Eccentric force excitation of a reinforced concrete building to assess torsion amplification

Dynamic analysis should be preferred for seismic design of buildings and other structures instead of a static one. However, in several current building codes seismic static design of buildings requires an amplification of torsion moments (or story eccentricities) computed from lateral forces statically applied at floor levels. Discrepancy of factors used to estimate torsion amplification among codes suggests further study on this subject. The main objective of this paper is to show estimations of the building dynamic torsion. These estimations were obtained from experiments conducted in a four-story reinforced concrete building without accidental eccentricity that was excited with a dynamic force generator placed at the roof. The exciter was operated at several frequencies and it was placed at two roof positions. By assuming three degrees of freedom for each building slab and using acceleration records from tests, equations of motion of the system were used to estimate dynamic story torsional moments. Results support the idea that building torsional response also depends on coupling between the excitation force frequency and the building modal frequencies associated with torsion (either pure torsion or translation coupled with torsion).