Double Angle Members Research Articles

Experimental and numerical investigations of block shear failure in gusset plates welded to double angle members

Double angle braces are used extensively in structures to resist lateral loads such as wind, earthquake, and blast loads. A possible failure mode in the gusset plate when all-around weld is used for the angles is the block shear failure. To date, only limited test data on welded gusset plates failing in this mode have been reported in the literature, and all of them only considered concentrically loaded gusset plates. This paper investigates the block shear failure in welded gusset plates experimentally and numerically when connected to double angle members with possible in-plane load eccentricity effects. The test results, including failure loads, fracture sequences, and load-displacement responses are reported in detail. Results showed that tensile fracture was initiated adjacent to the angle heel weld and then propagated along the tension plane. Due to existing in-plane load eccentricity, shear planes did not rupture simultaneously and the shear plane adjacent to the heel weld failed earlier. Following the testing, a numerical investigation using nonlinear finite element analyses with ductile damage capability was performed on the tested specimens. Subsequently, a parametric study was carried out to generate further numerical data over a wide range of gusset and angle dimensions, welding configurations, and connection lengths. Accordingly, the existing design equations for block shear strength as given by AISC specification were evaluated. It was shown that AISC provides excessively conservative and scattered failure loads because of ignoring stress triaxiality and using shear yield instead of shear rupture in its strength equations. In addition, equations specified for welded gusset plates by previous studies were considered and shown to be unsafe for general use. To address these shortcomings, an improved block shear strength equation is proposed in this paper and shown to predict the gusset strength in block shear with improved accuracy.

Influence of torsional stiffness in double-angle open-web joist and joist girder chords

The double angle chords of open-web steel joists are often designed to efficiently support loading with relatively slender sections, which introduces buckling instabilities that are treated differently by various design standards. This study investigates the buckling behavior of double angles, focusing on the importance of flexural–torsional buckling. A theoretical inelastic buckling investigation considering current design practices and known steel behavior demonstrates how flexural–torsional buckling does not control for all double angle configurations. Subsequent investigation focuses on double angles theoretically susceptible to flexural–torsional buckling, while implementing finite element modeling of full joist structures and isolated chord segments. The resulting elastic eigenbuckling analysis and geometrically and materially nonlinear analysis with imperfections captures significant twisting of individual angles in the built-up section; however, global flexural–torsional buckling was not observed to control. Most double angle configurations are controlled by flexural buckling due to an increased flexural–torsional buckling capacity. This result is attributed to the observed increased torsional stiffness of the chords, which is typically not considered in design. A supplementary investigation indicated this torsional stiffness was primarily a result of torsion being transferred through the stiff chord-to-chord connections, which do not exist in all double angle members. Accounting for the additional torsional stiffness in a theoretical inelastic buckling calculation leads to flexural–torsional buckling not controlling the design of most double angle cross sections considered in this study. Overall, this study indicates that designing double angles in joist structures by only considering flexural buckling with local buckling is reasonable with current industry practices.

Elastic compressive buckling resistance for back-to-back double angle assemblies

The present study investigates the elastic buckling resistance of compression members consisting of non-slender hot-rolled double angles connected back-to-back with discrete interconnectors. Towards this goal, the study (a) formulates a thin-walled beam finite element buckling solution that treats both angles as independent members away from the intermediate connector locations while constraining the displacements for both angles at interconnector locations, and (b) develops an extension of the Vlasov thin-walled beam theory, originally intended for monolithic members, to determine the effective torsional/warping sectional properties for back-to-back double angle members. The validity of the thin-walled beam finite element model is then verified against experimental results and shell finite element models and its predictions are shown to asymptotically converge to the Vlasov theory extension developed in the present study as the number of interconnectors is increased. A systematic parametric study consisting of 1250 finite element runs is then conducted and the database of results generated is used to propose a simple equation to predict the elastic compressive buckling resistance of double angle compression members.

Seismic evaluation of a steel braced frame using NBCC and ASCE 41

The article presents the seismic evaluation and retrofit of a 10-storey steel building located in Vancouver, British Columbia, designed following the provisions of the 1980 NBCC and the CSA-S16.1-M78 steel design standard. Lateral resistance is achieved by tension-only X-bracing with back-to-back double angle members. Seismic evaluation is first performed in accordance with the recommendations of the User's Guide to NBCC 2010 using response spectrum analysis. A Tier 3 systematic evaluation according to ASCE 41-13 is then carried out using both linear and nonlinear dynamic analysis procedures. The performance objectives are set same as those implied in NBCC 2010. The ASCE 41 approach results in much less corrections to the structure. Using the mean results from 10 ground motions is also less stringent than using the maximum of three records. For this structure, nonlinear dynamic analysis permitted to identify the concentration of inelastic deformations demands on the braces along the frame height. This behaviour induced considerable bending moment demands on the columns not captured by linear dynamic analysis. Finite element analysis of the structure columns showed that columns of braced steel frames can exhibit higher ductility compared to acceptance criteria specified in ASCE 41.

Experimental and numerical investigation of cold-formed steel double angle members under compression

This paper presents a numerical and experimental study of double angle members connected by batten plates under concentric and eccentric axial compression. The number of batten plates is varied to study the influence on the nominal axial strength. Numerical analyses are able to accurately predict the behavior and strength found in the experiments, except for long columns under eccentric axial compression where the composite section failed in major-axis flexural buckling. Two design hypotheses are compared to the results obtained: (i) non-composite action (no interaction between angles), with only local, flexural, and flexural-torsional buckling considered; and (ii) composite action (full interaction between angles), and the only considering local and minor-axis flexural buckling of the pair of angles. The two design hypotheses ignore load eccentricity. Numerical and experimental results for angles connected by bolted batten plates fall in between the design curves defined by methods (i) and (ii), while angles connected by welded batten plates have greater strength than the design curve defined by method (ii). The use of batten plates significantly increases the strength of the system, especially for members under eccentric compression. However, the strength remains constant after a certain number of batten plates are connected, and after a minimum batten plate width is reached.

Theoretical and experimental study of cold‐formed steel double angle members under compression

Theoretical and experimental study of cold‐formed steel double angle members under compression

Inhibiting Steel Brace Buckling Using Carbon Fiber-Reinforced Polymers: Large-Scale Tests

A new method is proposed for inhibiting the buckling response of steel braces. The strengthening technique involves attaching a core comprised of mortar blocks to the braces and then wrapping the entire system with carbon fiber-reinforced polymer (CFRP) sheets. An advantage of this method is that the technology can be applied in situ by a single individual with minimal training. Test results of seven specimens subjected to reversed axial loading show that it is feasible to achieve buckling restrained response up to 2% interstory drift. Other than the number of longitudinal fiber layers, it is observed that performance of the strengthening scheme depends upon the size of the core material, the presence of bond between the steel plate and core material, the existence of extra stitch plates for double angle members, and the presence of transverse CFRP layers at the member ends. It is also observed that double angle braces benefit more from the strengthening scheme than single angle braces, which are prone to premature out-of-plane buckling.

Fracture toughness of structural aluminum alloys

Some of the procedures available for considering the fracture behavior of structural aluminum alloys in design are described. For the higher strength aluminum alloys, particularly when used in fairly thick sections, plane-strain fracture-toughness concepts are useful; values of the critical plane-strain stress intensity factor, K 1c are summarized and typical values and ranges of expected values are presented. For highstrength alloys in relatively thin sections, and medium-strength alloys in thicker sections, a thickness-related K c approach seems to have merit; representative values of K c are presented for a range of thicknesses of some alloys. For the lower strength, very tough alloys or for relatively mild stress raisers, the results of tests of small structural assemblies are relied upon; the results of tensile tests of single- and double-angle members are presented as an illustration of this type of analysis.