing, with its own core and perimeter
columns, buttresses the others via a six-sided core or hub.
• Each tier of the building steps back in a spiral pattern.
• The architectural and structural design is by the Chicago office of Skidmore
Owings and Merrill.
Burj Dubai

Spinal Wall Systems
Burj Dubai, Schematic Plan

Wall Systems –
Outrigger and Belt Wall System

• The structural arrangement for this system consists of a main concrete core
connected to exterior columns by relatively stiff horizontal members such as a
one or two-story deep walls commonly referred to as outriggers.
• The core may be centrally located with outriggers extending on both sides, or
it may be located on one side of the building with outriggers extending to the
building columns on one side.
• The basic structural response of the system is quite simple. When subjected to
lateral loads, the column-restrained outriggers resist the rotation of the core,
causing the lateral deflections and moments in the core to be smaller than if
the freestanding core alone resisted the loading.

• The external moment is resisted not by bending of the core alone, but also by
the axial tension and compression of the exterior columns connected to the
outriggers.
• As a result, the effective depth of the structure for resisting bending is
increased when the core flexes as a vertical cantilever, by the development of
tension in the windward columns, and by compression in the leeward
columns.

Outrigger and belt wall system with centrally located core

Outrigger and belt wall system with an offset core

Vierendeel Frames Acting as

Haunch Girders as Outriggers

Cap Wall System
Cap wall system: (a) Plan and (b) Schematic section

Deflection (Case 1: Outrigger Wall at the Top)
Outrigger located at top, z = L

Deflection (Case 2: Outrigger Wall
at Quarter-height from the Top)
Outrigger at quarter-height from top, z = 0.75L

Deflection (Case 3: Outrigger Wall at Mid-height)
Outrigger at mid-height, z = 0.5L

Deflection (Case 4: Outrigger Wall at
Quarter-height from the Bottom
Outrigger at quarter-height from bottom, z = 0.25L

Optimum Location of a Single Outrigger Wall
Deflection index verses outrigger and belt wall location

Optimum Locations of Two Outrigger Walls
Deflection index vs. belt wall and outrigger locations

Recommendations for Optimum Locations
Optimum location of outriggers, (a) single outrigger, (b) two outriggers,
(c) three outriggers, and (d) four outriggers

Wall Systems - Flat Slab-
frame with Shear Walls

• Frame action provided by a flat slab–beam and column interaction is generally
insufficient to provide the required strength and stiffness for buildings taller
than about 10 stories.
• A system consisting of shear walls and flat slab-frames may provide an
appropriate lateral bracing system.
• Coupling of walls and columns solely by slabs is a relatively weak source of
energy dissipation.
• When sufficiently large rotations occur in the walls during an earthquake,
shear transmission from the slab into wall occurs mainly around the inner
edges of the wall.
Flat Slab-frame with Shear Walls

Flat Slab-frame with Shear Walls
Flat slab-frame with shear walls

Tube System with
Widely Spaced Columns

• The term tube, in usual building terminology, suggests a system of closely
spaced columns e.g., 8–15 ft. on center (2.43–4.57 m), tied together with a
relatively deep spandrel.
• However, for buildings with compact plans, it is possible to achieve tube action
with relatively widely spaced columns interconnected with deep spandrels.
• As an example, the plan of a 28-story building constructed in New Orleans is
shown in the next slide.
• Lateral resistance is provided by a perimeter frame consisting of columns 5 ft.
(1.5 m) wide, spaced at 25 ft. (7.62 m) centers, and tied together with a
spandrel 5 ft. (1.53 m) deep.
Tube System with Widely Spaced Columns

Tube System with Widely Spaced Columns
Tube building with widely spaced perimeter columns

Tubular Systems –
Frame Tube System

• In this system, the perimeter of the building consists of closely spaced columns
connected by deep spandrels.
• The system works quite efficiently as a hollow vertical cantilever. However,
lateral drift due to the axial displacement of the columns (commonly referred
to as chord drift) and web drift, caused by shear and bending deformations of
the spandrels and columns, may be quite large depending upon the tube
geometry.
Frame Tube System

• If the plan aspect ratio is large, e.g., much in excess of 1:2.5, it is likely that
supplemental lateral bracing may be necessary to satisfy drift limitations.
• The economy of the tube system therefore depends on factors such as spacing
and size of columns, depth of perimeter spandrels, and the plan aspect ratio of
the building.
• This system should, however, be given serious consideration for buildings taller
than about 40 stories.
Frame Tube System

Frame Tube System
Frame Tube Building. (a) Schematic plan and (b) isometric view

• Consider 5 models starting from a closed tube structure and openings at each
floor gradually increased towards finally a frame structure.
• Equal concentrated horizontal load is applied on top nodes of all 5 models as
shown in the next slides.
• Three parameters are checked and compared:
– Horizontal Displacement at top node
– Moment M22
– Maximum Moment
From Frame to Tube

Tube Frame

Ux = 0.17
Ux = 0.20 Ux = 0.31 Ux = 1.27
Ux = 15.3
Tube Frame
Displacement at Top Corner

30 Ksi
Tube Frame
133 Ksi
32 Ksi 34 Ksi 46 Ksi
Maximum Moment

11 Ksi
Tube Frame
61 Ksi
12 Ksi 13 Ksi 14 Ksi
Moment M22

• Consider the figure, in which columns of a tubular
building are noted as T and C. T denotes a column in
tension while C denotes a column in compression.
• When column C, is under compression, it will tend to
compress the adjacent column C1 because the two
are connected by the spandrel beams.
• The compressive deformations of C1 will not be
identical to that of corner column C since the
connecting spandrel beam will bend. The axial
deformation of C1 will be less, by an amount
depending on the stiffness of the connecting beam.
• The deformation of column C1 will, in turn, induce
compressive deformations of the next inner column
C2, but the deformation will again be less.
• Thus, each successive interior column will experience
a smaller deformation and hence a lower stress than
the outer ones. The stresses in the corner column
will be greater than those from a pure tubular action,
and those in the inner columns will be less. The
stresses in the inner columns lag behind those in the
corner columns, hence the term shear lag.
Understanding Shear Lag
Shear lag in framed tube

Shear Lag Effects in Frame Tube System
Shear lag effects in a hollow tube structure: (a) cantilever tube subjected to lateral loads,
(b) shear stress distribution, and (c) distortion of flange element caused by shear stresses.

Frame Tube System
Axial stress distribution in a square hollow tube with and without shear lag

Free-form Tubular Configurations

Shear Lag Effects in T-beams Flanges
Shear lag effects in T-beams flanges: (a) Cross-section of T beam. (b) Horizontal shear stresses
between beam web and flange. (c) Non-uniform distribution of compressive stresses in flange

Tubular Systems –
Irregular Tube

• The framed tube concept can be executed with any reasonable arrangement
of column and spandrels around the building parameter
• However, non-compact plans and plans with re-entrant corners considerably
reduce the efficiency of the system.
• For framed tubes, a compact plan may be defined as one with an aspect ratio
not greater than 1.5 or so.
Irregular Tube

• Elongated plans with larger aspects ratios impose considerable premium on
the system because of the following reasons:
– In wind-controlled design, the elongated building elevation acts like a sail
collecting large wind loads.
– The resulting shear forces most usually require closer spacing and/or
larger columns and spandrels parallel to the wind.
– Shear lag effects are more pronounced, especially for columns oriented
perpendicular to the direction of wind.
• In a similar manner, a sharp change in the tubular form results in a less
efficient system because the shear flow must pass around the corners solely
through axial shortening of the columns.
Irregular Tube

Irregular Tube
Secondary frame action in an irregular tube;
schematic axial forces in perimeter columns

Tubular Systems –
Exterior Diagonal Tube

• A trussed tube system improves the efficiency of the framed tube by
increasing its potential for use in taller buildings and allowing greater spacing
between the columns.
• This is achieved by adding diagonal bracing at the faces of the tube to virtually
eliminate the shear lag in both the flange and web frames.
• By applying structural principles similar to those of a trussed steel tube, it is
possible to visualize a concrete system consisting of closely spaced exterior
columns with blocked-out windows at each floor to create a diagonal pattern
on the building facade.
• The diagonals carry lateral shear forces in axial compression and tension, thus
eliminating bending in the columns and girders.

Exterior diagonal braces in a tall steel building

An example of exterior diagonal tube: Onterie Center, Chicago, IL.
123

Tubular Systems - Bundled Tube

• The underlying principle to achieve a bundled tube response is to connect two
or more individual tubes into a single bundle.
• The main purpose is to decrease shear lag effects.
• A bundled tube typically consists of a number of individual tubes
interconnected to form a multi-cell tube, in which the frames in the lateral load
direction resist the shears, while the flange frames carry most of the
overturning moments.
• The cells can be stopped at selected heights without diminishing structural
integrity.
• The torsional loads are readily resisted by the closed form of the modules.
Bundled Tube

Bundled Tube
Bundled tube: schematic plan

Bundled Tube
Schematics of bundled tubes

One Magnificent Mile, Chicago, IL; Structural system
128

Miscellaneous Systems

• Buildings with a high plan aspect ratio type tend to be inefficient in resisting
lateral loads because of shear lag effects.
• However, by introducing a limited number of interior columns (three at every
other floor in the building shown), it is possible to reduce the effect of shear
lag, and thus efficiency for resisting lateral loads is increased.
Cellular tube with interior vierendeel frames

Structural concept for super tall buildings
• The system shown consists of a service
core located at each corner of the building
interconnected by a super diagonal in-fill
walls.
• The service core at each corner acts as a
giant column carrying a majority of the
gravity load and overturning moments.
• The eccentricity between the super
diagonals and exterior columns is a
deliberate design strategy to enhance the
ductility of the lateral bracing system for
buildings assigned to high seismic design
category.
• The ductile response of the links is
anticipated to help in dissipating seismic
energy, thus assuring the gravity-carrying
capacity of the building during and after a
large earthquake.

Steel Structural Systems

Selection of Structural Systems

Selecting a System using “Rich Pictures”
• Consider as many factors as possible not just structural considerations
Column Layout
Grid
Typical Span
Length
No. of Bays:
X and Y
Slab Levels / Steps
Span Variation
Aspect Ratio of
Panels
Peripheral Beam-
Girders
Internal Beams/
Girders
Column Size
Support System

Basic Slab System
Components
Overall Geometry

• The “final” or detailed
design is carried out
after system selection
and preliminary
design
• Decision in system
selection affect the
performance and
value more than the
detailed design
refinements

Rich picture Diagram of professionals’
interaction while designing a building

Rich Picture Diagram

Knowledge Model for System Selection
142
• Architecture
• Building Services
• Construction Engineering
• Value Engineering
• Aesthetics
• Ergonomics Engineering
• Structural Engineering
• Knowledge Engineering
• Economics
• Artificial Intelligence
• System Engineering
• Common Sense

Different professionals involved and
their interrelationship with structural
engineer during the selection of
structural systems
143

• The Analytical Hierarchy Approach
– A weighted importance and suitability value analysis to determine the
comparative value of a system or option
Determining System Suitability
Value of
an
Option
Global
Importance
Weights and
Scores
Sub
Importance
Weights and
Scores
Suitability
Value and
Score














  
ijk
p
k
ijklij
n
j
iji
m
i
il SCSBSAV
111

• The Suitability Equation
– A weighted importance and suitability value analysis to determine the
comparative value of a system or option
Evaluating System Suitability














  
ijk
p
k
ijklij
n
j
iji
m
i
il SCSBSAV
111

Using the Suitability Equation
Evaluating System Suitability
Slab
Systems
Criteria Weights and Scores
System
Value
(V)
Main Criteria Ai Am
Sub Criteria Bij Sub Criteria Bin Bmn
Item k Item p Item k Item p Item
Wt Score Wt Score Wt Score Wt Score
p
Score
System – 1
System – l Cijkl Sijkl Cijnl Sijpl Cinkl Sinkl Cinnl Sinpl Smnpl
System - q

Score or Weight Representation of Suitability
10 Most important, most suitable, most desirable, essential
8, 9 Very important, very suitable, very desirable
6, 7 Important, suitable or desirable
5 May be or could be important, suitable or desirable
3, 4 May not be important, suitable or desirable
1, 2 Not important, not suitable, not desirable
0 Definitely not required, definitely not suitable, ignore
Assigning Suitability Values

Latest Techniques
• Genetic Algorithms (GA)
• Artificial Neural Networks (ANN)
• Fuzzy Logic
• Expert Systems (ES)
• Linear/Nonlinear Programming
• Value Engineering
• Analytic Hierarchy Process (AHP)

• Function has considerable effect on the selection of structural system
• Based on Function/Occupancy of Tall Buildings:
– Residential Buildings
• Apartments
• Hotels
• Dormitories
– Office and Commercial Buildings
– Mixed Occupancy – Commercial + Residential
– Industrial Buildings and Parking Garages
Selection of Structural System

• Known location of partitions and their load
• Column lines generally matches architectural layout
• Typical spans 15-22 ft.
• Tall buildings economy in achieved using the thinnest slab
• One way precast or flat slab – popular
• Lateral load resistance provided by frame or shear walls
• More or less fixed M/E system layouts
Typical Characteristics of Residential Building

• Unknown location of partitions and their load
• Typical spans 20-35 ft
• Need for flexible M/E layouts
• Post-tension or ribbed and flat slab with drop panel – popular
• Ideal balance between vertical and lateral load resisting systems: sufficient
shear walls to limit the resultant tension under gravity plus wind
• Lateral load resistance varies significantly
Typical Characteristics of
Office and Commercial Building

Selection of Slab System

• Basic Consideration
– Span length: small, medium, long
– Panel aspect ratio: square, rectangular, oblong
– Loads: light, medium, heavy
– Ducts and piping: electrical, mechanical, water supply
– Openings: size and location
– Architectural consideration: aesthetics, clearance, etc.
– Special elements: drop panel, column capital, beams
– Construction considerations: form work, time, case of considerations
Selection of Layout and Type of Slab

• Flat plates
– Relatively light load < 500 Kg/m2 ( 5 kPa )
– Common for residential buildings
– Used where Drop panel or Column capital is undesirable
– Easy construction. Low overall height of building
• Flat Slabs
– Medium Spans ( 6.0 to 9.0 m)
– Relatively heavy load > 500 Kg/m2 ( 5 kPa )
– Common in industrial floors, parking areas, etc.

• Waffle Slab
– Large spans ( 7.5 – 12 m )
– Relatively heavy loads
– Common for public buildings. More aesthetic appearance
– Difficult to construct. Requires special form work
• Beam-Slabs/Beam and Slabs
– Medium to large spans ( 5 to 10 m)
– Relatively economical in concrete and steel cost
– Greater depth: Increase in building height

• Band Slab
– Medium spans ( 5 to 9 m)
– More economical than flat slab
– Common in industrial floors, parking areas, etc.
• One-way Joist and Beams
– Large spans ( 7.5 – 12 m )
– Easier to construct than waffle slabs
– Suitable for high-rise office building
– Beams on shorter side, joist on longer side

Selection of Lateral Load
Resisting System

Lateral Load Resisting System Selection
0
10
20
30
40
50
60
70
80
90
100
Frame Fram-Shear Framed Tube Tube-in-Tube Multi-Cell
Framed Tube
Stories

Framed Systems: Resistance to Lateral load

Framed Systems: Resistance to Lateral load
160
• (OMRF = Ordinary Moment-Resisting Frame)
• (SMRF = Special Moment-Resisting Frame)
Flat Plate Flat Slab Waffle Slab Beam Slab Beam & Slab
Low wind
No seismic
Up to 6
floors
Up to 10
floors
Up to 10
floors
Up to 15 floors Up to 20 floors
Medium wind
Low seismic
Not used
Up to 6
floors
Up to 10
floors
OMRF
Up to 15 floors
OMRF
Up to 20 floors
High wind
Medium seismic
Not used Not used Not used
SMRR
Up to 15 floors
Up to 15 floors
SMRF
High seismic Not used Not used Not used Not used
Up to 15 floors
SMRF

Combination of Vertical and Lateral Systems
Frame Braced Dual System
OMRF IMRF SMRF SW DB SWF OBF SBF
Flat Plate X
Flat Slab X X X
Waffle Slab X X X X X
Beam Slab X X X X X X X
Beam & Slab X X X X X X X X

Examples

Options

Example Structure
164
Structural system study: 62-story building. (a) Plan and (b) elevation

• The structural schemes for this example structure can be broadly
classified into three categories.
– Cross-bracing schemes
• Exterior-braced tube
• Interior-braced tube
• Braced and framed tube combination
– Framed tubes with deep spandrels and closely spaced columns
• Framed Tube
• Twin Tube
– Assortment of schemes utilizing shear wall-frame interaction
• Shear wall frame interaction
• Moment frames and braced cores
• Outrigger and belt walls
• Jumbo column scheme
Classification of Structural Schemes
(for this example)

Category 1 – Cross Bracing Schemes
Isometric View
Exterior Braced Tube: Plan
Interior Cross Bracing
System: Plan
Isometric View
Option 1
Option 2

Category 1 – Cross Bracing Schemes
Isometric view of framed tube
and brace frame
Interacting framed tube and braced frame
Option 3

Category 2 – Framed tube systems
Option 1
Option 2
Framed Tube
Twin Tube

Category 3 – Non-tubular schemes
Option 3 : Outrigger and belt truss system
Option 2 : Moment frame and braced core systemOption 1 : Shear wall frame interacting system
Option 4 : Jumbo column scheme

• Taranath, B.S. (2010). Reinforced Concrete Design of Tall Buildings. Taylor
and Francis Group, LLC.
• Powell, G. H. Modeling for Structural Analysis.
• Wilson, E. L. Three-Dimensional Static and Dynamic Analysis of Structures
• Notes from various workshops conducted by Dr. Naveed Anwar
References

CE 72.32 (January 2016 Semester) Lecture 4 - Selection of Structural Systems

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CE 72.32 (January 2016 Semester) Lecture 4 - Selection of Structural Systems