Download III. Design Data And Creating Model

International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Investigation on Performance Levels of 8-Storey
RC Building with Pushover Analysis
Aung Pyae Phyo, Dr. San Yu Khaing

Abstract— This paper presents eight-storied regular shaped
reinforced concrete building. The structure is located in high
seismic risk zone, Mandalay. Its height is 106 ft above ground
level. The total length and width are 80 ft and 68 ft respectively.
High wind speed 80 mph is used. This structure is composed of
special moment resisting frame and it is designed out by using
SAP 2000 software. The structural elements are designed base
on UBC-97. For earthquake and wind forces, loading data were
referenced from UBC-97. Dead loads and live loads are used
according to ACI code. The load combinations required for the
whole structure is used according to UBC-97. In this study, the
performance level of structure are determined by using
Push-over analysis. The static pushover analysis is becoming a
popular method of predicting seismic force and deformation
demands for the purpose of performance evaluation of existing
and new structures. It will provide much useful information that
cannot be obtained from elastic static or dynamic analysis
procedures. The displacement control analysis is used and it
value is considered as Case I (2% of total structure height) and
Case II (4% of total structure height). The lateral displacement
of 25 in and 50.88 in is subjected to roof level joint 65
respectively. The performance level of Case I and Case II are
compared and also give out the values of base shear and roof
displacement for both cases.
Index Terms— SAP 2000, special moment resisting frame,
performance level, push-over analysis, lateral displacement.
I. INTRODUCTION
Myanmar is still a developing country and there is a
tendency to raise population in future. So, the high-rise
building is the only answer to solve the problem of population
dense. In establishing the new modern developed country, the
grand and modern high-rise buildings are reflect the scientific
and technological achievements of the age. A high-rise
building is a building in which structural system is modified to
make it sufficiently safe and economical. But, from a
structural engineer's point of view the multi-storeyed building
or tall building can be defined as one that, by virtue of its
height, is affected by lateral forces due to wind or earthquake
or both to an extent that they play an important role in the
structural design. It is necessary to investigate the earthquake
effect by using high technological analysis. Seismic
Manuscript received Oct 15, 2011.
Aung Pyae Phyo, Department of Civil Engineering, Mandalay
Technological University, (e-mail: [email protected]).
Mandalay, Myanmar, 09-43207740
Dr. San Yu Khaing, (Associate Professor), Department of Civil
Engineering, Mandalay Technological University, Mandalay, Myanmar,
09-2132780 (e-mail: [email protected]).
performance of a structure is dependent upon the performance
characteristics of its critical components.[1] A performance
objective is a goal that a building achieves a certain level of
performance for a special level of seismic ground shaking
hazard.[1] The static pushover analysis is a partial and
relatively simple intermediate solution to the complex
problem of predicting force and deformation demands
imposed on structures and their elements by severe ground
motion.
II. PUSHOVER ANALYSIS
A. Introduction
Nonlinear static analysis, or pushover analysis, has been
developed over the past twenty years and has become the
preferred analysis procedure for design and seismic
performance evaluation purposes as the procedure is
relatively simple and considers post-elastic behavior.
However, the procedure involves certain approximations and
simplifications that some amount of variation is always
expected to exist in seismic demand prediction of pushover
analysis.
Although, pushover analysis has been shown to capture
essential structural response characteristics under seismic
action, the accuracy and the reliability of pushover analysis in
predicting global and local seismic demands for all structures
have been a subject of discussion and improved pushover
procedures have been proposed to overcome the certain
limitations of traditional pushover procedures.
Traditional pushover analysis is widely used for design and
seismic performance evaluation purposes, its limitations,
weaknesses and the accuracy of its predictions in routine
application should be identified by studying the factors
affecting the pushover predictions. In other words, the
applicability of pushover analysis in predicting seismic
demands should be investigated for low, mid and high-rise
structures by identifying certain issues such as modeling
nonlinear member behavior, computational scheme of the
procedure, variations in the predictions of various lateral load
patterns utilized in traditional pushover analysis, efficiency of
invariant lateral load patterns in representing higher mode
effects and accurate estimation of target displacement at
which seismic demand prediction of pushover procedure is
performed.
The static pushover analysis is becoming a popular tool for
seismic performance evaluation of existing and new
structures. The expectation is that the pushover analysis will
provide adequate information on seismic demands imposed
by the design ground motion on the structural system and its
components.
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
B. Description Of Pushover Analysis
Pushover analysis is an approximate analysis method in
which the structure is subjected to monotonically increasing
lateral forces with an invariant height-wise distribution until a
target displacement is reached.
Pushover analysis consists of a series of sequential elastic
analyses, superimposed to approximate a force-displacement
curve of the overall structure. A two or three dimensional
model which includes bilinear or trilinear load-deformation
diagrams of all lateral force resisting elements is first created
and gravity loads are applied initially. A predefined lateral
load pattern which is distributed along the building height is
then applied. The lateral forces are increased until some
members yield. The structural model is modified to account
for the reduced stiffness of yielded members and lateral forces
are again increased until additional members yield. The
process is continued until a control displacement at the top of
building reaches a certain level of deformation or structure
becomes unstable. The roof displacement is plotted with base
shear to get the global capacity curve.
C. Performance-Based Plastic Design
Pushover analysis is performed by displacement coefficient
method or capacity spectrum method. The Capacity
Spectrum Method (CSM), a performance-based seismic
analysis technique, can be used for a variety of purposes such
as rapid evaluation of a large inventory of buildings, design
verification for new construction of individual buildings,
evaluation of an existing structure to identify damage states,
and correlation of damage states of buildings to various
amplitudes of ground motion. The procedure correlation of
damage states of buildings to various amplitudes of ground
motion. The procedure compares the capacity of the structure
(in the form of a pushover curve) with the demands on the
structure. Objective of Displacement Coefficient Method
(DCM) is to find target displacement which is the maximum
displacement that the structure is likely to be experienced
during the design earthquake. It provides a numerical process
for estimating the displacement demand on the structure, by
using a bilinear representation of capacity curve and a series
of modification factors, or coefficients, to calculate a target
displacement.
D. Necessity Of Non-Linear Static Pushover Analysis (NLSA)
The existing building can become seismically deficient
since seismic design code requirements are constantly
upgraded and advancement in engineering knowledge.
Further, Myanmar buildings built over past two decades are
seismically deficient because of lack of awareness regarding
seismic behavior of structures. The widespread damage
especially to RC buildings during earthquakes exposed the
construction practices being adopted around the world, and
generated a great demand for seismic evaluation and
retrofitting of existing building stocks.
Figure 1: Global Capacity (Pushover) Curve of a Structure
Pushover analysis can be performed as force-controlled or
displacement-controlled. In force-controlled pushover
procedure, full load combination is applied as specified, i.e.;
force-controlled procedure should be used when the load is
known (such as gravity loading). Also, in force-controlled
pushover procedure some numerical problems that affect the
accuracy of results occur since target displacement may be
associated with a very small positive or even a negative lateral
stiffness because of the development of mechanisms and
P-delta effects.
In displacement-controlled procedure, specified drifts are
sought (as in seismic loading) where the magnitude of applied
load is not known in advance. The magnitude of load
combination is increased or decreased as necessary until the
control displacement reaches a specified value. Generally,
roof displacement at the centre of mass of structure is chosen
as the control displacement.
The internal forces and deformations computed at the target
displacement are used as estimates of inelastic strength and
deformation demands that have to be compared with available
capacities for a performance check.
E. Advantages Of Pushover Analysis
The advantage of the pushover analysis is that it applies
equally to the evaluation and retrofit of existing structures as
to the design of new ones. A comprehensive evaluation of a
lateral system would require the execution of a series of
nonlinear time history analyses of the structure subjected to a
representative suite of earthquake ground motions. This
should be the emphasis for evaluation procedures of the
future.
The pushover is expected to provide information on many
response characteristics that cannot be obtained from a linear
elastic static or dynamic analysis. The following are examples
of such response characteristics:
- Force demands on potentially brittle elements, such as
axial force demands in columns, force demands on brace
connections, moment demands on beam-to-column
connections, shear force demands in deep reinforced
concrete spandrel beams and in unreinforced masonry
wall piers, etc.
- Estimates of deformation demands for elements that have
to deform inelastically in order to dissipate the energy
imparted to the structure by ground motions.
- Consequence of strength deterioration of individual
elements on the behavior of the structural system.
- Identification of critical regions in which the deformation
demands are expected to be high and that have to become
the focus of thorough detailing.
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
- Identification of strength discontinuities in plan or
elevation that will lead to changes in dynamic
characteristics in the inelastic range.
- Estimates of interstory drifts, which account for strength
or stiffness discontinuities and may be used to control
damage and evaluate P-delta effects.
- Estimates of global drift, which may be used to assess the
potential for pounding.
- Verification of completeness and adequacy of load path,
considering all elements of the structural system, all
connections, stiff nonstructural elements of significant
strength, and the foundation system.
(1)
(2)
(3)
(4)
(5)
Extensive (near complete) structural and nonstructural
damage
Significant potential for injury but not wide scale loss of
life
Extended loss of use
Repair may not be practical
Losses greater than 30% [1]
The last item is perhaps the most important one, provided
the pushover analysis incorporates all elements, whether
structural or nonstructural, that contribute significantly to
lateral load distribution.
The pushover is most useful for the evaluation at
performance levels that are associated with large inelastic
deformations (e.g., collapse prevention level). The method is
applicable and useful, however, for evaluation at any
performance level at which inelastic deformations will occur.
F. Performance Levels Of Structure
There are five levels of global structural response
depending on the permissible amount of damage suffered by
the structure when push-over analysis is performed. These are
(1) Operational (O) level,
(2) Immediate Occupancy (IO) level,
(3) Live Safety (LS) level,
(4) Collapse Prevention (CP) level,
(5) Collapsed (C) level. [1]
Operational (O) level
In the Operational level, the following facts can occur in the
structure;
(1) Negligible structural and nonstructural damage
(2) Occupants are safe during event
(3) Utilities are available
(4) Facility is available for immediate re-use
(5) Losses less than 5% of replacement value. [1]
Immediate Occupancy (IO) level
In the Immediate Occupancy level, the following facts can
occur in the structure;
(1) Negligible structural damage
(2) Occupants are safe during event
(3) Minor nonstructural damage
(4) Building is safe to occupy but may not function
(5) Limited interruption of operations
(6) Losses less than 15% [1]
Live Safety (LS) level
In this level, the following fact can occur in the structure;
(1) Significant structural damage
(2) Some injuries may occur
(3) Extensive nonstructural damage
(4) Building not safe for re-occupancy until repaired
(5) Losses less than 30% [1]
Collapse Prevention (CP) level
In this level, the following fact can occur in the structure;
Figure 2: Performance Level of Structure [2]
where,
IO = Intermediate Occupancy
LS = Life Safety
CP = Collapse Prevention
D = Damage Level
E = Emergency Level
III. DESIGN DATA AND CREATING MODEL
The proposed building for study is eight-storied reinforced
concrete building at Mandalay. It is considered high seismic
hazard near Sagaing fault is situated in the seismic zone 4.
The case study of proposed building is shown in Table I.
Type
Table I
Case Study of Proposed Building
Eight-storey regular shaped
RC building
Occupancy
Residential
Location
Mandalay
Size
Length = 80 ft
Width = 68 ft
Height
Typical storey height = 11
ft
Bottom storey height = 12 ft
Total height = 106 ft
Elevator
1 number
The material properties of structure are the strength of
concrete is 2500 psi and yield strength of concrete is 40000
psi. Dead loads and live loads are used according to ACI
code. The values of dead load and live load are shown in
Table II.
Exposure type is Type B and wind speed 80 mph is used.
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
The structure is situated in seismic zone 4, soil profile type is
SD and seismic source type is A. [5]
Table II
Value of Dead Load and Live Load ( UBC-97 )
Dead Load
Unit weight of concrete
150 pcf
9″ thick brick barrier wall
100 pcf
4.5″ thick brick barrier wall
50 pcf
Unit weight of water
62.5 lb
Weight of lift
3 tons
Weight of ceiling and
finishing
Live Load
Live load on residential area
25 psf
Live load on lobby
60 psf
Live load on stair case
100 psf
Live load o roof
20 psf
Case I
For lateral load, lateral displacement of the building is
considered only in X-direction. The method of displacement
control analysis is used. The 2% of total structure heights
(25in) is used for displacement control value.[2] Built-in
default hinges are used and only moment hinges are
considered. Hinges location are 5% from the column faces.[4]
The control option is Push to Displacement Magnitude which
is target displacement for this building. This start after the
gravity load case. Monitored storey and direction is the same
with the gravity load. Minimum number of saved steps is 10,
maximum number of null steps is 50, maximum number of
total steps is 200, iteration tolerance is 1.000E-04 and event
tolerance is 0.01.[3] The figures 4 to 8 show the results for
performance levels of the structure.
40 psf
Figure 4 .Iteration step=1, Performance Level = O,
Base shear = 9.045 K , Displacement = 0.101 in
Figure 3: 3D View of Proposed Building
IV. PUSHOVER ANALYSIS RESULTS
In this study, to determined performance levels of structure,
the following data are used. Nonlinear Static (Push-over)
Analysis method is used to check performance levels of
structure.[3] To perform pushover analysis, there need to
define at least two types of static nonlinear case and the first
case must be for gravity load and the other for lateral load. [3]
For gravity load, appropriate option for control is load to
level defined by pattern. It is zero initial condition and
therefore the structure has zero displacement and velocity.
And all elements are unstressed and there is no history of
nonlinear deformation. Monitored storey is roof level and in
Ux direction. Loading pattern are dead load and some portion
(mostly 25%) of live load. Minimum number of saved steps is
1, maximum number of null steps is 50, maximum number of
total steps is 200, maximum iteration/steps are 10, iteration
tolerance is 1.000E-04 and event tolerance is 0.01. [3]
Figure 5. Iteration step = 5, Performance Level = IO,
Base shear = 294.357 K , Displacement = 8.842 in
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Figure 9.shows the relationship between Base shear and
monitored displacement. Figure 10, 11, 12 and 13 show
Capacity Spectrum Curves for zone 2A, 2B, 3 and 4
respectively.
Figure 6. Iteration step = 8, Performance Level = LS,
Base shear = 347.66 K , Displacement = 16.91 in
Figure 9.Push-over Curve for Displacement and Base Shear
Figure 7. Iteration step = 9, Performance Level = CP,
Base shear = 359.897 K , Displacement = 19.534 in
Figure 8. Iteration step = 12, Performance Level = C,
Base shear = 373.701 K , Displacement = 24.997 in
Figure 10.ATC-40 Capacity Spectrum Curve for Zone 2A
Figure 11.ATC-40 Capacity Spectrum Curve for Zone 2B
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Figure 12.ATC-40 Capacity Spectrum Curve for Zone 3
Figure 13.ATC-40 Capacity Spectrum Curve for Zone 4
Table III
PUSH-OVER VALUES FOR DISPLACEMENT AND BASE SHEAR
Step
0
1
2
3
4
5
6
7
8
9
10
11
12
Displacement
in
0.000551
0.100544
1.985903
4.563420
6.054389
8.842094
11.674602
14.204222
16.909797
19.538913
22.228202
24.724109
24.996551
BaseForce
Kip
0.000
9.045
152.801
242.890
270.132
294.357
315.451
332.829
347.660
359.897
368.952
373.392
373.701
AtoB
BtoIO
IOtoLS
LStoCP
CPtoC
CtoD
1754
1750
1546
1358
1298
1244
1234
1204
1184
1168
1152
1150
1147
4
8
212
400
460
440
282
220
181
128
102
72
74
0
0
0
0
0
74
242
334
297
256
215
202
199
0
0
0
0
0
0
0
0
96
200
185
122
106
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
6
104
212
232
Although the structure is analyzed with seismic zone 2A, it
is considered to resist high seismic zone 4. Figure 10. shows
that the spectrum curve is reached at Immediate Occupancy
Level in zone 2A with performance point of base shear
(277.614K) and roof displacement(6.915in) and it is situated
in the elastic limit state. In Figure 11.the spectrum curve is
also reached at Live Safety Level in zone 2B with base shear
(294.666K) and roof displacement (8.884in). In Figure 12.the
spectrum curve is reached at Live Safety Level in zone 3 and
performance point at base shear (327.491K) and roof
displacement (13.427in). Both of Graph is situated in the
plastic limit state. In Figure 13.the spectrum curve is reached
at Collapse Prevention Level in zone 4 and it is situated
beyond the plastic limit state. Base shear and roof
displacement of figure 13 is 349.412K and 17.286in
respectively. Even small amount of load is added to this
structure, it will be reached at Collapse Level.
Case II
For lateral load, lateral displacement of the building is
considered only in X-direction. The method of displacement
control analysis is used. The 4% of total structure heights
(50.88in) is used for displacement control value.[2] Built-in
default hinges are used and only moment hinges are
considered. Hinges location are 5% from the column faces.[4]
The control option is Push to Displacement Magnitude which
is target displacement for this building. This start after the
gravity load case. Monitored storey and direction is the same
with the gravity load. Minimum number of saved steps is 10,
maximum number of null steps is 50, maximum number of
total steps is 200, iteration tolerance is 1.000E-04 and event
tolerance is 0.01.[3] The figures 14 to 19 show the results for
performance levels of the structure.
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Figure 14 .Iteration step=1, Performance Level = O,
Base shear = 9.045 K , Displacement = 0.101 in
Figure 17. Iteration step = 7, Performance Level = CP,
Base shear = 367.195 K , Displacement = 21.579 in
Figure 15. Iteration step = 5, Performance Level = IO,
Base shear = 312..97 K , Displacement = 11.336 in
Figure 18. Iteration step = 10, Performance Level = C,
Base shear = 375.438 K , Displacement = 28.274 in
Figure 16. Iteration step = 6, Performance Level = LS,
Base shear = 346.42 K , Displacement = 16.67 in
Figure 19. Iteration step = 19, Performance Level = C,
Base shear = 374.029 K , Displacement = 36.048 in
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Figure 20.shows the relationship between Base shear and
monitored displacement. Figure 21, 22, 23 and 24 show
Capacity Spectrum Curves for zone 2A, 2B, 3 and 4
respectively.
Figure 23.ATC-40 Capacity Spectrum Curve for Zone 3
Figure 20.Push-over Curve for Displacement and Base Shear
Figure 21.ATC-40 Capacity Spectrum Curve for Zone 2A
Figure 24.ATC-40 Capacity Spectrum Curve for Zone 4
Figure 22.ATC-40 Capacity Spectrum Curve for Zone 2B
Although the structure is analyzed with seismic zone 2A, it
is considered to resist high seismic zone 4. Figure 21. shows
that the spectrum curve is reached at Immediate Occupancy
Level in zone 2A with the performance point of base
shear(277.479K) and roof displacement(6.96in) . In Figure
22. the spectrum curve is also reached at Immediate
Occupancy Level in zone 2B with base shear(294.552K) and
roof displacement(9.065in) . Both of Graph is situated in the
elastic limit state. In Figure 23. the spectrum curve is reached
at Live Safety Level in zone 3 and the base shear is 326.174K
and roof displacement is 13.443in. It is situated in the plastic
limit state. In Figure 24. the spectrum curve is reached at
Collapse Prevention Level in zone 4 and it is situated beyond
the plastic limit state. Base shear and roof displacement of
figure 24 is 349.139K and 17.316in respectively. Even small
amount of load is added to this structure, it will be reached at
Collapse Level.
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International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Table IV
PUSH-OVER VALUES FOR DISPLACEMENT AND BASE SHEAR
V. CONCLUSIONS
The proposed building for study is eight-storied reinforced
concrete building at Mandalay. The structure is analyzed and
designed according to SAP 2000 software, UBC-97
specifications. Beams, columns, slabs and stairs are designed
with concrete compressive strength of fc′= 2500 psi and
reinforcing yield strength of fy = 40000 psi. When the Static
analysis is complete, the roof displacement is subjected to
Roof level. For gravity plus earthquake load is only
considered. Lateral displacement of the building was
considered X-direction only. In Case I, the displacement of
25in is subjected to Roof level joint 65. The performance
level is reached from step 1 to step 12. From step 1 to 4 ,
plastic hinges are found and the structure is reached in the
Operational Level. At step 5, 6 and 7, the structure is reached
in the Immediate Occupancy Level. The Life Safety Level is
reached at step 8. The structure is reached from step 9 to 11,
the performance level is in the Collapse Prevention Level.
In Case II, the displacement of 50.88in is subjected to Roof
level joint 65. For gravity plus earthquake load is only
considered. Lateral displacement of the building was
considered X-direction only. The performance level is
reached from step 1 to 19. From step 1 to 4, plastic hinges are
found and the structure is reached in the Operational Level. At
step 5, the structure is reached in the Immediate Occupancy
Level. The Life Safety Level is reached at step 6. The
structure is reached from step 7 to 9, the performance level is
in the Collapse Prevention Level. From step 10 to 19, the
structure is reached in Collapse Level. In the study, more
plastic hinges are found in beam than in column face.
From step 1 to 4, Case I and Case II have the same base
shear and roof displacement. From step 5 to the final step, the
base shear and roof displacement of Case II is greater than
that of Case I. But the Collapse Level of Case I and Case II
have the same roof displacement and base shear. So, the
displacement control value of 2% and 4% of the total
structure heights are not very differ. The performance level of
Collapse Prevention Level in Case I and Case II, base shear is
nearly 346K and roof displacement is nearly 16.7in in both
case. They have the same performance level of final overall
results. Based on the analytical results, the performance level
of the proposed building can be distinguished clearly with
pushover analysis. Therefore it is very effective in building
structures in order to know the effects of lateral force when
the seismic intensity reach in high level.
REFERENCES
[1]
[2]
[3]
[4]
[5]
ASCE/SEI 41-06, Seismic Rehabilitation of Existing Buildings,
ASCE, American Society of Civil Engineering, 2007.
ATC, 1996. Seismic Evaluation and Retrofit of Concrete Buildings,
Volume 1.
Pushover Analysis Manual in Structural Analysis Program (2000)
Version 14.0.0, CSI, Computer and Structure Institute.
FEMA 356, Federal Emergency Management Agency , Washington
D.C. (2000)
Uniform Building Code, Volume 2. Structural Engineering Design
Provisions 1997, 8th International conference of Building officials.
(1997)
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