Structural Robustness Computational Modeling

Two basic modeling approaches can be considered for collapse analysis of structural systems: (1) detailed modeling, which uses highly refined solid and/or shell element meshes to represent nonlinear material behavior and fracture, and (2) reduced modeling, which uses a much smaller number of beam and spring elements. Reduced models can be analyzed much more rapidly than detailed models, making them well suited for collapse analysis of complete structural systems. Because of the complexities of the nonlinear behavior that connections exhibit under collapse scenarios, physical tests are indispensable for establishing confidence in both detailed and reduced modeling approaches.
 

Steel Gravity Frames with Composite
Floor Systems

Both detailed and reduced models have been used to study the susceptibility to collapse of steel gravity frame systems (i.e., frames designed to carry only vertical loads) with simple shear connections under column loss. The composite floor slab was found to significantly enhance the capacity of the system relative to the bare steel framing system. However, even accounting for this enhanced capacity, the composite floor systems in the prototype buildings were found inadequate to sustain the applicable gravity loads under sudden column loss. Additional reinforcement in the floor slab was found to be effective at preventing collapse under column loss, and required tie force levels were investigated.

Related publications:

• Main, J.A. and Sadek, F. (2012). "Robustness of steel gravity frames with single-plate shear connections." NIST Technical Note 1749.
• Alashker, Y., El-Tawil, S., and Sadek, F. (2010). "Progressive collapse resistance of steel-concrete composite floors." Journal of Structural Engineering, 136(10), 1187-1196. doi:10.1061/(ASCE)ST.1943-541X.0000230
• Sadek, F., El-Tawil, S., and Lew, H.S. (2008). "Robustness of composite floor systems with shear connections: Modeling and evaluation." Journal of Structural Engineering, 134(11), 1717-1725. doi:10.1061/(ASCE)0733-9445(2008)134:11(1717)
  

Steel Moment Frames

 

The response of steel beam-column assemblies with moment connections under monotonic loading conditions simulating a column removal scenario has been investigated computationally. Two beam-column assemblies were analyzed, which incorporate (1) welded unreinforced flange, bolted web (WUF-B) connections and (2) reduced beam section (RBS) connections. Detailed models of the assemblies have been developed, which use highly refined solid and shell elements to represent nonlinear material behavior and fracture. Reduced models were also developed, which use a much smaller number of beam and spring elements and are intended for use assess the susceptibility of complete structural systems to disproportionate collapse. Computational results were compared with the results of full-scale tests described in the companion paper, and good agreement was observed, demonstrating that both the detailed and reduced models are capable of capturing the predominant response characteristics and failure modes of the assemblies, including the development of tensile forces associated with catenary action and the ultimate failure of the moment connections under combined bending and axial stresses.

Related publications:

• Sadek, F., Main, J. A., Lew, H. S., and El-Tawil, S. (2012). "Performance of steel moment connections under a column removal scenario. II: Analysis." Journal of Structural Engineering, in press. doi:10.1061/(ASCE)ST.1943-541X.0000617
  
• Sadek, F., Main, J. A., Bao, Y., and Lew, H. S., (2011). "Testing and analysis of steel and concrete beam-column assemblies under a column removal scenario." Journal of Structural Engineering, 137(9), 881-892.
• Sadek, F., Main, J.A., Lew, H.S., Robert, S. D., Chiarito, V, and El-Tawil, S., (2010). "An experimental and analytical study of steel moment connections under a column removal scenario." NIST Technical Note 1669.

Cast-in-Place Concrete Moment Frames

 

The response of cast-in-place concrete beam-column assemblies under monotonic loading conditions simulating a column removal scenario has been investigated computationally. Two beam-column assemblies were analyzed; one assembly was part of an intermediate moment frame and the other was part of a special moment frame. Two types of models were developed: (1) detailed models with highly refined solid and beam elements to represent the nonlinear material behavior of concrete and reinforcement, and (2) reduced-order models with significantly fewer beam and spring elements to represent the nonlinear behavior of structural components. The computational results were compared with experimental data from full-scale tests, and good agreement was observed, which demonstrates the capability of the detailed and reduced models to capture the primary response characteristics and failure modes, including the successive development of compressive arching action and catenary action in the beams and the fracture of reinforcing bars at the beam-column interface.

Related publications:

• Bao, Y., Lew, H.S., and Kunnath, S.K. (2013). "Modeling of reinforced concrete assemblies under a column removal scenario." Journal of Structural Engineering, in press. doi:10.1061/(ASCE)ST.1943-541X.0000773

• Sadek, F., Main, J. A., Bao, Y., and Lew, H. S., (2011). "Testing and analysis of steel and concrete beam-column assemblies under a column removal scenario." Journal of Structural Engineering, 137(9), 881-892. doi:10.1061/(ASCE)ST.1943-541X.0000422

• Lew, H.S., Bao, Y., Sadek, F., Main, J.A., Pujol, S., and Sozen, M.A. (2011). "An experimental and computational study of reinforced concrete assemblies under a column removal scenario." NIST Technical Note 1720.