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This paper presents the behavior of hollow fiber reinforced polymer–concrete–steel (HC-FCS) columns under axial compressive loading. The typical HC-FCS column consists of a concrete shell sandwiched between an outer fiber reinforced polymer (FRP) tube and an inner steel tube. The inner steel and outer FRP tubes provide continuous confinement for the concrete shell; hence, the concrete shell achieves a significantly higher strain, strength, and ductility compared to the unconfined concrete in conventional columns. The HC-FCS column represents a compact engineering system; the steel and FRP tubes act together as stay-in-place formworks. The effect of the fiber orientation and the steel tube diameter-to-thickness ratio ( D i / t s ) on the compressive behavior of HC-FCS columns was investigated. Ten HC-FCS cylinders with different steel tube D i / t s ratios and three concrete-filled fiber tubes (CFFTs) were manufactured and tested under static cyclic axial compressive loading in addition to three empty steel tubes. The behavior of the HC-FCS columns was complicated and related mainly to the stiffness of the FRP and steel tubes, which controlled the direction of the concrete dilation under axial load. HC-FCS columns with FRP tubes made with fibers oriented at ±45° showed low axial compressive strengths and high ultimate strains. HC-FCS columns with wet lay-up FRP tubes that had ±45° and 0° (hybrid FRP) exhibited high axial strengths and strains. The failure of the HC-FCS columns with hybrid FRP tubes consisted of two stages. The first stage was the rupture of the unidirectional FRP tube (outer tube), and the second stage was the reorientation of the oriented FRP tube exhibiting high axial strains. [ABSTRACT FROM AUTHOR]

Concrete-filled steel tube (CFST) member is widely used for building, bridge and foundation structures because of its excellent performance. When a CFST member is subjected to axial loads, the filling concrete is confined by the steel tube, resulting in a tri-axial state of compression that improves its strength, stiffness and ductility. However, the cracking of concrete in tension zone would decrease this enhancement when the CFST member is subjected to flexure, especially when it is used as a major flexural member with large-scale section in bridges. To overcome this weakness, the prestressed CFST concept is investigated in this paper. Eight prestressed CFST beams with large-scale section (300 × 450 mm) were tested under bending. Two concrete strengths (C50 and C60) and two different degrees of prestressing (0.26 and 0.40) were studied in the experimental program. The full vibration and grouting method was introduced to gain a good performance of specimens. The perfobond rib shear connector was adopted to achieve the composite action. The flexural behaviors were verified by comparing with predictions from a proposed model considering the confinement effects. A simplified method is proposed to determine the ultimate moment capacity based on the plastic stress block hypothesis. Both experimental and analytical results show that the prestressed strands could significantly enhance the confinement effect of the core concrete under bending, which, in turn, improves the prestressed CFST beam performance in strength, stiffness and ductility. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on seismic behavior of square fiber reinforced polymer (FRP)–concrete–steel columns. Six hybrid double-skin tubular columns (DSTCs) manufactured using high-strength concrete (HSC) were tested under combined axial compression and reversed-cyclic lateral loading. The main parameters under investigation were the axial load level, size of inner steel tube, provision (or absence) of a concrete-filling inside the steel tube, and the column aspect ratio. The results indicate that, in general, square DSTCs exhibit very ductile behavior under combined axial compression and reversed-cyclic lateral loading. However, the important influence of the axial load level on the column behavior is evident, with an increase in the load level leading to a significant decrease in the lateral deformation capacity of DSTCs. The results also indicate that a DSTC with a larger inner steel tube exhibits lower lateral displacement capacity than that of a companion DSTC with a smaller inner steel tube. It is shown, however, that provision of a concrete-filling inside the inner steel tube leads to a significant increase in the lateral deformation capacity of a DSTC with a larger inner steel tube to a level that is higher than that seen in a companion DSTC with a smaller hollow inner steel tube. Experimental results are presented together with accompanying discussions on the influence of the investigated parameters on the seismic behavior of square DSTCs. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study that investigated the application of a corner strengthening technique to square and rectangular concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs). Effects of the strengthening technique were investigated through axial compression tests undertaken on a series of CFFT specimens having different corner radii and sectional aspect ratios. The results indicate that the compressive behavior of square and rectangular CFFTs can be improved through corner strengthening, which significantly reduces or completely eliminates temporary post-peak strength softening observed in some CFFTs. The results also indicate that the premature FRP rupture at corners due to stress concentrations can be prevented through sufficient strengthening of the specimen corners. It is found that increasing the amount of corner strengthening leads to an increase in the confinement effectiveness of CFFTs and results in a more uniform confining pressure distribution along specimen cross-section. It is also found that corner strengthening provides a more significant improvement in the compressive behavior of CFFTs with lower original confinement effectiveness caused by a smaller corner radius or a larger aspect ratio. [ABSTRACT FROM AUTHOR]

A significant number of macroscopic constitutive models is available to simulate the behaviour of unreinforced masonry (URM) in compression, but they produce different predictions of sectional response to eccentric compressive loading. In this paper, nonlinear moment–curvature analysis of rectangular URM cross sections is presented to explore the influence of alternative constitutive models and ultimate limit state assumptions in terms of post-peak strength degradation and strain ductility. Theoretical moment–curvature diagrams are quantitatively compared to experimental data at multiple levels of load eccentricity through goodness-of-fit measures. Different constitutive models are found to produce a good simulation of experimental moment–curvature behaviour, depending on whether experimental data are disaggregated in terms of masonry type or not. Moment–curvature diagrams are derived by keeping constant either the magnitude or the eccentricity of axial load. Finally, the effects of tensile properties and model error of theoretical moment–curvature diagrams are assessed. [ABSTRACT FROM AUTHOR]

The tubed-reinforced-concrete (TRC) column is a kind of confined reinforced-concrete (RC) columns using the outer encasing thin-walled steel tube which discontinues at the beam-column joints and thus does not carry any direct axial load. Although this composite column has been used in some practical applications in China, there is still a lack of sufficient experimental data and design methods for the TRC columns subjected to eccentric compression. In this paper, 18 short circular TRC columns are tested under axial and eccentric compression considering parameters of eccentricity and diameter-to-thickness ratio of the steel tube. Failure mode, axial load-carrying capacity, and stress in the steel tube of the specimens are discussed in detail. The test results indicate that the average confining stress in the columns with small eccentricity is close to that in the columns under axial load at the peak load. To validate the employed stress–strain relationship for the confined concrete, a fiber-based model is developed and the predicted results agree well with the test results. Based on the strain energy equivalence principle, a regression formula to estimate the ultimate compressive strain of concrete confined by the steel tube is proposed. The equivalent concrete stress block parameters of circular TRC members are theoretically studied and the modified values are purposely suggested for the calculation of capacity interaction diagram. [ABSTRACT FROM AUTHOR]

• Three welding schemes are examined for strengthening circular steel tubes. • The effects of service load, welding scheme and gaps are revealed. • Welding-induced deformation and stress re-distribution are evaluated. • Thermal-mechanical FE models prove effective to reproduce the tests. This paper investigates the strengthening by welding method for axial-loaded circular steel tubes under service load, with a particular focus on the effects of service load and welding scheme. One un-strengthened member and four strengthened members were tested; the varying parameters included three welding schemes and two service load levels. Complementary coupled thermal–mechanical finite-element (FE) models and pure mechanical FE models accounting for no welding process were developed; only the former can faithfully reproduce the full-range behavior of the strengthened member. From the tests and FE parametric studies, the major conclusions included: (1) Notable axial shortening and deflection were observed during strengthening, reflected by obvious plateaus on the deformation curves; (2) Compared to continuous welding which led to the largest axial shortening and lateral fluctuation, intermittent welding and hybrid welding effectively reduced the welding-induced deformation and therefore are better choices; (3) Welding generated a field of intense tensile stress in and around the welds, and correspondingly, between the welds existed the compression zone; the compressive stress in the original member was larger than that in the strengthening member, and could also reach yield strength under high service load; (4) Welding-induced deformation increased with the service load level, with the dependence of the axial shortening being largely linear while that of the deflection being nonlinear; the ultimate resistance decreased with the increasing service load level; (5) The asymmetric gap pattern has a noteworthy effect on the performance of the strengthened member, primarily resulting from the asymmetric welding heat input. [ABSTRACT FROM AUTHOR]

• Rocking and racking mechanisms of traditional heavy timber frames are pointed out. • A semi-mechanism-based hysteretic model is proposed for traditional heavy timber frames with forked column foot joints and various timber panel infill. • Influence of infilled wood panels and vertical compression load is investigated based on the proposed model. Frames are essential lateral load-resisting elements in traditional Chinese timber structures. This paper presents a semi-mechanism-based hysteretic model for the lateral load-displacement relationship of traditional heavy timber frames with forked column foot joints and various wood panel infill. The backbone curve of the proposed model was determined based on the rocking and the racking mechanisms of the frames. Key points of the backbone curve, including the elastic limit point and the peak load point, and the corresponding tangent stiffness were expressed by the geometrical and physical parameters of the frames. Reloading and unloading paths were defined based on the elastic limit load and the elastic stiffness. Experimental results of typical frames were utilized to verify the proposed model regarding the backbone curve, hysteretic behavior and energy dissipation. Good agreement was achieved. Model-based parametric study was further conducted. The influence of infilled wood panels and vertical compression load was investigated. It is found that the lateral resistance contribution of the wood panels with a door opening is slightly greater than that of wood panels with a window opening, and the performance of frames under higher vertical compression load levels (without causing compression failure of the columns, which is rare) are superior to those under lower levels, e.g., a reduction of 50% in the vertical load can lead to a drop in the peak load and the cumulative dissipated energy of the frame of 13% and 30%, respectively. [ABSTRACT FROM AUTHOR]

• The interfaces among contacts between leaves of multi-leaf masonry walls play a main role on global behaviour of structure. • Interface characterization and failure scenario evolution enable to understand load-transfer mechanisms. • FE models were developed and calibrated by model updating procedure to assess the interface interaction between leaves. • Non-linear static analysis was compared with experimental performances of multi-leaf masonry panels. • This study enables to analyse the sensitivity of load-transfer mechanisms to core stiffness and interface effectiveness. Masonry heterogeneity and manufacturing imperfections affect structural behaviour of constructions; these unpredictable aspects are amplified in multi-leaf masonry walls by interfaces which play a main role on load transfer mechanisms and on global behaviour. In this paper, the performances of multi-leaf masonry walls subjected to compressive loads are investigated using experimental and numerical approaches. 9 experimental tests were performed on three-leaf specimens constituted by external leaves made of a regular pattern of brick elements and mortar joints, and a disaggregate internal core made of brick potsherds and mortar. FE models were developed to assess the interface interaction among layers through model updating procedure based on dynamic parameters experimentally identified at incremental load conditions, keeping fixed the mechanical parameters of external layers. FE models were updated by 13 parameters: i) unidirectional stiffnesses of 12 springs located between external layers and core; ii) elastic modulus of core. Updated models were tested through non-linear static analysis and results were compared with structural performances of multi-leaf masonry panels. Interface effectiveness regulated by different stiffness hierarchies among layers and between bricks and mortar joints handles the load transfer mechanisms and failure scenario. Compressive load is transferred through out-of-plane mechanisms triggered by maximum tensile stresses by 20% of failure load. In the plane the load is transferred with maximum compressive stresses at the ends (30% of failure load) decreasing centrally. Tangential stresses among layers are maximum near to load application and are linearly distributed in the plane of panels. [ABSTRACT FROM AUTHOR]

• Two novel precast steel reinforced concrete (SRC) composite columns are presented. • Cyclic test and analysis on ten short column specimens were conducted. • A novel analytical model is proposed to calculate the shear capacity of SRC columns. Two innovative precast steel reinforced concrete columns are presented in this paper. The partially precast steel reinforced concrete (PPSRC) column was composed of a precast outer part and a cast-in-place inner part, and the hollow precast steel reinforced concrete (HPSRC) column kept the column core hollow to further reduce the column deadweight. In this paper, six PPSRC column specimens and four HPSRC column specimens with low aspect ratios were subjected to reversal cyclic load and constant axial compression to explore the cyclic behavior, which was evaluated by the failure mode, hysteresis characteristic, stiffness degradation, energy dissipation capacity and displacement ductility. Meanwhile, the effects of section shape, axial compression, ratios of longitudinal and transverse reinforcement and concrete strength of the inner part were critically investigated. The test results indicated that the column specimens suffered flexural-shear failure mode and shear-critical failure mode according to different ratios of longitudinal reinforcement, and, in general, the PPSRC columns exhibited more satisfactory cyclic behavior than that of the HPSRC columns. When the other parameters were the same, a higher stirrup ratio and lower axial compression led to a more satisfactory energy dissipation capacity, stiffness degradation and higher displacement ductility, and the load-bearing capacity increased with increasing inner concrete strength, stirrup ratio and axial compression. Based on the test results, an analytical model was established to calculate the shear capacity of test specimens, and the proposed model was verified to be valid using a test database consisting of 66 shear-critical SRC column specimens. [ABSTRACT FROM AUTHOR]

Unbonded fiber-reinforced elastomeric isolators, due to their light weight, low cost and easy installation, are viable devices for seismic mitigation purposes, both for housing and building applications in highly seismic areas, even in the developing world. Important aspects of these isolators are that they do not have thick base plates, they are not bonded to the top and bottom structure, and their reinforcements are flexible. These features allow fiber-reinforced isolators to experience roll-over deformation under lateral loads. Roll-over deformation is stable, if the stabilizing moment due to vertical forces is higher than the overturning moment due to horizontal forces; however, if this condition is not verified, the isolator experiences a “roll-out instability.” This paper proposes a model for prediction of roll-out instability based on the applied forces equilibrium condition and on the assumption that a triangular distribution for the compressive stresses acts on the isolator. The model is developed considering that the isolator contact area varies with the applied displacement, and consequently the compressive stresses vary. An expression for the isolator instantaneous stiffness is proposed. Results are presented for roll-out tests performed on fiber-reinforced isolators that have varied aspect ratios and are subjected to different compression levels. A comparison is made between the measured displacement at roll-out and the results obtained from the aforementioned model. The comparison proves that the model accurately and reliably predicts roll-out displacement of elastomeric isolators subjected to horizontal and vertical forces. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study on the buckling behaviour of gusset plate connections under compression. The main focus is given to the post-buckling resistance of relatively slender gusset plate connections, cases which are commonly found in thin gusset plates in light-weighted structures, and also potentially, as driven by the recent development in steel manufacture industry, in high-strength steel (HSS) gusset plates. The numerical modelling strategy was carefully validated using the results of eight full scale tests previously conducted by the authors and co-workers, and an extensive parametric study was subsequently performed covering a reasonably wide range of geometric configurations and material properties of practical gusset plate connections. Stable post-buckling equilibrium paths were observed for the models with relatively thin gusset plates, and the use of HSS further promotes remarkable post-buckling resistance. The influences of initial imperfection and material strain hardening on the buckling behaviour of gusset plate connections have also been discussed in detail. Based on the results from the numerical study, two design approaches were proposed, namely, Column Buckling Approach, which is based on a modified column analogy from the current design practice, and Plate Buckling Approach, which is based on a plate analogy and modified Winter formulae. While both methods can be used by practicing engineers, the latter may take account of the plate action of gusset plates in a more reasonable manner, and it is also more consistent with the common design rules for plated structures. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on empty and concrete-filled circular hollow section (CHS) X-joints with curved chord under axial compression. A total of 16 specimens was fabricated by hot-rolled bending the chord members into curvature with three different radii to apply axial compression force to the brace members, in which 6 specimens were tested with empty curved chord, 6 specimens were tested with concrete filled in the curved chord only, and 4 traditional empty and concrete-filled CHS X-joints with straight chord were also tested for comparison. The effects of curvature radius of chord member and concrete filled in the chord member on the joint strength and behavior were evaluated. It is shown from the comparison that the ultimate strengths of the CHS X-joints with curved chord are generally larger than those of the traditional CHS X-joints with straight chord. In addition, the ultimate strengths of the concrete-filled CHS X-joints are larger than those of the empty CHS X-joints, and the enhancement of the ultimate strengths of the traditional CHS X-joints with straight chord is much more pronounced than that of the CHS X-joints with curved chord. [ABSTRACT FROM AUTHOR]

Hybrid FRP–concrete–steel double-skin tubular columns (DSTCs) are a new form of hybrid columns, consisting of an outer tube made of fiber reinforced polymer (FRP) and an inner tube made of steel, with the space between filled with concrete. In hybrid DSTCs, the three constituent materials are optimally combined to achieve several advantages not available with existing columns, including their excellent corrosion resistance and seismic performance. Although two existing studies have examined the seismic performance of circular hybrid DSTCs, they have been limited to normal strength concrete, small column dimensions, and small void ratios (ratios between the inner diameter and the outer diameter of the annular concrete section). Against this background, this paper presents the first experimental study on hybrid DSTCs filled with HSC subjected to axial compression in combination with cyclic lateral loading. A relatively large column section (with a diameter of 300 mm) was chosen to allow reliable experimental modeling of real columns. The test results indicate that hybrid DSTCs possess excellent ductility and seismic resistance even when high strength concrete with a cylinder compressive strength of around 120 MPa is used; column damage is concentrated in a small plastic hinge region near the column bottom end; and the performance of hybrid DSTCs can be enhanced by filling the inner void in the plastic hinge region. The test results provide valuable data needed for the formulation and verification of a theoretical model for the hysteric behavior of hybrid DSTCs, particularly when they are filled with HSC. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to present a proposal for the design of steel structures sensitive to buckling due to compression in order to fill in the gaps in the current Standard EN 1993-1-1 guidelines for obtaining the magnitude of the geometric equivalent imperfection. The proposal generalizes the approach provided in Clause 5.3.2(11) of EN 1993-1-1 (EC-3) for cases in which a torsional or flexural–torsional buckling mode may occur. The extension of the procedure also allows designers to obtain the magnitude and shape of the imperfection as well as the worst direction of the imperfection due to the external loads applied. It also identifies the cases in which it is necessary to consider the shape of the imperfection given by higher buckling modes. [ABSTRACT FROM AUTHOR]

Thin-walled columns with section are used widely as columns in aluminum alloy framed structures, offering high strength-to-weight ratios and convenience in connection with maintaining walls. In this paper, thin-walled aluminum alloy columns with section were studied experimentally and numerically to investigate the buckling behavior and to assess the accuracy of current design methods. A finite element model (FEM) was developed and used to perform parametric studies after being verified by tests. Effects of plate thickness on elastic buckling stress was studied using finite strip method (FSM) and to find the potential buckling failure mode at a given length. Tested ultimate strengths were compared with those predicted by the current American, European and Chinese specifications on aluminum alloy structures and the Direct Strength Method (DSM) on thin-walled structures. Following reliability analysis, the design strength predicted by current design specifications were found to be generally conservative, whereas DSM offered more accurate results. [ABSTRACT FROM AUTHOR]

This paper presents the results of cyclic loading tests on steel reinforced recycled concrete (SRRC) columns, including nine short columns and one long column. The main objective of this research is to evaluate the seismic behaviour of short columns based on the seismic tests of 10 SRRC columns under low cyclic loads with vertical axial force. The main design parameters of the columns in the tests are the recycled coarse aggregate (RCA) replacement percentage, axial compression ratio, stirrups ratio and shear span ratio. The crack status, failure modes, hysteresis loops, skeleton curves, energy dissipation capacity and ductility of SRRC columns are presented and analysed. The influence of the design parameters on the seismic behaviour of the columns is also investigated in detail. The test results show that the SRRC short column has poor ductility, as demonstrated by its brittle shear failure, and that the long column has excellent ductility, as demonstrated by its ductile flexural failure. The ductility and energy dissipation capacity of the short columns decreases as the increasing magnitude of RCA replacement percentage whilst the seismic behaviour of SRRC short columns is effectively improved by the appropriate design of the axial compression and stirrups ratio. Although the bearing capacity and stiffness of SRRC columns decreases dramatically as the shear span ratio increases, the ductility increases considerably. Based on the test and analysis results, a modified ACI design method is proposed to calculate the nominal shear strength of SRRC short columns. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study, which investigated the compressive behavior of square fiber reinforced polymer (FRP)–concrete composite columns through the tests of 40 column specimens. 24 FRP–concrete–steel double-skin tubular columns (DSTCs), four concrete-filled FRP tubes (CFFTs), and 12 CFFTs with inner voids (H-CFFTs) were tested under axial compression. The majority of the specimens were manufactured using high-strength concrete (HSC). The key parameters examined included the influence of the strength of the concrete, cross-sectional shape (i.e. circular and square) and dimension of the inner steel tube, and presence (or absence) of a concrete filling inside the steel tube. The results of the DSTCs with circular inner steel tubes indicate that concrete-filling inner tubes results in an increase in the ultimate axial stress but a decrease in the ultimate axial strain of concrete compared to those seen in DSTCs with hollow inner steel tubes. It is observed that concrete in hollow DSTCs manufactured with square inner steel tubes develop significantly lower ultimate axial stresses and strains than concrete in companion hollow DSTCs with circular inner steel tubes. It is found, however, that the performance of these specimens improves dramatically when the square inner steel tube is filled with concrete. Comparisons of the results indicate that concrete in filled DSTCs develops larger ultimate axial stresses and strains than concrete in companion CFFTs. Finally, the results demonstrate that H-CFFTs perform significantly worse than DSTCs and CFFTs, and their performance further degrade as the diameter of the inner void increases. [ABSTRACT FROM AUTHOR]

The paper focuses on the numerical prediction of the compressive response of masonry by means of detailed micro-modeling techniques. In such a model, the material constituents (mortar and units) and the unit–mortar interfaces are separately described by means of specific constitutive equations. The available modeling choices are evaluated through a literature review of related case studies. A systematic approach is proposed and is corroborated by the numerical simulation of a large number of experimental cases from various sources. A total of fifty experimental results are simulated resulting in an overall good prediction of the compressive strength and elastic modulus of the masonry composite, as well as a realistic depiction of the failure mode. This study focuses on the numerical prediction of the compressive strength of masonry, extending to issues pertaining to global stiffness, failure mode, hardening and softening behavior of masonry and their numerical simulation. The masonry typology considered consists of solid units, primarily brick, and mortar of mostly lower strength and higher deformability. [ABSTRACT FROM AUTHOR]

This paper presents an experimental investigation on the structural performance of Dou-Gong brackets of Yingxian Wood Pagoda under vertical load. Two scaled models of typical Dou-Gong brackets of the pagoda were designed, manufactured and tested. Both models behaved similarly to that of a wood component under compression perpendicular to grain. Four failure modes of the components were identified, these included yield of component under compression perpendicular to grain, fracture of component under tension perpendicular to grain, fracture of component due to shear and breaking-off of component under bending. The structural performance of the model Dou-Gong brackets was revealed from test observations, and the initial stiffness and the load-carrying capacity of the prototype Dou-Gong brackets of the pagoda were derived based on the similitude theory. This study provides reference, from a structural point of view, for the preservation of cultural heritage structures such as Yingxian Wood Pagoda. [ABSTRACT FROM AUTHOR]

• Full-scale test on slender CFTST beam-columns under eccentric load. • Equivalent confinement factor to overall quantify the confinement effect in CFTSTs. • Fiber element model for CFTSTs. • Effects of cross-section dimensions, material properties, slenderness and loading angle on CFTST behavior. • Theoretical model predicting the interaction curves of CFTST beam-columns. This paper presents an experimental and theoretical study on concrete-filled T-shape steel tubular (CFTST) beam-columns. Four full-scale slender beam-columns with slenderness ratio of 33.7–39.1 were tested under uniaxial eccentric compressive loads. In addition, three stub columns with the same cross-sections were axially compressed to investigate the confinement effect. Failure mode, axial displacement, lateral deflection, axial strain and hoop strain were obtained to understand the structural behavior of CFTST beam-columns. For beam-columns bended about the symmetric axis, torsion was not observed but a slight rotation of the neutral axis that was no longer parallel to the centroid axis was found by analyzing the experimental results. A fiber element model for CFTST beam-columns was developed and verified for parametrical analysis. The effects of key parameters, such as material properties, cross-section dimensions, loading angle and slenderness ratio, on the behavior of stub and slender beam-columns were investigated. Finally, theoretical models were proposed to estimate the interaction curve and the load-carrying capacity of CFTST beam-columns. The prediction matches well with the experimental results. [ABSTRACT FROM AUTHOR]

Diagonal bracing is a common method used in South Africa for the bracing of timber trussed roofs with spans of less than 9m. In the typical diagonally braced roof system, the brace is placed on the underside of the compression chord, which is as far as it can be placed away from the battens that brace the trusses not directly connected to the diagonal brace. Some of the stiffness of the bracing is lost because the low torsional rigidity of the top chord as well as the batten to top chord nailing is all part of the structural system. In this paper the authors investigate bracing of the entire roof system and contend that it is better to connect the bracing member closer to the battens as this improves the stiffness and bracing ability of the system. The authors study both the diagonal brace as well as the alternative bracing method that uses criss-cross metal strapping, sometimes called speed brace. The speed brace is nailed to the top of the compression chord and is therefore much closer to the battens. Two different truss spans were studied to ascertain the buckling length of the top chord for both the diagonal bracing as well as speed bracing. By moving the bracing system closer to the battens the authors show that the speed brace gives a stiffer system which thereby reduces the buckling length of the top compression chord. [ABSTRACT FROM AUTHOR]

• Welded H-section stainless steel members were tested under combined constant compression and uniaxial cyclic bending. • The hysteretic behavior of welded H-section stainless steel beam-columns was investigated. • The cyclic hardening and effects of loading history were fully discussed. • The applicability of current design codes was evaluated. Previous studies have shown that structural stainless steel material has significant cyclic hardening characterized by strain amplitude dependence and unsaturation under cyclic loadings, which could have a remarkable impact on the seismic performance of stainless steel structures. Whether the current stainless steel structure design codes based on their static performance can accurately predict the seismic capacities of stainless steel structural members is worth studying. This paper investigates the hysteretic behavior of welded H-section stainless steel members under combined constant compression and uniaxial cyclic bending. A total of 24 specimens were tested, which covered a variety of test variables including stainless steel grade, section slenderness, axial compression ratio and loading history. Residual stresses of the welded H-sections considered in this study were measured. Then, the failure mode, hysteretic response, ductility coefficient and energy dissipation of welded H-section stainless steel beam-columns were fully obtained from the tests. Cyclic hardening was observed at the structural member level in the tests. The effect of loading history on seismic behavior was thoroughly discussed. It was found that various loading histories can obviously influence the strengths and deformation capacities of welded H-section stainless steel beam-columns resulting in their discrete seismic behavior, mainly due to the unsaturated cyclic hardening feature of stainless steels. Moreover, the current design codes including European Code, American Specification, Australia/New Zealand Standard and Chinese Specification underestimate the seismic capacities of welded H-section stainless steel beam-columns. [ABSTRACT FROM AUTHOR]

• Existing dissipators of tension-only braces are prohibited in China. • A new passive mechanical device, namely asymmetrical friction damper (AFD), was designed, fabricated and tested. • An asymmetrical constitutive model is proposed. Equal energy dissipation capacity principle is chosen to modify parameters. • AFD co-works well with slender braces under different levels of earthquake. In this paper, a passive asymmetrical friction damper (AFD), which significantly improves the hysteretic behavior of tension-only braces (TOBs) is introduced. The AFD carries force and dissipates energy under tensile loading and carries nearly zero force under compressive loading without causing TOB slackness. As a result, the AFD prevents the TOBs from buckling and from the impact seen in systems with significant slackness. In this study, twelve tests were conducted. Based on the results, an asymmetrical constitutive model was proposed to describe the mechanical properties of the AFD. In addition, to optimize model parameters, an equal energy method was proposed and verified. Furthermore, nonlinear time history analysis was operated on a six-story benchmark model with comparisons between the AFDs and fluid viscous dissipators (FVDs) systems. With the maximum reduction of 58.1% on the 5th floor and an average 23.8% reduction over six floors under the rare earthquake, the displacement-dependent as well as velocity-dependent AFD proved itself better performance in energy dissipation and making the building deform evenly to redistribute the internal force and avoid local failure. [ABSTRACT FROM AUTHOR]

• Shear behavior of pre-damaged RC beams strengthened with the TRM was investigated. • The effects of the damage degree and the sustaining load level were analyzed. • TRM strengthening could completely restore the mechanical properties of pre-damaged RC beams and increase their shear capacity. • A revised model was proposed to predict the shear capacity of TRM-strengthened beams. An experimental investigation of pre-damaged reinforcement concrete (RC) beams strengthened in shear with textile-reinforced mortar (TRM) is reported in this paper. Eight RC beams, including a control beam and seven beams strengthened with U-shaped TRM jacket, were constructed and tested under four-point bending monotonic load. The investigated parameters included (a) the reinforcement ratios of carbon textiles, (b) the pre-damaged degrees of the RC beam before strengthening, and (c) the levels of the sustaining load on RC beams during the strengthening process. The test results indicated that (1) TRM strengthening could completely restore the mechanical properties of the pre-damaged RC beams, and the shear capacity of the beams was increased by 18.32–67.45% after strengthening. (2) The shear capacity of the pre-damaged strengthened beams was smaller than that of the non-damaged strengthened beams with identical strengthening scheme. 3 As the sustaining load on the beams to be strengthened increased, the failure of the specimens changed from shear-compression mode to diagonal-tension mode, and the shear capacity of the strengthened beams decreased. Finally, a modified model was proposed to predict the shear capacity of TRM-strengthened beams based on the current code. This model considered the effects of the pre-damaged degree and the sustaining load level. [ABSTRACT FROM AUTHOR]

• The failure mechanism of high-strength welded hollow spherical joints is clarified. • The basic mechanical properties of Q460 steel are obtained through tests. • SWDM fracture model is developed to investigate the bearing capacity of the WHSJ. • Four WHSJ full-scale specimens made of Q460 high-strength steel are tested. • New design formulae for the bearing capacity of high-strength WHSJ are proposed. High-strength large-span reticular shells have proliferated in recent years in engineering projects. Thus, appropriate rules should be developed to ensure the safe and efficient design of high-strength welded hollow spherical joints (WHSJ). However, to date, even the compressive failure mechanism of normal-strength WHSJ has not been fully recognized yet. In this paper, the failure mechanism and load bearing capacity of high-strength WHSJ are experimentally studied with four Q460 steel full-scale specimens under uniaxial loads. Also, three-dimensional refined finite element (FE) models are built, and their numerical results are shown to be in good agreement with experimental data. In addition, a micromechanical ductile fracture model, which can account for the effects of both stress triaxiality and Lode angle, is incorporated in FE models to improve the accuracy of joint failure prediction. Furthermore, focusing on the rib stiffener, the effects of two parameters, including the diameter-to-thickness ratio of the spherical joint and the ratio of spherical joint and chord diameters, on the performance of high-strength WHSJ are comprehensively studied. It was found that the failure of WHSJs subjected to uniaxial tension load is the punching shear rupture due to fully developed plastic strain, while the collapse of the WHSJs subjected to uniaxial compression load is caused by elasto-plastic buckling. Finally, new design formulae for the bearing capacity of high-strength WHSJ are proposed and validated using parametric analysis. It will be shown that these formulae help eliminate the potential risk in current specification provisions. [ABSTRACT FROM AUTHOR]

• A new type of DSC slab with stiffening ribs and tie plates (SDSC slab) is proposed. • The buckling stress, bearing capacity and ductility of the SDSC slabs are enhanced. • Formulas for the bending capacity and service behavior of the SDSC slab are proposed. • A modified theoretical model is established for predicting the buckling stress. • The design process of the steel plate in SDSC slabs is illustrated in detail by two examples. Double steel plate concrete (DSC) composite slabs consist of two outside steel plates and a concrete core that are connected by welded studs or cross tie bars. If weak restraints are designed for the interface between the steel plates and concrete core, the surface steel plate of the DSC slabs may suffer local buckling when subjected to a compressive load. In this paper, a new type of DSC slab with stiffening ribs and tie plates (SDSC slab) is proposed to improve the stability of surface steel plates. Two groups of bending tests are carried out to investigate the buckling behavior of both SDSC slabs and ordinary DSC slabs (ODSC slabs). Furthermore, the influence of the B/t ratio, that is, the ratio between the stud spacing and the steel plate thickness, is intensively analyzed. The experimental results reveal that compared with that of the ODSC slabs, the buckling behavior of steel plates in the SDSC slabs is delayed or even prevented owing to the additional stiffening ribs and tie plates. Thus, the bending capacity and ductility of SDSC slabs are both enhanced. According to the test results, calculation formulas are derived to predict the bending capacity of the ODSC slabs and SDSC slabs, respectively. In addition, design models for buckling analysis are proposed to determine the buckling stress and critical B/t ratio for different types of DSC slabs. The accuracy of the theoretical methods is validated against the test data, and two examples are provided in detail to illustrate the design process of the steel plate in SDSC slabs. [ABSTRACT FROM AUTHOR]

An increased interest in the strengthening and rehabilitation of deficient or deteriorated structures took place in the past few decades to maintain the sustainability and integrity of existing structures. Different strengthening techniques have been recently studied and used to retrofit or strengthen concrete elements. One of these techniques is the use of Shape Memory Alloys (SMA), taking advantage of its shape memory effect property. This property allows the application of an initial confining pressure, called active confinement, when the material is restrained. Such pressure is developed when the material undergoes phase transformations upon heating to a certain temperature. As a result of their unique properties, different types of SMA for different structural engineering applications have been studied. In this paper, an experimental program was conducted to evaluate the behavior of concrete columns confined with iron-based SMA (Fe-SMA) strips under uniaxial compressive loading. A total of 25 circular columns were tested, and the results were analyzed to identify the influences of selected parameters: 1) presence of internal reinforcement, 2) initial confining pressure, and 3) ratio of external SMA confinement. The test results indicated great improvement in the overall behavior of the confined columns, especially their axial deformation capabilities. In addition, an analytical approach was proposed to model the axial load-deformation behavior of the actively confined columns. The proposed expressions showed to adequately capture the axial behavior of the columns, and the obtained results were in good agreement with the experimental test results of columns confined with Fe-SMA strips. [ABSTRACT FROM AUTHOR]

• Investigation of shear-sliding failure of masonry through different test methods. • Shove test – issues in the determination of vertical compressive stress. • Test phases, boundary conditions, dilatancy affect the shove test results. • Assessment of vertical compressive stress contributions for different test phases. • Proposals of improvements of testing procedures and result interpretation. The shove test (ASTM Standard C1531) is an experimental technique aimed at studying the shear-sliding behavior of brick masonry. It can be executed according to various testing methods that differ in the way the vertical compression load is applied and in the way bricks and/or joints are locally removed for inserting jacks. One of the most critical aspects is the correct evaluation of the compressive stress state on the sliding brick. The objective of the present paper is to investigate the capability of the shove test in determining the shear strength parameters of brick masonries and to highlight the main advantages and disadvantages of the various testing methods. To this aim, nonlinear numerical simulations of the shove test were performed by adopting a brick-to-brick modeling strategy. The 2D numerical model was calibrated and validated through comparisons with experimental results of triplet tests and shove tests. The numerical analyses allowed to understand the influence the different testing methods and the masonry mechanical properties, such as dilatancy, may have on the test results. Based on the numerical outcomes, correction factors were calibrated for the proper evaluation of the compressive stress state on the sliding brick. Improvements with regards to the experimental procedures, i.e. additional test phases and measurements, were also proposed to enhance the results interpretation. [ABSTRACT FROM AUTHOR]

Abstract: Recently more attention has been paid to the energy absorption capability of novel structures in the retrofit of impact or blast protection structure. As the most versatile components, thin-walled metal tubes with different cross-sections are used and specifically explored by many researchers. One method for improving crashworthiness performance under quasi-static axial crushing is to vary the cross-sectional shape with only convex polygons. As an alternative, it is also necessary to develop tubes with concave polygon sections. In this paper, four types of geometries are studied experimentally. They are hexagon, octagon, 12-sided and 16-sided star, respectively. Experimental data are then compared with those predicted from FE simulations using ABAQUS. It is shown that the experimental and the corresponding numerical results are in agreement with each other. The increase in the number of inward corners demonstrates a promising improvement in energy absorption, but to a certain extent. It is found that the 12-sided star shape has the best energy absorption capability when D/t ratio is less than 50, where D is notional diameter and t is the thickness. The 16-sided star shape performed poorly compared to the others studied. [Copyright &y& Elsevier]

Abstract: Glass elements are widely used in modern buildings as structural columns, beams, stiffeners. Nevertheless, due to accidental eccentricities, additional loads, boundary conditions or imperfection factors, glass beam–columns are frequently subjected to in-plane compressive actions combined with bending moments. Because of this reason, the paper focuses on the buckling resistance of glass elements subjected to eccentric compressive loads. Based on large experimental numerical results available in literature for monolithic and laminated glass columns or beams in out-of plane bending, verification criteria for these simple loading conditions are firstly recalled from previous contributions. Subsequently, numerical static incremental simulations are performed with a finite-element model able to predict the buckling strength of imperfect glass elements eccentrically compressed. In it, also the effects of various geometrical imperfection shapes are deeply investigated. Finally, an analytical interaction curve is proposed for a conservative and rational buckling verification of monolithic and laminated structural elements. [Copyright &y& Elsevier]

Abstract: Current design practice of concrete-filled steel tube (CFST) columns uses different formulas for different section profiles to predict the axial load bearing capacity. It has always been a challenge and practically important issue for researchers and design engineers who want to find a unified formula that can be used in the design of the columns with various sections, including solid, hollow, circular and polygonal sections. This has been driven by modern design requirements for continuous optimization of structures in terms of not only the use of materials, but also the topology of structural components. This paper extends the authors’ previous work [1] on a unified formulation of the axial load bearing capacity for circular hollow and solid CFST columns to, now, including hollow and solid CFST columns with regular polygonal sections. This is done by taking a circular section as a special case of a polygonal one. Finally, a unified formula is proposed for calculating the axial load bearing capacity of solid and hollow CFST columns with either circular or polygonal sections. In addition, laboratory tests on hollow circular and square CFST long columns are reported. These results are useful addition to the very limited open literature on testing these columns, and are also as a part of the validation process of the proposed analytical formulas. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental research on 32 columns tested with unequal eccentricities at the ends. The purpose of this research is twofold: first, the contribution to the knowledge of these components and second, to calibrate and check simplified approaches. The analyzed variables are: the concrete strength (normal and high strength), the slenderness, the amount of longitudinal reinforcement, the ratio between eccentricities at the ends and the relative eccentricity applied. Ultimate strength and deformed shape of the column have been analyzed and simplified approaches proposed by EC-2 (2004) and by ACI-318 (2008) have been compared with experimental results. In general, the accuracy of the simplified methods proposed in the EC-2 (2004) and the ACI-318 (08), for HSC columns is lower than for NSC columns. Ductility factor (μΔ ) according to (EC-8 (2005)) has been assessed among the experimental tests in order to compare the displacement capacity. In general, ductility increases when the relative eccentricity (e 2/b), the ratio between eccentricities (β), and the amount of longitudinal reinforcement (ρl ) increase, and when the slenderness of the column (λg ) and the concrete strength (fc ) decreases. [Copyright &y& Elsevier]

• A new methodology for unification of confinement model for circular and square concrete columns with full/partial confining system is introduced. • Consideration of non-uniform concrete lateral expansibility at the horizontal and vertical directions improves model predictive performance. • A new methodology for reflecting the confinement path effect in confinement-induced improvements is proposed. • Proposed analytical model demonstrated a good predictive performance. Even though the usage of Fiber-reinforced polymer (FRP) full confinement arrangement is a more reliable and efficient strengthening technique than a partially confining strategy, it might not be cost-effective in real cases of strengthening. Experimental researches have demonstrated that confinement strengthening strategy is more effective for the case of circular columns compared to its application on square columns. This paper is dedicated to introducing a new unified model for determining the concrete confinement characteristics of FRP fully/partially confined circular/square concrete columns subjected to axial compressive loading. Through unification, the variations of the key parameters can be evaluated more-widely based on a unified mathematical framework. Consequently, it leads to the continuity in the predictions of FRP confinement-induced improvements for the different types of columns, contrary to those obtained from models only applicable to a specified cross-section or confining system. The substantial influence of non-homogenous concrete expansion distribution at the horizontal and vertical directions is taken into account in the determination of confinement pressure, besides arching action, by following the concept of confinement efficiency factor. Since the confinement-induced improvement is a function of its confining stress path, a new methodology is proposed to predict global axial stress–strain relation of FRP confined concrete columns considering confinement path effect, based on an extensive set of experimental results including 418 test specimens. The predictive performance of the developed model is assessed by simulating experimental tests reported in the literature. [ABSTRACT FROM AUTHOR]

• A spherical rolling isolator based on rubber balls is experimentally studied. • Both cyclic and shake table tests are performed. • The sphere deforms to an oval shaped body and this significantly alters they response of the system. • The rolling friction coefficient depends on the vertical load, and for the cases considered ranges within 6–18%. • The rubber spheres do not deteriorate even after of tens of tests. • It seems possible to use the bearing for isolating low rise masonry dwellings. This paper presents an experimental study of a low-cost seismic isolator that can be used for the protection of low-rise houses in low-income countries. The isolator is based on rolling of a rubber sphere on flat or spherical concrete surfaces. Using a closely spaced grid of such spheres may allow for avoiding of the diaphragm slab at the isolation level, or of reducing its thickness. Avoiding the cost of this extra slab is crucial for building seismically isolated low-rise dwellings economically feasible in low-income regions of the globe. The effects of the geometry of the rolling surface (i.e., flat or concave), of the diameter of the rolling sphere (i.e., 25, 50 or 100 mm), and of the applied compressive load on the seismic behavior of such isolation bearings were investigated. Initially, the rubber isolators were subjected to monotonic uniaxial compression to examine their behavior under vertical loading. Subsequently, cyclic tests were performed to obtain the lateral force–displacement diagram of the isolation system. Finally, a slab supported by 4 isolators was subjected to a group of 61 ground motions. Eight different configurations were tested, leading to a total of 488 dynamic tests. It was found that the restoring force of such systems can originate not only from the curvature of the concrete surface, but also from the vertical motion induced by the drastic change of the shape of the rubber spheres, as well. In fact, for the configurations tested, the vertical motion sourcing from shape of the deformed sphere was much larger than the vertical motion sourcing from the curvature of the concrete surfaces. Moreover, equations offering the coefficient of friction as a function of the vertical load, the size and the deformation of the sphere were derived. This is crucial for design, as the governing parameter for the design of the rubber spheres is not material failure, but excessive compressive deformation that leads to undesirably high rolling friction. Dynamic tests proved that the proposed low-cost rolling rubber isolators can substantially reduce the accelerations transmitted to the superstructure The cost of the tested 25 mm, 50 mm and 100 mm diameter natural rubber spheres is 0.6 $, 3 $ and 17$, respectively. [ABSTRACT FROM AUTHOR]

• According to the test results, the winding direction and the number of CFRP layers have a great influence on the overall buckling load of the CFRP-STCM, and the longitudinal fiber reinforcement effect of each CFRP layer is about 12.10%, while the circumferential fiber and 45° fiber is less than 5%. • The failure modes of the composite members are also closely related to the fiber direction of the composite member after overall buckling occurs. Among them, fiber breaking occurs in the members reinforced with longitudinal fibers, matrix cracking occurs in the specimens reinforced with circumferential fibers, and torsion occurs in the specimens reinforced with 45° fibers. • The results of parametric finite element analysis show that the thickness of the adhesive layer, the number of CFRP layers, the diameter-thickness ratio of the steel pipe, and the initial defect value all have a great influence on the overall buckling capacity of the composite member. The above factors should be considered in the calculation of the load-bearing capacity. • Based on the modified Perry-Robertson formula, the theoretical calculation method can reasonably predict the overall buckling bearing capacity of the axial compression composite member, which is convenient for engineering application. Compared with the experimental results, the maximum deviation of the proposed theoretical formula is 10.7%, the average value is −3.21%, and the coefficient of variation is −0.015. This paper presents an experimental and numerical study on the mechanical characteristics of an improved CFRP steel tube composite members (CFRP-STCMs) under axial compressive loading. The CFRP-STCM was reinforced with a thick-walled steel tube at both ends to prevent local buckling of CFRP retrofitted equal thickness circular steel tube under axial compressive load. The main focus of the lab test was the effects of fiber winding directions and the test results reveal that the related specimen mainly suffers from overall buckling failure modes. The remaining failure characteristics of the test specimens vary with the change in the fiber winding direction. The longitudinal fiber winding direction has the most obvious reinforcement effect on the load-bearing capacity of the composite member. The ±45° fiber winding direction leads to obvious torsion failure modes after the overall buckling of the test specimen. The numerical modeling was carefully validated using the test results and an extensive parametric analysis was conducted covering a reasonably wide range of geometric configurations. Parameterized numerical analysis reveals that the initial geometric imperfections (initial curvature) and the geometric configurations of the composite member other than the thickness of the adhesive layer have a significant influence on the load-bearing capacity of the composite member. A modified theoretical method for predicting the critical load of the composite member is proposed based on the Perry-Robert stability formula. The proposed simplified formula can accurately predict the critical buckling load of CFRP-STCM with errors and coefficients of variation are −2.71% and −0.014, respectively. [ABSTRACT FROM AUTHOR]

• The hysteretic friction properties of Curved Surface Sliders isolators are assessed. • The instantaneous friction coefficients at the two sliding surfaces are estimated. • A nonlinear constrained estimation approach is employed. • An estimator design model is derived. • Demonstrated effectiveness by means of comparative studies. One of the most challenging task for earthquake engineers is the accurate prediction of the dynamic response of structures implementing sliding seismic isolators, like the curved surface sliders. Indeed, the force–displacement behaviour of these devices strictly depends on the coefficiens of friction developed at the two sliding surfaces whose values vary instantaneously depending on variable compression load, sliding velocity, and contact temperature developed during the seismic motion. However, only the overall (or effective) friction coefficient of the isolator can be estimated, as weighted average of the values at the two sliding surfaces, through tipycal displacement controlled prototype experimental tests. This information is not suitable for the calibration of predictive isolator models (e.g. FEM analyses) or to characterize the tribological behaviour of potentially different friction pads at the two sliding surfaces. In this paper, an estimation approach, based on a Constrained Unscented Kalman Filter (CUKF) integrated with a Random Walk Model technique (RWM), that is capable to identify the two distinct friction coefficients, and their time-variations during displacement-controlled test is presented. The proposed tool allows the identification of the distinct frictional properties without any a-priori knowledge about the design properties at the two sliding surfaces that can widely differ in terms of adopted sliding materials, presence of lubrificant, and radius of curvature. The developed tool is firstly validated through the comparison with the results of FEM analyses and then applied on experimental tests carried out on a full-scale isolator prototype demonstrating its suitability for the assessment of the actual friction coefficients. [ABSTRACT FROM AUTHOR]

• Strengthened reinforced concrete columns with ultra-high performance concrete jacket. • Behavior of UHPC strengthened columns under eccentric loading. • Prediction of failure load of UHPC strengthened RC columns. The paper presents an experimental and numerical evaluation of reinforced concrete (RC) columns strengthened with ultra-high-performance concrete (UHPC). Fifteen columns were tested under static eccentric loading. The variables considered in the experimental program are the thickness of UHPC and the loading eccentricity. A three-dimensional (3D) numerical model was developed using ABAQUS software and validated with the experimental results. The validated numerical model was utilized to develop additional results of the strengthened RC column considering UHPC strength as a variable. Finally, an analytical model was proposed to predict the failure load and moment for rectangular RC columns strengthened with UHPC. The results confirmed the effectiveness of UHPC jacketing to strengthen RC columns under eccentric loading. The degree of enhancement is proportional to UHPC jacket thickness and inversely proportional to the nominal eccentricity. The proposed analytical model accurately predicted the failure load and moment for rectangular RC columns strengthened with UHPC. [ABSTRACT FROM AUTHOR]

• Cold-formed member-rotated T-joints of S960 steel were tested for static resistances. • Chord face failure, combined failure, and chord crown failure modes were observed. • Member-rotation had an obvious impact on the joint resistances and load-deformation behaviour of T-joints. • Static joint resistances were compared with the design rules available in the literature and EC3. • Generally, existing design rules were found not accurate and reliable. Brace-rotated and bird-beak joints are novel tubular joint configurations, which are obtained by the rotation of rectangular and/or square hollow section (RHS and/or SHS) members along their respective longitudinal axes by angles corresponding to their cross-sectional diagonal planes. The load vs deformation behaviour and static joint resistances of these novel tubular T-joints are presented in this study. In total, 30 cold-formed high strength steel (CFHSS) member-rotated tubular T-joints were tested. In this paper, member-rotated joints include brace-rotated (BR), square bird-beak (SBB) and diamond bird-beak (DBB) configurations, where RHS and SHS were used as braces and SHS were used as chords. The steel grade of RHS and SHS members was S960. The joint resistances of test specimens were determined by applying axial compression loads through the braces and supporting the chord ends on rollers. The automatic gas metal arc welding process was used to connect braces and chords. The brace width-to-chord width ratio (β) ranged from 0.33 to 0.83, effective brace width-to-chord width ratio (β ′) ranged from 0.25 to 0.88, chord width-to-chord thickness ratio (2 γ) ranged from 25.3 to 38.9 and brace thickness-to-chord thickness ratio (τ) ranged from 0.67 to 1.28. Three failure modes were observed in this investigation, which include chord face failure (F), combined failure (F + W), and chord crown failure (C). The joint resistances were compared with the nominal resistances obtained from the literature and EC3 (2005, [1]). It has been demonstrated that the design rules available in the literature are generally not suitable for the design of CFHSS brace-rotated and bird-beak T-joints made of S960 steel. Further, the effect of change of member-rotation angle (ω) on the joint resistances of CFHSS T-joints made of S960 steel was observed by comparing the joint resistances of BR, SBB and DBB T-joints of this investigation with the joint resistances of traditional RHS T-joints (Pandey and Young (2019), [2]) made of identical tubular member dimensions and steel grade. [ABSTRACT FROM AUTHOR]

• Experimental and numerical investigations on high strength steel circular hollow section X-joints were conducted. • The X-joints were made up of cold-formed high strength steel with nominal 0.2% proof stress up to 1100 MPa. • Effects of the geometrical ratios on the joint strengths were investigated. • Strength predictions by existing design provisions were generally unconservative. • A new design equation was proposed which provides more accurate strength predictions. This paper presents experimental and numerical investigations on cold-formed high strength steel (CFHSS) circular hollow section (CHS) X-joints. The CFHSS CHS members had nominal 0.2% proof stress up to 1100 MPa. The geometric parameters of the X-joints were designed by varying the ratios of β , 2 γ and τ. Seventeen X-joint tests were conducted by applying axial compressive load through the braces without preloading in chords. Non-linear finite element model (FEM) was then developed for the CFHSS CHS X-joints. After successful validation of ultimate strengths, failure modes and load-deformation curves, parametric studies were performed by using the verified FEM. The chord plastification failure of the X-joints was mainly found in both test and numerical studies. The relationship between the joint strengths and the variation of geometric ratios were investigated. The strengths of the X-joints obtained in this study together with the test strengths collected from the literature were used to assess the strength predictions by CIDECT and EN-1993–1-8, as well as those from the literature. It was found that the current predictions generally provided unconservative predictions. A new equation that considers the effects of geometric ratios on the strengths was proposed based on both the test and numerical results. By adopting the newly proposed equation, the predictions are improved and provide the least scattered results when compared with the other predictions. [ABSTRACT FROM AUTHOR]

• 162 experimentally verified nonlinear finite element pile cap models are developed. • Influences of various connection zone configurations are quantified. • Helical pile connections are found to govern the pile cap capacities under uplift loads. • The tension load capacities are found to be only 54% of their compressive counterparts. • Undesirable design configurations are identified, and resilient design configurations are proposed. Although significant research has been conducted on helical piles, there is a lack of research and official design guidelines on how to create resilient helical pile-to-pile cap connections, especially in tall and light structures where tensile uplift loads may govern the foundation design. The objective of this study is to advance the current understanding, quantify the influence of helical pile-to-pile cap connection detailing on the global system behavior, and propose recommendations for their resilient design. For this purpose, high-fidelity nonlinear finite element models are developed, experimentally verified, and 162 response simulations of helical pile cap systems are conducted to quantify the influences of: termination bracket types, bracket embedment depths, reinforcement ratios, shear span-to-depth ratios, and loading types. The results are analyzed in terms of the load, deformation, cracking, and failure behaviors. The analysis of variance and the factorial design methods are employed to quantify the percentage contribution of each parameter, as well as multi-parameter interactions, on the system capacity. The results, which are also applicable to micropile connections, demonstrate that the helical pile-to-pile cap connection capacity may govern the system capacity for the load conditions involving tension components. The tension load capacities of the pile cap systems (all of which are doubly and symmetrically reinforced) are found to be only 54% of their compression load capacities due to connection zone failures. This result is in contrast with the results from the traditional sectional analysis methods, which are not intended for the analysis of the connection zones and thus calculate the tension and compression capacities as equal. This paper presents the studies undertaken, conclusions reached, and recommendations made to create resilient helical pile-to-pile cap connections. [ABSTRACT FROM AUTHOR]

• Cold-formed tubular X-joints of S900 and S960 steels were tested for SCF. • Traditional, brace-rotated, square and diamond bird beak X-joints were examined. • Member-rotation has an obvious impact on the SCF values of tubular X-joints. • Test SCF values were compared with SCF values calculated from CIDECT and literature. • Existing equations over-predicted SCF for high strength steel tubular X-joints. This paper describes a test programme to investigate the stress concentration factors (SCF) of cold-formed high strength steel (CFHSS) traditional (without brace and/or chord rotation) and member-rotated tubular X-joints. The braces of traditional tubular X-joints were made of square, rectangular and circular hollow sections (SHS, RHS and CHS), whereas chords were made of SHS and RHS. The braces of member-rotated tubular X-joints were made of SHS and RHS, whereas chords were made of only SHS. The member-rotated tubular joints include brace-rotated (BR), square bird-beak (SBB) and diamond bird-beak (DBB) tubular joints. The tubular members were cold-formed from thermomechanically control processed (TMCP) S900 and S960 steel grades plates. A total of 15 tests was conducted to investigate the SCF of CFHSS traditional and member-rotated tubular X-joints subjected to brace axial compression loads. The brace and chord members were joined together using the automatic gas metal arc welding process. The measured values of brace width-to-chord width ratio (β) ranged from 0.33 to 0.74, effective brace width-to-chord width ratio (β') ranged from 0.25 to 0.85, chord width-to-chord thickness ratio (2 γ) ranged from 20.5 to 38.7 and brace thickness-to-chord thickness ratio (τ) ranged from 0.66 to 1.27. The experimentally obtained SCF values were compared with the SCF values calculated from the CIDECT [1] and literature [2,3]. It has been demonstrated that the existing SCF parametric equations are generally quite conservative and over-predicted the SCF values for cold-formed traditional and member-rotated tubular X-joints of S900 and S960 steel grades. Moreover, in order to observe the effect of member-rotation(s), the maximum SCF values of the brace and chord members of CFHSS member-rotated tubular X-joints were also compared with the maximum SCF values of CFHSS traditional tubular X-joints. [ABSTRACT FROM AUTHOR]

• Eight wood-frame walls were tested under high gravitational load and bending moment. • Wood-frame walls with high gravitational load are stronger and stiffer. • Better designs could be performed when considering gravitational load. In light wood-frame buildings, the gravitational and lateral force-resisting systems are composed of floor diaphragms and shear walls. During an earthquake, these walls are subjected to the simultaneous action of in-plane vertical force, shear force, and in-plane bending moment. In a mid-rise building, these internal forces can reach large magnitudes, especially on the lower stories, and could have an important influence on the lateral behavior of the walls. The historical use of light wood-frame construction has been in low-rise buildings. Consequently, few investigations have analyzed the effects of high gravitational forces or in-plane bending moment on the lateral behavior of wood shear walls designed for multi-story buildings. This paper presents an investigation of the cyclic lateral behavior of light wood-frame shear walls, designed for mid-rise buildings, subjected to large axial compressive load and in-plane bending moment. Eight wall specimens were experimentally tested with a cyclic lateral displacement protocol, a constant compressive load, and a cyclic in-plane bending moment. The effects of axial compressive load and in-plane bending moment were analyzed. Also, the wall length and the spacing of sheathing nails were varied to study the effects of these variables on the response. A numerical study was performed to show how these effects could influence the response of mid-rise timber buildings. An improvement in the lateral performance of the walls was observed compared to walls tested without compressive force nor bending moment, showing an increase in stiffness, load-carrying capacity, and dissipated energy. [ABSTRACT FROM AUTHOR]

• 26 cold-formed S900 and S960 steel grades non-90° X-joints were tested. • Material properties from transverse compression and weld heat affected regions were also investigated. • Chord face, chord side wall and combination of these two failure modes were observed. • Static resistances were compared with the predictions of CIDECT and EC3. • Existing design rules of CIDECT and EC3 are generally not accurate and reliable. This paper presents a test programme to investigate the static joint resistances and load vs deformation behaviour of cold-formed high strength steel (CFHSS) tubular non-90° X-joints. The investigation presented in this study predominantly focused on the chord failure of non-90° X-joints under brace axial compression loads. The experimental programme comprised of 26 tubular non-90° X-joints, which were fabricated with three included angles (θ 1), i.e. 30°, 50° and 70°. Circular and square hollow sections were used as braces, while chords were made up of rectangular and square hollow sections. The nominal yield strengths of the tubular members were 900 and 960 MPa. The brace and chord members were connected using automatic gas metal arc welding process. The measured value of brace-to-chord width ratio (β) ranged from 0.53 to 1, brace-to-chord thickness ratio (τ) ranged from 0.52 to 1.01, chord width-to-thickness ratio (2 γ) ranged from 20.5 to 38.7 and chord side wall slenderness ratio (h 0 / t 0) ranged from 15.4 to 38.9. A total of three failure modes was observed, including chord face failure, chord side wall failure and combination of these two failure modes. The static joint resistances obtained from the tests were compared with the nominal joint resistances of X-joints given in the CIDECT and Eurocode 3. It is shown that the existing design rules in these specifications are not suitable for the design of S900 and S960 steel grades tubular non-90° X-joints. Further, the impact of the included angle (θ 1) on the joint resistances of X-joints was observed by comparing the joint failure resistances of tubular non-90° X-joints obtained in this investigation with the joint failure resistances of tubular 90° X-joints of identical nominal tubular member dimensions and steel grades. In addition, tensile material properties from the weld heat affected regions of S960 steel grade tubular members of different thicknesses were also investigated. [ABSTRACT FROM AUTHOR]

[Display omitted] • Microcapsule cracking or microcapsule/matrix interface decohesion is considered. • Microcapsule size effect is discussed. • Buckling and post-buckling behavior are investigated. This paper shows an analytical approach to investigate buckling behavior of a carbon fiber reinforced resin (CFRR) cylindrical shell covered with a damaged porous microcapsule coating subjected to axial compressed loads under hygrothermal environment. The formulas are in view of the Donnell's shell theory considering the von Kármán geometric nonlinearity. The material parameters of microcapsule coating are predicted with the self-consistent model and Mori-Tanaka model. By utilizing Galerkin approach, the buckling behavior for simple-support damaged double-layered cylindrical shell are determined. The influence of temperature and moisture, interface debonding damages, material properties, geometry parameters of microcapsules and porosity upon the buckling behavior of the double-layered cylindrical shell are investigated. Results show moisture and geometry size of microcapsules are propitious to the critical buckling and post-buckling compressive load. In addition, the micro-structure damage greatly affects temperature and moisture. The geometric parameters of microcapsules significantly affect the buckling and post-buckling behavior of double-layered cylindrical shell. [ABSTRACT FROM AUTHOR]

• The efficacy of the FRP layers (number, length, material, and orientation) and the connection geometry (τ , β , and γ) on the static behavior is investigated. • The initial stiffness, ultimate resistance, resistance ratio, and failure shapes of the reinforced connections are investigated. • A theoretical formula is proposed to determine the ultimate resistance of the X-joints reinforced with FRP. In the present paper, the initial stiffness, the ultimate capacity, the capacity ratio, and the failure mechanisms of the tubular X-joints reinforced with Fiber Reinforced Polymer (FRP) under compressive load are investigated. For these aims, a finite element (FE) model was generated and verified with the available experimental data. Afterward, 109 finite element (FE) models were created to investigate the efficacy of the FRP layers (length, number, and orientation) and the connection geometry (β , τ , and γ) on the static performance of the reinforced X-joints. In the FE models, the effects of the weld profile and the contact between the FRP and the members (chord, weld, and braces) were considered. Results showed that the FRP can remarkably enhance the stiffness, ultimate capacity, and improve failure mechanisms. Despite the notable effect of the FRP on the performance of the joints, there is not any study on the X-joints with FRP. Hence, after the parametric study, the FE results were used to propose a theoretical formula, based on the yield body model, to predict the ultimate strength of X-joints with FRP. In addition, the proposed formula is confirmed by the acceptance criteria of the UK Department of Energy. [ABSTRACT FROM AUTHOR]

Based on our completed experimental research, the fire resistance of Q550 high-strength steel (yield strength of f y = 550 N/mm2) hollow columns under axial compression is explored by a finite element numerical simulation and theoretical analysis. First, an accurate analysis model of high-strength steel columns under fire exposure is established using ABAQUS software, and a numerical simulation analysis is carried out. Based on verified finite element model, a parametric analysis of the fire resistance of high-strength steel columns is conducted. The effects of the section area A , section height-thickness ratio H / t , section height-width ratio H / B , overall bending defect amplitude e 0 , slenderness ratio λ and load ratio n on the fire resistance of high-strength steel columns are investigated. Finally, based on the parametric analysis results and different limit state judgement criteria, the calculation formulas for the critical temperature T cr and maximum axial displacement u max of Q550 high-strength steel columns with rectangular sections under axial compression are proposed. The correctness of the formulas is verified by comparing the calculation results with the test results. The research of this paper provides a theoretical basis for improving the fire resistance design of high-strength steel columns at elevated temperatures, and greatly promote the healthy development and good application of high-strength steel in structural engineering applications. [ABSTRACT FROM AUTHOR]

• Induction heating (IH) is employed to raise the steel material strength. • A curved and partially strengthened brace treated by IH is innovatively proposed. • The untreated region yields first, while the treated one can still be elastic. • The curve shape mitigates the overall buckling and adjusts the yield displacement. • Cyclic loading tests on 2 proposed braces and 1 conventional brace are conducted. This paper presents an experimental and numerical study on a novel brace which is curved and partially strengthened by induction heating technology (induction-heated curved brace, IHCB). Induction heating, comprised of heating and quenching processes, is an advanced technology to raise steel material strength to 2–3 times. Because of the partial strengthening, the untreated region yields first while the induction-heated region is still elastic before larger deformation, so the post-yield stiffness of the whole brace increases. Besides, the initial curved deformation along the brace length, which is due to the uneven heating, is also flexibly employed to delay the yielding behavior and stabilize the compressive behavior. Tensile coupon tests and Vickers hardness tests on the untreated normal-strength specimens and induction-heated high-strength specimens show that the induction heating technology effectively improves the material strength to 2.2–2.6 times. Cyclic loading tests on CBB (conventional buckling brace) and IHCB series show that under tension, IHCBs show approximately 57% lower initial stiffnesses compared to that of CBB but reach the yield loads at the cycle twice as large as that of CBB owing to the initial deformation. After yielding, although the stiffness of CBB dramatically drops to 0.9% to its initial stiffness, IHCBs succeed to maintain higher post-yield stiffnesses, of which the ratios to their initial stiffnesses are larger than 30%. Under compression, IHCBs show the smooth transition into flexural behaviors without apparent buckling behaviors, and the stable compressive loads (the load at −0.5% axial strain) become 1.49 and 1.67 times larger than that of CBB. The data obtained from the strain gauges and displacement transducers show that the strain and transverse deformation of IHCBs are uniformly distributed along the brace length rather than locally concentrated at the limited region as seen in CBB. The numerical analysis conducted by ABAQUS and the proposed design equations capture the experimental results well. [ABSTRACT FROM AUTHOR]

• A fiber model for square ultra-high-strength CFST slender beam-columns is presented. • The numerical model incorporates the interaction of local and global buckling. • The stress–strain constitutive law of ultra-high-strength concrete is described. • The influences of parameters on response of square CFST columns are investigated. • Eurocode 4 is assessed for the design of square ultra-high-strength CFST slender columns. The application of ultra-high-strength concrete in a concrete-filled steel tubular (CFST) column is an emerging topic recognizing the greater member capacities and stiffness of high strength concrete. The current designing codes for composite structures have restricted the use of ultra-high-strength concrete. This is because very little research has been conducted on the structural response of eccentrically loaded square ultra-high-strength CFST slender beam-columns. Set against this limited research, a numerical model implementing the fiber element formulation has been developed in this paper to fill the knowledge gap on the behavior of square ultra-high-strength CFST slender beam-columns under combined compression-bending conditions. The fiber-based model accounts for important features including local and global interaction buckling, triaxial confinement mechanism, member imperfection, and geometric and material nonlinearities. The stress–strain relationships for normal- and high-strength concrete do not reflect completely the brittleness of ultra-high-strength concrete in the post-peak branch. The existing material model is modified for simulating the material response of ultra-high-strength concrete. A comparison is made between the experimental data and the numerical simulation to assess the accuracy of the fiber-based model. The model has shown to provide a good agreement between the test data and the simulation. The parametric investigation indicates that the ultra-high-strength concrete may be effectively and hence more economically used in short and intermediate steel–concrete composite columns subjected to the compression load with a small eccentricity. It is also noted that the steel section with yield stress up to 550 MPa can be utilized with the ultra-high-strength concrete. The design code, Eurocode 4, accurately predicts the bending strength of ultra-high-strength square CFST slender beam-columns subjected to uniaxial bending. [ABSTRACT FROM AUTHOR]

• GA and DSM used for cross-section optimisation of anti-symmetric CFS beam-columns. • Manufacturing constraints and practical construction limitations were imposed. • 132 CFS elements were optimised using different lengths and load eccentricities. • Capacity improved by 62%, 92% and 188% for short, medium and long length elements. • FE models are developed considering material nonlinearity & geometric imperfections. Flexibility in the manufacturing of cold-formed steel (CFS) cross-sectional shapes provides a unique opportunity to improve the load-carrying capacity of these elements, leading to more efficient and economic structural systems. This paper presents a practical constrained optimization methodology for pin-ended anti-symmetric CFS beam-columns members with different lengths subjected to various combinations of axial compression and bending moment. The optimization process is carried out using a Genetic Algorithm (GA) with respect to buckling resistance of CFS elements determined according to the Direct Strength Method (DSM). In total, 132 CFS beam-columns with three different lengths (1000, 2000 and 3000 mm) and eleven different cross-sections are optimized under concentrically compressive loads with different levels of eccentricities varying from 0 to 30 mm. Each cross-section is formed using a certain number of fold-lines of steel plate, while the length and angle of the constituent elements of the cross-section are considered as the main design variables. To provide more practical beam-column elements, end-use constraints as well as a range of practical manufacturing and construction limitations are imposed on the selected cross-sections during the optimization process. A standard commercially available anti-symmetric Z section is taken as a starting point of optimization and used to assess the efficiency of the optimized sections. The results show that, for the given plate width and thickness, the adopted optimization process can significantly increase the strength of beam-column members on average 62%, 92%, and 188% for the short, medium and long length elements, respectively, compared to those with the standard section. It is also demonstrated that using more complex cross-sectional shapes does not necessarily provide higher strength for beam-column members. Finally, to verify the efficiency of the optimized sections, detailed nonlinear finite element models are developed using ABAQUS software. The developed models should prove useful for the efficient design of CFS beam-column elements in practice. [ABSTRACT FROM AUTHOR]