Page 74 - Jabalpur_EQ

Brick Masonry & Relinforced Concrete Buildings
Chapter 3

most of buildings in the area were not designed in accordance with Indian seis-
mic codes, the adequacy of code provisions can not be determined.

DISCUSSION OF STRUCTURAL BEHAViOUR

Despite the fact that the majority of buildings were not designed for seismic
loads, many of them survived the earthquake with minor damage. One and twwo
those
storey unreinforced masonry buildings performed satisfactorily, especially
which did not suffer from major layout or planning deficiencies and where the
quality of workmanship and material were good. The masonry infill walls in the
weak RC frames clearly enhanced the overall shear resistance of the structure
when the integrity of RC frames under lateral loads was doubtful and provided
a redundant load path.

It has been a difficult task to include the effect of infill walls into the analysis as
well as in the design because of the uncertainty in how the brick walls interact
with the surrounding frame. If RC frame alone is considered to be the structural
system, then it is deficient in connection details, strength, drift and compatibility
with brick infills. If the masonry infillis considered to be shear wal, then it lacks
continuity and is brittle and unreinforced. They should be either isolated from the
frame with adequate gap or should be designed as shear walls with adequate
strength and ductility.

Ordinary structures are not expected to resist seismic loads elastically and there-
fore, they are designed to undergo large inelastic deformations and to possess a
stable hysteretic behaviour, i.e., designed to be ductile. The earthquake-resistant
design derives much of its success from the provisions which ensure inelastic
deformability of structural components and energy dissipation capacity and less
from the concept of lateral load coefficients and response spectra. Such ductility
provisions specified in the IS:13920-1993 should be followed for all RC structures.

CONCLUSIONS

The damage caused by moderate-size, shallow earthquakes increases propor-
tionally with the growth of urban centers and population. This observation was
once again verified in the Jabalpur earthquake of May 22, 1997. An area of about
15 km in radius was severely affected by the earthquake, which caused extensive
damage to residential houses and to office buildings.

Conventional unreinforced masonry structures suffered the most damage near
the epicentral region, and some of them were damaged beyond repair. Mixed
construction types and RC frame buildings performed relatively better and the
majority of them survived with minor damage to the brick infill walls.

Jabalpur Earthquake of May 22, 1997 64