Torsional effects and regularity conditions in RC building

Earthquake damages at the perimeter of buildings are often the result of excessive deformations caused by torsion during the earthquake. Large torsional responses can be expected if the buildings have large eccentricity and low torsional stiffness. Modern codes, including Eurocode 8, recognise the importance of the torsional stiffness on the seismic response and include such information in its static torsional provisions. Eurocode 8 states that the static torsional provisions can be applied to torsionally unbalanced buildings with regularity in elevation that satisfies the set of conditions presented in the Annex B of the Code. Present paper shows an overview on the EC8 torsional provisions including the code background and the comparison with other codes (UBC, etc.). On the other side the EC8 provisions could be improved during the revision process. In fact recent studies on the static torsional provisions of Eurocode 8 show that the static torsional provisions generally are satisfactory in order to limit additional demands when the building has a large torsional stiffness, but the provision seems to be deficient when applied to torsional flexible models. Moreover, the behaviour of buildings designed in High Class of ductility (“HD”) is more complex. This paper compares most of the results existing in the literature, suggests proposals of modification and underlines the importance of further studies in order to evaluate a condition of minimum torsional stiffness.