Effective Rigidity of Reinforced Concrete Elements in Seismic Analysis and Design

Seismic analysis and design of reinforced concrete structures are performed based on linear response, however it is universally accepted that under severe earthquakes inelastic response and cracking is accepted. Therefore element properties should reflect this condition and inertias of beams and columns should be reduced accordingly. Several procedures are suggested to considered effective rigidity: Priestley (2003), ACI 318S-05, FEMA 356, Paulay and Priestley (1992). In this work the convenience to consider the effective stiffness of elements is demonstrated. However since there is not a uniform criteria, a comparison of these procedures was performed. Most world seismic standards do not establish effective stiffness for seismic analysis, although all of them accept inelastic incursions. Therefore it is useful to find a common or reasonable criterion to reduce inertias. Priestley (2003) procedure is applied to find the effective rigidity of elements, which are dependent on element strength. A direct consequence of reduced inertias is larger elastic displacements. On the other hand seismic standards specify displacements are computed by factoring elastic displacements from analysis, therefore using reduced inertias a substantial increase of estimated displacements would occur produced turning analysis into over conservative.