IT IS NECESSARY TO COMPUTE ADDITIONAL SHEAR AT EACH COLUMN LINE AND STOREY DUE TO TORSION MOVEMENT OF THE UNSYMMETRICAL BUILDING , PLANNED FOR THE SEISMIC ZONES.

TORSION IN BUILDING :-- Building frames are generally unsymmetrical in plan as well as in elevation.This leads to horizontal twisting of frames when subjected to wind or earthquake forces .The magnitude of horizontal tortional movement depends upon the distance between centre of mass and center of rigidity which is called eccentricity and the design eccentricity is taken 1.5 times the computed eccentricity in accordance with the provision under IS :1893-1984.Some other national building codes require that the design eccentricity is taken as 1.5 times the computed eccentricity or 0.05 times the maximum plan dimension at that level , which ever is greater .The additional storey shear due to horizontal torsional movement may be calculated approximately by assuming the vertical elements (ie columns and the shear walls) at each storey to be fixed at their ends to parallel rigid plates .The torsional shear force acting on each element may then be taken as proportional to its lateral stiffness and it's distance from the centre of rigidity of the storey under consideration.The flexural action of the vertical elements resists the horizontal torsion .Centre of mass is the centre of gravity of the mass associated with a storey.The shear due to torsion along each axis resisted by a particular element will then be proportional to the lateral stiffness of the element, relative to the total rotational stiffness of the storey and its distance from the centre of rigidity.The net storey shears are known,lateral forces can be computed at each floor along the column line under consideration.The member forces in a frame be determined using either of the method.These member forces should be superimposed on those obtained from the gravity analysis to get the design forces.The frame members and the joints can be then designed in accordance with the limit state design method.