Analytical Load and Dynamic Increase Factors for Progressive Collapse Analysis of Building Frames

Abstract

Proposed analytical expressions for the load increase factor (LIF) and dynamic increase factor (DIF) are compared with the empirical formulae recommended in the Unified Facilities Criteria (UFC) guidelines UFC 4-023-03 in this study. Static and incremental dynamic analyses are carried out for two moment-resisting frames under sudden column loss. The LIF and DIF of the column-removed frames are estimated from their load-displacement envelopes. They are used to evaluate the adequacy of the analytical LIF and DIF expressions in comparison with the empirical formulae. The study results indicate that the proposed analytical expressions may be an alternative in estimating the LIF and DIF for progressive collapse analysis. INTRODCUTION Development of practical and efficient approaches for protecting building structures from progressive collapse under accidental loadings has been an imperative issue in the last decade. In general, tie force, alternative load path, integrity provisions, and specific local load resistance are recommended as feasible measures for reducing the vulnerability of building structures to progressive collapse (Abruzzo et al. 2006; Mohamed 2006; Nair 2006). Since the cause, reoccurrence, and intensity of abnormal loadings for triggering the progressive collapse are difficult to predict precisely, provision of threat-independent alternative paths for loading transfer has been an acceptable and popular solution among those proposed measures. Alternative load paths of a damaged building structure may be verified by assessing its load transfer ability under stipulated column loss scenarios. Linear static (LS), nonlinear static (NS), and nonlinear dynamic (ND) methods may be used for evaluating the alternative load paths. Particularly, a detailed step-by-step LS analysis procedure has been issued by US General Service Administration (GSA) (2003). Also, all the LS, NS, and ND analysis approaches are recommended in the Unified Facilities Criteria UFC 4-023-03 by Department of Defense (DoD) (2005). In both the LS and NS procedures, dynamic loading simulation is replaced with a constant dynamic amplification factor (DAF) equal to 2. In the latest issued UFC 4-023-03 guidelines (2009), two different load magnification factors, namely the load increase factor (LIF) and dynamic increase factor (DIF), are suggested for considering the dynamic effect in the LS and NS analysis approaches, respectively. Empirical formulae based on ND analysis results of various frame models subjected to column loss are recommended in the guidelines. The empirical formulae provide a simple but practical solution for