STEPS FOR SAFE DESIGN AND CONSTRUCTION OF MULTISTOREY REINFORCED CONCRETE BUILDINGS - PART 2
2.5 Heavy Water Tanks on the Roof:
Heavy water tanks add large lateral inertia forces on the building frames due to the so called ‘whipping’ effect under seismic vibrations, but remain unaccounted for in the design. See the fall of such water tank in
Fig.10 - 5 storey R.C., collapse of open plinth, water tank at top dislocated (Bhuj)
All projected systems above the roof top behave like secondary elements subjected to roof level horizontal earthquake motions which act as base motions to such projecting systems. To account for such heavy earthquake forces, IS:1893-2002 (Part 1) provides in clause 7.12 that their support system should be designed for five times the design horizontal seismic co-efficient Ah specified in clause 6.4.2. Similarly any horizontal projections as the balconies or the cantilevers supporting floating columns, the cantilevers need to be designed for five times the design vertical co-efficient as specified in clause 6.4.5 of IS: 1893-2002 (Part 1)
2.6 Lack of Earthquake Resistant Design:
Many buildings in Gujarat were not designed for the earthquake forces specified in IS:1893, which was in existence from 1962, revised in 1970, 1976 & 1984. The applicable seismic zoning in Gujarat had remained the same as adopted in 1970 version. It is the same even in 2002 version of IS:1893 (Part I).
Fig.11:- Lateral Strength of Building Frame
Inspite of that, the structural designers ignored the seismic forces in design. It may also be stated that most buildings are designed against lateral load in the transverse direction. Hence they collapse in the longitudinal directions.
Proper arrangement of columns is shown in Fig. 11 which would give adequate seismic resistance along both axes of the building.