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A limited amount of data exists on the cyclic behavior of concrete-filled steel deck diaphragms, and the few historical testing programs that have been conducted used specimens that may not be representative of modern floor assemblies typical in North America. As part of a larger research project, the Steel Diaphragm Innovation Initiative, an experimental program was conducted to fill critical knowledge gaps related to the behavior and design of concrete-filled steel deck diaphragms leading to a better understanding of the seismic performance of steel buildings utilizing these types of diaphragms. The experimental program included eight diaphragm specimens that were approximately 4 m by 5 m, tested in a cantilever test setup wherein one edge is restrained against translation while the opposing edge is subjected to an in-plane cyclic displacement history. Parameters that were varied included deck height (51 mm and 76 mm), concrete cover over the deck (51 mm to 115 mm), concrete type (normal weight and lightweight), configuration of perimeter headed shear studs, and the presence and amount of reinforcing steel. Specimen behavior was quantified including initial stiffness, peak shear strength, ductility, and energy dissipation and qualitatively connected to observed cracking and deformations. The results of this experimental program, combined with tests from the literature were used to develop proposed expressions for peak shear strength and initial stiffness. The proposed expressions reasonably capture the experimental results and are suitable for design. [ABSTRACT FROM AUTHOR]

The design of steel frames by advanced analysis can result in more efficient structures while maintaining satisfactory levels of reliability by incorporating nonlinear structural analysis. The structural reliabilities of a series of two-story, two-bay steel frames designed by both load and resistance factor design (LRFD) and advanced analysis methods are compared. Yield strength and gravity loads are modeled as random variables. Strength distributions and probabilities of failure are calculated by Monte Carlo simulation and first-order approximation for two failure criteria—plastic collapse and first plastic hinge. The results indicate that LRFD methods successfully enforce the target reliability on first plastic hinging. In contrast, design by advanced analysis results in acceptable reliabilities against plastic collapse, but relatively large probabilities of plastic hinging at service load levels. Specifications based on advanced analysis may require greater attention to serviceability criteria. Resistance factors for advanced analysis are calculated based on the strength distributions for two levels of target reliability. System-based failure criteria, such as plastic frame collapse, present a fundamental difficulty in determining resistance factors applicable to a wide-range of structures. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate the behavior, and provide recommendations on, the design of open cross-section thin-walled cold-formed steel members that employ complex stiffeners. Complex stiffeners are formed when the free edge of an open section is folded multiple times, as opposed to simple stiffeners which employ a single fold (a lip). Simple stiffeners become ineffective when long lips are required to stabilize the flange, as the lip itself initiates the instability. In the elastic buckling regime, complex stiffeners are shown to hold a distinct advantage in local buckling over simple stiffeners. Closed-formed solutions for both local and distortional buckling of members with complex stiffeners can provide conservative and reliable solutions, but remain cumbersome and somewhat limited in applicability, therefore numerical solutions are preferred. Nonlinear finite element analysis is used to examine the post-buckling and ultimate strength regime. Complex stiffeners are shown to provide improved ultimate strength performance over simple stiffeners, but with a slight increase in imperfection sensitivity. Further, for an equivalent amount of material, complex stiffeners only provide advantages for specific stiffener lengths, though global optimal designs (maximum strength while minimizing material) still favor complex stiffeners over simple stiffeners in the investigated examples. The cold-formed steel design specification in current use is shown to be a poor predictor for the ultimate strength of bending members with complex stiffeners. However, the direct strength method, recently adopted as an alternative design method in the North American Specification for cold-formed steel members is shown to be a reliable predictor of ultimate strength. The direct strength method is recommended for design and optimization of members with complex stiffeners. [ABSTRACT FROM AUTHOR]