Twisting of buildings is one of the factors that cause severe damages during earthquakes. This commonly occurs in buildings due to stiffness irregularities in plan and elevation of buildings, and mass irregularity in plan and elevations of buildings.
So, when an earthquake occurs, the seismic forces would cause greater movement at the side of a building in which the mass of the floor is greater or the vertical members are not distributed with the same concentration compare with the other side.
Causes of Building Twisting During Earthquake
1. Stiffness Irregularity in Plan
Irregularity in stiffness in the plan occurs due to the use of columns of different sizes, presence of structural wall on one side of buildings, or presence of staircase or elevator core at one corner of buildings, Fig. 1.
Buildings with unequal vertical members such as columns, or walls, or a combination thereof, when shaken at its base in certain directions due to seismic forces, its floors twist about a vertical axis and displace horizontally as shown in Fig. 2.
Similarly, buildings that have walls only on two sides or one side and thin columns along with the other, twist when shaken at the ground level, Fig. 3.
2. Mass Asymmetry in Plan of Buildings
If the mass on the floor of a building is more on one side, then that side of the building moves more due to ground movement Fig. 4. This building moves such that its floors displace horizontally as well as rotate.
For instance, one side of a building may have storage, or a library, or water tanks at the rooftop. Water tanks with large mass of water are usually placed at corners of buildings. This affects the distribution of mass in the plan. These irregularities in mass in plan causes twisting of buildings during earthquake shaking due to mismatch of the center of mass and center of rigidity.
3. Stiffness Irregularity in Elevation
Stiffness irregularities come up in buildings as a result of structural and architectural considerations. Buildings that are irregular shapes in plan tend to twist under earthquake shaking. For example, in a propped overhanging building Fig. 6, the overhanging portion swings on the relatively slender columns under it. The floors twist and displace horizontally.
Effect of Twisting on Buildings
Twisting 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 columns and walls on the side that moves more Fig.
Many buildings have been severely affected by this excessive torsional behavior 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 vertical members).
If this twist cannot be avoided, special 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 such calculations. But, for sure, buildings with a twist would perform poorly during strong earthquake shaking.