] [Thread] [Date] [Author]

["A. Sarvghad" <sarvgha(--nospam--at)mcmail.CIS.McMaster.CA>]

> 
> I'm interested in the change of lateral forces distribution by mode shapes
> during pushover analysis.
> Would you explain the process in more detail ?
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The lateral load distribution used in the pushover can affect the accuracy
of the results. Ideally, the distribution should represent the lateral
inertial force distribution that causes the most severe responses on the
building. Such distribution depends on, among other variables, the
severity of the shaking, the dynamic characteristics of the building and
the frequency contents in the ground motion. There is no general agreement
on the best choice of lateral load distribution to be used in the pushover
process. Currently, two types of load distribution are used. They are the
fixed load distribution, and the variable load distribution.

In the fixed load distribution, the distribution is determined a priori
and remains unchanged during the pushover. Some of the fixed distributions
used are:

--A single concentrated horizontal force at the top floor. 
--Uniform load distribution on all floors.
--Triangular or standard code load distribution.
--A load distribution proportional to the product of the mass vector and
the fundamental mode shape. 
--Lateral force distribution based on a linear elastic dynamic analysis or
response spectrum analysis of the building.

To allow for the changes of inertial forces with the level of inelastic
deformation, some researchers have proposed adaptive load patterns to be
used in the pushover. The load distribution changes as the building is
deformed to larger and larger displacements. Some of the variable load
distributions are:

--A distribution proportional to the product of the mass vector and the
fundamental mode shape is used initially until first yielding takes
place. Then for each load increment beyond yielding, the forces are
adjusted to be consistent with the deflected shape in the inelastic state.
--A load distribution is based on the product of the current floor
displacements and masses.
--A distribution based on mode shapes derived from secant stiffnesses at
each load step.
--A distribution proportional to story shear resistances at each step.

These adaptive load distributions require more computational effort.
However, according to the following reference, their superiority over the
more simple fixed load distributions has not been established.

**Applied Technology Council."Guidelines for the Seismic Rehabilitation of
**Buildings" Volume I, Guidelines, and Volume II, Commentary. FEMA
**273/274-Ballot Version. Prepared by the Applied Technology Council for
**the Building Seismic Safety Council; published by the Federal Emergency
**Management Agency, 1996.
--------
Abdi (A.S. Moghadam)
sarvgha(--nospam--at)mcmaster.ca
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