Incremental Response Spectrum Analysis IRSA

An alternative nonlinear static procedure was proposed by Aydinoglu (2003). The procedure, called Incremental Response Spectrum Analysis (IRSA) does not properly belong to the pushover methods. The procedure is displacement-based, it uses the equal displacement rule and the structure nonlinear behavior is modeled as piece-wise linear. The initial modes are computed and the elastic response spectrum for the initial structure is carried out. In the first stop the spectral ordinates are scaled to the formation of the first plastic hinge, which corresponds in the piece-wise linear capacity curve, to the first change of stiffness. The updated modal quantities are computed and additional spectral ordinates are computed up to the formation of the second plastic hinge. The procedure continues until the entire spectral ordinates are applied. The method does not require any transformation to an equivalent SDOE system. 

Nonlinear performance assessment of bridges with Incremental Response Spectrum Analysis (IRSA) procedure... 

Incremental Response Spectrum Analysis (IRSA) is a multi-mode pushover procedure, in which the incremental response is assumed piecewise linear at each pushover step in between the formation of two consecutive plastic hinges. The key point in IRSA is modal scaling, which is applied at each step to modal displacement increments, to identify their proportions in different modes (Aydmoglu 2003, 2007). 

Based on limited observations made above, it appears that approximate Incremental Response Spectrum Analysis (IRSA) procedure, which is the natural extension of the traditional Response Spectrum Analysis (RSA), proves to be a reliable analysis tool for deformation-based seismic assessment and design of bridges in the general framework of performance-based design. 

Aydinoglu, M.N. 2004. An improved pushover procedure for engineering practice Incremental Response Spectrum Analysis (IRSA). International Workshop on Performance-based Seismic Design Concepts and Implementation . Bled, Slovenia.PEER Report 2004/05 345-356. 

Aydinoglu, M.N. 2007. A Response Spectrum-Based Nonlinear Assessment Tool for Practice Incremental Response Spectrum Analysis (IRSA). Special Issue Response Spectra (Guest Editor M.D. Trifunac), ISET Journal of Earthquake Technology 44(1), No. 481. 

Multi-mode pushover analysis procedure IRSA (Incremental Response Spectrum Analysis) has been introduced by the first author to enable the two and three dimensional nonlinear analyses of buildings and bridges (Aydmoglu 2003). The practical version of the procedure (Aydmoglu 2004, 2007) works directly with smoothed elastic response spectrum and makes use of the well-known equal displacement rule to scale modal displacement increments at each piecewise linear step of an incremental application of linear Response Spectrum Analysis (RSA). In this paper, main steps of IRSA are summarized and its performance is evaluated on two example bridges under three different ground motions. 

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