Keywords: progressive collapse, catenary action, structural ties, punching shear, flat slab.

This paper highlights the lack of stiffness in flat slab structures that could lead to difficulties in applying the alternate path approach for progressive collapse mitigation in flat slab and flat plate structures. The paper presents recommendations to improve the performance of flat slab structures when an event triggers progressive collapse. In order to effectively mitigate progressive collapse in flat slab construction, the paper advocates the importance of designing and detailing a system of reinforcement ties that is capable of developing a continuous load path in three dimensions to transfer gravity loads redistributed after initiation of progressive collapse. The unified facilities criteria [1] includes recommendations for the calculation of design forces for these ties. A system of reinforcement ties must be provided in the plane of the flat slab and must be mechanically spliced and placed in two orthogonal directions, in addition to reinforcement ties along all vertical members. The horizontal reinforcement ties are placed along column strip lines at both top and bottom of the slab. Internal top and bottom reinforcement ties in column-strips must be anchored to peripheral ties. Bottom reinforcement placed along column strip lines should have the ability to resist stress reversal caused by loss of vertical support. Top and bottom reinforcement that are continuous and mechanically spliced will have the ability to mitigate or eliminate punching shear at the slab-column interface that are caused by large rotations associated with progressive collapse failure. The significance of reinforcement ties in forming catenary action in flat slabs was tested by Regan [2]. Vertical ties are designed to transfer the pull exerted on the vertical supports while flat slabs deform to form catenary action. The system of ties ensures that if a flat slab structure is damaged, integrity would still be maintained to allow evacuation and rescue activities. The improved performance does not include failure initiated by loss of corner columns. Corner columns must be particularly protected using methods such as the enhanced local resistance design approach, as discussed by Mohamed [3,4].

1

Unified Facilities Criteria UFC 4-023-03, "Design of Buildings to Resist Progressive Collapse", US Department of Defense, 2010.

2

P.E. Regan, "Cantenary Action in Damaged Concrete Structures", Special Publication, American Concrete Institute, SP48-9, 191-224, 1975.

3

O. Mohamed, "Strategies for Mitigation of Progressive Collapse of Corner Panels in Reinforced Concrete Buildings", Transactions, Safety and Security Engineering II, WIT Press, 161-169, 2007. doi:10.2495/SAFE070161

4

O.A. Mohamed, "Solutions for Progressive Collapse Mitigations in New Designs", in B.H.V. Topping, L.F. Costa Neves, R.C. Barros, (Editors), "Proceedings of the Twelfth International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 219, 2009. doi:10.4203/ccp.91.219

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