Page 71 - Jabalpur_EQ

Brick Masonry & Reinforced Concrete Bulldings
Chapter 3

and ductility and may need to be strengthened. Second, infills can be treated as
loads
shear walls and, in that case, their inadequate shear capacities for in-plane
and flexural capacities for out-of-plane ioads need to be corrected (BSSC 1992b,
Rosenblueth 1980).

PERFORMANCE OF STAIRCASE ENCLOSURES

Frequent failures of the walls enclosing the stairs at the roof level of many build-
motions at
ings can be attributed to the large amplification of the ground upper
floors and to excessive slenderness of the enclosing walls. It has been observed
that stairwells attract relatively large amount of seismic forces due to the bracing
action provided by inclined members (slab or beam) to support stairs. Further-

more, walls enclosing stairwells are cantilevered structures projecting above the
roof located around a discontinuity in diaphragms (floors and slabs) which act as
stress-raisers. Damage to staircase enclosures, called mumty in the local parlance,
was widespread and was seen in structures which otherwise would have sur-
vived the shaking without any significant damage.

Dynamic instability of the slender walls and poor anchorage with the roof slab
were two primary reasons for such a poor performance of numties. Figure 3-25
shows a typical arrangement of a mumty. For most of mumty walls, the slender-

One & half storey
high insufficiently
braced brick wall
and jali

H

H

Figure 3-25. Typical
arrangement of stairwell
in low rise buildings in
the Jabalpur area.

Jabalpur Earthquake of May 22, 1997 61