Page 52 - Jabalpur_EQ

Brick Masonry & Reinforced Concrete Buildings
Chapter 3

structures resisted the in-
The load bearing brick walls in unreinforced masonry
plane shear imposed by lateral loads during the earthquake. The brick walls were
often weakened by the openings for doors and windows. The system of wall piers
and spandrels thus created was observed to fail in the shear, developing familiar
X or diagonal cracks. There were many out-of-plane failures due to the dynamic

instability of slender walls.
The region also enjoys the popularity of mixed construction, which employs RC
floor slabs, cantilevers (balconies and porches), and a few columns, while load
bearing masonry walls carry most of the gravity loads. Damage statistics point
out the limited success of this type of construction for buildings taller than twvo
storeys. However, when construction practices were sound and the building suf-
fered no major irregularities of any kind, this type of construction fared well for
most of the two storey houses.

The affected RC buildings were located within the town and, when properly con
structed, they exhibited superior behavior in comparison with non-engineered
structures. Most of these structures were undamaged or suffered minor non-
structural damage. However, such buildings close to the epicentral area suffered
extensive damage to the infill walls and/or partitions. Infills failed in in-plane
shear in addition to knocking out-of-plane due to lack of anchorage in the sur-
rounding frame. Failures in RC beams or columns were rare.

The type of construction that constitutes the majority of new building stock can
be best described as weak RC frame with solid brick infills The beam-column frame
is designed for gravity loads, or is expected to carry only such loads, and there-
fore, are weak in resisting lateral loads. Moreover, such frames may not be
present in both directions of the structure. Often, RC portal frames are used to
support the floor over the open bays in one direction while load bearing brick
walis are used in the other direction. Unreinforced brick infills are used for exte-
rior and interior curtain walls and partitions. Very high in-plane shear stiffness
and a significant amount of shear strength before the masonry begins to crack are
characteristics of infill wall. Being a stiffer element in the load path, these walls
attract more seismic forces than the flexible and weak RC frames. Since masonry
walls are able to accommodate only small movements within the mortar joints,
they begin to crack at relatively low seismic force levels. Severe cracking of infill
walls was observed and in some cases, the bricks shed off the frame posing a seri-
ous falling hazard.

DESCRIPTION OF STRUCTURAL DAMAGE

It is difficult to establish precisely what caused the collapse or failure of most of
structures. In most cases, the collapse or failure occurred due to errors in the con-
ceptual design (i.e. not conforming to sound earthquake-resistant design prac-
tice) and/or construction errors that are recognized and accounted for by many
building codes worldwide, including Indian Standard.. A description of damage

Jabalpur Earthquake of May 22, 1997 42