Concluding Remarks

An introduction to methodologies available for computing the membrane tension capacity of concrete – steel composite slab systems was provided. A hypothetical event whereby an interior column in a typical structural steel framing system is rendered ineffective was outlined and a methodology for computing the load carrying capacity of the steel floor framing system after this event was described. Inelastic static structural analysis was conducted using bilinear connection characteristics typical of simple framing connections in steel systems. The analysis conducted included bilinear connection modeling for axial load-deformation response and moment-rotation response within the structural steel framing system.

A static nonlinear analysis of the typical 30-ft by 30-ft framing system that included nonlinear connection behavior consistent with that of web-cleat connections was conducted. The analysis indicated that while it is doubtful that the typical structural steel framing system could support the GSA-level dynamic loading estimates, one can say with certainty that the typical structural steel framing system can resist progressive collapse in the event an internal column is rendered ineffective. This statement is supported by the fact that the point-in-time loading of 1.0,0 + 0.25L can be supported through catenary and flexural action in the structural steel framing and membrane action in the composite concrete-steel deck system with moderate levels of dynamic amplification reserve.

The analysis conducted suggests that it is better to have smaller moment capacity and flexural stiffness for connections distributed throughout the floor framing system (as is typically found in structural steel interior framing arrangements). When the moment capacity is low, there is a smooth transition between the formation of the flexural mechanism and the catenary tension behavior that is essentially secondary after the initial collapse. If the moment capacity is too large, there will likely be snap-through-type behavior whereupon a significant magnitude of vertical displacement will rapidly take place prior to the formation of catenary action.

In general, a balance between flexure and catenary action in the steel grillage can be attained when the following axial and moment characteristics are met in regard to the connections at the ends of the beams and girders in the structural steel system;

It is interesting to note that this behavior is nearly approached for typical structural steel framing systems. Axial capacity, axial stiffness, and rotational stiffness characteristics needed are easily attained by providing web-cleat connections (double angle connections) that “fill up” the web of the connected beam or girder. The moment capacity recommendations may be able to be attained if better modeling of the connection limit states is performed. Foley et al. (2006) found that 1/2-inch thick web connection angles that fill up the web of the connected member can approach 0.2Mpb.

The results of the study suggests that typical structural steel framing systems have significant levels of inherent robustness and general structural integrity without providing any special design effort. Furthermore, if slight increases in connection angle thickness and the number of bolt rows used in these connections are provided, the toughness of the system in response to abnormal events can be significantly enhanced.

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