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Buildings should hold ground
7/26/2006
 

          The behaviour of a building during earthquakes depends critically on its overall shape, size and geometry, in addition to how the earthquake forces are carried to the ground. Hence, at the planning stage itself, architects and structural engineers must work together to ensure that unfavourable features are avoided and a good building configuration is chosen.
A desire to create an aesthetic and functionally efficient structure drives architects to conceive wonderful and imaginative structures. However, each of these choices of shapes and structure has significant bearing on the performance of the building during strong earthquakes.
The wide range of structural damages observed during past earthquakes across the world is very educative in identifying structural configurations that are desirable versus those which must be avoided.
Size of the buildings: In tall buildings with large height-to-base size ratio, the horizontal movement of the floors when the ground shakes is large. In short but very long buildings, the damaging effects during the tremor are many. And, in buildings with large plan area such as warehouses, the horizontal seismic forces can be excessive to be carried by columns and walls.
Horizontal layout: In general, buildings with simple geometrical plans have performed well during strong earthquakes. Buildings with re-entrant corners, such as U, V, H and cross-shaped in plan, have sustained significant damage.
Mostly, the bad effects of these interior corners in the plan of buildings are avoided by conceiving the buildings in two parts.
For example, an L-shaped plan can be broken up into two rectangular plan shapes using a separation joint at the junction. Often, the plan is simple, but the columns/walls are not equally distributed in the plan. Buildings with such features tend to twist during an earthquake.
Vertical layout: Earthquake forces developed at different floor levels in a building need to be brought down along the height to the ground by the shortest path; any deviation or discontinuity in this load transfer path results in poor performance of the building.
Buildings with vertical setbacks (such as hotel buildings with a few storeys wider than the rest) cause a sudden jump in earthquake forces at the level of discontinuity. Buildings that have fewer columns or walls in a particular storey or with unusually tall storey tend to get damaged or collapse.Manybuildings with an open ground storey intended for parking collapsed or were severely damaged in Gujarat during the 2001 earthquake.
Buildings on sloping ground have unequal height columns along the slope, which causes twisting and damage in shorter columns. Buildings with columns that hang or float on beams at an intermediate storey and do not go all the way to the foundation have discontinuities in the load transfer path.
Adjacent buildings: When two buildings are too close to each other, they may pound each other during a strong quake. With increase in height, this collision can be greater.
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