Continuous composite beams utilizing stand-off screws to support offset load bearing cold-formed steel stud framing.

摘要

An experimental research study was conducted at South Dakota State University (SDSU) to evaluate the structural performance of concrete/cold-formed steel (CFS) composite load distribution members (LDM) used to support offset wall stud framing. Twenty-one specimens divided into five assemblies were fabricated and tested. The measured load, deflection, and strain data was used to assess the serviceability and strength of the composite LDM. The composite action was developed by stand-off screws installed in the CFS track which were encapsulated by a cast in-place concrete beam. Each specimen was a three-span continuous beam with twenty-four inch spans. Four assemblies included stand-off screws and top reinforcement. One assembly did not utilize stand-off screws or top reinforcement.All specimens were tested with a concentrated load (point load) applied to the midpoint of each span. The specimens were loaded by increasing the increments of vertical displacement until failure. Within each assembly, one specimen was loaded to failure by increasing the increments of vertical displacement cyclically. The specimens were instrumented to measure strain data in several elements of the specimen during load testing.This research included a literature review, fabrication of test specimens, instrumentation of test specimens, and load testing of each specimen to failure. Upon completion of experimental testing, an evaluation of the structural performance of the test specimens was completed. The data measured was used to compare the observed performance to theoretical models for strength and deflection.Seventeen of the specimens were built-up of CFS wall panels and composite LDMs. These specimens were evaluated with respect to the strength and axial stiffness of the CFS wall studs by AISI Standard code provisions. The deflections for these systems were primarily related to the axial shortening effects of the wall studs. The strength was limited to the buckling strength of the CFS wall studs.Four of the specimens were composite LDMs supported by rigid, roller supports. These specimens were analyzed by applying the strut-and-tie method from ACI Building Code, Appendix A code provisions. The results indicate that for a given stand-off screw arrangement, the strut-and-tie method provides a reasonable approach for analyzing the members of the composite LDM. The measured deflections were below deflection limits L/180 and L/240 at the peak loads. This indicated that deflections were negligible for the composite LDM. The strength of the composite LDM was determined to be sufficient to support the nominal strength of three different CFS wall stud sections currently used by NUCOR in CFS structures.