Quantitative seismic safety checking with un-scaled records using demand and capacity factor design

Quantitative safety-checking is an essential part of performance-based design and retrofit of new and existing construction. The intensity-based Demand and Capacity Factor Design (DCFD) is a practical closed-form analytical safety-checking format, derived based on a set of simplifying assumptions that lends itself quite well to visual interpretation. Various sources of uncertainty can be considered within the frame of DCFD –albeit in a simplified manner. Adopting the critical demand to capacity ratio as a global damage measure directly within DCFD safetychecking, and thus skipping the engineering demand parameter, facilitates the identification of the onset of the prescribed limit states. A nonlinear dynamic analysis procedure known as the Cloud Analysis can effectively be used due to its simplicity and relatively small number of un-scaled records employed. It is to note that the Cloud Analysis is perfectly compatible with the underlying assumptions of DCFD safety-checking format. A modified version of the Cloud Analysis, referred to as Modified Cloud Analysis, has the capability of explicitly addressing the cases of global instability or numerical non-convergence. Modified Cloud Analysis is going to be particularly useful for vulnerability assessment of structures with degrading behavior. Although the modified Cloud Analysis relaxes some of the underlying assumptions of the original Cloud Analysis; namely, the Lognormal fragility curve and constant standard error of regression, it can still be implemented within the DCFD safety-checking format by adopting equivalent Lognormal statistics. Safety-checking is performed by using the system-level damage measure expressed in terms of a critical demand to capacity ratio. The critical demand to capacity ratio permits the mapping of damage at the component level to the system level. In other words, it makes it possible to adopt a compatible definition for limit states’ thresholds at the components and the system level. The longitudinal frame of an older RC building located in Van Nuys (CA) is employed as a case-study. The structure in question has suffered severe damage due to Northridge 1994 Earthquake. Herein, a two-dimensional nonlinear model of the frame is constructed in OpenSees, in which the interaction between flexure, shear and the axial forces and the rigid end rotation due to bar slip are explicitly modelled. It can be observed that DCFD safety-checking leads to an extremely efficient yet vigorous and accurate safety-checking.