Search

This study investigated the seismic performance and self-centering behavior of a three-story steel frame equipped with self-centering brace (SCBs) using pseudodynamic tests aided by OpenSees version 2.5.7 and OpenFresco version 2.7.0. The three-story self-centering braced frame (SCBF) was simplified to a three-story shear building model, and a generic experimental element with three nodes with horizontal degrees of freedom was used to establish a two-dimensional (2D) numerical model. The input ground motions were scaled to a service level earthquake (SLE), design basis earthquake (DBE), maximum considered earthquake (MCE), and a very rare earthquake (VRE) with 63%, 10%, 2%, and 0.5% probability of exceedance in 50 years, respectively. The test results demonstrated the excellent seismic performance and self-centering behavior of the SCBF. The maximum story drift ratio and maximum residual story drift ratio of the SCBF under a VRE were 1.91% and 0.119%, respectively, below the thresholds of 2.0% and 0.5%. The SCBs provided more than 90% of the initial lateral stiffness of the SCBF and dissipated approximately 55% of the seismic input energy. The seismic performance of the SCBF satisfied the no-damage criterion under an SLE and DBE. Although the columns in Story 1 yielded under an MCE and VRE, no visible residual deformation of the column flanges was detected in Story 1 after the tests. In addition, the maximum residual drift ratio of Story 1 was only 0.066%. Therefore, the columns in Story 1 only required simple and quick reinforcements before being usable. Unexpected factors, such as the fit-up tolerances around the pins, out-of-sync activation of the SCBs in the same story, residual deformation of the SCBs, and the out-of-sync activation of the two groups of disc springs in the same SCB, did not affect the seismic performance and self-centering capacity of the SCBF. [ABSTRACT FROM AUTHOR]

This study presents a novel approach to enhancing the progressive collapse resistance of a fully welded joint (FWJ) by introducing sliding inner cores within the joint, which is referred to as the FWJ with sliding inner cores (FWJS). Two specimens were prepared and designed with additional consideration of composite action of the slab. Quasi-static tests and refined numerical simulations were conducted to analyze the damage mode, deformation behavior, strain distribution patterns, internal forces, and resistance development in both specimens. The test results revealed that the first fracture occurred in the tensile beam flanges for both FWJ and FWJS specimens, whereas the FWJS specimen exhibited delayed crack of the tensile beam flange compared to the FWJ specimen. The FWJS specimen demonstrated better utilization of the catenary mechanism during the flexural–catenary combined stage. Additionally, in the elastic, elastic–plastic, plastic, and flexural–catenary combined stages, the addition of the sliding inner cores in the FWJS specimen significantly enhanced its resistance by 15.6%, 22.3%, 23.4%, and 70.2%, respectively. It is also revealed that the response of the sliding inner core consisted of bending, transition, and full tension stages, which was shown to effectively replace the fractured beam flange in redistributing the internal forces. Based on the analysis of the working mechanism and numerical parametric analysis, optimal values for the 10 design parameters of the FWJS are suggested, and a comprehensive design procedure for the FWJS is proposed. [ABSTRACT FROM AUTHOR]

In the current research work, the shear capacity of the cold-formed steel (CFS) three-bolted clip angle (CA) beam-to-column moment connection (3BM) is investigated with 45 laboratory test results. The 3BM connection comprises a three-bolted CA connector that connects the web portions and a bolted flange cleat (FC) that connects the flange portions of the beam and column; in comparison, the three-bolted shear connection (3BS) connects only the web portions of the beam and column. The presence of the FC improved the shear strength of the 3BM connection over the 3BS connection and it is quantified using a coefficient called the "rigidity factor (β)." A shift in the failure mode of CAs from shear local buckling in a 3BS connection to tearing failure in a 3BM connection was observed as the FC resists beam torsional deformation and delays the early local buckling of the CA. The existing design shear equation of the 3BS connection is improved by expressing it as a function of CA failure modes. Following the same trend, the existing design shear method for the CFS moment connection is improved by incorporating the failure modes of the CA boundary conditions along with the geometric and material properties of both the CA and FC for the present 3BM connection. In addition, an empirical shear design equation is suggested for the 3BM connection. The design factors of the proposed 3BM shear equations for load and resistance factor design, limit state design, and allowable strength design methods were evaluated from the reliability study. [ABSTRACT FROM AUTHOR]

The brittle behavior traditional directly welded connections of steel beams to concrete-filled steel tube columns is caused primarily by excessive out-of-plane deformation of the tube walls at the level of the beam tension flange. To address this issue, this study proposes a reference connection that involves fillet welding a built-up steel beam directly to the corner of a diamond-shaped concrete-filled steel tube (DCFST) column. Two options are suggested for enhancing the seismic performance of the reference connection: a predrilled hole with a 12-mm diameter at the midwidth of each beam flange, or a rib stiffener reinforcement on the outside of each beam flange. The two proposed specimens and a traditional directly welded connection were subassembled and investigated experimentally. The results showed that the reference DCFST connection significantly improved the seismic response of the directly welded connection in terms of initial stiffness, strength, ductility, and energy-dissipating capacity. Specifically, the initial stiffness and strength capacity increased by over 150% compared with that of the traditional connection counterpart. Furthermore, the rib stiffeners contributed to further enhancement of the seismic response by shifting the beam plastic hinge away from the column face. Overall, these findings suggest that the use of DCFST connections is reliable for composite moment-resisting frames (C-MRFs), and the proposed enhancements can improve the seismic performance of these connections significantly. [ABSTRACT FROM AUTHOR]

This paper presents the study of a single-sided column–tree connection with a replaceable buckling-restrained cover plate (BRCP) at a bottom flange to achieve rapid postearthquake repair for connections in the presence of concrete slabs. During major earthquakes, the bottom flange core plate of the BRCP yields and dissipates energy, and the connection rotates around the center of the top flange connection plate. The slip and yield moments of the proposed connection were theoretically calculated, and the design principle was proposed. Quasi-static loading tests were performed on four specimens. Moreover, the hysteresis behavior, failure modes, force–deformation hysteresis curves, force–deformation backbone curves, strain responses, energy dissipation capacity, and neutral axis position were obtained. Additionally, the influence of restrained forms, unbonded material, and neutral axis position on the hysteresis performance were analyzed. The results show that the two-piece restraint plate leads to extreme local bending of the core plate, the unbonded material on the restraint plate has little effect on the seismic performance, and the core plate undergoes larger deformation when the neutral axis is close to the top flange. Finally, finite-element models of the test specimens were established and validated using the test hysteretic curves and the failure modes to further investigate the test results. [ABSTRACT FROM AUTHOR]

This study investigates the performance and viability of a horizontal stiffener detailing (HSD) to (1) be used at the link-to-column connection in a D-braced eccentrically braced frame layout where conventional link-to-column connections show poor performance, (2) minimize the overstrength factor of shear links caused by the hardening of link flanges, and (3) mitigate an undesirable failure mechanism previously observed in shear links fabricated using ASTM A992 steel. HSD and conventional stiffener detailing (CSD) were tested using W460×60 sections under two loading protocols, including one simulating near-collapse excitation. While the flanges of CSD shear links are greatly stiffened by vertical stiffeners, webs of HSD shear links exhibit inelastic buckling at large rotation angles. This allows the flanges to freely deform, thereby reducing the likelihood of brittle fracture at the flange connections, as well as undesirable link overstrength induced by the flange contribution. This study shows that the horizontal stiffener configuration developed a ductile failure mechanism resulting from gradually increasing local buckling in the web. The strength degradation of links with horizontal stiffener detailing was much more gradual due to the ever-increasing web buckling, unlike the sudden brittle fracture at the flange-to-column face of links with CSD. This research shows that the HSD is a viable and economic alternative for shear links, which has the potential to decrease welding in the shear link while exhibiting adequate seismic performance. HSD also maintains a low overstrength factor and utilizes a simplistic design approach. Finally, the bolted extended end-plate connection and its details at the link ends used in the test experiment provided a viable solution for replaceable links. [ABSTRACT FROM AUTHOR]

Wide-flange columns are commonly used for the construction of special moment frames and braced frames. For special moment frames, design engineers routinely use deep sections to meet the stringent code-specified story drift limits in order to achieve economy in recent years. Columns with shallow and stocky sections are common for braced frame construction. Observation made from two recent experimental studies involving both deep and shallow columns under axial compression and cyclic drifts showed a variety of buckling modes. A classification with three cyclic buckling modes is proposed in this paper; subsets of each buckling mode are also presented. A parameter measuring the relative rotational restraint that the web provides to each flange is derived. After calibrating this parameter with the test results, a practical procedure that can predict the governing cyclic buckling mode is proposed. [ABSTRACT FROM AUTHOR]

This paper presents the evaluation of two methods for manually estimating the elastic lateral-torsional buckling (LTB) resistance of general doubly and singly symmetric I-section members having a linear variation in the web depth. The study focuses on unbraced lengths having idealized torsionally simply supported (fork) end conditions. The first method uses cross-sectional properties at the midspan of the unbraced segment along with a moment gradient modifier equation, written in terms of the flange stresses, to develop its strength approximation. This approach is employed in the first edition of the AISC-MBMA Design Guide 25 (DG 25). The second method is an extension of an approach that has been shown to be accurate and efficient for prismatic doubly and singly symmetric I-section members and is recommended in the second edition of DG 25. This approach uses cross-sectional properties at a critical section within the unbraced length, defined as the cross section where the ratio of the moment demand to the base LTB resistance in uniform bending is maximum, along with a moment gradient modifier equation written in terms of the above maximum ratio and the corresponding ratios at the quarter points of the unbraced length. Furthermore, Method 2 employs an additional calibration factor that accounts for an apparent destabilizing effect of the inclination of the shear center with respect to the axis of torsional restraint. The predictions from these methods are evaluated by an extensive parametric study encompassing the full design space for I-section members with linearly-varying web depth, as defined by DG 25. The second method is shown to provide improved accuracy, avoiding undesirable nonconservative errors in certain situations. The calibration factor developed as part of the second method is shown to eliminate undesirable errors in the first method as well, providing streamlined and accurate buckling estimates for linearly tapered member unbraced lengths. [ABSTRACT FROM AUTHOR]

In this study, the flexural behavior of noncompact and slender concrete-filled U-section (CFU) beams was investigated focusing on local buckling of U-section walls and shear transfer across the beam-to-slab interface. Seven large-scale CFU beam specimens were tested under negative bending. The width-to-thickness ratios of U-section walls (i.e., flanges and webs) and stiffening details for preventing local buckling were considered as the test variables. The tests showed that, due to excessive slab reinforcement, all specimens were failed by shear failure at the composite interface of beam and slab after flexural yielding. The ultimate strength and deformation capacity of CFU beams were degraded by flange and web local buckling. Buckling modes of U-section walls were affected by the stiffening details. The ultimate strengths of noncompact and slender CFU beams were compared with the nominal strengths calculated in accordance current standards. In addition, the flexural behavior of CFU beams affected by the local buckling and partial debonding of U-section walls and slip behavior at the composite interface was further investigated through fiber-based section analyses based on the effective stress–strain models of materials. [ABSTRACT FROM AUTHOR]

Six full-scale specimens were tested to examine the cyclic-loading performance of steel moment connections between an I-section beam and I-section column. The specimens represented the current practice in Japan in materials, dimensions, detailing, and fabrication. Three of the specimens had the beam welded to the web of the column; the other three had the beam welded to the flange of the column. Some of the specimens had doubler plates welded to the column web. Regardless of the framing orientation, the specimens displayed similar strength and stiffness. All specimens met the performance requirement for special moment frames according to US seismic provisions except for one specimen, which had the beam welded to the column web, and which failed prematurely by fracture of the continuity plate. Yielding of the continuity plate was found to be an important factor that can lead to such failure. Therefore, by making the continuity plates stronger than the beam flanges, either by adopting a sufficiently strong steel or thicker plate, moment connections to the column web, fabricated according to the current Japanese practice, may behave very similarly to moment connections to the column flange. [ABSTRACT FROM AUTHOR]

Some of Louisiana's bridges built in the 1950s and 1960s used two-girder or truss systems, in which transverse floor beams are carried by main longitudinal members and continuous (spliced) beams supporting the deck are supported by the floor beams. The main longitudinal members are either two edge (fascia) girders or trusses. Continuous-beam bottom flanges are in compression in the negative moment region, which could result in lateral torsional buckling. When the continuous beams are load rated, Cb is calculated in accordance with a standard specification that does not account for potentially beneficial bracing effects from a noncomposite concrete deck and could therefore underestimate actual flexural strength. As a result, the rating may require a restrictive bridge posting or closure. This issue affects bridges that are key parts of Louisiana's highway system, i.e., longer, multispan crossings with high average daily traffic (ADT) counts. The calculated load rating could require expensive, and possibly unnecessary, bridge rehabilitation or replacement with significant traffic disruption. This research reassessed the methodology behind load rating these continuous beams, with efforts focusing on deriving more realistic values for Cb. Its main objective was to evaluate the capacity of bridges built with continuous beams and noncomposite decks and to develop a new approach for rating those beams by more accurately representing the moment gradients using Cb. An experimental study was conducted to evaluate lateral torsional–buckling resistance of a reduced-scale, two-span grillage system that included three lines of continuous beams supporting a noncomposite concrete deck. A significantly higher moment gradient factor as compared to the existing codes and standards was recommended by accounting for the bracing effect provided by a noncomposite deck. [ABSTRACT FROM AUTHOR]

A warping-inclusive kinematic coupling method to be used in finite-element analysis of members featuring wide-flange cross sections is proposed in this paper. This coupling method is used in mixed-dimension macromodels that combine continuum and beam-column elements to reduce the computational cost of purely continuum finite-element models. The proposed coupling method, utilizing either linear or nonlinear constraint equations, is implemented and validated in a commercial finite-element software; the source code is made publicly available. Case studies indicate that including warping in the coupling formulation is critical for components that may experience coupled local and lateral-torsional buckling. Also highlighted is the potential of macromodels to reduce the total degrees of freedom by up-to about 60%, and computational memory use by up to around 80%, while retaining solution fidelity for beam, column, and panel zone components in steel moment-resting frames. The case studies show that the linear constraint equation formulation may not be suitable for all problems; however, it may still yield acceptable results as long as the level of twisting is insignificant and lateral-torsional buckling is not critical. [ABSTRACT FROM AUTHOR]

A finite-element formulation is presented for the elastic distortional buckling analysis of doubly symmetric wide flange beams. The formulation characterizes the distribution of the lateral displacement along the web height through Fourier modes and is equipped with features to capture the load height effect and to enforce user-defined kinematic constraints. The formulation is used to investigate the load height and bracing height effects and develop a database of over 20,000 runs on practical beam cross-sections to characterize (1) the reduction in critical moment capacity due to web distortion; and (2) the effect of a load height on the elastic critical moments. A novel indicator characterizing web distortion along the beam span is proposed and integrated within the finite-element formulation in a procedure devised to optimize the location of transverse stiffener(s) along beam spans so as to reduce cross-sectional distortion and maximize the elastic critical moment attainable. The procedure is then applied to optimize the design of beams in two case studies. [ABSTRACT FROM AUTHOR]

A steel delta girder (SDG) is formed by welding two inclined plates in the form of a V from the web to the compression flange of an I-section. Analytical, numerical, and experimental research on SDGs has shown that they possess enhanced flexural and shear capacities over traditional I-sections with comparable cross-sectional areas. In this paper, three-dimensional nonlinear inelastic finite-element models that take into consideration the effects of initial geometrical imperfections and residual stresses are developed to investigate the inelastic lateral-torsional buckling and shear behavior of simply-supported SDGs under uniform bending and pure shear. Using European H- and I-sections as base cross sections, the analyses cover a wide range of SDG dimensions and inclined stiffener configurations. The results of these analyses demonstrate that the flexural capacities of SDGs can be represented by two of the existing EuroCode 3 (EC3) buckling curves. Design equations for the shear capacity of SDGs are then developed and proposed in accordance with EC3 provisions. Finally, design recommendations for selecting the proper inclined plate dimensions and configurations within the delta region of SDGs are provided. [ABSTRACT FROM AUTHOR]

When a wide-flange section is exposed to close-in detonation, local and global deformation modes are developed due to a complex nonuniform pressure environment. The local deformation includes local distortion and rotation of the flanges, as well as deformation of the web, or even breaching near the web/flange junction. Experimental studies are presented to investigate the response of wide-flange sections to close-in detonations with and without stiffener plates and loaded about their weak and strong bending axes. The effectiveness of the stiffeners in mitigating damage is studied, and although they do have a positive effect, in some cases, the stiffeners do not contribute to the member resistance. Numerical models, validated against the test results, are used for a parametric study on the effect of the angle of incidence between the charge and the section, and the web-to-flange thickness ratio. It is found that a member needs to be analyzed for loading directions between the weak and strong bending axes to account for the maximum possible flange distortion. The thickness of the flange is irrelevant for the local web deformation but important for the global behavior of the member. [ABSTRACT FROM AUTHOR]

Recent full-scale testing of steel wide-flange columns under axial compression and cyclic lateral drifts for special moment frame applications showed that deep, slender columns could experience significant flexural strength degradation due to plastic hinge formation with buckling modes and considerable axial shortening within the hinge. Test results showed that the interaction between web and flange local buckling played a significant role for the observed degradation, even when the cross-sectional elements met the highly ductile limiting width-to-thickness ratios specified in the current seismic design standard for steel frames. These observations were also confirmed by numerical simulations. Enhanced limiting width-to-thickness ratios for the web of a wide-flange column for both special and intermediate steel moment frames are proposed to limit the severity of strength degradation and axial shortening. [ABSTRACT FROM AUTHOR]

This study experimentally and numerically demonstrated that the traditional basic shear method is not applicable for estimating the shear stresses in tapered beams with corrugated steel webs (CSWs). The vertical components of the inclined flange forces (which are zero in the prismatic case) may decrease or increase the effective shear force on the tapered CSWs, which are reflective of the positive and negative influences of the Resal effect. In addition to the CSWs, the inclined flange also participates in resisting the shear force in a tapered beam with CSWs, and the shear forces borne by the CSWs and inclined flanges are mainly adjusted by the bending moment. Accordingly, an improved method, which is referred to as the modified shear method, was proposed in this study by considering the web shear modification of CSWs and the shear resistance of the inclined flange. The study also revealed that the effective shear force on the CSWs of regular tapered beams is actually larger than the total shear force, especially in sections that are subjected to larger moments. Thus, the calculated shear stresses in the CSWs predicted by the basic shear method are smaller than the actual values. The shear stresses that act on the corrugated webs of the reverse tapered beams with CSWs would be overestimated using the basic shear method. Parametric finite-element (FE) analysis showed that the proposed modified shear method can reflect the actual stress condition of tapered beams with CSWs. The modified shear method can be analytically reduced to the traditional basic shear method when applied to prismatic cases. This study is expected to provide a novel strategy and theoretical guideline for rapidly and efficiently obtaining the shear stresses in a variable cross-section beam with CSWs. [ABSTRACT FROM AUTHOR]

This paper presents a novel experimental investigation into the pre- and postcritical behavior of steel-plate girders in shear. We have quantitatively scrutinized the deformation behavior of the web shear buckling using an optical measurement system. We reveal that the postcritical phase consists of two distinct stages, which refutes the belief of seemingly conflicting modeling approaches between the North American and the European design codes. We show that the first stage ends with the onset of a temporary instability of the main shear buckling that caps the girder's shear resistance and corresponds to the shear capacity of the Basler model from the North American design code. We also show that the second stage, however, confirms the modeling approach of the Eurocode, because this stage exhibits (after stabilizing secondary buckling develops) a recovery of the shear resistance and terminates with the onset of a failure mechanism with plastic hinges in the flanges. While restoring the validity of the Basler model, we uncover the need for further experimental and numerical investigations to quantitatively assess the range of applicability of the Eurocode model. [ABSTRACT FROM AUTHOR]

Lip stiffened flanges in cold-formed steel compression members may undergo local or distortional buckling depending on the slenderness of the flange and rigidity of the lip. In distortional buckling, the flange-lip assembly moves perpendicular to the flange. The decrease in stiffness (or depth) of the lip increases the vulnerability of such elements to distortional buckling. In the limiting case of zero lip depth, the plate behaves as an unstiffened element. Hence the behavior of the unstiffened element may be intuitively understood as the limiting case of distortional buckling of lip stiffened element as the lip depth tends to zero. In this study, experimental and numerical results are used to show that both elastic buckling and ultimate strength behavior of a uniformly compressed unstiffened element may also be represented as distortional buckling of the lip stiffened element of zero lip depth. In addition to the effective width method, the modified direct strength method equation developed for distortional buckling is also shown to accurately estimate the ultimate strength of uniformly compressed unstiffened elements. [ABSTRACT FROM AUTHOR]

Hollow structural sections (HSS) are used as columns in frame systems because of their ability to efficiently resist multiaxial loads. While the majority of previous studies of special moment frames (SMFs) focused on wide flange columns, HSS columns can provide a means of increasing the versatility of SMFs. In order to fully explore the use of HSS columns in SMFs, the collapse behavior of HSS columns subject to combined axial and lateral loading is computationally studied. Seventeen different HSS profiles are selected to cover a wide range of local and global slenderness ratios, including width-to-thickness ratio (b/t), depth-to-thickness ratio (h/t), and the global slenderness ratio about the weak axis (L/ry). Detailed finite element studies show that the behavior of HSS columns is highly dependent on the local slenderness ratios and the level of initial applied axial load (P/Py). A regression analysis is performed leading to a proposed design aid for HSS columns in SMFs along with updated highly ductile local slenderness limits. The proposed highly ductile limits suggest that the current AISC limits for HSS columns are conservative when considering the width-to-thickness ratio and potentially unconservative when considering the depth-to-thickness ratio. [ABSTRACT FROM AUTHOR]

This paper presents a reduced-order numerical modeling approach for the analysis of heat transfer in composite floor slabs with profiled steel decking exposed to fire effects. This approach represents the thick and thin portions of a composite slab with alternating strips of shell elements, using a layered thick-shell formulation that accounts for both in-plane and through-thickness heat transfer. To account for the tapered profile of the ribs, layered shell elements representing the thick portion of the slab adopt a linear reduction in the density of concrete within the depth in the rib. The specific heat of concrete in the rib is also proportionally reduced to indirectly consider the heat input through the web of the decking, because the reduced-order model considers thermal loading only on the upper and lower flanges of the decking. The optimal ratio of modified and actual specific heat of concrete in the rib is determined, depending on the ratio of the height of the upper continuous portion to the height of the rib. The reduced-order modeling approach is validated against experimental results. [ABSTRACT FROM AUTHOR]

A shake-table test of a 1:3 scale model of a thin, lightly reinforced concrete (RC), 5-story coupled wall, representing a typical apartment building with a high wall-to-floor area ratio, was performed. The wall consisted of two T-shaped piers, faced to each other by the webs, which were connected at each story level by the coupling beam and slab. The wall was subjected to a series of uniaxial and biaxial seismic excitations. Two different structural details providing different degrees of confinement were used at the free edges without boundary columns. The lightly confined free edges of the flanges (using hairpin transverse reinforcement) behaved satisfactorily in the case of moderate seismic demand. Due to the beam-slab interaction and the slab, which was considerably ticker than in typical buildings, the coupling beams were much stronger than expected, causing brittle shear failure of the overloaded wall piers under bidirectional excitation. The analyzed wall developed substantial strength and low ductility. The main observations of the experiment were confirmed by the results of postexperimental analytical studies performed using a three-dimensional (3D) multiple vertical-line element model, which has been incorporated into a wellknown open-code program system. [ABSTRACT FROM AUTHOR]

Based on the condition of static equilibrium and the equivalent law of shearing stress of an infinitesimal segment, this paper provides a strict derivation of the general formula for shearing stress in a nonuniform box girder with corrugated steel webs in the elastic stage. The derived formula is applied to investigate the stress distribution in concrete flanges and corrugated steel webs. Additional shear stress was found to be caused by the bending moment, and the axial force should be included in calculations from the effect of variable cross sections, which are quite different from that of the uniform cross sections. Moreover, because the additional shear stress of the bending moment is self-balanced, the shear force distribution can be adjusted between the concrete slabs and steel webs. Research shows that shear stress decreases markedly in the webs, whereas the shear stress apparently increases in the inclined bottom flange in the action of the bending moment. This study also found and explained the phenomenon that shear stress in the lower surface of the inclined bottom flange is not zero. In the elastic stage, the proposed formula is in good agreement with the results of the three-dimensional (3D) finite-element analysis for a cantilever beam. This example shows that the classic method of material mechanics is no longer suitable for the calculation of a nonuniform beam. Finally, because the formula is too complex to be extensively executed by engineers, two simplified calculation methods are put forward considering the equivalent force principle, and the simplified calculation methods are proved to be valid and applicable according to the contrasting results with the nonuniform mathematical formula. [ABSTRACT FROM AUTHOR]

The axial compressive strength of flange-damaged steel columns is studied using an equilibrium-based analytical approach. Compared to a related investigation that employed a variational technique, the presented model shows similar trends regarding severely reduced axial compressive strength associated with local flange damage. An empirical equation based on a parametric study performed using the developed technique is proposed as a tool that can be used by practicing engineers to determine a quick and reasonable approximation of the capacity of locally flange-damaged steel columns. The developed equation is validated using detailed finite-element analysis. [ABSTRACT FROM AUTHOR]

The NIST Building and Fire Research Laboratory developed 29 recommendations for research based on the findings from the investigation of the World Trade Center (WTC). This paper discusses the results of a collaborative research effort conducted to address recommendation R9.1 from that report, namely to "Develop and validate analytical tools, guidelines, and test methods necessary to evaluate the fundamental behavior and fire performance of components and the structure as a whole system." Specifically, it describes the development of a closed-form analytical model to represent moment-curvature-thrust-temperature (M-Φ-P-T) behavior of steel beam-columns in strong or weak-axis bending. The model considers uniform temperature distributions through the cross sections, and it is developed from parametric studies using a fiber-based analytical approach calibrated to experimental and finite-element results. The section-constitutive model was developed to provide a computationally efficient alternative to fiber-based or finite-element analysis (FEA) models and facilitate development of analytical approaches that do not require sequentially coupled thermal-mechanical analyses. [ABSTRACT FROM AUTHOR]

AASHTO LRFD and AISC specifications have adopted Basler's interaction equation, which was formulated for noncompact sections without considering shear buckling. AASHTO LRFD specifications, however, have completely neglected the interaction effect of bending on shear strength since the 3rd edition in 2004. AISC LRFD specifications followed suit with the 2005 edition. This study was aimed at investigating interaction behaviors in noncomposite I-girders and developing new interaction equations without considering shear buckling. Also, a simple and universally applicable methodology was proposed for web panels whose failures were associated with shear buckling. The interaction should not be ignored when web failures are governed by yielding because of combined bending and shear. Otherwise, the shear capacity of the web panel could be grossly overestimated, thereby compromising the safety of the girder. [ABSTRACT FROM AUTHOR]

At ambient temperature, estimations of the postbuckling strength of steel plates (web and flanges) in wide-flange beams are based on the assumption that the stress at the edge of the plate equals the yield stress of the material. However, at elevated temperatures material behaves in a nonlinear manner beginning at very small strains. The work presented in this paper has shown that at elevated temperatures the ultimate buckling load occurs when stresses at the plate edge are smaller than the yield stress, which are typically defined at large strains such as at 2%. Hence, the current expressions for plate buckling strength at ambient temperature cannot be directly applied at elevated temperature. By taking into account the nonlinear behavior of steel at elevated temperatures, a new postbuckling strength equation for webs and flanges in wide-flange beams that correlates well with finite-element studies at elevated temperatures is proposed. [ABSTRACT FROM AUTHOR]

A LiteSteel beam (LSB) is a new cold-formed steel hollow flange channel section produced using a patented manufacturing process involving simultaneous cold-forming and dual electric resistance welding. The LSBs are commonly used as floor joists and bearers with web openings in residential, industrial, and commercial buildings. Their shear strengths are considerably reduced when web openings are included for the purpose of locating building services. However, no research has been undertaken on the shear behavior and strength of LSBs with web openings. Therefore, experimental and numerical studies were undertaken to investigate the shear behavior and strength of LSBs with web openings. In this research, finite-element models of LSBs with web openings in shear were developed to simulate the shear behavior and strength of LSBs including their buckling characteristics. They were then validated by comparing their results with available experimental test results and used in a detailed parametric study. The results showed that the current design rules in cold-formed steel structures design codes are very conservative for the shear design of LSBs with web openings. Improved design equations have been proposed for the shear capacity of LSBs with web openings based on both experimental and parametric study results. An alternative shear design method based on an equivalent reduced web thickness was also proposed. It was found that the same shear strength design rules developed for LSBs without web openings can be used for LSBs with web openings provided the equivalent reduced web thickness equation developed in this paper is used. This is a significant advancement as it simplifies the shear design methods of LSBs with web openings considerably. [ABSTRACT FROM AUTHOR]

This paper examines the accuracy of equations specified by the North American and Australasian steel structures codes for determining the net section tension capacity of a channel brace. It points out that there are three distinct factors affecting the net section efficiency of a cold-formed steel channel brace bolted at the web. These factors include (1) the in-plane shear lag associated with stress concentration around a bolt hole that is also present in flat sheets, (2) the out-of-plane shear lag that is also present in an I-section bolted at the flanges only, and (3) the bending moment arising from the connection eccentricity with respect to the neutral axis. Each of the relevant factors is explicitly incorporated into the equation proposed in this paper for determining the net section tension capacity of a cold-formed steel channel brace bolted at the web. The proposed equation is demonstrated through laboratory tests on low-ductility channel braces having practical aspect ratios that were bolted onto flexible plates, to be more reliable than the code equations and those existing in the literature. [ABSTRACT FROM AUTHOR]

This work aims to enhance the energy-dissipation capacity of classical rectangular T-stubs by proposing an hourglass shape for the T-stub flange according to the approach usually adopted for added damping and stiffness (ADAS) devices. A new type of axial damper is developed. First, a mechanical model of the device is set up and a finite-element model is carried out in ABAQUS code. The accuracy of both models is verified through comparison with experimental results. Next, on the basis of cyclic tests, the improvement of the energy-dissipation capacity of classical T-stubs provided by the proposed approach is quantified, and the low-cycle fatigue curves are determined with reference to the case of both T-stubs on rigid support and of coupled T-stubs. The results of the work also represent a useful tool for designing a dissipative double split tee connection. [ABSTRACT FROM AUTHOR]

This paper contains detailed analyses of an experimental study conducted to evaluate the ductility, stiffness degradation and energy dissipation characteristics of rectangular, flanged, and end-confined reinforced masonry (RM) shear walls failing in flexure. The test program consisted of seven two- and three-story RM shear walls, with aspect ratios of 1.5 and 2.2, tested under reversed cyclic lateral displacements simulating seismic loading effects. Documentation of the compressive strains at the wall toes, wall base curvatures, and ductility levels attained are presented. The paper focuses on determining the extent of plasticity over the wall height, evaluating the contribution of flexure and shear deformations to the overall wall lateral displacements, identifying the trend of stiffness degradation, and quantifying the amount of energy dissipation. The rectangular walls displacement predictions at ultimate loads using Canadian Standards Association (CSA) S304.1 were in better agreement with the experimental results compared to the Masonry Standards Joint Committee (MSJC) code predictions. However, both the MSJC code and the CSA S304.1 significantly overestimated the test results for the flanged and end-confined walls. Analysis of the measured displacements showed that the contribution of shear displacement to the overall wall displacement was, on average, 21 and 25% of the total displacement for the walls with aspect ratio of 2.2 and 1.5, respectively. The relationship between the energy dissipation and the ratio of the postyield to the yield displacements was found to be almost linear for the test walls. In addition, the wall stiffnesses degraded rapidly to about 60% of their gross stiffness at very low drift levels (0.1% drift). Measured compressive strain at the wall toes were almost double those specified in both North American codes. Extent of plasticity over the wall height was about 75% of the wall length. The data presented in this paper is expected to facilitate better understanding of RM wall behavior under in-plane load to researchers, practicing engineers, and code developers. This study aimed at presenting the flanged and end-confined categories as cost-effective alternatives to enhance the seismic performance of midrise RM construction in North America. [ABSTRACT FROM AUTHOR]

The subject of the paper is cold-formed thin-walled channel beams with open or closed profile of drop flanges. The geometric properties of two C-sections are described including warping functions and warping inertia moments. The analytical solutions of problems of global and local stability of thin-walled beams are presented. The theoretical solutions of elastic buckling problems of these beams are numerically found using FEM and are experimentally verified. The results of analytical and numerical investigations are compared and presented in the tables and figures. [ABSTRACT FROM AUTHOR]

In this paper, seven reinforced concrete-block shear walls with aspect ratios of 1.5 and 2.2 (two- and three-storey high) were tested under displacement-controlled cyclic loading. The response of rectangular, flanged, and end-confined walls, designed to have the same lateral resistance when subjected to the same axial load, is discussed. In general, high levels of ductility accompanied by relatively small strength degradation were observed in all walls with a significant increase in ductility and displacement capabilities for the flanged and end-confined walls compared to the rectangular ones. For both aspect ratios evaluated, the drift levels at 20% strength degradation were 1.0, 1.5, and 2.2% corresponding to the rectangular, the flanged, and the end-confined walls, respectively. The ductility values of the proposed flanged and end-confined walls were, respectively, 1.5 and 2 times those of their rectangular wall counterparts (with the same overall length and aspect ratio). In addition to the enhanced ductility, a saving of more than 40% in the amount of vertical reinforcement was achieved using the proposed alternative strategies while maintaining the same lateral wall resistance. Existing design clauses were used to predict the wall capacities using the American and the Canadian masonry codes and showed excellent agreement. This will facilitate adoption of the new construction categories with minimal modifications to existing code clauses. The test results indicate that higher ductility than the currently endorsed values by North American codes should be used for rectangular walls. Moreover, higher values should be expected when the proposed strategies are adopted which would significantly reduce the seismic demand on reinforced concrete-block shear wall construction. [ABSTRACT FROM AUTHOR]

The monolithic structural action of various types of walls of flanged cross section and walls with engaged stiffeners or returns (nonrectangular sections) is critically dependent on the shear capacity of the interface between the components making up the section. An assessment of the shear capacity of the interface may also be required as part of the elastic or inelastic analysis of structural elements. Observation of past earthquake events has confirmed that effectively connected flanges also directly influence the seismic performance of nonrectangular sections by providing a more robust and seismically resistant structural element. To analyze the significance of the parameters that influence the vertical shear resistance of interfaces of flanged sections of masonry obtained using different types of bonding and bonding patterns (header units, shear connectors, and wire ties), a series of specimens with “H-shaped” cross sections was tested. The vertical shear capacity of the interface at the flange-web intersection was assessed in each case and compared to predicted code capacities. Considerable strength reserves and shear ductility were observed in almost all cases. Following the experimental study, a numerical investigation was carried out using a simplified micromodeling finite element approach. It showed that both the size of the specimen as well as the boundary and applied loading conditions on the flanges can significantly influence the observed vertical shear resistance of the interfaces of specimens with the same bonding pattern. Using the results of the numerical investigation, the critical parameters for a test to determine the vertical shear capacity of brick masonry nonrectangular cross sections are assessed and the governing parameters for a suitable shear test suggested. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP) pultruded profiles are produced by a number of manufacturers worldwide in similar, but nonstandard, wide-flange, I, angle, and tubular profiles. At present there is no American National Standards Institute approved design code in the United States for structural design with pultruded FRP profiles. Manufacturers of pultruded profiles each provide their own design equations, design methods, material properties, and safety factors for their pultruded products. There is a need for standardization of production and design of pultruded profiles to enable mainstream use of these profiles in structural engineering practice. The purpose of this paper is twofold: (1) to provide appropriate resistance factors ([lowercase_phi_synonym] factors) for wide-flange pultruded columns that are compatible with ASCE 7 load factors and (2) to provide a unified analytical equation for local and global buckling of concentrically loaded axial members, which may be appropriate for a future design code. The resistance factors are provided for different target levels of structural reliability, β, and for different nominal design properties of the pultruded materials. The resistance factors were determined using Monte Carlo simulation based on the results of 75 tests of full-scale pultruded columns that have been reported in the literature. In addition, resistance factors and structural reliabilities were calculated for the design equations provided by the manufactures in their design codes. The paper demonstrates that a unified design equation for pultruded columns can be developed for LRFD with reliability indices that are similar to those used for conventional materials. The paper also shows that markedly different reliability indices are obtained for the different manufacturer-provided equations even though identical allowable stress design safety factors are recommended by all manufacturers. [ABSTRACT FROM AUTHOR]

This paper describes research results for an innovative curved highway bridge girder system, which uses I-shaped steel girders with hollow rectangular tubes as flanges. The increased torsional stiffness provided by the tubular flanges dramatically improves the structural behavior of the curved girders, resulting in substantially reduced deflection, cross section rotation, and stress compared to conventional curved I-shaped steel plate girders. In this paper, finite-element (FE) models for systems of curved tubular-flange girders are described. The models consider material inelasticity, second-order effects, initial geometric imperfection, and residual stresses. The girder systems are comprised of curved hollow tubular-flange girders (CHTFGs), cross frames between the CHTFGs, and a concrete deck. A parametric study is performed using the FE models to study the effects of web stiffeners, tube diaphragms, geometric imperfection, and residual stresses on the load capacity of three-girder systems with CHTFGs. Then, the results for the CHTFG systems are compared with results for corresponding conventional curved I-girder systems. The effects of the curvature, cross section dimensions, number of cross frames, and a concrete deck are investigated. The results indicate that the CHTFG systems are more structurally efficient than the corresponding curved I-girder systems. [ABSTRACT FROM AUTHOR]

The flexural capacity of a new cold-formed hollow flange channel section known as LiteSteel beam (LSB) is limited by lateral distortional buckling for intermediate spans, which is characterized by simultaneous lateral deflection, twist, and web distortion. Recent research has developed suitable design rules for the member capacity of LSBs. However, they are limited to a uniform moment distribution that rarely exists in practice. Many steel design codes have adopted equivalent uniform moment distribution factors to accommodate the effect of nonuniform moment distributions in design. But they were derived mostly based on the data for conventional hot-rolled doubly symmetric I-beams subject to lateral torsional buckling. The effect of moment distribution for LSBs and the suitability of the current steel design code rules to include this effect for LSBs are not yet known. This paper presents the details of a research study based on finite-element analyses (FEAs) of the lateral buckling strength of simply supported LSBs subject to moment gradient effects. It also presents the details of a number of LSB lateral buckling experiments undertaken to validate the results of FEAs. Finally, it discusses the suitability of the current design methods and provides design recommendations for simply supported LSBs subject to moment gradient effects. [ABSTRACT FROM AUTHOR]

Two series of tests with different depth wood I-joists were conducted to study the effects of circular and square web openings and the placement of steel angle retrofits at openings. Finite-element analysis of all test configurations was used to improve understanding of how web openings and retrofits affect the stiffness, stress distributions around openings, and ultimate failure mechanisms. Test results show failure loads well above prescribed design loads; however, the presence of web openings affected the type of failure modes. Control joists failed at the tension flange, whereas joists with web openings failed in shear through web openings or by means directly attributable to the presence of an opening. Ultimate capacity and stiffness was generally reduced by the presence of a web opening, with openings located closer to high shear areas producing worse results. Square web openings lowered the ultimate capacity approximately 10% more than circular openings when located at the same location. Steel angle retrofits improved capacity if an adequate retrofit length was used to redistribute stress concentrations. Finite-element models accurately predicted decreases in stiffness due to web openings; however, the models predict an improvement in stiffness due to the presence of steel retrofits that was not ascertained in experimental results. [ABSTRACT FROM AUTHOR]

This study brings light to the source of plastic hinge-like failure modes often observed in tests. It is found that such failure modes, accompanied by severe flange deformations, are not due to the anchoring action of the flanges, but are due to direct shear force acting on the flange cross sections. This study also examines why the horizontal anchoring mechanism cannot develop in interior panels or in end panels with a heavy end stiffener. The results obtained in this study, along with the findings reported in the companion paper (Part I), reveal that the anchoring mechanism is virtually nonexistent in normal plate girders. That is to say, the postbuckling strength of the web panel in normal plate girders is mostly attributable to the postbuckling mechanism developed by means of lateral supports provided by the flanges and the transverse stiffeners. It is also found that a tension field can develop in the end panel and it is recommended, therefore, that any restriction contrary to this observation found in current design codes be repealed. [ABSTRACT FROM AUTHOR]

This study revisits the anchoring mechanism believed to be the source of the postbuckling strength of plate girder web panels. In classical tension field theories, it has been implicitly assumed that a tension field cannot develop in the web panel without recourse to the anchoring mechanism of the flanges and/or adjacent panels. The contribution of the flange anchoring mechanism in practical plate girders differs from theory to theory. It has been found during this study that the anchoring mechanism hardly develops, no matter how heavy the flanges are, without transverse stiffeners that have sufficient axial stiffness necessary to keep the flanges from moving the web panel inwards during the anchoring action. In practical plate girders, the contribution of the flange anchoring mechanism to the postbuckling strength is negligibly small not because the flanges are too flexible to function as anchors but because the transverse stiffeners are axially too flexible to support the flanges. Taking advantage of the flange anchoring mechanism in practical designs is beyond the realm of possibility because it requires an inconceivably high axial stiffness of the transverse stiffeners. [ABSTRACT FROM AUTHOR]

A closed form solution for elastic global buckling of twin girder systems interconnected with cross frames is derived. Current design specifications for such systems only consider individual girder buckling between cross frames. The solution, which is suitable for design specifications, was developed for a uniform moment loading condition. Finite-element analyses (FEAs) were used to verify the closed form solution and extend it to more practical loading conditions. FEA showed that the load height condition had only a minor effect for twin girders compared to the published effects on single girders. Both singly and doubly symmetric sections were studied and showed that the girder spacing and the in-plane moment of inertia of the girders are the principal variables controlling global buckling of twin girders. The number and size of the intermediate cross frames had little effect. A method for improving the global buckling capacity through the use of a partial top flange lateral bracing system is presented along with a design example. [ABSTRACT FROM AUTHOR]

Adequate shear strength between the web(s) and the flange(s) in flanged masonry shear walls is essential for the wall to provide the required stiffness and strength. In this regard, an experimental investigation was conducted to study the behavior of flanged concrete block walls. The study focused on investigating different specimen configurations to study the shear transfer mechanism between wall flanges and webs. Seven preliminary specimens with different configurations and different flange-web connections were tested in order to choose the most appropriate specimen configuration for the main set of 15 specimens. The ultimate capacity, deformation characteristics, and crack propagation were investigated. In addition, proposed analytical models based on shear-friction failure mechanism were compared with the experimental results and showed good agreement. [ABSTRACT FROM AUTHOR]

The behavior of I-shaped steel girders with flanges made from concrete-filled tubes is investigated experimentally, focusing on concrete-filled tubular flange girders (CFTFGs) in positive bending regions where the concrete-filled tube is the compression flange. The investigation included CFTFGs that are either composite or noncomposite with a concrete deck. For the composite CFTFGs, girders with rectangular tubes and either a flat web or a corrugated web are studied, and for the noncomposite CFTFGs, girders with round tubes and flat webs are studied. The objectives are: (1) to provide experimental data demonstrating the load carrying capability of CFTFGs and the advantages of CFTFGs over conventional I-girders; and (2) to verify the adequacy of analytical models by comparing experimental and analytical results. The experimental results demonstrate the improved lateral torsional buckling capacity of CFTFGs, and their capability to carry factored design loads under construction conditions and in the final constructed condition. The comparison between experimental and analytical results shows the adequacy of simple analytical models for CFTFGs. [ABSTRACT FROM AUTHOR]

Unstiffened, unbraced steel beams used in bridge falsework are subjected to patch loads from timber or steel posts. This paper investigates the critical web limit states in these types of beams experimentally and through finite-element analysis. Existing equations were appropriate for calculating the web yielding and crippling capacity, as observed in experiments, except that a 1:1 stress gradient through the flange and fillet of the web is found to be more appropriate and conservative for web yielding than the 2. 5:1 gradient assumed in the current AISC specifications. Lateral deformation of a beam flange, due to buckling of the web, was also observed experimentally. The capacity of the web for resisting lateral buckling can be calculated assuming that the web of the beam acts like a column with an appropriate effective area and length factor. The effect of accidental eccentricity between the flange and post is not significant if limited to three times the web thickness. Blocking is approximately 50% effective in increasing the web yielding and crippling capacity. Bracing or stiffeners should be used to prevent lateral buckling of a web where necessary. [ABSTRACT FROM AUTHOR]

Tests conducted on oriented strand board (OSB) webbed wood I-joists with web openings revealed that the typical failure mode was that the OSB fractured from tension zones around an opening, with cracks developing diagonally toward the joist flanges. The joist collapsed when the cracks reached the flanges. Finite-element models were developed using ABAQUS to simulate joist behavior. Material nonlinearity, crack propagation, and joist failure were dealt with by a user-defined subroutine. Good correlation was obtained between the test and the analytical results. The finite-element models were then used to investigate the effect of opening size and location on the behavior of joists. The lower bound and the upper bound of the load bearing capacity of joists with openings were found by finite- element modeling and were empirically expressed by polynomials. [ABSTRACT FROM AUTHOR]

A lateral bracing system is installed at the top flange level of a steel tub girder to form a quasi-closed box, thereby increasing torsional resistance and retaining the original box shape. A single diagonal (SD) type or crossed diagonal type is typically adopted along with intermediate internal bracing systems (also called internal cross frames) that are placed at either even or odd numbered spacing. A significant coupling action between SD bracing members and internal cross-frame members has been found to occur in the case of a SD lateral bracing system and internal cross frames placed at odd numbered panel spacing. The member forces developed in lateral bracing are 25–30% higher than those in structures with internal cross frames installed at even numbered panel spacing. Matrix equations were formulated to compute the brace forces developed in both SD lateral bracing and K-shaped internal cross-frame members subjected to bending action. Member forces computed from the new equations were compared with those from three-dimensional finite-element analysis, and an excellent agreement between the two solutions was found. [ABSTRACT FROM AUTHOR]

This paper revisits a fundamental assumption used in most classical failure theories for postbuckled web plates under shear, namely that the compressive stresses that develop in the direction perpendicular to the tension diagonal do not increase any further once elastic buckling has taken place. This assumption naturally led to a well-known theory that tension field action in plate girders with transverse stiffeners must be anchored by flanges and stiffeners in order for the webs to develop their full postbuckling strength. However, a careful examination of the results of the nonlinear finite-element analyses carried out for this study reveals that the diagonal compression continuously increases in close proximity to the edges after buckling, thereby producing in the web panel a self-equilibrating force system that does not depend on the flanges and stiffeners. These findings provide a fuller understanding of the actual mechanics of tension field action. [ABSTRACT FROM AUTHOR]

This paper reports a detailed investigation concerning the local-plate and distortional elastic postbuckling behaviors of cold-formed steel lipped channel columns with web and flange intermediate stiffeners (the corresponding unstiffened lipped channel column postbuckling behaviors are often used as reference). This investigation relies on results obtained through geometrically nonlinear analyses based on a recently developed and numerically implemented generalized beam theory (GBT) formulation that incorporates (1) conventional (no shear deformation), (2) shear (nonlinear warping), and (3) transverse extension deformation modes. The numerical results shown provide the evolution, along a given local-plate or distortional postbuckling equilibrium path, of the column (1) deformed configuration and (2) relevant displacement profiles and/or stress distributions—mostly for validation purposes, some of them are compared with values yielded by shell finite element analyses performed by means of the code ABAQUS. In order to assess the influence of the member end support conditions, the paper also includes a comparison between the distortional postbuckling behaviors of columns with pinned / free-to-warp and fixed / warping-prevented end sections. Taking full advantage of the GBT unique modal features, all the above results are discussed in great detail and it becomes possible to unveil, explain, and/or shed some new light on several interesting and scarcely known behavioral aspects. In particular, one is able to provide illuminating and structurally (mechanically) meaningful explanations for the qualitative differences exhibited by the local-plate and distortional postbuckling behaviors of plain and stiffened (web and flanges) lipped channel columns. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to investigate the seismic behavior of reduced beam section (RBS) moment connections to a deep wide-flange column. The test matrix for the experimental program consisted of six full-scale interior RBS connections, where the column for the specimens ranged in depth from a W24 to a W36 wide flange section. Five of the specimens had a composite floor slab, with the remaining specimen having no floor slab and a supplemental lateral brace at the end of the RBS. The results from the study show that a composite floor slab provides restraint to the beams, reducing the magnitude of beam top and bottom flange lateral movement in the RBS, column twist, and strength degradation due to beam instability in the RBS. The performance of each of the test specimens was found to meet the seismic connection qualification criteria in Appendix S of the AISC seismic provisions. The results of the experimental study were used to develop a procedure for predicting the torque applied to a column from an RBS connection. [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]