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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]

Steel multitiered concentrically braced frames (MT-CBFs) not specifically detailed for seismic resistance with a response modification factor of R=3 are permitted in moderate seismic regions. Past research studies, however, have raised concerns regarding the seismic performance of MT-CBFs and CBFs designed using R=3 provisions, namely the stability of columns in MT-CBFs and connection limit states in R=3 CBFs. This paper examines the seismic response of R=3 MT-CBFs and proposes a design method to improve their seismic performance. A set of 16 prototype R=3 MT-CBFs, ranging from 8 to 20 m tall with two to five tiers and located in a moderate seismic area, was selected and designed in accordance with 2016 US design guidelines. Nonlinear response-history analyses were then performed to examine the seismic response of these frames using fiber-based numerical models. R=3 MT-CBFs designed to 2016 design guidelines are shown to experience premature fracture in brace-to-beam/column connections in a single tier, which imposes high in-plane bending demands on columns and in some cases, leads to column instability or a tier-sway mechanism. New seismic design recommendations, including requirements for intermediate struts, columns, and tier drift, are proposed. The design method aims to prevent column instability or a tier-sway mechanism and protect brace connections from excessive deformation. [ABSTRACT FROM AUTHOR]

The noncompact web-slenderness limit in current design specifications is based on an assumed value of the plate buckling coefficient for a web subjected to flexure that is between the values for fixed and simply-supported edge conditions. While the limit provided in current specifications is a good representation in many situations, there is evidence that the limit overestimates the magnitude of the restraint provided by smaller flanges. This paper shows that the ratio of the area of the compression flange to the area of the web in compression is a good indicator of the restraint provided to the bend-buckling of the web plate. The overestimation of the plate buckling coefficient can result in the classification of cross sections having relatively small flanges as noncompact web sections when their behavior is more akin to that of slender web sections. Slender web sections have a maximum possible flexural resistance, referred to as the plateau strength, less than the yield moment Myc, because of load shedding from the web. Furthermore, the St. Venant torsional stiffness contribution to the lateral torsional buckling resistance is neglected for these sections due to web distortional flexibility. This paper evaluates the appropriate noncompact web-slenderness limit via test simulation of a targeted set of short girder specimens in which the calculated resistance is governed by the plateau strength and proposes a modified equation to rectify the problem. The impact of this change on the prediction of a broad range of member limit states is illustrated via comparison to other test simulations and to experimental results. [ABSTRACT FROM AUTHOR]

The lateral torsional buckling (LTB) equations in some current standards are based on unified equations that are a fit to a large body of experimental test data. However, these curves tend to overestimate the capacities from finite-element (FE) simulations based on typical idealized end conditions, residual stresses, and geometric imperfections. Uniform moment tests conducted on various types of cross sections have led to proposed modifications of the current specification for LTB equations. This paper investigates and recommends, by means of FE test simulations, the use of the modified equations with additional improvements for moment gradient cases. The emphasis of the studies is on rolled I-section members. Specifically, the paper demonstrates an inelastic Cb effect that can be particularly significant for beams in which the maximum moment occurs within the unbraced length rather than at a braced point. A new way of applying the LTB modification factor, Cb, in the inelastic LTB region for these cases is recommended for ordinary application of current standard resistance equations. Also, it is shown that this effect can also be captured seamlessly via an explicit inelastic buckling analysis using stiffness-reduction factors (SRFs) derived from the design resistance curves. [ABSTRACT FROM AUTHOR]

The objective of this paper is to employ the results from the extensive instrumentation installed on recently tested full-scale cold-formed steel (CFS)-framed buildings to reveal a deeper understanding of the behavior of the building under seismic excitations. In particular, this paper complements a companion paper that focuses on system-level design and response. Here, utilizing strategically located string potentiometers, strain gauges, and accelerometers, the responses of the walls and diaphragms are isolated from the overall building response and studied. The interaction of shear walls along a wall line, as well as across stories is studied through measured data on strains in hold-down anchors, strains on floor-to-floor strap connecting shear-wall chord studs, and displacements across shear-wall sheathing and openings. The behavior of the floor diaphragm is studied through displacements measured perpendicular to the plane of one wall of the building and accelerometers throughout the floor of the building. The results serve to highlight the subsystem-level outcomes that accompany the complex system-level response as CFS-framed buildings undergo seismic excitations. [ABSTRACT FROM AUTHOR]

This study is concerned with the strength limit state of serial bolted connections in structural steel plates. It points out that the ultimate load capacity of a serial bolted connection failing in combined bearing and shear-out cannot be computed as the simple sum of the respective ultimate bearing and ultimate shear-out capacities, which is implicitly permitted in design specifications worldwide. Based on the laboratory test results of 10 hot-rolled steel plate specimens composed of three different grades with nominal thicknesses ranging from 5 to 8 mm, the paper first establishes the ultimate bearing coefficient of a 20-mm bolted connection in a structural steel plate to be 3.5. Coupled with the shear-out equation previously derived, a design equation where the shear-out capacity of the downstream bolt varying quadratically with the end distance is then proposed to determine the combined bearing and shear-out capacity. The proposed equation is demonstrated through verification against independent laboratory test results involving 5-mm plates of three different grades to be significantly more accurate than the simple sum. Explanation for the unexplained results obtained by another researcher using his own equation is provided in this paper. [ABSTRACT FROM AUTHOR]

This paper is the second part of a two-part report describing the behavior of headed stud-SFRCC (steel-fiber-reinforced cementitious composite) shear connection. The first paper presented an experimental study of headed stud-SFRCC shear connections, whereas this paper focuses on the strength evaluation of those shear connections. This paper first presents a finite-element model of the connections, which was validated using the experimental results. An updated empirical equation considering the contribution made by the weld collar is proposed to estimate the shear strength of a single headed stud. Finite-element models were developed to evaluate the shear strength of each stud in a densely arranged group of headed studs. The results reveal that the interaction between headed studs mitigated the confining effect on studs, and the studs thus sustained less shear strength than those with larger stud spacing. Studs placed in different positions sustain different shear forces, and this difference is greater when smaller stud spacing is used. Formulas with which to estimate the reduction in the strength of headed studs in SFRCC are suggested according to a parametric study of the finite-element analysis. [ABSTRACT FROM AUTHOR]

Cold-formed steel lipped channel beams (LCBs) are used extensively in residential, industrial, and commercial buildings as loadbearing structural elements. Their shear capacities are considerably reduced when the web openings are included for the purpose of locating building services. Past research has shown that the shear capacities of LCBs were reduced by up to 70% due to the inclusion of these web openings. Hence there is a need to improve the shear capacities of LCBs with web openings. A cost-effective way of eliminating the detrimental effects of large web openings is to attach suitable stiffeners around the web openings and restore the original shear strength and stiffness of LCBs. Hence detailed experimental studies were undertaken to investigate the behavior and strength of LCBs with stiffened web openings subject to shear, and combined bending and shear actions. Both plate and stud stiffeners, with varying sizes and thicknesses, were attached to the web elements of LCBs using different screw-fastening arrangements. Simply supported test specimens of LCBs with aspect ratios of 1.0 and 1.5 were loaded at midspan until failure. Numerical studies were also undertaken to investigate the strength of LCBs with stiffened web openings. Finite-element models of LCBs with stiffened web openings under shear, and combined bending and shear actions, were developed to simulate the behavior of the tested LCBs. The developed models were then validated by comparing their results with experimental results and were used in further studies. Both experimental and finite-element analysis results showed that the stiffening arrangements recommended by past research and available design guidelines are not adequate to restore the original shear strengths of LCBs. Therefore, new stiffener arrangements were proposed based on screw fastened plate stiffeners. The details of the research reported in this paper are presented. [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]

The paper presents the experimental results and proposed design method for the compression strength of the cantilevered leg of cold-formed steel clip angle connectors. The variations of the test specimens included the plate aspect ratio, material thickness, and material strength. The test results were compared to three existing standard design methods for cold-formed steel members similar to the clip angles, but no good agreement was found. A new design method was therefore developed to calculate the nominal compression strength of the cantilevered leg of clip angles. The new method was based on the elastic buckling solution of thin-walled plate columns. The allowable-strength design safety factors, load and resistance factor design, and limit-state design resistance factors were also produced to support the proposed design method. [ABSTRACT FROM AUTHOR]

This paper examines the accuracy of design equations specified in the North American, European, and Australasian codes for cold-formed steel structures in determining the ultimate tilt-bearing capacity of single-shear bolted connections without washers in flat steel sheets. It points out that all the code equations do not properly distinguish the tilt-bearing failure mode from the conventional bearing failure mode. While the latter takes place downstream of the bolt, the former takes place upstream. Unlike the conventional bearing capacity, the tilt-bearing capacity is affected by the width of the connected sheet and does not vary linearly with either the sheet thickness or the bolt diameter. Furthermore, it is not affected by material ductility. Based on the test results of 156 specimens composed of G2 and G450 sheet steels having various dimensional configurations, this paper proposes a design equation that is dimensionally consistent and that is considerably more accurate than all the code equations. The proposed equation was also verified against single-shear single-row bolted connections tested by independent researchers which failed in the tilt-bearing mode. The verified thicknesses ranged from 0.92 to 3.0 mm, and the bolt diameters ranged from 6.4 to 16 mm. An additional finding is that the tilt-bearing capacity is not significantly affected by the orientation of the bolt head or nut. A resistance factor of 0.75 is recommended for use with the proposed equation for determining the tilt-bearing capacity of single shear single-row bolted connections in cold-reduced steel sheets. [ABSTRACT FROM AUTHOR]

The paper presents experimental and numerical investigation of cold-formed steel (CFS)-framed walls sheathed with corrugated steel sheets. The new wall technology is considered as an alternative shear wall configuration for midrise buildings at strong earthquake and high-wind regions because of its attributes of noncombustibility and higher structural performance than the conventional shear wall types. However, design provisions of such corrugated sheathed CFS-framed shear walls have not been included in the current design standards. To establish comprehensive database information and to observe the performance of the bearing wall for the first time, a test program of full-scale wall assemblies using corrugated steel sheathing was conducted. The test specimens included both shear wall specimens and bearing wall specimens under lateral and vertical/gravity loading. In accordance with the full-scale tests, a numerical model was developed in a computer program, and seismic performance assessment was evaluated through nonlinear time history analysis, i.e., incremental dynamic analysis using a methodology proposed by FEMA. The paper presents the results of the experiments and numerical simulations and produces appropriate seismic performance factors for this new lateral-resisting system. [ABSTRACT FROM AUTHOR]

This study developed a special energy dissipation X-bracing for cold-formed steel (CFS) shear walls to improve the ductility of the corrugated steel sheathing. In previous research, CFS shear walls with corrugated sheathing demonstrated poor ductility under cyclic loading. The paper presents a potential solution by adding a special energy dissipation bracing (EDB) to improve the ductility of the corrugated sheet–sheathed shear walls. The EDB can be tuned according to the CFS shear wall base performance to achieve the highest postpeak deflection capacity, thereby significantly improving the seismic ductility. To prove the concept and analyze the feasibility of the new EDB, a test program of nine full-scale shear walls was conducted. The experimental results showed that with the tuned EDB, the corrugated steel sheet–sheathed shear walls had improved ductility with no considerable reduction in the initial stiffness and shear strength. Installing the proposed EDB can be an efficient approach to enhance the seismic performance of CFS-framed shear walls. Details of the test program and discussion are presented. [ABSTRACT FROM AUTHOR]

This paper presents the experimental investigation on the compressive behavior of cold-formed high-strength steel tubular stub columns. In this paper, the nominal 0.2% proof stress of the high-strength steel was 700, 900, and 1,100 MPa. A total of 25 stub column tests on circular, rectangular, and square hollow structural sections were conducted. Geometric imperfections and load-deformation histories were reported and assessed. The experimental results were compared against the design values calculated from the Australian, European, and North American codes, and the corresponding compactness criteria were assessed. A finite-element model, which incorporated the varying material properties, was described, and the influence of geometric and material imperfections was also evaluated. The finite-element results successfully captured the experimental observations and can be adopted for future parametric studies. [ABSTRACT FROM AUTHOR]

Concrete-filled steel tube (CFT) beam-columns are categorized as compact, noncompact, or slender depending on the governing slenderness ratio (width-to-thickness b/tb/t or D/tD/t ratio, λλ) of the steel-tube wall. The current AISC specification recommends the bilinear axial force-bending moment (P-MP-M) interaction curve for bare steel members for the design of noncompact and slender CFT beam-columns. This paper compiles the experimental database of tests conducted on noncompact and slender CFT beam-columns, and demonstrates the overconservatism of the AISC P-MP-M interaction curve. This paper also presents the development and benchmarking of detailed 3D finite-element models for predicting the behavior and strength of noncompact and slender CFT members. The benchmarked models are then used to evaluate the fundamental P-MP-M interaction behavior of CFT beam-columns, and the influence of material and geometric parameters such as the tube slenderness ratio (λλ), material strength ratio (Fy/f'c), member length-to-section depth ratio (L/D), and axial load ratio (P/Po). The parametric analyses indicate that for L/D ratios up to 20, the P-M interaction curves are governed by the relative strength ratio (csr=AsFy/Acf'c). The parametric analysis results are used to propose revisions to the current standard's interaction equations for designing noncompact and slender CFT beam-columns. [ABSTRACT FROM AUTHOR]

This paper concludes a two-part paper, which addresses the reliability of simplified approaches for rack design. The first part presents a parametric study on medium-rise, unbraced racks differing for components, geometry, and flexural stiffness degree of beam-tocolumn joints and base-plate connections. The evaluation of the elastic critical load multiplier, which is required to select the analysis method and check member instability, is considered through a simplified approach that is adequately calibrated for the more traditional steel structures. This second part focuses on seismic design, which is governed by the value of the fundamental period of vibrations (T1). Simplified approaches from the literature on typical steel frames and recommendations to predict period T1 are applied to two beam formulations differing in the absence or presence of warping. Research outcomes are discussed, considering the limits of the simplified approaches, and improvements to increase the accuracy of fundamental period prediction are proposed. For reproduction purposes, this paper details the data used in the application of the considered simplified approaches and the previously discussed approaches. [ABSTRACT FROM AUTHOR]

This paper proposes a new kind of composite waffle slab that consists of orthogonal steel girders and flat RC slab. The steel beams help decrease the cracking that occurs with concrete, and using prefabricated RC slabs reduces the framework and speeds up the construction. An experimental investigation of steel-concrete composite waffle slabs is conducted, and the specimens are proved to have excellent ductility and load-bearing capacity. Then a mixed finite-element (FE) model combining the beam elements and shell elements is proposed to predict the behavior of composite waffle slab. Based on the numerical model, a series of parameter analyses are conducted, and the influences of different parameters on the mechanical behavior of the composite waffle slab are intensively investigated. Simplified formulas for the ultimate capacity of the composite waffle slab are derived based on the test observations and numerical analysis. Reasonable recommendations are proposed to help engineers design the composite waffle slab effectively. [ABSTRACT FROM AUTHOR]

Recycled aggregate concrete-filled steel tube (RACFSTs) are attracting increasing attention from academics as a solution to use recycled aggregate concrete (RAC) as main load bearing members in multilevel or even high-rise buildings. Despite their promising future for structural use, limited attention has been devoted to investigate the time-dependent behavior of such elements. This paper reports on an investigation of how the incorporation of recycled aggregates (RAs) affects the long-term responses of these composite specimens. To achieve this, 21 RACFST specimens with different aggregate replacement ratios have been prepared for the long-term tests. The accuracy of currently available RAC concrete models in predicting the creep behavior of the RACFST specimens has been investigated based on the related experimental results. Replacing natural coarse aggregates with recycled aggregates can increase the long-term deformation of the composite columns by 22% but barely affects the rate of creep development of the specimens. The observations from the research reported in this paper indicate that it is reasonable to account for such effects by multiplying an amplification factor to the available concrete models for normal composite specimens. Benchmarking analyses have shown that the available amplification factors proposed for RAC members can be used to predict the long-term responses of RACFST specimens by considering a nil exposed perimeter. Parametric analysis has been carried out to evaluate the time effects on the static responses of RACFST specimens. [ABSTRACT FROM AUTHOR]

The aims of this study are to generate experimental data and develop numerical models for aluminum alloy continuous beams, and to utilize the results to underpin the development of revised design methods for indeterminate structures. This paper presents an experimental program and finite-element (FE) analyses for two-span continuous beams (i.e., five-point bending) of square and rectangular hollow sections (SHSs and RHSs). The experimental program comprised 27 five-point bending tests with three different positioning of loads. The testing procedures and key results are reported. The test specimens were manufactured by extrusion, with 18 of grade 6061-T6 and 9 of grade 6063- T5 heat-treated aluminum alloys. The test specimens were nonslender sections, and mostly of Class 1 proportions. Generally, the specimens failed by the formation of a collapse mechanism comprising three plastic hinges. The distances between the supports and the loading points were varied in order to form the first plastic hinge in different locations, to achieve different load levels between the first hinge and collapse, and to change the rotation demands on the first hinge that formed. The FE models were developed and failure was defined as either when a plastic collapse mechanism was formed or the material fracture strain was reached on the tension flange, whichever occurred first. The numerical models were first validated against the experimentally obtained load-deflection responses, as well as the failure modes. The experimental and FE ultimate loads were both found to be beyond the theoretical loads corresponding to the formation of the first hinge as well as the calculated plastic collapse loads. A key characteristic of aluminum alloy, strain hardening, is shown to be particularly significant in both the experimental program and the numerical investigation. The validated FE models are used to generate numerical results through parametric studies in the companion paper. The development of design rules for indeterminate aluminum alloy structural systems is then described. [ABSTRACT FROM AUTHOR]

This paper describes an experimental program investigating the local-global interaction buckling of stainless steel I-section beams. Three alloys were included: austenitic S30401, ferritic S44330, and lean duplex S32101. Extensive coupon tests were conducted to determine the specific properties of stainless steel sheets, including nonlinearity, anisotropy, and strain hardening. The test program encompassed six laterally braced tests and 24 unbraced tests with spans ranging between 1.9 and 4.0 m. The unbraced specimens were tested in a specially designed setup featuring clearly defined loading and support conditions. Interaction buckling was successfully achieved in the tests. The geometric imperfections, bearing capacity, and sectional and global deformations of each specimen were carefully recorded before or during the tests. Key factors affecting the test conditions were identified. The research addressed the lack of experimental data for local-global interaction buckling of stainless steel beams with open cross section and establishes the foundation for further theoretical study, as described in a companion paper. [ABSTRACT FROM AUTHOR]

Based on the previous research results of the authors, this paper presents an accurate and consistent equation for determining the ultimate shear-out capacity of a structural steel bolted connection. The equation is verified against independent laboratory test results obtained by other researchers around the world. Comparisons against alternative equations found in the design specifications and literature are also included. The paper explains why certain equations appear to be accurate for particular configurations, but are grossly inaccurate for others. This paper describes the various assumptions embedded in the existing equations, some optimistic and others pessimistic. It shows that the current code equations lead to very significant errors on either side of conservatism, while the proposed equation is consistently accurate for all test specimens known to fail in shear-out. A resistance factor of 0.85 is recommended for the proposed equation in order to achieve a reliability index of 4.0. The use of the proposed equation instead of the current AISC specification's equation will facilitate structural designs that are more economical yet reliable. [ABSTRACT FROM AUTHOR]

Steel pins are an attractive and efficient alternative technology to screws when connecting cold-formed steel or other materials to cold-formed steel members. However, unlike screws, pins remain an essentially proprietary connection technology based on manufacturer claims of special pin shank design. As such, current cold-formed steel design standards and specifications do not include provisions for their direct use and designers have no simple, rational means for making informed design decisions beyond information provided by manufacturers. This paper reviews the current design model for screws and examines the basic tension pullout/withdrawal limit state of helical knurled steel pin connections for cold-formed steel thicknesses common in residential construction. Included in this paper is a determination of material properties using a nondestructive test method as an alternative to traditional methods for estimation of steel base metal thickness. A simple lower bound friction model is proposed for helical steel pins and the model is calibrated using data from 32 physical pin tests. The calibrated model is shown to provide an acceptable degree of reliability and safety consistent with that of screws in current cold-formed steel design standards and specifications. [ABSTRACT FROM AUTHOR]

Double-skin composite (DSC) walls consist of a thick concrete infill sandwiched in between two steel faceplates on the exterior surfaces. DSC walls used in high-rise buildings have higher reinforcement ratios and are subjected to larger axial-force ratios as compared to DSC walls used in safety-related nuclear facilities. This paper presents the results of experimental and numerical investigations conducted to evaluate the cyclic in-plane shear behavior of DSC walls for high-rise buildings, and the influence of higher reinforcement ratios and axial-force ratios. The DSC wall specimens were designed with a reinforcement ratio of 6.4%, and with flange walls designed as boundary elements to ensure that the walls would be shear critical. The wall specimens failed by cyclic yielding and local buckling of the steel faceplates in the web walls, and eventual crushing of the concrete infill. The steel faceplates prevented spalling of the crushed concrete and as a result, the wall specimens had stable hysteretic loops and large shear-deformation capacity. Using vertical stiffeners and tie battens as connectors further increased the shear-deformation capacity of thewall specimens, with the ultimate shear strain reaching 3%. A mechanics-basedmodel (MBM) was used to analyze the in-plane shear response of the wall specimens. The experimental and analytical investigations indicate that axial compression has limited influence on the shear strength, but decreases the shear-deformation capacity of the DSC walls. Analytical parametric studies indicate that for DSC walls made using normal-strength concrete and steel, high reinforcement ratios (of over 7.5%) and high axial-force ratios (exceeding 0.40) can potentially lead to crushing of the concrete infill prior to yielding of steel faceplates, and thus nonductile failure modes. Finally, the design equations specified in various codes are verified using experimental results of 42 specimens from past tests and from this experimental program. Those code equations provide reasonable and conservative estimations of the shear strength of DSC walls, with the ratio of experimental-to- calculated values equal to approximately 1.30 on average. [ABSTRACT FROM AUTHOR]

Tension field theory assumes that after elastic shear buckling has occurred, compressive stresses in the web plate do not increase; therefore, postbuckling shear strength solely results from the development of tensile stresses within a defined diagonal tension field. Finite-element analyses have found this fundamental assumption to be invalid and a new theory (compression theory) was proposed that bases the development of postbuckling shear strength mostly on the compressive response of the plate (tension plays a secondary role). Previous work has validated compression theory against a wide variety of published experimental data at ambient temperature. This paper shows that compression theory can predict the ultimate postbuckling shear strength of steel web plates up to temperatures of 1,100°C. This finding is significant because steel plate girders subjected to fire loading are highly susceptible to web shear buckling. Experimentally validated finite-element models are used to develop temperature-dependent multiplicative parameters to permit the use of compression theory to calculate ultimate postbuckling shear strength at elevated temperatures. Comparisons with published experimental data show that predictions of ultimate postbuckling shear strength from compression theory closely agree with the literature. [ABSTRACT FROM AUTHOR]

Steel-plate composite (SC) walls can be anchored to reinforced concrete (RC) foundations or connected to RC walls using lapsplice connections. These lap-splice connections consist of steel rebar or dowels that are fully developed (Ld) in the RC portion and embedded (Lemb) in the SC wall. Tension forces are transferred from the steel faceplates of the SC wall to the steel rebar or dowels using stud anchors and ties weLded to the steel faceplates. The efficiency-of-force transfer depends on the dowel embedment length (Lemb), the number of stud anchors and ties engaged (n), the eccentricity or distance between the steel faceplates and dowel bars (α), and the tie bar reinforcement ratio (ρt). This paper presents the results of experimental investigations conducted in Japan and in the United States to evaluate the pullout (tension) strength and ductility of these lap-splice connections. The parameters included in the tests were Lemb, n, α, and ρt. The effects of eccentricity (offset distance between dowel bars and steel faceplates) on the axial tension behavior of SC-to-RC lap-splice connections were also investigated. Detailed numerical models were developed to gain additional insight into the behavior and force transfer in lap-splice connections. The results from the experimental and numerical investigations were used to propose design and detailing recommendations for lap-splice connections. [ABSTRACT FROM AUTHOR]

The fatigue life of steel bridges can be extended by externally bonding carbon fiber-reinforced polymer (CFRP) patches to the damaged elements to arrest the crack propagation. In this paper, the influence of the patch pretension level, stiffness, and fiber orientation on the stress intensity factor levels are investigated. A three-dimensional finite-element model of the double-sided CFRP patch-strengthened specimen is used to study the fracture behavior of an inclined edge crack under different combinations of loading Modes I and II. The introduction of a compressive stress by pretensioning of the CFRP patch prior to bonding produced a significant reduction of up to 70% in stress intensity factors (SIFs) for different crack lengths, which led to increasing the remaining fatigue life of the steel member. The increase in the CFRP patch-to-steel axial stiffness ratio reduced the SIFs by up to 30%. The optimum benefit of using the CFRP patch to reduce mode I SIF was accomplished when the patch axial stiffness was about 50% of or below the steel plate stiffness. Furthermore, orienting the fibers perpendicular to the crack direction is not necessarily the most effective in arresting the crack propagation. The effectiveness of the fiber orientation in the composite patch depends mainly on both the loading direction and the crack inclination angle. [ABSTRACT FROM AUTHOR]

The paper presents the outcomes of an experimental program developed and carried out with the aim of characterizing the behavior of two types of moment-resisting joints in multistory frames of concrete-filled high-strength steel rectangular hollow section (RHS) columns and mild carbon steel beams. The paper describes the design approach and displays the specific detailing for the two joint typologies: with reduced beam section (RBS) and with cover plates (CP). In spite of the various past and recent studies which were focused on the evaluation of full-strength welded beam-to-concrete-filled-tube (CFT) column joints, particular design procedures are missing at the European level. The objectives of the current research were to (1) development a design procedure for welded beam-to-CF-RHS joints, (2) experimental validation of the of the design procedure, and (3) assessment of the seismic performance. The monotonic and cyclic tests evidenced a good conception and design of the RBS and CP joints. In particular, the current study proved the feasibility of using higher steel grades in columns and joint components, and the use of an external diaphragm for the transfer the loads from the beam to the side walls of the RHS steel tube. The seismic performance of the joints was proved adequate, the joints sustaining rotation capacities larger than the 40 mrad at the significant damage performance level. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive parametric study on the ultimate strength of perforated cold-formed steel columns using the finite element (FE) package ABAQUS. FE models were developed for columns predisposed to local (L), distortional (D), and global (G) buckling failures. The parametric study involved five cross-section types (i.e., C, stiffened C, Z, rack, and hat sections), five widths of holes, four lengths of holes, four different sizes of hole spacings, three material properties, two magnitudes and various directions of geometric imperfections, and three buckling modes (i.e., L, D, and G) and all of the possible interactions between them (i.e., LD, LG, DG, and LDG). Ultimate strengths, as well as failure modes, were obtained from the analyses and are plotted against the current codified direct strength method (DSM) in AS/NZS 4600. Significant discrepancies between the numerical results and the DSM-predicted strengths were observed due to the influence of holes, the interaction of buckling modes, and occasionally the inherent limitations of the DSM itself. The effects of column length, material properties, and centroid shift due to perforation are also discussed. A data pool including 146,207 column-ultimate strengths were obtained, most of which were used to formulate new design equations for perforated thin-walled columns based on the direct strength method, as presented in a companion paper. [ABSTRACT FROM AUTHOR]

The parametric study of the ultimate strength of perforated cold-formed steel columns is presented in a companion paper. The parameters include five cross-section types (i.e., C, stiffened C, Z, rack, and hat sections), five hole widths, four hole lengths, four hole spacings, three material properties, two magnitudes and various directions of geometric imperfections, and three buckling modes (i.e., L, D, and G) and all the possible interactions between them (i.e., LD, LG, DG, and LDG). Based on the extensive range of strength data obtained, four design methods based on the direct strength method (DSM) are evaluated, including the existing AS/NZS 4600 DSM (Australia Standard 2005), Design Option 4 by Moen and Schafer (2011), and two sets of proposed methods. All these methods are complemented by a reliability analysis deriving appropriate resistance factors. The proposed methods are based on Design Option 4 and include DG interaction. They only require the inputs of elastic local and distortional buckling loads based on gross section properties, which is a significant simplification compared to the methods based on net section properties. Additionally, Proposed Method 1 uses a uniform penalty factor to ensure the satisfaction of the overall resistance factor of 0.85, while Proposed Method 2 adopts a factor based on a regression analysis involving parameters such as the dimensions of holes and cross-sections. Proposed Method 2 outperforms all the other methods in terms of a reduced overall scatter in prediction and an increased overall reliability. This method can also be used for nonperforated columns with a better performance than the current AS/NZS 4600 DSM (Australia Standard 2005). [ABSTRACT FROM AUTHOR]

Concrete-filled stainless steel tubular (CFSST) columns have attracted increasing research interests in recent years; however, It seems that the behavior of this type of innovative column under cyclic loading has not been addressed sufficiently so far. This paper reports test results of 10 CFSST columns under constant axial load and cyclically increasing flexural loading. The main parameters varied in the experiments were axial load level, cross-sectional type, and concrete type. The influences of these parameters on strength, ductility, stiffness, and energy dissipation were investigated. It was found that CFSST columns exhibited excellent energy dissipation and ductility, even when the specimens were subjected to high axial loads. The hysteretic behavior of the tested CFSST columns was compared to that of their carbon steel composite counterparts reported in the literature. Several existing design codes were used to predict the ultimate strength and flexural stiffness of the test specimens, and some suggestions are proposed accordingly for designing CFSST columns. [ABSTRACT FROM AUTHOR]

Strengthening of deteriorating structural members by fiber-reinforced polymers (FRPs) is an increasingly common and validated technique; however, concerns over means to evaluate the long-term durability of these retrofits exist. This paper explores a novel approach to overcome this concern through the use of a novel self-sensing composite material. Specifically, the objective of this paper is to provide a proof of concept for an integrated strengthening and sensing methodology for structural steel members achieved via infusing more-traditional composites with carbon nanotubes (CNTs). To assess the strengthening and sensing capabilities of the CNT-based composite, a set of unidirectional tensile tests were conducted. The experimental results show stiffness increases and strain reductions due to the application of the CNT-based sensing composites that were in close agreement with both analytical and finite-element models. The sensing aspect was also validated by a corresponding linear change in resistance of the CNT-based sensor with increasing load up to the point at which debonding of the adhesive layer occurred. The nanotube sensing layer is able to capture the strain in the member as well as the onset and extension of interfacial debonding. [ABSTRACT FROM AUTHOR]

Fracture is an extreme limit state in steel structures, often precipitating structural failure or serious loss of function. Methods to predict fracture in civil structures include traditional approaches developed in other disciplines (mechanical or aerospace engineering) subsequently adopted in structural engineering, as well as approaches to characterize earthquake-induced fractures originating within civil engineering. Developed over nearly six decades, the state of the art is composed of theories and models that address fracture over multiple scales and are targeted toward disparate application scenarios. The paper examines these approaches from a structural engineering standpoint, considering trade-offs in accuracy and expense, while identifying areas for improvement. Traditional approaches (including linear elastic and elastic plastic fracture mechanics) are presented, followed by newer local approaches that are better suited for scenarios where traditional approaches are inapplicable. By simulating micromechanisms such as microvoid growth as well as granular cleavage, local approaches address fracture under large-scale yielding, ultralow-cycle fatigue (which occurs during earthquakes), and low-stress triaxiality, all of which are important in civil structures. The physical basis for these approaches is outlined, with a summary of best practices for calibration and application. However, these local approaches have limitations as well, and often require substantial resources for successful implementation. With this background, optimal fracture assessment strategies are outlined for common structural scenarios, considering accuracy and cost. Limitations of the entire fracture modeling framework are summarized because they pertain to mainstream adoption within structural engineering research and practice. As the profession moves toward accurate performance characterization, it is anticipated that research will accelerate to overcome these limitations. [ABSTRACT FROM AUTHOR]

The I-section member lateral torsional buckling (LTB) resistance equations of the unified provisions underlying the current AISC and AASHTO specifications are a fit to a large body of experimental test data. It has been observed that finite-element (FE) test simulations commonly predict smaller capacities than the AISC/AASHTO LTB equations, especially in the inelastic LTB region. One reason for this disconnect is the fact that the residual stresses and geometric imperfections are often approximated by conservative nominal values in test simulations. Another reason for the discrepancy is the common lack of consideration of inelastic effective length effects within calibrations to experimental test results. This paper recommends improvements to the current LTB resistance equations to address these shortcomings. The impact of inelastic end restraint in representative beam LTB experimental tests is illustrated through FE test simulations. In addition, extensive test simulation results are presented, based on reduced residual stresses and geometric imperfections determined in separate research, which illustrate the quality of the proposed prediction equations for I-section members of various cross section types subjected to uniform moment. [ABSTRACT FROM AUTHOR]

This paper presents a new design methodology for equal-width rectangular hollow section (RHS) X-joints failing by sidewall buckling. In the new approach, a slenderness parameter is defined based on the elastic local buckling stress of the sidewall, idealized as an infinitely long plate under patch loading. A Rayleigh-Ritz approximation is thereby used to obtain a closed-form solution. The proposed design equation is verified against experimental results over a wide range of wall slenderness values obtained from the literature and complemented by a brief experimental program carried out by the authors. It is demonstrated that the new design equation yields excellent results against the experimental data. Finally, a reliability analysis is performed within the framework of both the Eurocode and the AISI standards to ensure that the proposed design equation possesses the required level of safety. The newly proposed equation strongly outperforms the current Comité International pour le Développement et l'Etude de la Construction Tubulaire (CIDECT) design rule for sidewall buckling and also further extends the range of applicability to a wall slenderness ratio of up to 50. [ABSTRACT FROM AUTHOR]

This paper summarizes the results of experimental investigations on steel composite beams with simple connections at elevated temperatures. In the United States, three different types of simple connections are most commonly used in building construction with steel composite beams: (1) double-angle connections; (2) single-angle connections; and (3) shear-tab connections. A testing program was conducted to examine the structural behavior of all three of these connections at elevated temperatures. Each specimen consisted of a steel composite beam attached to a loading frame with simple connections. The specimens were subjected to controlled heating and cooling curves while the loading frames were fire protected and designed to remain in the elastic range. The shear-tab connections showed the largest connection rotation, but they fractured during the cooling phase of the tests. The prying of the angles in the double-angle and single-angle connection during the tests provided ductility to prevent fracture during the cooling phase of the experiments. The double-angle connections demonstrated the least connection rotation and excellent performance during the tests. [ABSTRACT FROM AUTHOR]

This paper presents numerical and experimental investigations of the in-plane strength and design of pin-ended circular arches having a sinusoidal corrugated web under combined in-plane loads. Finite-element models are developed that account for the effects of the corrugated web, initial geometric global and local imperfections of the arch and its web and flanges, residual stresses, the included angle and curvature of the arch, and different combined load cases. These are validated by test results and used together with the experiments to investigate the failure modes and strengths of such arches. It is found that an I-section arch with a corrugated web may fail in a global mode or in a web shear buckling mode. There are two types of global failure modes for arches under combined loads. In most cases, corrugated arches may fail in an elastoplastic buckling mode. However, when wind load plays an important role in the combined loads, corrugated arches may fail in a plastic yielding mode. An interaction design equation is proposed for predicting the global in-plane strength of steel arches with a sinusoidal corrugated web under combined axial and bending actions. The design equation provides lower bound predictions for the strengths of corrugated arches. General procedures are also proposed for the practical strength design of steel I-section arches with a sinusoidal corrugated web. [ABSTRACT FROM AUTHOR]

Gap between the outer steel tube and core concrete is a type of initial imperfection that may exist in concrete-filled steel tubular (CFST) columns. This paper is devoted to investigate the influence of the gap on the performance of eccentrically loaded CFST columns. Twelve CFST column specimens and two reference hollow steel tubes were tested under eccentric compression. The main test parameters included the gap type (circumferential and circular-segment) and gap ratio. A finite element model was established to simulate the behavior of the CFST columns with gaps, and the model was validated against the experimental results in terms of failure mode, load versus deformation response, and ultimate strength. Based on the experimental and numerical investigations, the performance of eccentrically loaded CFST columns with gaps was analyzed. Parametric studies were then conducted to reveal the influence of different parameters on the ultimate strength of the CFST columns with gaps. Finally, the maximum allowable gap ratio in practice was proposed for the eccentrically loaded CFST columns with circumferential gaps, and a simplified model was proposed to predict the ultimate strength of the composite columns. [ABSTRACT FROM AUTHOR]

This paper presents a numerical analysis on the postfire behavior of steel-reinforced concrete (SRC) column to axially and rotationally restrained SRC beam joints. A finite-element analysis (FEA) model was developed to simulate the reaction of the composite joints under an entire loading and fire phase, including ambient temperature loading, heating, and cooling with constant loads and postfire loading. For validation, the proposed modeling method was used to simulate the results of SRC joint postfire tests and SRC column and joint fire-resistance tests. The comparison demonstrates the accuracy of the proposed FEA model. Based on the validated FEA model, the failure modes of SRC joints with restrained beams were investigated, and the joint deformation and internal force development during the entire loading and fire phase were analyzed. Afterward, two important parameters, namely, residual joint strength index and residual stiffness index, were defined based on the joint moment versus relative rotation angle curve, to quantify the postfire performance of the composite joints. Through regression analysis, formulas were developed to calculate the residual joint strength index and residual stiffness index. [ABSTRACT FROM AUTHOR]

This paper studies the postfire performance of steel-reinforced concrete (SRC) columns subjected to an entire loading and fire phase, including ambient temperature loading, heating and cooling with constant external loads, and postfire loading. A sequentially coupled thermal-stress analysis module was employed in a finite-element analysis (FEA) program to establish temperature field analysis and structural-analysis models of SRC columns subjected to the entire loading and fire phase. To improve the precision of the FEA model, the influence of fire-induced concrete explosive spalling (CES) was considered by using element change technique. Besides the existing fire-resistance test data of SRC columns, to further validate the accuracy of the FEA model in the postfire phase, a set of fire tests on SRC columns, including a fire-resistance test and postfire test subjected to the entire loading and fire phase, were conducted. Compressive failure and bending failure were observed in the test. Comparison between predicted and test results indicates that the accuracy of the FEA model was acceptable, and then the FEA model was extended to simulate a full-scale SRC column in a real project subjected to the entire loading and fire phase. The temperature distribution, load versus axial deformation relations, and load redistribution of the full-scale SRC column during the entire loading and fire phase were investigated. It was found that the postfire peak load of the calculated column can be 26.7% lower than that at ambient temperature. The load carried by shaped steel rises from 20% to the peak value of 93% during the heating and cooling phase, indicating the significant contribution of shaped steel to resist the external loads during fire exposure. [ABSTRACT FROM AUTHOR]

This paper deals with the effects of geometric imperfections on flexural inelastic buckling resistance of axially loaded columns with equally spaced lateral braces and presents a simplified way of forming the critical geometric imperfection for finite-element analyses, which leads to rational results for buckling resistances. First, laterally braced columns with geometric imperfections formed by the traditional single half sine wave or the eigenbuckling modes were analyzed by FEM to illustrate the effect of different imperfection shapes. Second, geometric imperfections of columns acquired from measurements in laboratory tests were adopted, and the statistical characteristic and probability distribution of initial deflections along the column length were obtained; then, extensive numerical results from finite-element analyses were obtained through random imperfection simulations. Last, the comparison was made between statistical results based on the random imperfections and those based on eigenbuckling modes to propose a critical geometric imperfection shape as a linear superposition of a limited number of scaled eigenbuckling modes, making it possible to obtain a rational buckling resistance result by one-time analysis, which is safe but not too conservative. The commonly used lowest eigenbuckling mode or equivalent sine shape for initial geometric imperfections do not provide a rational prediction for the inelastic buckling resistance of laterally braced columns. The critical geometric imperfection shape presented in this paper can be used in advanced analysis for inelastic buckling resistance of braced columns. [ABSTRACT FROM AUTHOR]

This paper presents the behavior of axially loaded steel columns subjected to thermal gradients through their cross sections. Experimental tests were conducted on full-scale wide-flange steel columns with W8 × 35 and W14 × 53 sections made from standard grade 50 steel. The experimental investigations confirmed that variations in fire protection thickness had significant influence on the thermal gradient developing through the steel cross section. Thermal gradient along the flanges caused bowing of column specimens toward the hotter side. Thermal bowing introduced second-order moments and adversely affected stability, leading to failure of column specimens by inelastic flexural column buckling. The experimental behavior and results were compared with those obtained from detailed nonlinear finite-element analyses of the tested specimens. These nonlinear finite-element models utilized standard (temperature-dependent) material properties, and reasonably predicted the axial load-temperature-deformation behavior and failure temperature of column specimens subjected to nonuniform heating. [ABSTRACT FROM AUTHOR]

Thin-walled cold-formed steel (CFS) clip angles are commonly used to connect CFS framing members or attach CFS members to the main building structure. The design provisions for CFS clip angle connectors have not been included in the AISI design specifications and standards. This paper presents a research project aimed at developing a design method for the thin-walled cold-formed steel load bearing clip angles subjected to shear forces. An extensive test program was conducted to investigate the behavior and shear capacity of clip angles with various dimensions and thicknesses. It was found that the local buckling and the distortional buckling could dominate the failure mechanism dependent on the plate slenderness ratio. Based on the test results, two design methods were developed for determining the nominal shear strength of load bearing clip angles with and without consideration of the clip deformation, respectively. The ASD safety factors and the LRFD resistance factor were also produced to support the proposed design provisions. [ABSTRACT FROM AUTHOR]

The roofing systems of low-rise buildings are commonly made of thin, high-strength steel roof sheeting and battens. These light-gauge steel roofing systems are subjected to high-wind uplift loads during tropical cyclones and thunderstorms. In recent times, roof batten-to-rafter or truss connection failures have been identified as a major cause of severe roof failures during high-wind events. Most of these connection failures are in the form of localized pull-through failures of the bottom flanges of steel roof battens at their screw connections. The design practice at present is based on expensive and time-consuming full-scale tests or inappropriate small-scale tests. Hence, there is an urgent need to develop suitable test methods to determine the pull-through capacity of roof battens. This paper presents the details of an experimental study undertaken to investigate pull-through failures of light-gauge steel roof battens using both full-scale and small-scale tests. The investigation showed that the design capacity equations and test methods given in current cold-formed steel design standards for single-screw connections cannot be used for roof batten pull-through failures. This paper recommends suitable small-scale test methods and associated calculation methods to determine the pull-through capacities of roof battens. [ABSTRACT FROM AUTHOR]

This paper discusses the impact of hinged connectors (a common connection in folding structures) on the behavior of sandwich panels (with fiber-reinforced polymer faces and foam core). A sandwich panel is subjected to uniform loading and tested when restrained by hinged connectors in compression and in tension. The measured results are compared to finite-element numerical models, focusing on global behavior (displacements and strains at center) and local behavior (strains near connectors). Parametric studies using these validated numerical models investigate the impact of the number, size, and relative placement of hinged connectors. These studies culminate in guidelines for the design of structures comprised of hinged, folding panels. Ultimately, this paper addresses a research gap in understanding the behavior of sandwich panels connected by hinges. [ABSTRACT FROM AUTHOR]

The direct analysis method is the primary means of assessing system stability within a standard specification. This method, and in particular its use of reduced stiffness, has been thoroughly validated for use in frames consisting of structural steel members. However, appropriate stiffness reductions have not yet been established nor has the method as a whole been validated for frames with steel-concrete composite columns. Through comparisons between second-order inelastic analysis results and results from the design methodology on a parametric suite of small frames, the current design provisions are evaluated in this paper. The results indicate that while the current design provisions are safe and accurate for the majority of common cases, there exist cases in which the current provisions result in high levels of unconservative error. Modifications to the current design provisions are proposed to address these issues. [ABSTRACT FROM AUTHOR]

An experimental investigation of simply supported built-up section beams with different sectional configurations has been conducted under both four-point bending and three-point bending. Intermediate stiffeners were employed to the webs of built-up sections to improve the buckling strength that is susceptible to local buckling and/or distortional buckling. In this study, the built-up sections were assembled by self-tapping screws from two channels with web stiffeners, which were brake-pressed from high-strength, zinc-coated grades G500 and G550 structural steel sheets with nominal 0.2% proof stresses of 500 and 550 MPa, respectively. The screws were located either at the flanges or webs, and the built-up sections include both open sections and closed sections. The moment capacities and observed failure modes of the beam tests are presented, and no failure was found in the screws. The local and distortional buckling behavior of the built-up section beam specimens were found to be different from the single profiles. Finite-element (FE) models were developed, and numerical analysis that includes material nonlinearity, geometric nonlinearity, and contact nonlinearity was performed. The FE and the experimental results were found in good agreement in terms of ultimate moments and failure modes. The verified FE models are used to perform parametric studies in the companion paper. The structural behavior and design rule for built-up section beams with intermediate stiffeners is then explored and presented. [ABSTRACT FROM AUTHOR]

This paper investigates the global in-plane failure and local web shear failure mechanism and strength of steel I-section circular arches with a sinusoidal corrugated web. In reference to a flat web that can resist both the shear and axial forces, the sinusoidal corrugated web can resist the shear force only. As a result, the sinusoidal corrugated web may fail in an elastic-plastic shear buckling mode. This study considers pin-ended circular steel arches with a sinusoidal corrugated web under a uniform radial load or a uniform vertical load to elucidate numerically their different failure modes. It is found that local web failure occurs suddenly without warning, and all aspects pertaining to the local web shear failure are investigated thoroughly with an equation for the ultimate shear-carrying capacity of nonuniformly sinusoidal corrugated webs being proposed. It is also found that the effects of the shear deformations of corrugated web on global in-plane buckling and the strength of steel arches are significant. A strength design equation for arches under nominal uniform axial compression and an interaction equation for arches under a uniform vertical load are developed. Strength design procedures for steel arches with a sinusoidal corrugated web against global failure and web shear failure are proposed. All of the equations proposed for global in-plane buckling, local web shear buckling, global in-plane strength, and web shear strength agree with finite-element results. [ABSTRACT FROM AUTHOR]

This paper presents the flexural-torsional buckling resistance and design of steel circular arches subjected to uniform compression with elastic end bending restraints by using finite element (FE) numerical analyses. Firstly, effects of geometric and mechanical parameters such as initial imperfections, section types, material properties, slenderness, rise-to-span ratios, and end restraints on flexural-torsional buckling resistances of arches are investigated and are found to be eliminated to a large extent by introducing the normalized slenderness. Then, on the basis of extensive numerical results, a design method is proposed to predict the flexural-torsional buckling resistances of circular arches in uniform compression with elastic end restraints by the column curves according to the normalized slenderness and a specific section type, namely curve 'a' for hollow sections, curve 'b' for welded box sections, and curve 'c' for welded I-sections. Next, the flexural stiffness of an arch is studied, taking the destabilizing effect of the axial force into account to calculate the end restraining provided by adjacent arch segments to the adverse segment in a laterally-braced arch in uniform compression to obtain the flexural-torsional buckling resistance using the normalized slenderness and the column curve analytically. The result shows a good agreement with that gained from finite element numerical analyses, and proves it very conservative when the adverse segment is assumed to be hinged without any end restraining. [ABSTRACT FROM AUTHOR]

Steel moment frame (MF) is one of the most prevalent and well researched steel seismic force resisting systems worldwide. However, efficiency and feasibility of using MF diminishes for large spans. In this paper, a recently developed steel seismic force resisting system, named buckling-restrained knee-braced truss moment frame (BRKBTMF), is suggested as a better alternative system. The BRKBTMF uses buckling-restrained knee-braces with open web trusses. In this paper, a four-story prototype office building was designed using both MF and BRKBTMF systems. Performance of the prototype building with different span lengths was systematically studied. The results show that BRKBTMF uses less structural steel, yet it performs better with lower structural damage, repair costs, and probability of collapse as compared with the MF for all span lengths and under all earthquake intensities considered. Hence, it is concluded that BRKBTMF is a viable and effective alternative seismic force resisting system for long-span applications. [ABSTRACT FROM AUTHOR]

A set of seismic analysis and design requirements are presented to improve seismic stability of steel two-tiered eccentrically braced frames (EBFs) with I-shaped link beams. The proposed requirements include strength, stability, and stiffness provisions for braces, intermediate beams, and columns. In particular, these requirements aim to make use of intermediate beams to limit out-of-plane deformation of diagonal braces, torsionally brace the link beam in the intermediate level using diagonal braces, utilize column stiffness and strength to brace the intermediate beam out-of-plane, estimate and account for in-plane bending demands of the columns due to uneven yielding of the links, and control inelastic link rotation. The proposed special analysis and design requirements for two-tiered steel EBFs prevent excessive out-of-plane deformation of the intermediate beam and columns, promote sequential yielding of link beams, and limit inelastic rotation of the link in the tier experiencing the largest lateral deformation. The proposed requirements are demonstrated for a case study two-tiered EBF. Nonlinear static and dynamic analyses are performed to evaluate the seismic response of the case study frame against the frame designed without special design provisions and to validate the proposed requirements. [ABSTRACT FROM AUTHOR]