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IITK-BMTPC Earthquake Tip 7

How Buildings twist during Earthquakes? page 2
Once more, let us consider the rope swing on the
tree. This time let the two ropes with which the cradle
is tied to the branch of the tree be different in length.
Such a swing also twists even if you sit in the middle
(Figure 4a). Similarly, in buildings with unequal
structural members (i.e., frames and/or walls) also the
floors twist about a vertical axis (Figure 4b) and Earthquake
Ground
displace horizontally. Likewise, buildings, which have Shaking

walls only on two sides (or one side) and flexible Figure 5: One-side open ground storey building
frames along the other, twist when shaken at the twists during earthquake shaking.
ground level (Figure 4c).
What Twist does to Building Members
Twist in buildings, called torsion by engineers,
makes different portions at the same floor level to
move horizontally by different amounts. This induces
more damage in the frames and walls on the side that
moves more (Figure 6). Many buildings have been
severely affected by this excessive torsional behaviour
during past earthquakes. It is best to minimize (if not
completely avoid) this twist by ensuring that buildings
have symmetry in plan (i.e., uniformly distributed
mass and uniformly placed lateral load resisting
systems). If this twist cannot be avoided, special
(a) Swing with unequal ropes calculations need to be done to account for this
additional shear forces in the design of buildings; the
Indian seismic code (IS 1893, 2002) has provisions for
Vertical Axis about such calculations. But, for sure, buildings with twist
which building twists
will perform poorly during strong earthquake shaking.

Earthquake
Ground Earthquake

Ground
Movement Movement

(b) Building on slopy ground These columns are more vulnerable
Figure 6: Vertical members of buildings that move

Wall more horizontally sustain more damage.

Wall Reading 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-
Wall Structural Seismic Hazard, John Wiley & Sons, Inc., USA
Columns
Beam Beam Columns Authored by:

(c) Buildings with walls on two/one sides (in plan) C.V.R.Murty
Indian Institute of Technology Kanpur
Figure 4: Buildings have unequal vertical Kanpur, India

members; they cause the building to twist Sponsored by:
about a vertical axis. Building Materials and Technology Promotion

Buildings that are irregular shapes in plan tend to Council, New Delhi, India
twist under earthquake shaking. For example, in a
propped overhanging building (Figure 5), the This release is a property of IIT Kanpur and BMTPC New
overhanging portion swings on the relatively slender Delhi. It may be reproduced without changing its contents
and with due acknowledgement. Suggestions/comments
columns under it. The floors twist and displace may be sent to: nicee@iitk.ac.in. Visit www.nicee.org or
horizontally. www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.

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