Page 98 - Jabalpur_EQ

Chapter 6 Lifelines

forces. However, even this level of resistance is inadequate for more advanced
seismic codes, such as Uniform building Code (ICBO 1996) in similar seismic risk
areas. It is believed that seismic forces were indeed large when the tank was
nearly full on the morning of the earthquake. If the designer had provided only
the code level strength, the damage would have been more severe, possibly

endangering its lateral stability of the entire structure. The slender staging that
results from the low design forces specified in IS:1893 1984, are very unfavour-
able feature for seismic areas.

Critical Review of Is:1893-1984 for Seismic Design
Forces for Water Tanks

The supporting structure (staging) of an overhead reservoir may look like that
used in the building-like structures but its behaviour under seismic loads is very
different. Unlike buildings, in overhead reservoirs most of the mass is concen-
trated at the top of the staging. Moreover, the staging does not have redundancy,
a fundamental characteristic for good seismic performance, which is present in
the multiple bays and frame-lines of a building framing system. This lack of

redundancy is more serious in circular shaft type staging where lateral stability of
the structure depends on only single element, i.e., shaft, and failure of which
would severely jeopardize the lateral stability ot the entire structure. Addition-
ally, such structures lack the damping and additional strength which is present in
building systems due to non-structural and non-considered elements.

For the above reasons, the non-building systems such as stagings need to be
designed for seismic forces much larger than that would be needed for a building
with similar "dynamic" (ie., mass and period) properties (SEAOC 1990). The
observation is affirmed by advanced building codes such as UBC:96 which spec-
ifies larger design forces for non-building systems.

IS:1893-1984 prescribes design forces for elevated tower-supported tanks at 1.5
times of the most ductile moment frame system for ordinary buildings. This
increase in forces is due to the importance factor, not because of the structure per-
formance factor K. The value of K for water tanks is taken as 1.0 which is same as
specified for the most ductile building framing system. In comparison, the Uni-
form Building Code (ICBO 1996) prescribes the importance factor of 1.25 for
water tanks but specifies different values for the structure performance factor to
indicate their lack of redundancy. As a result, IS:1893-1984 prescribes forces
which are much smaller for elevated tower-supported tanks compared to
UBC:96, which can not be justified. The damage observed in the Jabalpur earth-
quake illustrates that the design forces are currently being underestimated at
least by a factor of 3.
For the seismic zone III, the IS:1893-1984 design spectrum curves for a building
system (e.g. moment resisting frame) and a non-building system (e.g. a water
tank) are compared with that of UBC:96 in a similar seismic risk zone (i.e., Zone
II of UBC:96) in Fig. 6-4.

Jabalpur Earthquake of May 22, 1997 88