other occasions both shape and structural system work together to make the structure a marvel. 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 Buildings

In tall buildings with large height-to-base size ratio (Figure 1a), the horizontal movement of the floors during ground shaking is large. In short but very long buildings (Figure 1b), the damaging effects during earthquake shaking are many. And, in buildings with large plan area like warehouses (Figure 1c), the horizontal seismic forces can be excessive to be carried by columns and walls.

Horizontal Layout of Buildings

In general, buildings with simple geometry in plan (Figure 2a) have performed well during strong earthquakes. Buildings with re-entrant corners, like those U, V, Hand + shaped in plan (Figure 2b), have

(a) Simple Plan :: good (b) Corners and Curves : : poor (c) Separation joints make complex plans into simple plans Figure 2: Simple plan shape building do well during earthquakes

sustained significant damage. Many times, the bad effects of these interior corners in the plan of buildings are avoided by making 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 (Figure 2c). Often, the plan is simple, but the columns/walls are not equally distributed in plan. Buildings with such features tend to twist during earthquake shaking. A discussion in this aspect will be presented in the upcoming IITK-BMTPC Earthquake Tip 7 on How Buildings Twist During Earthquakes?

Vertical Layout of Buildings

The 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 (like the hotel buildings with a few storeys wider than the rest) cause a sudden jump in earthquake forces at the level of discontinuity (Figure3a). Buildings that have fewer columns or walls in a particular storey or with unusually tall storey (Figure3b), tend to damage or collapse which is initiated in that storey. Many buildings with an open ground storey intended for parking collapsed or were severely damaged in Gujarat during the 2001 Bhuj earthquake.

Buildings on a sloping ground have unequal height columns along the slope, which causes ill effects like twisting and damage in shorter columns (Figure 3c). 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 (Figure 3d). Some buildings have reinforced concrete walls to carry the earthquake loads to the foundation. Buildings, in which these walls do not go all the way to the ground but stop at an upper level ,are liable to get severely damaged during earthquakes.
 

(a) Setbacks
		Unusually Tall Storey
(b) Weak or Flexible Storey
(c) Slopy Ground	(d) Handing or Floating Columns
		Reinforced Concrete Wall Discontinued in Ground Storey
(e) Discontinuing Structural Members
Figure 3: Sudden deviations in load transfer [path along the height lead to poor performance of buildings

Adjacency of Buildings

When two buildings are too close to each other, they may pound on each other during strong shaking. With

Figure 4: Pounding can occur between adjoining buildings due to horizontal vibrations of the two buildings

ncrease in building height, this collision can be a greater problem. When building heights do not match (Figure 4), the roof of the shorter building may pound at the mid-height of the column of the taller one; this can be very dangerous.

Building Design and Codes…

Looking ahead, of course, one will continue to make buildings interesting rather than monotonous. However, this need not be done at the cost of poor behaviour and earthquake safety of buildings. Architectural features that are detrimental to earthquake response of buildings should be avoided. If not, they must be minimised. When irregular features are included in buildings, a considerably higher level of engineering effort is required in the structural design and yet the building may not be as good as one with simple architectural features. Decisions made at the planning stage on building configuration are more important, or are known to have made greater difference, than accurate determination of code specified design forces.

Resource Material

Arnold, C., and Reitherman, R., (1982), Building Configuration and Seismic Design, John Wiley, USA.

Lagorio, H,J, (1990), EARTHQUAKES An Architect’s Guide to Non-Structural Seismic Hazard, John Wiley & Sons, Inc., USA.

 

Authored by: C.V.R. Murty

Indian Institute of Technology Kanpur, India

Sponsored by: Building Materials and Technology Promotion Council, New Delhi.

 

Suggestions/comments may be sent to eqtips@iitk.ac.in 

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