Keywords: collapse, linear, dynamic, torsion, shear, deformation, damage, protection.

Significant damage or loss of primary load carrying members of reinforced concrete framed structures can lead to disproportionate failure of large parts of the structure or even the entire structure. Many building codes and design standards requires the investigation of the capacity of structural systems to sustain the loss of critical structural elements without experiencing disproportionate damage. A review of progressive collapse mitigation provisions in selected building codes was given by Mohamed [1]. The design guide, UFC 4-023-23 [2], published by the United States Department of Defense, provides recommendations on the design of structures more than three stories high to reduce the potential for progressive collapse. The document classifies buildings based on the desired level of protection. Medium and high levels of protection categories require the use of the Alternate Path (AP) method to investigate the capability of the structural system to transfer loads safely from a notionally removed column to the remaining structural elements. Transfer of loads from a notionally removed corner column to adjacent structural elements can impose significant stress-deformation demand on structural elements. For example large torsion can be induced in beams supporting the adjacent panel when a corner column is removed. However, according to UFC 4-023-23, when the panel area exceeds the prescribed floor damage limits, the panel and its structural elements must be designed to support the additional load or the loads must be transferred to adjacent columns. This paper investigates the implementation of UFC 4-023-23 to protect against progressive collapse of frame structures when their dimensions exceed the damage limits. The effects of beam dimensions, reinforcement ratios, and distribution of reinforcement on structural response to sudden loss of critical columns was given by Hansen et al. [3]. This paper examines the effect of the presence of steel lateral bracing on progressive collapse potential and distribution of forces to structural members, including lateral bracing, after the loss of a corner column. The importance of three-dimensional analysis to simulate progressive collapse is emphasized. All members in the case study building are designed according to ACI 318-05 [4] and lateral bracing members are designed according to AISC-LRFD method [5].

1

O.A. Mohamed, "Progressive Collapse of Structures: Annotated Bibliography and Comparison of Codes and Standards", Journal of Performance of Constructed Facilities - ASCE, 20(4), 418-425, 2006. doi:10.1061/(ASCE)0887-3828(2006)20:4(418)

2

"Unified Facilities Criteria - Design of Buildings to Resist Progressive Collapse (UFC 4-023-03)", Department of Defense, United States of America, 2005.

3

E. Hansen, F. Wong, D. Lawver, R. Oneto, D. Tennant, M. Ettouney, "Development of an Analytical Database to Support Fast Running Progressive Collapse Assessment Tool", in "Proceedings the 2005 ASCE Structures Congress - Metropolis & Beyond", American Society of Civil Engineers, New York, 2005. doi:10.1061/40753(171)208

4

"ACI 318-5: Building Code Requirements for Structural Concrete and Commentary; American Concrete Institute", American Concrete Institute, Michigan, United States of America, 2005.

5

"Manual of Steel Construction - LRFD, 3rd Edition", American Institute of Steel Construction, Illinois, United States of America, 2001.

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