NonLinear Static Analysis for Buildings
Seismic analysis of buildings can be categorized depending upon the
sophistication of modeling adopted for the analysis. Buildings loaded beyond
the elastic range can be analyzed using NonLinear static analysis, but in
this method one would not be able to capture the dynamic response, especially
the higher mode effects. This is pushover analysis. There is no specific code
for NLSA. This procedure leads to the capacity curve which can be compared
with design spectrum/DCR of members and one can determine whether the building
is safe or needs strengthening and its extent.
The capacity of structure is represented by pushover curve. The most
convenient way to plot he load deformation curve is by tracking the bas shear
and the roof displacement. The pushover procedure can be presented in various
forms can be used in a variety of forms for the use in a variety of
methodologies. As the name implies it is a process of pushing horizontally,
with a prescribed loading pattern, incrementally, until the structure reaches
the limit state. There are several types of sophistication that can be used
over for pushover curve analysis.
Level1: It is generally used for single storey building,
whereat a single concentrated horizontal force equal to base shear applied at
the top of the structure and displacement is obtained.
Level2: In this level, lateral force in proportion to storey
mass is applied at different floor levels in accordance with IS:18932002
(PartI) procedure, and story drift is obtained.
Level3: In this method lateral force is applied in
proportion to the product of storey masses and first mode shape elastic model
of the structure. The pushover curve is constructed to represent the first
mode response of structure based on the assumption that the fundamental mode
of vibration is the predominant response of the structure. This procedure is
valid for tall buildings with fundamental period of vibration upto 1 sec.
Level4: This procedure is applied to soft storey buildings,
wherein lateral force in proportion to product of storey masses and first mode
of shape of elastic model of the structure, until first yielding, the forces
are adjusted with the changing the deflected shape.
Level5: This procedure is similar to level 3 and level 4 but
the effect of higher mode of vibration in determining yielding in individual
structural element are included while plotting the pushover curve for the
building in terms of the first mode lateral forces and displacements. The
higher mode effects can be determined by doing higher mode pushover analysis.
For the higher modes, structure is pushed and pulled concurrently to maintain
the mode shape.
Case Study of NonLinear Static Analysis using SAP2000
