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General Information:

Joseph Main
Principal Investigator

robustness@nist.gov

100 Bureau Drive, M/S 8611
Gaithersburg, MD 20899-8611

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Large-Scale Testing

Subassemblies representing portions of the framing system of the prototype buildings have been tested at full scale under simulated column removal (see Acknowledgments for partnering institutions and supporting organizations). An important objective of this testing was to provide experimental data for validation of computational models that have been developed to represent nonlinear behavior and failure modes of the connections.

Steel Moment Frames

Two steel beam-column assemblies were tested at full scale under simulated column removal. The two beam-column assemblies represent portions of the structural framing system of two 10-story prototype buildings. One test specimen was taken from an intermediate moment frame of prototype building 1, which was designed for seismic design category C, and this specimen had welded unreinforced flange, bolted web (WUF-B) connections. The other specimen was taken from a special moment frame of prototype building 2, which was designed for seismic design category D, and this specimen had reduced beam section (RBS) connections. Each beam-column assembly consisted of three columns and two beam spans. Both specimens were taken from exterior moment frames in the N-S direction, with beam spans of 6.10 m.

video steel beam 
Center column of intermediate moment frame assembly after connection failure

The specimens were subjected to monotonically increasing vertical displacement of the unsupported center stub column
to observe their behavior under a simulated column removal scenario. The vertical displacement of the center column was increased until the vertical load-carrying capacity was depleted. For both assemblies, the behavior was dominated by flexure in the early stages of the response. With increased vertical displacement of the center column, the beam connections exhibited yielding, and tensile axial forces developed in the beams, indicating catenary action. The test results show that the rotational capacities of the connections in both assemblies under monotonic column displacement are about twice as large as those based on seismic test data.

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Cast-in-Place Concrete Moment Frames

Two cast-in-place concrete beam-column assemblies were tested at full scale under simulated column removal. The two beam-column assemblies represent portions of the structural framing system of two 10-story prototype buildings. One specimen was part of an intermediate moment frame from a building designed for seismic design category C, and the other was part of a special moment frame from building designed for seismic design category D. Each beam-column assembly consisted of three columns and two beam spans. Both specimens were taken from exterior moment frames in the N-S direction, with beam spans of 6.10 m.

 Reinforced Concrete Assemblies under a Column Removal Scenario (NIST Technical Note 1720)
Special moment frame assembly subjected to large displacement

Both assemblies were subjected to monotonically increasing downward displacement of the unsupported center column, simulating a column removal scenario. Each test was terminated upon reaching the collapse mechanism of the assembly. For both assemblies, the behavior was dominated by flexure in the early stages of the response. With increased vertical displacement of the center column, resistance was provided through the development of compressive diagonal axial forces, or “arching action,” due to the restraint on axial elongation of the beams by the end columns. With further increase in the vertical displacement, tensile axial forces developed in the beams and the behavior was dominated by catenary action. The failure of both assemblies was characterized by: (1) crushing of concrete at the top of the beam section near the center column, (2) development of major flexural cracks (deepening and widening), and (3) fracture of one of the bottom reinforcing bars at a major crack opening near the center column. The test results show that the rotational capacities of the beam-to-column joints in both assemblies under monotonic column displacement are about seven to eight times as large as those based on seismic test data.

Related publications:

Precast Concrete Moment Frames

Two precast concrete moment-frame assemblies, each comprising three columns and two beams, were tested at full scale under simulated column removal. The two moment frame assemblies represent portions of the perimeter moment frames of two 10-story prototype buildings. One test specimen was taken from an ordinary moment frame (OMF), which was designed for seismic design category B, and the other specimen was taken from a special moment frame (SMF), which was designed for seismic design category D.

 Center column of special moment frame assembly after connection failure
Center column of special moment frame assembly after connection failure

The full-scale test specimens were subjected to displacement-controller vertical loading of the unsupported center column to simulate a column removal scenario. The tests were continued beyond the ultimate capacity of the assemblies to characterize the failure modes and collapse mechanisms that were developed. The failure of both the OMF and SMF assemblies was characterized by (1) fracture of the bottom anchorage bars at the welded connection to the center column and (2) diagonal cracking and shear deformation of the end columns under outward forces generated by arching action in the beams. Other failure modes that were observed included shear stud failure for the OMF specimen and bond failure of anchorage bars for the SMF specimen.

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