WIT Press

Consideration Of Torsional Irregularity In Modal Response Spectrum Analysis


Free (open access)

Paper DOI






Page Range

209 - 218




724 kb


O. A. Mohamed, O. A. Abbass


Modal Response Spectrum Analysis (MRSA) is one of the most commonly used methods for determination of seismic response in building structures. In MRSA, the structure is idealized as a series of single-degree-of-freedom systems, each having its own mode shape and vibration period. MRSA is permitted for structures in any seismic design category (SDC) and many types of irregularities. Even the most traditional structures can have torsional irregularity types 1a or 1b as defined in ASCE 7–10. The presence of torsional irregularities implies seismic forces may be amplified due to eccentricity of seismic forces. The purpose of this paper is the discuss the determination of the effects of torsional irregularity on seismic response in accordance with ASCE 7–10, when MRSA is used for calculation of seismic forces and drifts. A common misconception is that use of MRSA incorporates different vibration modes, therefore, torsional irregularity effects need not be accounted for as many vibration modes already include twisting of the three-dimensional structure. This paper discusses why torsional irregularity must be accounted for, even when MRSA is used. A case study reinforced concrete structure is used to demonstrate an interpretation of the current code provisions for incorporation of torsional irregularity effects when MRSA method is used.


Modal Response Spectrum Analysis, Equivalent Lateral Force Analysis, torsional irregularity