Wind-Induced Torsional Loads and Responses of Buildings: Fid-Bau Portal

Wind-Induced Torsional Loads and Responses of Buildings

Isyumov, N. / Case, P. C.

Wind-induced loads on a building are invariably spatially non-uniform and some twisting about its vertical axis results. The magnitude of this wind-induced moment or torque depends on the shape of the building, the influence of surrounding buildings and the direction of the wind. Even for symmetrical plan forms such as a squares or a rectangular in homogeneous surroundings, torsional forces occur when the direction of the wind is not along a line of symmetry. The wind-induced torque is accentuated in the presence of eccentricities between the effective centres of rigidity, mass and aerodynamic force and for buildings with relatively long torsional periods of vibration. Torsional effects can substantially increase wind-induced loads on the main wind force resisting system; accentuate the racking of exterior walls and therefore influence the detailing of the curtain wall system; and add to the wind-induced motions, which if excessive can adversely affect occupant discomforts. Despite its importance, wind-induced torsion is overlooked or at best dealt with in a nominal fashion by most building codes. The National Building Code of Canada and more recently ASCE Standard 7 require designers to consider partial as well as full wind loading. In the partial load case, the wind loading on any portion of the building is reduced by 25% for buildings over 60 feet. This results in a wind- induced torque which corresponds to a base shear eccentricity of just under 4% of the building width. This is far short of actual eccentricities for some buildings, predicted from wind tunnel model tests and empirical data bases. While the effects of torque on the structural frame are seldom dominant, they can substantially alter the distribution of the wind-induced forces. If not allowed for in design, this has the effect of diminishing the effective safety index of some parts of the structural frame. Torsional load effects are not the domain of tall buildings alone and significant eccentricities of wind-induced forces also occur for low and intermediate height buildings. The effects of wind-induced torque are discussed for buildings of tall, intermediate and low height.