Abstract:
Many buildings in the present day scenario have irregular configurations in both plan
and elevation. Past earthquake studies show that buildings suffer severe damage
during earthquakes due to torsional irregularity. Response spectrum method is the
most commonly used tool in the seismic analysis of structures with torsional
irregularity. Number of retrofitting works have been increased in recent years because
it reduces the vulnerability of damage of an existing irregular structure in the event of
a future earthquake.
The study is conducted considering planar irregularity in ordinary moment resisting
frame (OMRF) only. Many structures with planar irregularity were designed without
considering torsional effects, require retrofitting. Considering this fact, the study
assessed design of structures by response spectrum method of seismic analysis, design
of retrofit schemes and estimates for these designs using PWD rate schedule.
In this study, 4-storied, 6-storied, 8-storied and 10-storied buildings with same
architectural plan are considered in soil type S1, S2 & S3 in seismic zone-2.
Equivalent static load method and response spectrum method have been adopted.
These irregular plans were modeled using Finite Element Method (FEM). BNBC code
is used to analyze and design structures. FEMA guidelines and methodology are used
for retrofitting works. The analysis was performed considering concrete strength 3500
psi, reinforcing steel strength 60 ksi, live load 63 psf and peripheral brick wall load.
From the study it is found that structures with torsional irregularity considered in this
study which is designed by equivalent static load method is vulnerable for response
spectrum analysis and retrofit is needed for them. Variations of design output for two
design methods increase with increasing building story height for three types of soils.
For a building of specific height outputs of these methods vary for different soil types.
However, soil variation effect is little for 4-storied building. It also has been observed
that almost two times extra cost is needed for retrofit compared to the extra cost for
earthquake resistant design of new buildings.
