Journal of Structural Engineering

Partially restrained composite connections (PR-CC) are partial strength connections that can be used to resist moments induced by live, wind, and earthquake loads in low and moderate height unbraced frames. PR-CC use the continuity and strength that are already built into the structures and are especially well suited for low-rise structures in areas of low-to-moderate seismic risk. Good hysteretic performance was the primary criterion used in the development of the analysis procedures and design criteria presented in this guide. These procedures are more detailed and conservative than those in the 1997 National Earthquake Hazards Reduction Program and American Institute of Steel Construction seismic specifications and apply to a wider range of structures. Trial designs indicate that the most efficient structures result when connection strength is about 75% of the noncomposite strength of the steel beams framing into the connections.

Experimental and analytical studies were conducted to investigate the behavior of fiber reinforced concrete–encased steel joist composite members subjected to reversed cyclic bending. This new and innovative structural system completely eliminates the need for any shear connectors between the steel joists and surrounding concrete as well as the need for conventional longitudinal and transverse reinforcement, all of which are quite labor intensive. All connections in the system are steel-to-steel; therefore, the system can be used with cast-in-place or precast construction. Five one-half-scale specimens were tested, each consisting of open web steel sections encased in concrete containing discrete steel fibers. In the test program, the most important parameter studied was the configuration of steel elements. The test results are very encouraging. The two materials (structural steel and fiber-reinforced concrete) interact to provide stable hysteretic behavior with excellent energy dissipation and ductility. The study indicates that the presence of steel joists as well as steel fibers can adequately inhibit the chances of brittle shear failure, thus making the system very suitable for seismic resistant structures. The specimen load-carrying capacity and its hysteretic response can be quite accurately predicted by the proposed analytical models.

This paper presents an experimental and analytical study on the behavior of short, concrete-filled steel tube columns concentrically loaded in compression to failure. Fourteen specimens were tested to investigate the effect of the steel tube shape and wall thickness on the ultimate strength of the composite column. Confinement of the concrete core provided by the tube shape was also addressed. Depth-to-tube wall thickness ratios between 17 <D/t< 50, and the length-to-tube depth ratios of 4 <L/D< 5 were investigated. Ultimate strength results were compared to current specifications governing the design of concrete-filled steel tube columns. Nonlinear finite-element models were developed and verified using experimental results. The analytical models were further used to investigate the adequacy of design specifications. Experimental results suggest that circular tubes offer substantial post-yield strength and stiffness, not available in most square or rectangular cross sections. Also observed by these results was that current design specifications were adequate to predict the yield load under most conditions for a variety of structural shapes.

Procedures for calculating the deflections of composite beams with web openings are described. Initially, a matrix formulation is used. Modeling assumptions are verified by comparison with experimental data, and recommendations for practical application of the matrix analysis procedures are made. The results of the comparison are used to develop a design aid for estimating the maximum deflection of beams with web openings and an expression for calculating the deflection across a web opening. The work demonstrates that, in most cases, a single web opening often has little effect on the total deflection of a composite beam. There are, however, important cases where the effect can be significant. The effects of an opening and of shear deflections are of the same order. Ignoring both the web opening and shear deformation can lead to significant error. The matrix stiffness method, the design aid, and closed form equations provide reasonable estimates of both total deflection and deflection across an opening.

Composite floor systems, consisting of reinforced concrete slabs and steel girders, are common in buildings and bridges. Partial restraints at the interface of the concrete slab and the steel girder have significant effects on the response of the composite beam. A few analytical models capable of accounting for these effects are available; however, none can be efficiently used for the nonlinear analysis of composite beams. In this paper, the basic governing equations are presented for a composite beam with deformable shear connectors under small displacements. Based on these equations, a new composite beam element is developed using the force method of analysis. The performance of this new element is compared with that of a previously developed displacement-based composite beam element. Both elements consist of two beam components connected through a deformable interface. A distributed spring model is used to account for the shear deformation at the interface. Two numerical examples on the nonlinear behavior of composite beams are solved using both displacement- and force-based elements. Comparison of the numerical results shows the superior performance of the newly developed force-based element over the displacement-based element.

This paper is concerned with calculating the maximum deflection of steel-concrete composite beams with partial shear interaction. Under the guidance of various current design codes, this deflection is related to the strength of shear connectors in the composite beam. In this paper, a shear connector stiffness based approach is developed. This approach is obtained based on the solution for a simply supported beam under uniformly distributed load. To validate this approach, the predicted maximum beam deflection using the proposed method is compared against the results from a linear-elastic finite-element analysis and some tests on composite beams. In both the finite-element analysis and the proposed method, uniform distribution of shear connection stiffness along the beam length is assumed. In the absence of a reliable way to calculate the shear connector stiffness, this paper suggests a simple procedure to obtain this value so that the proposed method may be used in practice.

This paper describes a series of bending tests to examine the influence of web slenderness on the rotation capacity of cold-formed rectangular hollow sections (RHS) for use in plastic design. The results indicate that the plastic web slenderness limits in design standards, which are based on tests of I-sections, are not conservative for RHS. Some sections, which are classified as compact by current steel specifications, do not demonstrate rotation capacity suitable for plastic design. The common approach in which the flange and web slenderness limits are given independently is inappropriate for RHS. There is considerable interaction between the webs and flange, which influences the rotation capacity, as shown by approximate isorotation curves. A proposal for a bilinear interaction formula between the web and flange slenderness limits for Compact RHS is given.

A new procedure for calculating the effective width of stiffened elements with multiple longitudinal intermediate stiffeners is presented. The method determines the critical stress for distortional buckling of the entire stiffened element as a unit, and local buckling of the subelement plates between stiffeners. Approximate expressions for calculating distortional buckling are verified via comparison to numerical methods. The effective width of the element is determined using Winter's equation based on the governing buckling stress. Reduced postbuckling capacity in the distortional mode is considered. Comparison to experimental data and numerical analysis shows the resulting method is a reliable predictor of the flexural capacity of cold-formed steel members with multiple longitudinal intermediate stiffeners in the compression flange.

This paper is concerned with the inelastic lateral buckling strengths of cold-formed channel section (CFC) beams. Code rules for designing steel beams against lateral buckling that are based on data for hot-rolled I-section beams may be unsuitable for CFC beams because they have different stress-strain curves, residual stresses, and initial imperfections. Distortion may reduce the resistance of a CFC beam to lateral buckling. In addition, due to the monosymmetry of the cross section, the effective shear center moves toward the web and the effective section becomes asymmetrical after yielding, so that a CFC beam under major axis bending is subjected to minor axis bending and twisting. Also, the minor axis bending and warping strain distributions and therefore the strengths of CFC beams are related to the twist rotation and minor axis deflection directions. This paper deals with these problems by developing an advanced finite-element model to investigate the elastic lateral-distortional buckling, inelastic behavior, and strengths of CFC beams with residual stresses and initial imperfections. The results are used to develop improved design rules for CFC beams. The effects of moment distribution and load height on the strengths are also studied.

When steel roof and wall cladding systems are subjected to wind uplift/suction forces, local pull-through/dimpling failures or pull-out failures occur prematurely at their screwed connections. During extreme wind events such as storms and hurricanes, these localized failures then lead to severe damage to buildings and their contents. An investigation was therefore carried out to study the failure that occurs when the screw fastener pulls out of the steel battens, purlins, or girts. Both two-span cladding tests and small-scale tests were conducted using a range of commonly used screw fasteners and steel battens, purlins, and girts. Experimental results showed that the current design formula may not be suitable unless a reduced capacity factor of 0.4 is used. Therefore, an improved design formula has been developed for pull-out failures in steel cladding systems. The formula takes into account thickness and ultimate tensile strength of steel, along with thread diameter and the pitch of screw fasteners, in order to model the pull-out behavior more accurately. This paper presents the details of this experimental investigation and its results.

Tests of four castellated beams are described, in which the buckling of the web-post between openings was the focus of interest. Evidence of web-post buckling was observed in all test beams, although one also underwent lateral displacements. Elastic finite-element analysis was used to predict the buckling loads. The test results showed that the buckling loads were not sensitive to moment-to-shear ratio, and the maximum test loads exceeded the finite-element predictions by margins varying between 4 and 14%. A finite-element parameter study was conducted, and general results that can be used for a range of hole geometries are presented.

This paper deals with the stability behavior of web-tapered beams subjected to bending loads. According to the current AISC specifications, a tapered member is treated as prismatic with modified length and cross section identical to that of the tapered member's smaller end. Modification factors are used to determine the modified length and also to account for the moment gradient within the unbraced length and the restraining effect for adjacent spans. In this paper, these factors are evaluated via a finite-element analysis and compared to the ones proposed by the current specifications. The current design specifications for steel tapered beams are reexamined, and modifications that cover a wider range of geometry and loading cases are proposed.

It is well known that a central elastic torsional restraint restricts the lateral buckling shape of an elastic I-beam and increases its elastic flexural-torsional buckling resistance. However, available information on the effects of torsional restraints on elastic buckling are either incomplete, or in some cases inaccurate. This paper investigates the effects of moment distribution and load height on the elastic flexural-torsional buckling of beams with central torsional restraints. The effects of off-center and continuous restraints are also studied, and design approximations and procedures are developed.