Seismic Performance of Collectors in Steel Building Structures

Collectors are key structural elements in the Seismic Force-Resisting System (SFRS), delivering the inertial forces that develop in the floor system to the primary (vertical plane) lateral force-resisting elements. In contrast to the vertical-plane SFRS elements (shear walls, braced frames, moment frames, etc.), no research efforts or experimental program to date has focused specifically on collectors. Instead, design procedures for collectors have traditionally applied special seismic load combinations in ASCE-7 involving the application of the System Overstrength Factor. The rationale for this stringent prescriptive rule is an attempt to ensure that collectors and their connections remain elastic, thereby maintaining the structural integrity of the SFRS. Also, design engineers use a variety of connections for the collectors, and practically no research has been conducted to validate the adequacy of these connection details.

As part of this research project, component-level testing that evaluates collectors and their connections is underway at Lehigh University, with associated analytical studies conducted at the University of Arizona. System-level testing is to be conducted at UCSD. Based on results from numerical simulation of the seismic response of a 12-story prototype building, a one-story test frame (see figures below) will be constructed tested. Since the objective of this research is to evaluate collectors and the load path in the diaphragm, not the SFRS, an innovative test technique has been developed that uses the test frame to simulate the expected absolute floor acceleration time response of a particular floor in the prototype. The test frame is kept essentially elastic through testing.

Cantilever columns in the test frame are to provide lateral resistance and all the other columns are designed to carry gravity loads. The test frame incorporates a full-scale composite slab with metal deck. Inertia forces from the reactive weight are to be transmitted through steel anchors, collectors, and their connections to the SRFS. Several collector types are incorporated in the test frame. The test frame will be carefully instrumented so that the load path can be evaluated.