Search

Fiber-reinforced elastomeric isolators (FREIs) have been proposed as a cost-effective solution for expanding the use of seismic isolation to normal-importance structures. By using lightweight fiber reinforcement and eliminating the attachment plates, FREIs reduce cost while improving the isolation efficiency and reducing tensile stresses in the rubber. However, the flexural flexibility of the fiber allows cross-sectional distortions (i.e., warping) to occur, which significantly impacts the stability of these devices. This paper evaluates the buckling of rectangular, circular, and annular FREIs, taking into account shear warping effects. A planar buckling theory previously proposed by the authors is adapted for the three-dimensional problem, and effective warping rigidities and warping-related areas are derived for the above bearing geometries, accounting for rubber compressibility. To assess the adequacy of the proposed buckling theory and derived warping properties in predicting the buckling of FREIs, a parametric finite element study is conducted. The critical load predictions of the proposed analytical formulation are found to be in excellent agreement with those of the numerical simulations. It is shown that traditional estimations of the buckling load that neglect warping are significantly unconservative. Finally, design recommendations and resources are provided for practice-oriented applications. [ABSTRACT FROM AUTHOR]

Graphene platelets (GPLs) exhibit outstanding mechanical and physical properties and therefore are employed as a reinforcement in advanced polymer composite structures. The purpose of this paper is to analyze the biaxial buckling of functionally graded graphene-reinforced sandwich plates with a soft orthotropic core. A new high-order three-layer theory is developed for the accurate modeling and analysis of the sandwich structure. The sandwich plate is divided into three layers including two face sheets and a core and a different third-order kinematic assumption is dedicated to each layer. The transverse flexibility of each layer as well as the displacements continuity at the interfaces are considered. Additionally, the continuity conditions and the conditions of zero transverse stresses in the whole structure are satisfied. The plate is subjected to a biaxial compressive loading and the governing equations are derived using the principle of minimum potential energy. Analytical solutions are presented for simply supported boundary conditions to obtain the critical buckling load. The influences of plate geometry and GPL properties on buckling load are investigated. The results obtained in specific cases are compared with the published results and the validity of the present results is confirmed. It can be drawn that the use of GPL increases the buckling load of graphene-reinforced sandwich plates. [ABSTRACT FROM AUTHOR]

This paper aims to evaluate the seismic response of multitiered buckling-restrained braced frames (MT-BRBFs), assess the design provisions specified by the 2022 AISC Seismic Provisions for multitiered BRBFs, and propose improvements to these provisions. A set of 17 frames is first selected by varying bracing configuration, frame height, number of tiers, and tier height ratio. The frames are then designed in accordance with the 2010 and 2022 AISC Seismic Provisions. A numerical parametric study is performed under scaled ground motion accelerations. The results of the parametric study show that when the frames are designed to the 2010 provisions, the frame inelastic deformation tends to concentrate in the tier(s) undergoing tensile yielding due to their lower postyield stiffness, compared to BRBs yielding in compression, which creates unequal story shear, contributed by braces, in adjacent tiers with BRBs in tension and compression and engages column flexure to compensate for unbalanced brace story shears between tiers. Columns experience yielding and even buckling in several cases due to combined flexural and axial load demands. MT-BRBFs designed to the 2022 AISC Seismic Provisions exhibit a more uniform deformation response between tiers and relatively lower flexural demands in their columns. However, these provisions may overestimate column in-plane flexural demands (on the order of 3), resulting in potentially uneconomical design solutions. On the basis of the numerical simulations, modifications are proposed to compression BRBs' adjusted strength to better estimate column in-plane flexure and tier deformation while achieving an economical column design. The proposed improvements are validated using dynamic analyses. [ABSTRACT FROM AUTHOR]

This technical note presents the optimal shape of an arch carrying a central point load to two pinned supports for maximum in-plane bifurcation buckling load. The arch shapes considered are triangular, parabolic, circular, catenary, and ogival. It is assumed that the arches are elastic and inextensible and their flexural rigidity is uniform throughout the arch length. The Hencky bar-chain model was used for elastic buckling analysis. It is found that the optimal arch shape depends on the arch apex height to length (h/L) ratio. The optimal arch shape changes from circular to catenary to parabolic and finally to triangular from h/L=0.1 to 1. This shows that the funicular triangular arch shape for the central point load is not the optimal arch shape against in-plane buckling when h/L is less than 0.528. For maximum buckling load, one may use either the circular, catenary, or parabolic arch with h/L approximately equal to 0.3. [ABSTRACT FROM AUTHOR]

A steel brace is effective to retrofit old reinforcement concrete (RC) buildings by increasing the horizontal load capacity of structures. However, concrete cracks that easily occur in the surrounding RC members under tension initiate early strength degradation, so this research aimed to develop a new self-centering brace (SCB) with the compression-only capacity such that no tension force is developed in the brace or to the adjoining RC members. Original SCBs have been developed to reduce the residual deformation of structures with a symmetrically flag-shaped hysteretic response. By altering the force transfer mechanism in the original SCB, a compression-only self-centering brace (C-SCB) can be developed under a symmetrically reversed cyclic loading, providing an alternative for the RC building retrofit. This paper first introduces the mechanics and deformation mechanism of the proposed C-SCB. A test program is then conducted on the cyclic tests of a buckling-restrained energy dissipating bar (EDB) to evaluate the energy dissipation. A 3,695-mm-long C-SCB is composed of posttensioning high-strength steel tendons, steel compression members, and buckling-restrained EDBs for the energy dissipation. The C-SCB in tests showed a good compression-only self-centering capability up to an axial displacement of 31 mm, with a maximum axial force of 1,530 kN. No damage of the steel tendons, compression members, or EDBs was found after three phase tests. Finite element analysis further validated the compression-only hysteretic response and mechanics of the C-SCB in a symmetrically reversed cyclic loading. [ABSTRACT FROM AUTHOR]

Angle profiles belong to the most common steel profiles used in several types of civil engineering structures, such as in lattice towers. Although, in such towers, they are usually considered as axially loaded (truss) members, an additional bending moment develops due to the eccentricity between the point of load application and the gravity center of the profile. Therefore, this paper focuses on the buckling resistance of single-angle members subjected to combined compression and bending. The investigations have been performed through numerical parametrical studies and have been compared to different normative documents where interaction equations are provided. [ABSTRACT FROM AUTHOR]

The paper presents a modified differential quadrature method (DQM) to solve buckling problems of beams and rectangular plates. The proposed method addresses the problem of applying multiple boundary conditions at each boundary point by developing a novel approach that constructs basis functions satisfying multiple boundary conditions. The weighting coefficient matrices of high-order derivatives in the DQM can then be calculated directly from the basis function, and additional equations about boundary conditions can be avoided. The method is first applied to derive the basis functions of beams with various boundary conditions and to solve beam buckling problems. Then, the method is extended to the buckling of rectangular plates, where the plate deflection is assumed as the sum of multiplications of two basis functions for beams with the corresponding boundary conditions. To verify the accuracy of the proposed method, several examples are presented, and very good agreement is observed between the results obtained by the proposed method and those reported in the literature as well as those obtained by the finite element method. [ABSTRACT FROM AUTHOR]

Large infrastructure projects can efficiently use large diameter bars as longitudinal reinforcement in reinforced concrete elements. In earthquake-prone zones, critical regions of these elements must be detailed for ductility and hysteretic energy dissipation. Longitudinal bars in these regions are expected to sustain large-amplitude strain cyclic reversals before bar fracture occurs, in a mode of failure defining the collapse prevention limit state. To date, no successful large strain-amplitude cyclic loading testing has been reported on large-diameter reinforcing bars to observe the large-strain amplitude fatigue life of such bars, which often involves plastic bar buckling. This paper describes a test program designed to determine the plastic buckling/straightening fatigue life of ASTM A706 Grade 60 No. 18 bars (Ø57 mm) reinforcing bars subjected to cycles of reversed plastic strains. The design and implementation of the loading apparatus, critical for the gripping of the bars, the metallurgical characterization of the bars, and several key test results are discussed in the paper. Finally, the paper discusses a general unidimensional fatigue damage index precursor to bar fracture following a few plastic buckling and straightening cycles. [ABSTRACT FROM AUTHOR]

The possible failure modes of cold-formed steel (CFS) lipped angle (LA) compression members are yielding, local, flexural torsional, or flexural buckling, and any possible interaction among these buckling modes. In general, the strength estimated by current design guidelines are conservative for these members when flexural-torsional buckling (FTB) is the first global buckling mode because the postbuckling strength of this mode is not accounted for in the global buckling strength equations. The initial part of this paper reports the results of an experimental and numerical study of CFS-LA members undergoing independent FTB. Modifications are suggested to global buckling strength equations based on these results. Subsequently, the reduction in the ultimate strength from strength corresponding to independent buckling modes for LA members undergoing interaction between buckling modes such as local-flexural torsional, flexural-flexural torsional, local-flexural, and local-flexural torsional-flexural are studied systematically using finite-element analysis results. A simple and more accurate interaction equation that accounts for these interactions among buckling modes in CFS-LA compression members is proposed. [ABSTRACT FROM AUTHOR]

Buckling and critical temperatures of steel columns are crucial parameters for structural fire safety design. However, the existing simplified design methods for buckling or critical temperature are based on column models with either pin ends or with large rotational restraints. Their applications on steel columns with limited rotational restraint stiffness ratios—the ones smaller than the critical rotational restraint stiffness ratio—is questionable. In this paper, parametric analyses on a validated numerical column model with rotational restraint stiffness ratio varying from 0.0001 to 10.0 were conducted. The effects of slenderness ratio, load ratio, and axial restraint stiffness ratio were also considered. Results show that for steel columns with limited rotational restraint stiffness ratios, prediction errors of existing simplified design methods were significant. Following the parametric analyses, new prediction formulas of buckling temperature and critical temperature are proposed for H-section steel columns. Results of proposed formulas aligned closely with those of parametric analyses in the full range of considered parameters. Furthermore, the proposed formulas were also validated by test results in the literature. [ABSTRACT FROM AUTHOR]

This study proposed novel designs for achieving self-centering (SC) behavior using superelastic shape memory alloy (SMA) buckling-restrained plates (BRPs). Compared with previous studies relying on small-diameter SMA bars, this study employed large-dimensional SMA plates that could provide sufficient strength for real construction projects. The design concept and configurations of SMA-BRPs were first introduced. Corresponding tests were then conducted. Three specimens were tested, including one that was unsatisfactory and two that were acceptable. Based on the initial examination of the behavior and failure mode of the unsatisfactory trial specimen, the corresponding modifications were made to achieve the improved performance of two acceptable specimens. Subsequently, the hysteresis and SC behaviors, energy dissipation capacity, and yielding strength degradation of the two acceptable specimens were investigated. Both specimens exhibited high-quality flag-shaped behaviors in their tension–compression cycles, with their strengths reaching around 200 kN under tension and more than 310 kN under compression, representing one of the largest strength values shown by a single SMA element in the literature. Asymmetric behavior was observed under tensile and compressive loadings, primarily due to the asymmetric behavior of the SMA base material, high-mode inelastic buckling under compression and friction between the SMA core plate and buckling-restraining device. After the tests, finite element models were developed to obtain an in-depth understanding of the detailed behavior of SMA-BRPs. The studies showed that SMA-BRPs successfully achieved the SC buckling-restraining design target. The design incorporating end restrainers was preferred due to its flexibility, which reduces application restraints. [ABSTRACT FROM AUTHOR]

A new type of buckling-restrained brace (BRB), called induction-hardened buckling-restrained brace (IHBRB), is proposed in this study. The IHBRB uses a round bar as the core steel and a hollow round section as the restrainer. The core steel undergoes partial induction hardening, which makes its strength lower in the middle but higher at the two ends. This strength difference along the length is expected to trigger energy dissipation in the middle but keep the ends undamaged. The restrainer has an inner diameter similar to that of the core steel, which minimizes the gap between them and avoids time-wasting grouting. This study introduces experimental and analytical research on four types of IHBRB. Tensile coupon tests revealed that the core steel of the IHBRB-600 series satisfied the target yield strength of 600 MPa in the middle and 1,200 MPa at both ends. The core steel of the IHBRB-800 series also satisfied the target yield strength of 800 MPa in the middle and 1,200 MPa at both ends. The ascending cyclic loading test showed that the IHBRB exerted a load capacity of over 1,000 kN as designed and exhibited tensile-compressive symmetric hysteresis behavior. IHBRB-600L, the specimen that failed by tensile fracture, showed that the high-strength brace end succeeded in remaining elastic as planned, while the low-strength brace middle showed a higher-order mode buckling. However, unexpected overall buckling at the joint occurred on other specimens. The low cycle fatigue specimens showed large cumulative inelastic deformation ratios of more than 200. A three-dimensional finite-element model of IHBRB-800L built by ABAQUS/Explicit (Dassault Systèmes Simulia Corp., Paris) captured the experimental results well. A parametric study revealed that the longer restrainer fixed at one side of the core steel end was one of the solutions to guarantee the great seismic performance of the IHBRB. [ABSTRACT FROM AUTHOR]

A new beam element only having the translational displacement degrees of freedom is developed to analyze fracture and buckling behaviors of bilayer beams or columns with delamination using the multilayer modeling method. By comparing their degrees of freedom, it is found that the present beam element is equivalent to the conventional Timoshenko beam element. The upper and lower layers are both divided into one or several sublayers, which are modeled by the developed beam elements. The element stiffness matrix, load vector, and geometric stiffness matrix using the linear interpolation polynomial are derived by the principle of minimum potential energy. To illustrate the effectiveness, accuracy, and efficiency, both the fracture and buckling analyses are conducted. Four typical fracture specimens (i.e., homogeneous and asymmetric double cantilever beams, single leg bending, and asymmetric end-notched flexure specimens) are analyzed using the developed beam element, and the obtained energy release rate and its components using the virtual crack closure technique are compared with those of analytical and conventional two-dimensional (2D) finite element solutions. Further, buckling analysis of bilayer beam-columns with delamination is conducted, focusing on the effect of the number of sublayers. The results show that the developed beam element is capable of effectively and efficiently calculating the convergent stress intensity factor ratio, total energy release rate and its components, and critical buckling load of delaminated bilayer beams or columns with relatively few elements, but without introducing the interface continuity condition in comparison to the conventional modeling method. [ABSTRACT FROM AUTHOR]

This paper investigates experimentally and numerically the buckling behavior of three-layered cross-laminated timber (CLT) panels under in-plane axial compression. The main aim is to assess the panels' ultimate limit load and their typical failure mechanisms. A total of 14 specimens were tested. Seven of them were homogeneous (HO series), entirely made of beech, whereas the others were hybrid (HB series), made of beech (outer layers) and Corsican pine (inner layer). The tests gave evidence of a rather stable column buckling capacity for CLT panels, with evidence of major failure mode due to bending. In some cases, rolling shear and delamination were highlighted as additional failure mechanisms. Finally, further assessment of experimental evidence is provided by extended analytical calculations (based on existing formulations, including the Eurocode 5 approach) and even finite-element (FE) numerical analyses for the examined three-layer CLT compositions. Comparative results are discussed in terms of structural performance, capacity, and weakness of analytical models for CLT solutions. [ABSTRACT FROM AUTHOR]

The building industry requires the development of a ductile lateral force resisting system made of mass timber to help mass timber to continue to grow in popularity as the primary structural building material. The OpenSees framework was used to develop a numerical model of a single-story timber frame with a timber buckling-restrained brace, which was validated by the results of a previously completed series of quasistatic cyclic timber buckling-restrained brace (TBRB) component and subassembly tests. The experimental validation included a model developed for the hysteretic behavior of the TBRB and the rotational capacity of beam–column joints made with mass ply lam (MPL) panels, two slotted-in steel plates, and a number of steel dowels. The single-story model TBRB braced frame was then expanded to a numerical model of an eight-story mass timber buckling-restrained braced frame. The TBRB frame was analyzed with static pushover, quasistatic cyclic loads, and earthquake simulations. During the simulation of eleven design-level earthquake ground motions, the building experienced a peak interstory drift at of 2.54% at the first-floor level and a peak floor acceleration of 1.7g at the roof. The numerical model developed in this research of a timber buckling-restrained braced frame with TBRBs is novel and could be used to design timber buckling-restrained braced frames as a ductile lateral force resisting solution for mass timber buildings in seismic regions. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study to model the thermal-induced pipeline buckling–soil interaction as a demonstration of the feasibility of modeling global buckling using an effective stress remeshing and interpolation technique with a small strain (RITSS) approach in 3D. The numerical results are compared with a previously published large-scale laboratory test, centrifuge, and numerical models. The influence of embedment depth, pipe weight, and pipe–soil interface roughness on the global pipe–soil response is investigated. The current study unveiled some of the key behaviors of partially embedded and buried pipes during thermal expansion. First, the expansion-induced displacements comprise vertical and lateral displacements. Heavy pipes dive deeper into the soil, whereas lightweight pipes rise above their initial position as they move laterally for partially embedded and buried pipes. Furthermore, expansion-induced displacements become insignificant as the burial depth increases. The soil in front of the buried pipe flows up and above to its rear end as the expansion-induced displacements increase. Moreover, lateral buckling occurs in buried pipes even though the lateral out-of-straightness of the pipe is within the recommended limit set by the Det Norske Veritas (DNV) guideline. Finally, the pipe–soil interface roughness significantly influences the undrained lateral resistance for partially embedded pipes. [ABSTRACT FROM AUTHOR]

Current design specifications for cold-formed steel incorporate the Direct Strength Method (DSM) to design cold-formed steel members under compression. Although the DSM has been used for quite some time, its buckling strength curves have not been experimentally assessed thoroughly. This study attempts to fulfill this objective. First, a thorough literature review was conducted to inspect the buckling strength curves in the DSM, and the issues are summarized in this paper. Second, experimental data associated with compression tests of 453 solid and 228 perforated cold-formed steel columns were collected from the literature. Design parameters (e.g., elastic critical buckling loads) of the columns were then determined and assembled with the experimental results in a database to be used as a source to assess the buckling strength curves in the DSM. Finally, the LRFD resistance factors (ϕc) of the DSM buckling strength curves were determined in accordance with the AISI S100. This paper highlights the level of accuracy of the buckling strength curves in the DSM. [ABSTRACT FROM AUTHOR]

The present study investigates the elastic lateral torsional buckling resistance for steel I-beams that are strengthened with steel cover plates while being subjected to loading. A variational principle was developed for the problem by accounting for the full sequence of prestrengthening load application, strengthening process, poststrengthening load application, up to the buckling response. The variational principle was then used to develop a finite-element formulation for the problem leading to a quadratic eigenvalue problem. The formulation successfully captures the effects of prestrengthening loads, the beneficial effects due to prebuckling deformation, and the interactions between both effects. The study documents the benefit of providing transverse stiffeners to maximize the critical moment capacity attained when cover plates are used for strengthening. The study determines the locations of cover plates that would maximize the gain in critical moments in strengthening scenarios where only part of the beam span is to be strengthened. A systematic parametric study was developed to characterize the gain in elastic critical moment strength attained by cover plate strengthening and associated prebuckling deformation effects, in terms of dimensionless parameters that characterize the effects of prestrengthening load magnitude, beam span, cross-sectional geometry, length of cover plates, and cover plate cross section. Other parameters investigated include the distributions of prestrengthening and poststrengthening loading. [ABSTRACT FROM AUTHOR]

Buckling of slender members due to gravity loading or thermal effects is influenced by the member's geometric imperfections, boundary conditions, and intermediate lateral supports. When assessing the capacity of such members, these parameters are often unknown (e.g., the rotational stiffness of end connections in a truss or the lateral support provided by the ties to a rail track), and conservative assumptions must be made resulting in conservative assessments. Distributed fiber optic sensors (DFOS) can potentially be used to determine these parameters with greater accuracy using strain measurements along the length of a member. A series of buckling experiments was conducted on a slender member instrumented with DFOS subjected to axial load with varying levels of lateral restraint or to increasing temperature. The distributed strain data were then used to evaluate the geometric imperfections, boundary conditions, and lateral support stiffness. These inputs were used to create a finite-element model to estimate the ultimate load response of the member using data acquired at service loads. [ABSTRACT FROM AUTHOR]

High-strength steels (HSSs) have been gaining increasing attention in recent years because of their potential application for lightweight structures. Hybrid structural steel I-sections, featured by dissimilar strength grades for constitutive plates, provide a more economical solution to suit bending loading scenario. This study conducted experiments on nine I-sections with Q690 flange plates under four-point loads, incorporating three different web strength grades including Q690, Q460, and Q355. Furthermore, the local buckling behavior of representative I-sections with various section geometries was analyzed using a validated numerical method to understand the role of web strength grade. Test and numerical analysis results revealed that the section with web steel in the strain hardening stage exhibited different levels of ductility among specimens with different strength webs. It was also observed that earlier yielding or inelastic buckling of the lower strength web plate in hybrid I-sections affects the local buckling behavior of I-girders, in terms of moment resistance and rotation capacity. In addition, the initial local imperfection mode used in numerical analysis was found to affect the rotation capacity of I-girders, but the effect of web strength grade remains consistent among I-sections with the same local buckling deformation mode. This study provides further insights into the local buckling behavior of hybrid I-sections subjected to uniform bending, laying the foundation for further development of design methods. [ABSTRACT FROM AUTHOR]

The bidirectional ductile end diaphragm concept uses energy-dissipating buckling restrained braces (BRB) as fuses located at the end of a bridge superstructure's floating spans. This system using BRBs can provide seismic resilient and damage-free bridges fully operational immediately after an earthquake. A shake-table testing program was conducted to subject a 1/2.5-scale specimen to series of ground motions. The specimen tested represents one span of a five-span bridge having BRBs connected to the abutment and the pier next to it. The purpose of these tests was to experimentally validate proposed connection details when subjected to the three-dimensional (3D) displacement histories (compared with the axis of the BRBs) that resulted from bidirectional ground motions and the fact that the connections must accommodate inclined BRB layouts. The test protocol included earthquake displacement histories that represent design demands, cycles of thermal excitations, and (to eventually make the BRBs fail) extreme motions. The testing program validated the effectiveness of the proposed concept and the ability of BRBs to sustain multiple ground motions before failure. [ABSTRACT FROM AUTHOR]

Buckling-restrained braced frames (BRBFs) are attracting interest worldwide by virtue of their extraordinary response reduction performance. The system relies strongly on the energy dissipation of buckling-restrained braces (BRBs). Therefore, the surrounding members (beams, columns, or gusset plates) must sustain the force imposed by a BRB. Notably, beams are not designed primarily to support the axial force. The sections consequently tend to become deep and slender to enhance the flexural strength and stiffness effectively, thereby leading to local buckling, which lessens the reaction force to the BRB. Moreover, the instability is activated by the compressive axial force transferred from the BRB. However, beams in the BRBFs generally become composite beams through an assemblage with a concrete slab by stud shear connectors. The concrete slab shifts the neutral axis of the beam, which imposes more strain on the bottom flange. The BRB axial force and composite effect therefore influence the steel beam buckling behavior. Eventually, the beam might be subjected to instability at a smaller-than-expected deformation. However, the rotation capacity of the composite beam in BRBFs has not been clarified yet. To address this concern, this study established a finite-element analysis (FEA) model referring to a former specimen and an experiment scheme. Additionally, a parametric study of the influential factors was conducted using the experimentally validated FEA model. This study examined a proposed evaluation equation of an apparent axial force issued from the composite effect based on the obtained results. An earlier evaluation index established for bare steel beams was ultimately modified to include the axial force influence. Eventually, the results demonstrated that the structural performance can be assessed appropriately using the existing evaluation equations proposed using a steel frame subassembly. [ABSTRACT FROM AUTHOR]

This paper presents a theory for the buckling of short beams considering cross-sectional distortions due to transverse shear (i.e., shear warping), based on the consistent linearization of a geometrically nonlinear planar beam. The proposed deformation field considers the warping amplitude as an independent kinematic field, and the hyperelastic material assumes that the stresses normal and tangent to the deformed cross section are linear with respect to their work-conjugate finite strains. An approximate closed-form solution to the resulting quartic equation for the critical load is provided to facilitate practical implementation. Theoretical differences giving rise to distinct buckling theories for higher-order shear beams are discussed in terms of (1) the assumed deformation field, (2) variational consistency, and (3) material constitutive relation. The proposed formulation is applied to evaluate the stability of infinite strip unbonded fiber-reinforced elastomeric isolators (FREIs) with moderate-to-high shape factor, for which shear warping is expected to have a major influence due to the flexural flexibility of the fiber reinforcement. A homogenization procedure is described to obtain effective isolator rigidities considering rubber compressibility and fiber extensibility. Next, a finite element parametric study of the buckling of unbonded infinite strip FREIs is presented, and the results are used as a benchmark to evaluate the adequacy of the proposed and existing formulations. The theory presented herein and its approximate solution exhibit the best match with the numerical results, and the latter is deemed adequate for practical application. [ABSTRACT FROM AUTHOR]

The North American Standard (NAS) and the European Standard (EC3) for cold-formed steel design provide limited guidelines for the strength predictions of built-up columns. The existing design equations are limited to flexural buckling and local-flexural interactive buckling. The present study evaluates the local-flexural interactive design equations for cold-formed steel built-up back-to-back I-section columns against the results of experimental and numerical studies. The results show that the existing design guidelines for local-flexural interactive buckling are unconservative by about 17%, 5%, and 15% for DSM-NAS, EWM-NAS, and EWM-EC3, respectively. Furthermore, a maximum reduction in the built-up column strength by local-flexural interaction is observed when its nominal local and flexural buckling loads are close. Based on the results and observations, new design equations are proposed by including the product of nominal local and flexural buckling loads for the North American and European standards, which capture the local-flexural interactive buckling in built-up I-section columns. The effect of fastener spacing on the ultimate load for local-flexural interactive buckling is also evaluated, and its effect is found to be small, unlike for global buckling. The minimum fastener spacing criteria of NAS are assessed, and suitable recommendations are provided for fixed-end columns. Overall, the present study provides insights into the local-flexural interactive buckling behavior of built-up I-section columns and proposes reliable design equations to predict the ultimate load. [ABSTRACT FROM AUTHOR]

The building industry requires the development of a ductile lateral force–resisting system made of mass timber to allow mass timber to continue to grow in popularity as the primary structural building material. A timber frame with mass ply lam (MPL) beams and columns with a timber buckling-restrained brace (TBRB) was subjected to a quasi-static cyclic loading protocol defined according to the qualification procedure of conventional buckling-restrained braces (BRBs). Two of the TBRBs in this study included a carbon fiber–reinforced polymer (CFRP) wrap to help confine the two ends of the mass ply panel (MPP) casing and improve the performance of the brace. The experiments showed that the TBRB without any CFRP wrap met the requirements for qualification of conventional BRBs, and the mass timber braced frame was able to reach a drift ratio of 3.0%. The TBRB with CFRP wraps met the requirements for qualification of conventional BRBs; the confinement provided by the CFRP wraps postponed failure of the TBRB by enabling the steel core to reach a maximum strain of 3.1% in both tension and compression and the mass timber braced frame to reach a maximum drift ratio of 4.5%. [ABSTRACT FROM AUTHOR]

Bridge design specifications include design provisions for ductile diaphragms to resist seismic excitations perpendicular to a bridge's axis. Buckling restrained braces (BRBs) can conveniently be used for this purpose. Bidirectional ductile diaphragms expand the concept to resist all-direction seismic forces, and nonlinear inelastic response history analyses (NL-RHAs) can be used to show that satisfactory seismic performance objectives can be obtained using this concept. However, a simple design procedure is needed to address the design of the BRBs in the bridge longitudinal direction. As a first step, to fill that need for common multispan simply-supported bridges, a relatively simple-to-use equivalent lateral force (ELF) procedure has been developed on the basis of results from "optimal designs" obtained using NL-RHAs. Findings indicate that the proposed ELF design procedure leads to design that meet the target design objectives. [ABSTRACT FROM AUTHOR]

This study presents an experimental investigation into the cyclic performance of a new tube-in-tube buckling restrained brace (BRB) configuration. In this study, reduced- and large-scale brace geometries are tested to quantify inelastic hysteretic response, understand core confinement parameters, and determine the influence of end connection constraints. A total of 11 reduced-scale brace specimens are cyclically tested, along with three large-scale braces representing two core geometries and two core constraint configurations. Results from the reduced- and large-scale brace testing suggest that the tube-in-tube BRB configurations are capable of achieving the required inelastic capacity for qualification in AISC 341-16 specifications. All scaled specimens survived cumulative inelastic demands that exceeded 240 times the yield displacement (with several surviving more than 400 times the yield displacement). Results indicate that tube-in-tube BRBs are capable of producing stable tension-compression hysteretic behavior during cyclic loading. Measured β values for the large-scale Specimen G2 were lower than 1.06 for all deformation cycles at or below 9.2Δby , while measured β values for the large-scale Specimen G3 were lower than 1.07 for all cycles at or below 3.6Δby. The peak β value for Specimens G2 and G3 was 1.16 and occurred during the 15.3Δby and 7.1Δby cycles, respectively. [ABSTRACT FROM AUTHOR]

A full-scale, two-tiered steel buckling-restrained braced frame (BRBF) was tested to evaluate experimentally the seismic behavior of steel multitiered BRBFs, namely, column stability response, column seismic demands, and tier deformations under a loading protocol representing earthquake ground motions. The test specimen consisted of diagonal braces oriented in opposing directions in the two adjacent tiers to create the most critical multitier response. The test frame was designed in accordance with the 2010 AISC Seismic Provisions as a lateral load-resisting system of a single-story building. The frame was subjected to a three-phase loading protocol consisting of lateral displacement time histories corresponding to a far-field ground motion record and a near-field ground motion record applied sequentially achieving total frame drifts in excess of 3.5%, followed by a final monotonic lateral displacement corresponding to 4.5% story drift. The test frame exhibited a stable response despite a non-uniform distribution of frame inelastic deformation between the tiers, which induced significant in-plane bending moments in the columns. Flexural bending, combined with a large axial compression force, led to partial yielding in the columns. Large deformation demands were also observed in the BRB yielding in tension and attracting the majority of frame lateral deformation. On the basis of test results, a displacement-based analysis approach was proposed to relate column in-plane bending and flexural stiffness to relative inelastic tier deformations. [ABSTRACT FROM AUTHOR]

The paper develops a stiffness reduction factor to be used in geometric nonlinear beam-element type elastic analysis of cold-formed steel structures. The factor accounts for the reduction in flexural and warping torsion rigidities resulting from local and distortional buckling as well as residual stresses, particular to cold-formed steel structures. The purpose of applying the factor is to accurately account for the geometric second order effects when predicting the internal distributions of moments of cold-formed steel structural frames. The stiffness reduction factor arising from local and distortional buckling is first determined followed by the stiffness reduction factor caused by residual stresses. Subsequently, the two effects are combined in a single expression, which is a format suitable for incorporation in the North American specification for cold-formed steel structures, AISI-S100. [ABSTRACT FROM AUTHOR]

Double-beam systems have found many applications in civil and mechanical engineering. Extensive works of research have been conducted on the investigation of double-beam systems in which some have studied the vibration characteristics of general double-beam systems without considering the effect of axial load, whereas others investigated the vibration and buckling of axially loaded double-beam systems with classical boundary conditions. A few previous studies have considered a general axially loaded double-beam system, but none of them have provided a general solution to calculate the critical buckling load of the system with arbitrary boundary conditions. In the present study, free vibration and buckling of an axially loaded double-beam system, which was composed of two nonidentical beams with generalized boundary conditions, were analytically studied by using the Euler–Bernoulli beam theory. A semianalytical method was proposed to calculate the natural frequencies and mode shapes of the system, and an explicit formula was derived to determine the critical buckling load of the system. Parametric investigations were performed to study the influences of the connecting stiffness of the interlayer and axial loads on the dynamic characteristics and critical buckling load of the system. Results show that the proposed method can be used for free vibration and buckling analyses of generalized double-beam systems subjected to axial loads. [ABSTRACT FROM AUTHOR]

The design of connections of gusset plates, beams, and columns of concentrically braced frame systems is typically based on the forces generated by brace actions without considering the frame actions. This may result in underestimating the design force demand on gusset plates, potentially leading to the failure of interface weld connections at higher lateral drift levels. The present study aims to estimate the force demands at the connection interfaces, typically about a 2% story-drift ratio (SDR), corresponding to the design-level ground motions for braced frame systems. The study considers both the forces of frame and brace action on the corner gusset plate in the design process. It also aims to develop suitable design recommendations to prevent premature weld failures at higher lateral drift. High-fidelity finite-element models of in-plane buckling (IPB) braced frame systems are developed and analyzed under various loading modes. A current standard design method to determine the interface weld size was found to be inadequate to prevent premature failures at the higher drift levels. This is primarily due to the accumulation of the plastic strain at the tip of the gusset at higher drift. The out-of-plane bending of the gusset plate due to geometric imperfection was also found to contribute toward the development of high plastic strain at the tips of gusset plates. Three different connection configurations are considered and analyzed under critical cyclic loading conditions. Finally, design recommendations are presented to avoid stress concentration and premature fracture of gusset plate connections of the IPB braced frame systems. [ABSTRACT FROM AUTHOR]

Concrete encasements are usually utilized to protect bridge above-ground piles from corrosion. These concrete encasements are usually square or round shapes reinforced by longitudinal rebars. However, the contributions of concrete encasements are not considered in traditional pile design procedures. On the other hand, rating engineers need to have realistic evaluations of the piles to estimate the capacity of the foundation for scour-critical bridges. Underestimation and overestimation of pile capacity can result in either potential bridge failure or uneconomical assessment. The objective of this paper is to investigate the effect of concrete encasement on pile buckling behavior subject to concentric load. To achieve this objective, four full-scale concrete encased pile specimens with various pile and concrete encasement lengths were fabricated and tested with concentric axial loading to measure the specimen ultimate capacity. In addition, the finite-element (FE) approach was utilized to simulate the buckling behavior and predict the ultimate capacities of the lab-tested specimens. The developed FE models were validated against the laboratory test data. Finally, the validated FE models were utilized in the parametric study to investigate the effect of the length and shape of the concrete encasement on the ultimate capacities of the steel H-piles. The results indicated that the concrete encasement could increase the buckling load of the pile, and ignoring the effect of the concrete encasement, as with the use of the equations suggested by the AISC, could result in a significantly underestimated prediction. It was also found that the concrete encasement increases the initial axial stiffness of the pile before the buckling load is achieved. [ABSTRACT FROM AUTHOR]

Mass timber buildings are increasing in popularity as the building industry aims to use more sustainable construction materials. A lateral force resisting system with a mass timber frame and a timber buckling restrained brace (TBRB) is presented as a possible solution to allow the expanded use of mass timber in buildings located in cities with high risk of natural hazards such as earthquakes and hurricanes. A series of nine quasi-static cyclic tests was completed to study the performance of the TBRB frame as well as the elastic performance of the bare mass timber frame. The variables studied included the level of axial force applied to the columns to simulate gravity load and the out-of-plane displacement of the TBRB frame. The mass timber frame was tested four times with a different TBRB. The four subassemblies achieved a drift ratio of at least 2.8% before failure of the TBRB due to weak axis buckling of the steel core. The maximum displacement ductility of the four TBRB frame subassemblies ranged from 3.1 to 3.5. The addition of the TBRB enhanced the energy dissipation capacity of the bare mass timber frame by 4.0 to 8.6 times after 14 cycles of lateral displacement. [ABSTRACT FROM AUTHOR]

Although it is often asserted that, in view of their reduced length, axially compressible beams have a higher buckling load than their inextensible counterparts, a detailed analysis demonstrates that this is not necessarily the case. The argument to arrive at this conclusion is made in terms of relatively straightforward concepts of elasticity and structural mechanics. It is shown that for certain classes of materials, the reduced prebuckling length is more than compensated for by a softening of the elastic response, leading to a reduction of the Euler critical load. [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]

For the first time, a two-node beam element with nine degrees of freedom (DOFs) was developed in this paper using a combination of modified couple stress and k th-order shear deformation beam theory to analyze the free vibration and static buckling responses of rotating bidirectional functionally graded porous (BDFGP) microbeams resting on variable elastic foundations, in which the whole structure was exposed to a magnetic field in the hygrothermal condition. The mechanical properties and the length-scale parameters varied in both the thickness and length directions, which in the novel porosity model and power-law indexes of the material were considered to be a function of the x - and z -coordinates. The present method's accuracy was determined by comparing it with published findings. Additionally, extensive research was conducted to evaluate the effect of various parameters on the mechanical responses of the rotating BDFGP microbeam. The numerical results showed that porosity coefficient, elastic foundation parameters, and hygrothermal environment all significantly affect the rotating BDFGP microbeam's free vibration and static buckling behavior. [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]

Piles are sometimes unavoidably embedded in slopes for supporting buildings on hills. In contrast to a pile on a horizontal ground, a pile in slope is acted on by additional sliding force from the slope, impairing its load-bearing capacity and buckling resistance. Nevertheless, proper assessment for the buckling of piles in slopes is difficult because of the simulation of distributed sliding forces and nonlinear pile–soil interactions. This paper refines the newly developed Euler–Bernoulli pile element formulation for the nonlinear buckling analysis of single piles in slopes by including the sliding force in element formulation, enabling a rigorous consideration of soil–pile interactions. Detailed mathematical derivations are provided. The numerical implementation is illustrated with a flowchart. Four examples for verification are demonstrated to reveal the robustness of the proposed method and the importance of considering the sliding force. [ABSTRACT FROM AUTHOR]

Offshore pipelines that deliver raw fluids usually work under high temperature/high pressure (HPHT) conditions to ease the flow and prevent the solidification of wax fractions. Under HPHT conditions, lateral buckling and axial pipeline walking can occur. Existing studies have shown that axial walking is prone to occur in short pipelines and lateral buckling tends to occur in long pipelines. However, there is no clear definition for distinguishing between short and long pipelines, nor are there criteria to determine the occurrence of these two behaviors under certain conditions. In this study, the relationship between the length of a pipeline and lateral buckling critical temperature was derived based on an analytical solution, considering the influence of tension forces and seabed slope angle. A theoretical analytical solution of axial walking was reviewed and summarized. The two solutions for axial walking and lateral buckling were combined, and a criterion based on critical pipeline length was proposed to estimate the possibility and priority of these two behaviors for a given pipeline under certain conditions. The influence of seabed slope and axial and lateral friction factors on the two behaviors was explored by carrying out parameter analysis, and the influence regularity of these factors on critical pipeline length and critical temperature was obtained. [ABSTRACT FROM AUTHOR]

The seismic performance of structures could be enhanced by the inclusion of supplemental components that dissipate the earthquake-induced energy. This is especially crucial for rocking structures that possess low energy dissipation properties due to their damage avoidance mechanism. Amongst variously developed yielding-type energy dissipaters (EDs), buckling-restrained energy dissipating carbon steel bars have received considerable attention as they make the most use of the inherent energy dissipation of steel and are easy to fabricate. However, maintenance, repair costs, and performance disruptions owing to corrosive environments have been mostly disregarded in past investigations. Employing stainless steel can be a viable solution to overcome such issues. The work described in this two-part study sheds light on various aspects of the buckling-restrained stainless steel EDs through experimental and finite element (FE) investigations. The mechanical properties of type 304L stainless steel including uniaxial monotonic response, strain sensitivity, and cyclic hardening are characterized. It is shown that the material possesses high ductility along with substantial hardening characteristics. In the first phase of testing, energy dissipating bars with different fuse diameters and lengths are designed. Load and strain capacities and bar-tube interactions of the buckling-restrained energy dissipation device are studied through quasi-static tests. In the second phase of testing, various EDs are designed, fabricated with stainless and mild steel, and tested under quasi-static loading to validate the findings of the FE investigations. The test results demonstrate a stable hysteresis response of the buckling-restrained stainless steel EDs with a cyclic average strain capacity of 10%. The FE modelling procedure, calibration, and parametric studies are presented in the companion paper as Part II. [ABSTRACT FROM AUTHOR]

To better understand the mechanical behavior of cold-formed steel (CFS) quadruple-limb built-up box-section (QBB) columns associated with flexural buckling, 12 axial compression tests with different slenderness ratios and screw spacing were conducted. The test results showed that, for a slender specimen (L=2,700 and 3,300 mm), when the screw spacing was reduced from 600 to 300 mm, the load-bearing capacity increased by about 6%–10%. When the screw spacing was reduced from 600 to 150 mm, the load-bearing capacity increased by about 14.6%–17.5%. Arranging end fastener groups can enhance the strength of the column, but it may be cumbersome and uneconomical. A finite-element model (FEM) was developed and calibrated against the test results. A series of parametric analyses was carried out to investigate the effects of the screw spacing and shear stiffness on the elastic buckling load and ultimate load-bearing capacity. Finally, a practical design method was proposed to evaluate the column's elastic buckling load and load-bearing capacity associated with flexural buckling. Compared with the simulation and test results, the proposed design method was accurate at predicting the load-bearing capacity of the specimens, with a relative difference of less than 6%. [ABSTRACT FROM AUTHOR]

A buckling-restrained braced frame (BRBF) reduces the response of buildings during a strong earthquake, during which the surrounding members such as beams and columns must resist the axial force imposed by the buckling-restrained braces (BRBs). However, the steel beams and columns are subjected to reversed axial force from the horizontal and vertical component of BRB axial force. The steel beams might then buckle under the compressive force or even yield under the subsequent tensile axial force because the beams are not designed primarily to sustain the axial force. Therefore, the rotation capacity becomes inconsistent with the typical loading protocols. Faced with this concern, this study examines a BRBF design with reasonable balance between the steel frame and the BRBs. Pushover analyses and nonlinear response history analyses (NRHAs) are undertaken to quantify the magnitude of the beam axial force. Furthermore, a detailed finite-element analysis (FEA) model is constructed based on results obtained from a previous experiment. The strain distribution and axial force transfer mechanism are identified using an experimentally validated FEA model. Results confirm that BRB axial force is transmitted primarily to the single side of the flange. Furthermore, the parametric study in terms of the beam section, BRB axial force, and loading protocols establishes a database of the structural performance. The results of this research are used to formulate a novel index and equations describing the ultimate strength ratio, rotation capacity, and energy dissipation capacity. [ABSTRACT FROM AUTHOR]

This study investigates the distortional buckling strength of a partially closed cold-formed steel (CFS) channel section with intermediate web stiffeners and outward lips under axial compression. Generally, a closed CFS cross section has higher torsional resistance when compared to an open cross section. However, an open cross section is more popular in the construction industry. Despite the popularity of CFS open sections, distortional buckling can reduce the load-carrying capacity of such open cross sections. To improve the distortional buckling strength of the proposed selected section, a simple spacer plate is added externally to the channel section in the transverse direction. Initially, a numerical model is developed using the finite element software ANSYS, which is then validated against the experimental results available in the literature. A detailed parametric study was also conducted to investigate the influence of thickness, depth, and spacing of spacer plates on the distortional buckling strength of stiffened CFS channel sections. From the results of the parametric study, it was found that the presence of spacer plates can increase the axial capacity of stiffened CFS channel sections by as much as 32%. Finite element results were compared with the design strengths calculated in accordance with the direct strength method (DSM). [ABSTRACT FROM AUTHOR]

Collapse in steel structures is often controlled by loss of load carrying capacity of steel columns due to interactive buckling, which involves interactions between local and global (i.e., lateral and lateral torsional) buckling. Commonly used concentrated plastic hinge or fiber-based elements do not simulate the physics of this response, potentially leading to inaccuracy in performance assessment. A nonlinear fiber-beam-column element [termed the Torsion Fiber Element (TFE)] to simulate monotonic interactive buckling in steel beam-columns is presented. The element, implemented in the OpenSees platform, incorporates St. Venant as well as warping torsion through enrichment of strain interpolation functions, in addition to axial and flexural deformation modes. Local buckling is represented through a softening multiaxial constitutive relationship. The efficacy of this approach is examined by comparing its results against those obtained from continuum finite element simulations as well as experimental data on beam-columns subjected to monotonic loading. The comparisons indicate that the element can functionally represent the physics underlying interactive buckling, resulting in effective prediction of the overall monotonic load-deformation response, as well as internal deformation and stress fields. Limitations of the element in its current form are summarized, along with prospective improvements. [ABSTRACT FROM AUTHOR]

The theoretical lateral-torsional buckling solutions for I-beams utilized in many design specifications may the overestimate lateral torsional buckling (LTB) resistance when significant shear is present due to web distortion. This paper investigates the effects of shear on the elastic LTB of doubly symmetric steel I-beams through computational models. A parametric study was conducted considering a range of geometric parameters and loading conditions commonly found in design applications in buildings and bridges. The results show that shear can have a substantial impact on the LTB resistance under many practical design circumstances. The impact of shear on LTB is more significant for sections with larger web slenderness ratios and smaller depth-to-flange width ratios. Postbuckling shear resistance does not significantly impact the reductions in LTB resistance due to shear. A practical design method that accounts for shear effects on the LTB behavior in stiffened and unstiffened beams was proposed and verified by comparison with computational solutions. Design examples demonstrating the proposed method are provided. [ABSTRACT FROM AUTHOR]

The durability of profiled metal-faced insulating sandwich panels (MFISPs) in terms of exposure to high temperatures and thermal cycles is critical for their use in civil engineering applications. In this paper, a total of 20 full-scale panels made with two different thicknesses were tested under instantaneous lateral pressure loading at ambient conditions as well as under various combinations of high temperatures, thermal cycles, and lateral pressure. The pressure was applied through a vacuum chamber whereas thermal blankets were used for sustained and cyclic thermal exposure of the panels. Each thermal cycle entailed 24 h of time duration with a temperature range of 20–80°C. The results show a consistent reduction in the initial stiffness, local buckling pressure, and strength of the panels when they were loaded at high temperatures. These reductions were more substantial (up to 24%) under the combined effects of high temperature (80°C), thermal cycles, and pressure loading. The study also shows that the thickness of the foam core can play an important role in controlling the local buckling capacity and strength of the MFISPs. A nonlinear finite-element analysis was also conducted with a relatively good correlation with the test results. The numerical results provide further insights into the structural response and explain certain aspects that could not be obtained from the tests. Given that MFISPs are typically used for roof panels, where the combination of high temperatures and wind load is inevitable at certain regions, the results presented in this work are recommended for consideration in their design. [ABSTRACT FROM AUTHOR]

This paper presents an improved design approach based on the direct strength method (DSM) in American Iron and Steel Institute (AISI) design specification S100 for cold-formed steel (CFS) built-up closed cross-section columns subjected to local, global, and local–global interaction buckling modes. Initially, the failure mode and load data corresponding to various design parameters such as intermediate fastener connection spacing, boundary conditions, material properties, slenderness, and cross sections from various researchers are collected, including the authors' test results. The design predictions using the current DSM design provisions are compared with the collected test results. The results show that the current DSM approach requires improvisation to adequately predict the axial compression load. The data are then analyzed to understand the behavioral features of the built-up closed cross-section column such as limit for interaction buckling, erosion of strength due to arrant interaction buckling, and increase in strength due to flange or web overlapping effect. The interpretation methods used to improve the current design approach are presented. The original DSM design procedure is improved by failure mode classification and modified slenderness to account for increase and erosion in strength. Finally, the safety (accuracy and reliability) of the design predictions from the improved DSM-based approach is assessed using the large amount of data collected. Since the introduction of the direct strength method to the North American Specification for the Design of Cold-Formed Steel Structural Members (AISI S100) in 2001, it has gained traction among design engineers due to its simplicity, advantages over other design methods, and specific design curves for individual buckling modes. To implement the DSM for CFS built-up members, several researchers have proposed different design approaches, and research is still happening. This study is also aimed toward the same objective but is specific to the local–global interaction in built-up members. It is demonstrated why the DSM prediction is unsafe for CFS built-up members with larger intermediate fastener connection spacing and excessively safe for CFS built-up members with closer intermediate fastener connection spacing. An intermediate fastener connection spacing limit that separates the local–global interaction buckling and element overlapping effect is determined. A design proposal for limiting the interaction buckling mode and considering the overlapping effect of elements in the DSM design method is suggested. More specifically, the proposed design method is in line with the current AISI approach so the designer need not confuse how to use this new design approach in practice. Therefore, for further ease of understanding, two design examples for built-up closed cross-section columns—(1) subjected to local–global interaction buckling; and (2) subjected to element overlapping effect—are included in this paper. It is also recommended that future research may focus on further classifying the AISI main specification sections, E2 and F2 (simple classifications based on the global failure modes), based on the interaction buckling modes in built-up cross-section members. [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]

There is a lack of reliable research regarding the current engineering specifications considering the broad applications of high-strength steel (HSS) in civil engineering. Axial compression tests were performed on 12 HSS welded H-section stub columns using Q420–Q960 grade steels to study their local buckling behavior. In addition, we performed a parametric analysis using a finite-element model (FEM). Prior to the tests, the initial geometric imperfections and material properties of the specimens were measured to ensure that they satisfy the relevant engineering specifications. Twelve specimens exhibited typical local buckling. The axial deformation and axial strain were analyzed, and the utilization rate of the steel strength decreases with an increase in the yield strength and width–thickness ratio. Furthermore, a close relationship was observed between the occurrence of local buckling in the flange and the web. The results also indicate that the postbuckling strength of the axially compressed HSS welded H-section stub columns with slender sections cannot be ignored. The test results were used to verify the accuracy of the FEM, which considers the initial geometric imperfections and welding residual stress. For the Q420–Q960 steel welded H-section stub columns, parametric analysis was performed using the verified FEM to evaluate the limits of the width–thickness ratio and ultimate axially compressed bearing capacity. To apply current European, American, and Chinese codes, the modification factors for the width-thickness ratio limit and ultimate bearing capacity are given. [ABSTRACT FROM AUTHOR]