Seismic torsional effects on multi‐storey steel buildings

During earthquakes, a structure may undergo torsional effects even if it is perfectly symmetrical. One of the causes is that seismic waves do not arrive in‐phase in each point of the infrastructure. Moreover, there are always eccentricities between the mass centre, where the seismic forces are considered to be acting, and the stiffness centre, around which the building rotates, this contributing as well to the occurrence of torsional effects. These eccentricities are generated by asymmetries of the structure itself, position of non‐structural elements, asymmetrically applied loading or other factors. Seismic design codes usually take into account torsional effects by means of an equivalent eccentricity that is added to the actual distance between the centres of mass and stiffness. In ordinary design, to analyse the behaviour of a structure subjected to seismic actions, when establishing the mass of the building, a fraction of the variable loads is considered to be uniformly distributed on each floor. This obviously does not happen in reality. The present paper focuses on estimating the limits of the possible actual variation of internal forces in structural members caused by torsional effects. To this aim, two eccentrically braced steel structures were considered. In each case, the variable loads were redistributed in the plan of the building according to different loading schemes, but keeping their total amount constant, equal to the value corresponding to the uniform arrangement on each floor.