BUILDINGS FRAMES LEADS TO HORIZONTAL TWISTING UNDER WIND OR EARTH QUAKE RESISTANT (FORCES)

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 on the distance between center of mass and center of rigidity which is called eccentricity. The design eccentricity is taken as 1.5 times the computed eccentricity in accordance with clause 4.2.4 of IS : 1893 -1984 .The location of the centre of rigidity or centre of rotation is determined by a method similar to that used to obtain the centre of rotation of a rivet group. Lateral stiffness of the vertical elements is used in place of the cross sectional area of the rivets .This means that 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 than 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 .Clause 4.2.4 of IS:1893 -1984 require that shear force should not be reduced on account of torsion .Once 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 can be determined and these member forces should be superimposed on those obtained from the gravity analysis to get the design forces .The frame members and joints can then be designed in accordance with the limit state design method .