Page 59 - Jabalpur_EQ

Chapter 3 Brick Masonry & Relntorced Concrete Buildings

Plan Asymmetry
Asymmetric distribution of mass and lateral stiffness caused considerable dam-
age in many buildings due to torsional forces. Significant torsion is expected
when distance between the centre of rigidity and the centre of mass exceeds 20%
of either major plan dimension (BSSC 1992a). Torsional forces tend to rotate the
diaphragm and displacement at the tops of the vertical elements induce a side-
sway moment which may have not been recognized gravity only design of ver-
in
tical elements.

5-Storey Himgiri Apartments, Wrighty Town
For example, in Fig. 3-10 which shows the ground floor plan of the 5-storey Him-
giri Apartments, the columns in the first storey were subjected to lateral dis
placements due to the rotation of the first floor which was caused by torsional
forces. The gravity only design of columns did not account for these sidesway
moments and column sections near the first floor failed (Fig. 3-8).

This building deficiency lies in its layout, and strength and stiffness of infill walls
and surrounding frames. Bracing the open bays or building shear walls will
reduce the eccentricity between the centre of mass and the centre of rigidity. One
should be very cautious when designing an asymmetrical layout for the vertical
elements, because they can be designed to meet any amount of torsional forces
but they are not likely to perform well during earthquakes.

Eccentricity

Centre of Centre of
Rigidity Mass

Lateral displacements
at the tops of columns

Figure 3-10. Asymmetrical arrangements of vertical elements results into eccen-
tricity causing torsional forces and displacements.

Jabalpur Earthquake of May 22, 1997 49