Comparison of symmetric and asymmetric steel diagrid structures by non linear static analysis

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 04 Issue: 05 | May-2015, Available @ http://www.ijret.org 486
COMPARISON OF SYMMETRIC AND ASYMMETRIC STEEL DIAGRID
STRUCTURES BY NON-LINEAR STATIC ANALYSIS
Prashant T G1
, Shrithi S Badami2
, Avinash Gornale3
1
P.G student, Department of Civil Engineering, Adichunchanagiri Institute of Technology, Chikkamagaluru, Karnataka,
India
2
Assistant Professor, Department of Civil Engineering, Adichunchanagiri Institute of Technology, Chikkamagaluru,
Karnataka, India
3
Assistant Professor, Department of Civil Engineering, Maharaja Institute of Technology, Mysore, Karnataka, India
Abstract
Diagonalized grid structures – “diagrids” - have emerged as one of the most innovative and adaptable approaches to structuring
buildings in this millennium. Diagrid is a particular form of space truss, it consists of perimeter grid made up of a series of
triangulated truss system. Diagrid is formed by intersecting the diagonal and horizontal components.
Construction of multi‐storey building is rapidly increasing throughout the world. Advance in construction technology, materials,
structural systems, various analysis and design software have facilitated the growth of various kinds of buildings. Diagrid
buildings are emerging as structurally efficient as well as architecturally and aesthetically significant assemblies for tall
buildings. Recently these diagrid structural systems have been widely used for tall buildings due to the structural efficiency and
aesthetic potential provided by the unique geometric configuration of the system. This paper presents a 12 storey steel diagrid
structure which is 36m in height. Symmetric and asymmetric structural configurations of diagrid structures were modelled and
analyzed using SAP 2000 by considering Dead load, Live load and Seismic Loads (IS 1893-Part-1, 2002). Then FEMA 356 hinges
(auto hinges) are assigned to the same structure and Nonlinear Static (Pushover) analysis is carried out by using seismic load as
the pushover load case to find out the performance points that is Immediate Occupancy, Life Safety, and Collapse Prevention of
diagrid elements using static pushover curve. At the same time spectral displacement demand & spectral displacement capacity as
well as spectral acceleration demand and spectral acceleration capacity is compared to know the adequacy of the design by using
ATC capacity spectrum method.
Keywords: Diagrid, Pushover analysis, Spectral displacement demand, Spectral displacement capacity, Spectral
acceleration demand, Spectral acceleration capacity
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1. INTRODUCTION
The evolution of tall-building structural systems, based on
new structural concepts with newly adopted high-strength
materials and construction methods, has been towards
„stiffness‟ and „lightness‟. Structural systems nowadays are
becoming stiffer and lighter. Diagrid, are known as a very
light structure and one of the strongest when it comes to
withstanding against lateral forces. The term “diagrid” is a
combination of the words “diagonal” and “grid” and refers
to a structural system that is single‐thickness in nature and
gains its structural integrity through the use of triangulation.
The lighter a structure is, the higher it can rise. On the other
hand, it is also easier to blow away a light object than a
heavy one. Diagrid can save from 20% to 30% the amount
of structural steel in high-rise buildings. Moreover, high-
strength material technology has come a long way since the
invention of modern high-rise buildings in 1930‟s, materials
themselves are stronger and lighter.
Diagrid structures carry lateral wind loads much more
efficiently due to their diagonal member‟s axial action
compared to the conventional orthogonal structures for tall
buildings such as framed tubes. Today‟s architects have
been losing interest in aesthetic expressions provided by
conventional braced tubes composed of orthogonal members
and large diagonal members because they always seek
something new and different.
2. NONLINEAR STATIC (PUSHOVER)
ANALYSIS
The use of the nonlinear static analysis came into practice in
1970‟s but the potential of the pushover analysis has been
recognized for last 10-15 years. This procedure is mainly
used to estimate the strength and drift capacity of existing
structure and the seismic demand for the structure subjected
to selected earthquake. This procedure can be used for
checking the adequacy of new structural design as well.
Pushover analysis is defined as an analysis where a
mathematical model directly incorporating the nonlinear
load-deformation characteristics of individual components
and elements of the structure shall be subjected to
monotonically increasing lateral loads representing inertia

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forces in an earthquake until a “target displacement” is
exceeded. Target displacement is the maximum
displacement (elastic plus inelastic) of the structure at top
expected under selected earthquake ground motion.
Nonlinear Static (Pushover) analysis is conducted to find out
the performance points that is Immediate Occupancy, Life
Safety, and Collapse Prevention of diagrid elements using
static pushover curve.
Fig.1: Static pushover curve
Response characteristics that can be obtained from the
pushover analysis are summarised as follows:
a) Estimates force and displacement capacities of the
structure and sequence of the member yielding and the
progress of the overall capacity curve.
b) Estimates force (axial, shear and moment) demands on
potentially brittle elements and deformation demands on
ductile elements.
c) Estimates global displacement demand, corresponding
inter-storey drifts and damages on structural and non-
structural elements expected under the earthquake ground
motion considered.
d) Sequences of the failure of elements and the consequent
effect on the overall structural stability.
e) Identification of the critical regions, where the inelastic
deformations are expected to be high and identification of
strength irregularities (in plan or in elevation) of the
building.
3. NEED FOR NON-LINEAR STATIC
(PUSHOVER) ANALYSIS
Conventionally, seismic assessment and design has relied on
linear or equivalent linear (with reduced stiffness) analysis
of structural systems. In this approach, simple models are
used for various elements of the structure, which are
subjected to seismic forces evaluated from elastic or design
spectra, and reduced by force reduction (or behaviour)
factors. This ensures displacements are amplified to account
for the reduction of applied forces. The reduced force-
amplified deformation linear elastic approach fails to fit
within the principle of failure mode control, which is part of
performance based assessment and design. This in turn has
led to an increase in the use of inelastic analysis as a more
realistic means of assessing deformational state in structures
subjected to strong ground motions. The pushover analysis
is a significant step forward by giving consideration to those
inelastic response characteristics that will distinguish
between good and bad performance in severe earthquakes.
The non linear static pushover analysis is a partial and
relatively simple immediate solution to the complex
problem for predicting forces and deformation demands
imposed on the structure and its elements due to ground
motions. The pushover is a part of an evaluation process and
provides estimates of demands imposed on structures and
elements. Hence, there is always a need of a method which
is rational and accurate and at the same time able to identify
seismic deficiencies correctly and that too in a correct order
of vulnerability. Pushover analysis is able to satisfy these
criteria satisfactorily and in a convenient way.
3.1 Capacity Spectrum Method (ATC 40)
In this method the maximum inelastic deformation of a
nonlinear SDOF system can be approximated from the
maximum deformation of a linear elastic SDOF system with
an equivalent period and damping. This procedure uses the
estimates of ductility to calculate effective period and
damping. This procedure uses the pushover curve in an
acceleration-displacement response spectrum (ADRS)
format. This can be obtained through simple conversion
using the dynamic properties of the system. The pushover
curve in an ADRS format is termed a “capacity spectrum”
for the structure. The seismic ground motion is represented
by a response spectrum in the same ADRS format and it is
termed as demand spectrum which is as shown in Fig. 1.
The equivalent period (Teq) is computed from the initial
period of vibration (Ti) of the nonlinear system and
displacement ductility ratio (μ). Similarly, the equivalent
damping ratio (βeq) is computed from initial damping ratio
and the displacement ductility ratio (μ). ATC 40 provides
the following equations to calculate equivalent time period
(Teq) and equivalent damping (βeq).
Fig-2: Schematic representation of Capacity Spectrum
Method (ATC 40)
4. PROBLEM DEFINITION
This unit presents the details about dimensions of building,
material used and type of analysis for the present study and
are as mentioned in TABLE 1. A 12 storey steel diagrid
structure having height 36m and lateral dimensions as
18mX18m is considered for the analysis. The dead load, live

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load and seismic loads and the default load combinations
were considered for the analysis and the structure is
modelled in SAP 2000 and Linear Analysis is conducted to
get the maximum bending moment, shear force and axial
force. Later the FEMA 356 Hinges were defined in the
model and Nonlinear Static (Pushover) Analysis has been
conducted using ATC-40 capacity spectrum method to
calculate Base Shear, Displacements, Effective time,
Spectral Displacement Capacity & Spectral Displacement
Demand and also Performance points of Diagrid Structure.
Table-1: Building Details Considered for analysis
Sl.
No
Building Details
1 Symmetric structure Model-1
2 Asymmetric structure Model-2
3
Plan dimensions of
building
18m X 18m
4 Height of building 36 m
5 No. of stories 12
6 Storey height 3 m
7 Type of structure Steel diagrid structure
8 Type of analysis
Nonlinear static
analysis
Fig-3: Plan of Diagrid Structure 18mx18m (Model-1)
Fig-4: 3D Diagrid Building Model (Model-1) in SAP 2000
Fig-5: Plan of diagrid structure 18m*18m (Model-2)

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Fig-6: 3D diagrid building model (Model-2) in SAP2000
5. RESULTS AND DISCUSSIONS
The modelled building is analysed using Nonlinear Static
(Pushover) analysis. This chapter presents Nonlinear Static
(Pushover) analysis results and its discussions. Pushover
analysis was performed first in a load control manner to
apply all gravity loads on to the structure (gravity push),
then a lateral pushover analysis in transverse direction was
performed in a displacement control manner starting at the
end of gravity push. The results obtained from this analysis
are checked by comparing spectral displacement demand
and spectral displacement capacity from the pushover curve.
5.1 Nonlinear Static (Pushover) Analysis
Nonlinear Static (Pushover) Analysis permits to identify
critical members likely to reach limit states during the
earthquake. Nonlinear Static Analysis is carried out after
assigning flexural hinges (FEMA 356 Auto hinges) using
ATC 40 Capacity Spectrum Method. Performance points &
levels (IO, LS, and CP) which were observed in different
pushover steps for symmetric structure are as shown in Fig
7, Fig 8 and Fig 9 and for asymmetric structure are as shown
in Fig 11, Fig 12 and Fig 13. Spectral Acceleration vs.
Spectral Displacement Graph which is obtained from the
analysis for symmetric structure is shown in Fig 10 and for
asymmetric structure is shown in Fig 14. Spectral
Displacement Demand & Spectral Displacement Capacity
which is calculated from Fig-10 and Fig 14 is shown in
Chart-1.
Fig-7: Pushover step -2 of Model-1

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Fig-10: Pushover Demand Capacity Curve (ATC 40) of
Model-1
Fig-11: Pushover step-1 of Model-2

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Fig-14: Pushover Demand Capacity Curve (ATC 40) of
Model – 2
For symmetric structure it can be observed from Fig 7 that is
pushover step 2 that the assigned hinges are in a state of
immediate occupancy. In the next steps which are shown in
Fig 8 and Fig 9 that is step 3 and step 4 one can observe that
some hinges shifts from state of immediate occupancy to
state of life safety due to the incremental increase in lateral
load.
For asymmetric structure it can be observed from Fig 11 that
is pushover step 1 that assigned hinges are in a state of
immediate occupancy. In the next step which is shown in
Fig 12 that is step 2 it can be observed that some hinges
shifts from immediate occupancy state to state of life safety
and from Fig 13 that is step 3 it can be seen that some
hinges shifts from life safety to collapse state.
0
0.05
0.1
0.15
0.2
0.25
MODEL 1 MODEL 2
Spectral
displacement
demand
spectral
displacement
capacity
Chart-1: Comparison between Capacity & Demand
6. CONCLUSION
It can be concluded that from the pushover analysis one can
know the state of the structure by observing changes in the
hinge states.
From Chart-1 it can be observed that spectral displacement
capacity of asymmetric structure is more than that of
symmetric structure by 63% for the same height and plan
area.
ACKNOWLEDGMENTS
The authors thank the Principal and Management of
Adichunchanagiri Institute of Technology, Chikkamagaluru
for the continued support and cooperation in carrying out
this research study.
REFERENCES
[1] Rohit Kumar Singh, Dr. Vivek Garg, Dr. Abhay
Sharma (2014), “Analysis and design of concrete
diagrid building and its comparision with
conventional frame building”, IJSET, pp1330-1337,
Vol 2, Issue 6, September 02, 2014.
[2] K. Moon (2009), “Design and construction of steel
diagrid structures”, NSCC, pp 392-405, 2009.
[3] Kyoung-Sun Moon, Jerome J. Connor and John E.
Fernandez(2007), “Diagrid structural systems for
tall buildings- characteristics and methodology for
preliminary design”, Wiley interscience, pp 205 -
230,2007.
[4] J. Kim, Y. Jun and Y.-Ho Lee (2010), “Seismic
Performance Evaluation of Diagrid System
Buildings”, 2nd
Specialty conference on disaster
mitigation, pp 1-9, June 9-12, 2010.
[5] Ravi K Revankar and R.G.Talasadar (2014),
“Pushover analysis of diagrid structure”,
International Journal of Engineering and Innovative

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Technology (IJEIT), pp168-174, Volume 4, Issue 3,
September 2014.
[6] IS 1893 (Part 1): 2002 - Criteria for Earthquake
resistant Design of structures.
[7] IS 800 – 1984 - Code of practice for General
construction in steel.
[8] CSI Analysis Reference Manual for SAP 2000.
BIOGRAPHIES
Prashant T G, is a Final Year PG Student,
pursuing his Master of Technology in
Structural Engineering from
Adichunchanagiri Institute of Technology,
Chikkamagaluru, Karnataka
Shrithi S Badami, is working as a
Assistant Professor in department of civil
engineering in Adichunchanagiri Institute
of Technology, Chikkamagaluru,
Karnataka. Her areas of research interest
include earthquake engineering, structural
engineering and concrete technology.
Avinash Gornale is working as a Assistant
Professor in department of civil
engineering in Maharaja Institute of
Technology, Mysore, Karnataka. His areas
of research interest include earthquake
engineering and concrete technology

Comparison of symmetric and asymmetric steel diagrid structures by non linear static analysis

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Comparison of symmetric and asymmetric steel diagrid structures by non linear static analysis