Performing Pushover Analysis in Etabs
Performing Pushover Analysis in Etabs
Performing Pushover Analysis in Etabs
About ETABS
ETABS is a software for linear and nonlinear , static and dynamic analysis and
design of buildings in 3D. It can create computer model of a building in the
same fashion as a real building.
Building Structure
Floor Diaphragm
Lateral Load Transmitting
Frame and Shear Walls Element
Lateral Load Resisting Elements
Floor Slab Elements
Gravity Load Resisting Elements
(e) 2D Frame
Various Ways
Fig. 1 Various Waysto Model a Real Structure
to Model a Real Struture
6 @ 3.0m
N-S beams: 45cm x 60cm
Slabs: 135mm
6.0m
3.75m
6.0m 5.0m 6.0m
Column Deformation Elastic Rotation < 0.01 radians Rotation < 0.02 radians
(FEMA 273 p. 6-20)
Beam Deformation Elastic Rotation < 0.02 radians Rotation < 0.025 radians
(FEMA 273 p. 6-19)
Allowable Drift Index 0.005 0.010 0.020
Note: Drift index is the ratio of the horizontal roof displacement to the height of the building relative to the ground.
One often asks: Can my building withstand a magnitude 7.2 earthquake? But,
the M7.2 earthquake causes different shaking intensities at different locations,
and the damage induced in buildings at these locations is different. Thus,
indeed it is particular levels of intensity of shaking that buildings and
structures are designed to resist, and not so much the magnitude. The peak
ground acceleration (PGA), i.e., maximum acceleration experienced by the
ground during shaking, is one way the engineers and scientists quantify the
severity of the ground shaking (seismic intensity).
Ca and Cv are the earthquake near-source effect that defines ground motion
range of the spectrum controlled range of the spectrum
Make sure that you read through the sections of the FEMA-273
Guidelines and Commentary or its updated version FEMA 356 before
you attempt to apply the NSP. The procedures discussed here automate
the process but you still need a thorough knowledge of the method to
ensure valid results. Finally, it is emphasized that the ETABS
documentation for the pushover is not intended to, and does not,
document the pushover analysis method, but rather is intended to
document the pushover analysis capabilities of ETABS.
The following ETABS screen shots outlines the basic flow from creating a
structural model to analysis and design, and to viewing the results of
nonlinear static pushover analysis.
1. Create an elastic building model just like you would for any other
analysis.
Orthogonal Effects in Spectral Analysis (ORTHOGONAL EFFECTS are the earthquake load effects on
structural elements common to the lateral-force-resisting systems along two orthogonal axes).
A well designed structure should be capable of equally resisting earthquake motions from all possible directions.
One option in existing design codes for buildings requires that members be designed for "100 percent of the
prescribed seismic forces in one direction plus 30 percent of the prescribed forces in the perpendicular direction".
However, they give no indication on how the directions are to be determined for complex structures. For structures
that are rectangular and have clearly defined principal directions, these "percentage" rules yield approximately the
same results as the SRSS method of directional combination. For rectangle building ETABS and SAP2000 can
handle the percentage rule automatically by ABS directional combination with orthogonal scale factor of 1. The
SRSS method can be both used in rectangular or arbitrarily shaped building.
The required design seismic forces may come from any horizontal direction and, for the purpose of design, they
may be assumed to act non-concurrently in the direction of each principal axis of the structure.
For three dimensional response spectra analyses, it has been shown that the "design of elements for 100 percent of
the prescribed seismic forces in one direction plus 30 or 40 percent of the prescribed forces applied in the
perpendicular direction" is dependent on the user's selection of the reference system. These commonly used
"percentage combination rules" are empirical and can underestimate the design forces in certain members and
produce a member design which is relatively weak in one direction. It has been shown that the alternate building
code approved method, in which an SRSS combination of two 100 percent spectra analyses with respect to any
user defined orthogonal axes, will produce design forces that are not a function of the reference system. Therefore,
the resulting structural design has equal resistance to seismic motions from all directions.
Scaling of Results
After dynamic base shear are calculated in the major and minor principal direction. The next step is to scale
the previously used spectra shapes by the ratio of static design base shear to the minimum value of the
dynamic base shear. This approach is more conservative than proposed by the current requirements because
only the scaling factor that produces the largest response is used. However, this approach is far more rational
because it results in the same design earthquake in all directions.
The current code allows the use of different scale factors for a design spectra in two different directions. This
will result in a design that has a weak direction relative to the other principal direction.
1. If a normalized or acceleration spectra was used to calculate the spectral results, the spectral
results must be scaled to match the site specific spectra and structure specific criteria.
2. The UBC sets minimum values for the design base shear.
1. Run and review the results of the basic linear and dynamic analysis to
verify the behavior of the building prior to running pushover analysis.
Note:
Refer to the ETABS User Manual how these overrides are used by ETABS.
Microscopic
F F F F
D D D D
KINEMATICORIGIN-ORIENTED
PEAK-ORIENTEDCLOUGH DEGRADING TRILINEAR
TAKEDA
Fiber Models
Fiber Section
Increasing level of
Distributed Nonlinearity
Microscopic
Pushover analysis will continue until any of the following three conditions is satisfied:
a) Cumulative base shear is less than or equal to the base shear defined by the user.
b) Displacement at the control joint in the specified direction exceeds specified
displacement.
c) The structure becomes unstable.
Click
1. Display the pushover curve to view the building responses according to the
performance point or to adjust the intensity of ground shaking and the
damping ratio to determine the appropriate levels of ground motion and
performance objectives for the building in order to meet the owners
expectation.
Capacity Pushover deformed shape shows that life safety is reached in step 7 and
collapsed in the second story beams in step 8.
Ductility - In the Pushover curve the severe event of earthquake design level
performance base shear is 4633 kN and the severe event of earthquake design level
inelastic roof displacement is 0.190 meter, while the Code design elastic base shear is
1467 kN and the Code design elastic roof displacement is 0.059 meter.
There is a possibility that the beams in the second story will be damaged in step 8 to an
earthquake intensity with earthquake near source effect Ca = 0.4 and Cv = 0.672, that
defines the ground motion response within the acceleration and velocity controlled
ranges of the design response spectrum.
Since the computed drift ratio (/H) = 0.012) when the specified earthquake magnitude
is exceeded is less than idealized inelastic drift index of 0.02 and the inelastic response
of the building in the earthquake design level shows life safety capacity, the design of
the building is within Life Safety and Collapse Prevention performance objectives.
The building reliable inelastic displacement of 0.190 meter has increased to
0.251 meter when the specified earthquake magnitude is exceeded, that is, when Ca and
Cv are 0.4 and 0.9 respectively.
The roof displacement corresponding to the severe event of earthquake design level is
0.190/0.059 = 3.22 times the roof displacement at Code elastic design level.
The overall result of the pushover analysis in X-direction indicated that the capacity and
ductility of the seven story RC building is adequate to sustain the severe earthquake
excitation, and some members of the moment resisting frame remain elastic and thus
maintain the stability of the building structure.
Analysis results for other load pattern will be demonstrated by running ETABS