Ultimate Resistance Research Articles

Study of the fire resistance of partially encased composite steel-concrete columns according to Eurocode 4

In this paper, the behavior of partially encased steel and concrete composite columns without fire protection exposed on four sides to the standard fire and under axial loading was studied using the simplified calculation model of the Eurocode4 (EC4) method. The main objectives of this study are: first to evaluate the temperature of the different components of the column section as well as the variation in the characteristics of the materials constituting them (strengths and Modulus of elasticity); and then analyze the influence of several parameters, such as the section size, the reinforcement rate, the buckling length and the mechanical strengths of the steel and concrete on the ultimate fire resistance. The results show that the dimensions of the section, the reinforcement rate and the buckling length have a significant influence on the fire resistance. On the other hand, the concrete strength, the structural steel grade and the reinforcing steel grade have a moderate influence.

Reliability-based capacity safety factor for G20Mn5 cast steel joints

Cast steel joints have complex geometries and great load-carrying capacity, and hence their resistances are commonly determined through FE simulations. Considering the significant variability in the material mechanical properties (that in fracture strains, in particular) and the model uncertainty inevitably involved in the FE analysis, a large safety factor (γR) should be adopted for the FE simulation-based design of cast steel joints. However, the γR-values specified by the current design specifications, e.g. the Chinese standard CECS 235: 2008 in which γR = 3.0, are deemed to be overly conservative. This paper examined the structural reliability of G20Mn5 cast steel joints to provide a rational γR-value for the practical joint design. The above-mentioned uncertainty of the material and geometric properties as well as the model uncertainty related to FE simulations are evaluated, and Monte Carlo simulations were conducted on nine representative joints to obtain the statistical characteristics of cast steel joints’ ultimate resistance, allowing for both failure modes of material fracture and instability caused by large-scale yielding. FORM was adopted to calculate the reliability index β for the assumed γR-values and different loading scenarios, and the recommended values for γR under target reliability indices are proposed. For snow load-dominant and wind load-dominant load combinations, the recommended γR corresponding to β = 4.2 are 2.29 and 2.16, respectively, and the recommended γR corresponding to β = 3.7 are 2.01 and 1.90, respectively.

Robustness analysis of dynamic progressive collapse of precast concrete beam–column assemblies using dry connections under uniformly distributed load

A series of dynamic progressive collapse tests using the uniformly distributed load (UDL) was conducted, to analyze the robustness of three precast concrete (PC) beam–column assemblies using dry connections of top-and-seat angles (TSA-D), strengthened top-and-seat angles (STSA-D), and high-ductility reinforcement (DSTSA-D), under a middle column removal scenario. Finite element (FE) models were also developed to accurately simulate the collapse process. Based on the tests and FE models, comparative studies were conducted on dynamic and static collapse responses of the PC assemblies with identical configuration, loading regime, and boundary conditions. The results showed that: the horizontal reaction force developments were similar under dynamic and static collapse scenarios; under dynamic collapse scenarios, the initial stiffness of the PC assemblies increased due to the strain rate effect, while the peak vertical reaction forces under the compressive arch action (CAA) were smaller than those under static collapse scenarios, attributed to dynamic damage; under both dynamic and static collapse scenarios under UDL, the beam deformed in a downward convex curved shape, with local material deformation becoming more concentrated under dynamic collapse scenarios. An energy-based method for calculating dynamic collapse resistance was evaluated and revised: 1) the traditional method, which converts static responses from static analysis into dynamic responses was found to underestimate the resistance under small deformations due to neglecting the strain rate effect, and to overestimate ultimate resistance by ignoring dynamic damage; 2) by using dynamic vertical reaction forces instead, a more accurate prediction of the dynamic collapse resistance was achieved with a single dynamic collapse analysis. Additionally, dynamic amplification factors for the PC assemblies were calculated based on FE models and static test results providing basic knowledge in whole PC structural level research.

Experimental Study on Horizontal Resistance Parameter of Gravelly Soil Considering Slope Gradient Factor

Horizontal resistance can be significantly different for gravelly soil slope sites with different gradients. The impact of slope gradient on the horizontal resistance of soil based on a three-dimensional physical simulation test has been investigated, and the displacement of the pile top and soil pressure for four piles under various gradients (slope gradient 0–45°) was analyzed. The research reveals that ① The soil resistance exhibits a linear increase stage, non-linear steep increase stage, and gradually stabilizing stage with the increase in load. ② The ultimate soil resistance is significantly affected by depth and displacement, and hyperbolic variation characteristics related to the displacement stage. It has a slope weakening effect, and the steeper the slope gradient, the lower the ultimate soil resistance of the pile sides, which effect is negligible when the depth exceeds 0.6–0.7 times that of the pile buried depth. ③ Based on the characteristics of horizontal ultimate resistance-displacement-depth variation in soil, a gradient factor, which is only related to the slope gradient, was used to describe the impact of gradient on the soil resistance modulus (kini) and ultimate resistance (pu). kini is reduced close to the ground surface and gradually increases with depth until it becomes equal to the value of level ground. The ratio between pu in slope ground and level ground was determined for slope angles. The above horizontal resistance parameters were expressed as a function based on the test data to calculate the lateral ultimate bearing capacity of gravelly soil considering the slope gradient factor. The research results developed the theory of foundation design for building structures, promoting the reliability evaluation of pile soil systems towards a more scientific and rigorous direction.

Axial compressive behavior of multi-celled corrugated-plate CFST walls: Tests and numerical simulations

In recent years, steel-concrete composite walls have been widely used in practical engineering, primarily attributed to their exceptional properties. This paper proposed a novel type of composite wall called the multi-celled corrugated-plate concrete-filled steel tubular (MC-CFST) wall, which consisted of corrugated cells and interval flat plates connected alternately. Axial compression tests involving 14 MC-CFST wall specimens were performed to investigate their compressive resistant behavior. According to the test results, the steel and concrete showed good composite action. It was found that all specimens exhibited ductile failure with residual resistance-to-ultimate resistance ratios varying within the range of 0.503 to 0.672. Local buckling occurred in the interval flat plates, corrugated steel plates, and boundary flat plates. In addition, a finite element (FE) model was established and validated. The results showed that the vertical normal stress of concrete in the trough section was obviously enhanced, and the load-bearing capacity of the MC-CFST wall was primarily carried by the concrete. Moreover, a parametric analysis was conducted to investigate the effects of geometric dimensions and material strengths. The numerical results were then compared with the calculation results of existing codes. It was found that the design formulas in existing codes could not accurately calculate the load-bearing capacity of the MC-CFST wall. Finally, a design formula with good prediction accuracy was suggested, which could be used to predict the ultimate resistance of the MC-CFST wall.

Estimation of breakout force of spudcan footing in structured clay

For a jack-up rig installed in cohesive soil, assessing the extraction force is critical to ensure the safe and successful retrieval of each spudcan footing. The extraction force depends on the preceding stages, including installation and operation of the rig, due to soil strength changes around the spudcan. To investigate the entire ‘penetration - operation - extraction’ process of spudcan, an effective stress large deformation finite element analysis is conducted, with a Structured Cam-Clay model being incorporated to describe the behaviour of structured soil. The large deformation approach is validated against existing centrifuge tests. Parametric studies are then carried out to explore the influential factors on breakout force, including operation period, vertical load ratio, penetration depth, destructuring rate and soil sensitivity. A unique curve is formulated to predict the evolution of the recovery degree of breakout force over the operation period in structured cohesive soil. The soil sensitivity is identified as the dominant factor that affects the ultimate net resistance ratio, which is the key parameter required in applying the prediction curve. A simple method is proposed to estimate the ultimate net resistance ratio and a procedure to predict the breakout force is suggested.

Nonlinear Analysis of the Mechanical Response of an Existing Tunnel Induced by Shield Tunneling during the Entire Under-Crossing Process

The safety of existing tunnels during the entire under-crossing process of a new shield tunnel is critically important for ensuring the sustainable operation of urban transportation infrastructure. The nonlinear behavior of surrounding soils plays a significant role in the mechanical response of tunnel structures. In order to assess the mechanical response of the existing tunnel more reasonably, this study attempts to propose a novel theoretical solution and calculation method by simultaneously considering the nonlinear characteristics of surrounding soils and the tunneling effects of a new tunnel during its entire under-crossing process. Firstly, the additional stresses acting on the existing tunnel stemming from the tunneling effects of a new shield tunnel during different under-crossing stages are calculated using the typical Mindlin solution, as well as the Loganathan and Poulos solutions. The influences of the additional thrust, friction force, and grouting pressure and the loss of surrounding soils are taken into account. Then, the nonlinear Pasternak foundation model is introduced to characterize the behavior of surrounding soils, and the governing differential equation for the mechanical response of the existing tunnel is derived using the typical Euler–Bernoulli beam model. Subsequently, a novel theoretical solution and calculation approach are established using the finite difference formula and the Newton iteration method for assessing the mechanical response of the existing tunnel. Finally, one case study is performed to illustrate the mechanical behavior of the existing tunnel during the whole under-crossing process of a new shield tunnel, and the validity of the developed solution is verified against both the computed result of finite element simulation and the field measurements. In addition, the influences from the ultimate resistance and reaction coefficient of surrounding soils and those from the vertical distance and intersection angle between existing and newly constructed tunnels are analyzed and discussed in detail.

Experimental and numerical investigations on cold-formed titanium-clad bimetallic steel angle and channel section stub columns

Titanium-clad bimetallic steel (TCBS) exhibits outstanding corrosion resistance, and can be advantageously applied in corrosive environments. This study investigated an innovative application of TCBS in cold-formed stub columns. During the cold-forming process, TCBS demonstrated no discernible separation at the bonding interface of the Q235 substrate layer and TA1 cladding layer, indicating the maintenance of synergistic deformation between the cladding and substrate layers. The compressive behavior and ultimate resistance of cold-formed TCBS angle and channel section stub columns were investigated using both experimental and numerical methods. These tests included material tensile flat and corner coupon tests, initial local geometric imperfection measurements, and stub column tests. The stub column test comprised four cold-formed TCBS angle section stub columns and three cold-formed TCBS channel section stub columns covering the non-slender and slender cross-sections. The experimental results were utilized to validate the finite-element (FE) model, which was then used in a parametric study to obtain further numerical data at different cross-sections. A comparative analysis of the ultimate resistance from the tests and numerical analyses with the predicted results from design approaches in EN 1993-1-1, AISI S100, and the direct strength method (DSM) was revealed. The evaluation findings generally showed that the EN 1993-1-1 design method forecasts for cold-formed TCBS channel section stub columns were dangerous. The non-slender cross-section classification coefficient, which was inapplicable to cold-formed TCBS channel steel stub columns, was the primary cause of the above unsafe prediction. As for the design approaches in AISI S100 and DSM for cold-formed TCBS angle and channel section stub columns, as well as those in EN 1993-1-1 for cold-formed TCBS angle section stub columns, they were judged safe but conservative. The above prediction error was mainly caused by ignoring the enhancement in strength properties of the TCBS after the cold-forming process. The considerations mentioned above led to the proposal and demonstration of modifying recommendations that offered safe, accurate, and consistent design approaches to ultimate resistance for the cold-formed TCBS angle and channel section stub columns.

A study of neural network-based evaluation methods for pipelines with multiple corrosive regions

In recent years, significant developments have been made in methods for assessing the remaining strength of corroded pipelines. However, existing methods have limitations as they mainly focus on the local impact of corrosion defects. This study explores evaluation methods using neural networks to predict the ultimate resistance of pipelines containing multiple corrosive regions. Firstly, based on the validated method, the study generates a dataset comprising 3,000 corroded pipeline models and pixelates the corrosion information of these models via digital images. Then, three neural network evaluation frameworks are constructed: a Multilayer Perceptron (MLP) using the overall corrosion matrix, an MLP based on corrosion feature parameters, and Convolutional Neural Networks (CNN) based on corrosion images. Following this, the study analyzes the relationship between various corrosion parameters and failure pressure, compares the training effectiveness of the three neural network methods, and validates the accuracy and applicability of the proposed approach. The results indicated that various corrosion features should be considered when evaluating corroded pipelines, particularly depth. In addition, all three neural network-based methods show improved applicability and reliability compared to traditional evaluation methods, with CNN-image having the highest evaluation accuracy (correlation coefficient = 0.9564, average error = 3.46%).

Axial compression behaviors of square steel tubes at low temperatures

This paper investigated the compression behaviors of square steel tube (SST) columns used in cold-region engineering structures. Twenty-eight SST columns were tested at low temperatures of 20, −30, −60, and − 80 °C. The studied parameters were low temperature (T) and slenderness ratio (λ) of SSTs. Under low-temperature compression, stub columns (λ = 5) failed in local buckling whilst slender columns (λ = 15 and 25) failed in global buckling. Low temperatures exhibited marginal effects on failure modes, and slightly improved (less than 12 %) the ultimate compressive resistance (Pu) of SSTs. Decreasing T and λ values increased the compressive resistance of SSTs. Finite element (FE) models were established to simulate compression behaviors of SSTs at low temperatures, with accuracy confirmed by 28 test results. The FE parametric studies indicated that decreasing the width-to-thickness ratio of SSTs generally improved their compression capacity and ductility. Moreover, different code prediction formulae in GB50017, AISC 360–16, and Eurocode 3 for SSTs were checked to predict the Pu. Finally, prediction equations were developed to predict the compression resistance of SSTs at low temperatures. The prediction equations provided reasonable estimations on Pu with an average discrepancy of 4 %.

To justification of concrete strength criterion at biaxial compression

Introduction. Numerous experimental data from Russian and foreign researchers indicate that the classical plasticity hypotheses do not take into account the different resistance to uniaxial tension and compression, the influence of the spherical tensor. At the same time, experiments show that the ultimate resistance depends on the type of stress state, and hydrostatic pressure increases the strength and plasticity of solids. Aim. To establish the dependence of the influence of the second component of stresses during biaxial compression of concrete on the parameters of the complete diagrams of deformation of the material σbR and εbR it is necessary to describe these diagrams, and to construct a closed curve on the plane of the main stresses (concrete strength criterion). Materials and methods. Based on the experimental materials of foreign and domestic researchers, including the experiments of the authors of the article, methods of mechanics of deformed solids, limit curves and a closed curve on the plane of the main stresses in the form of a chain line forming the smallest area strength surface in the form of a catenoid are proposed. Results. The article provides an analysis of the known strength criteria from the point of view of their geometric interpretation in the stress space. It is shown that these studies relate mainly to metals and metal structures, and for the design of reinforced concrete and steel-concrete structures in a complex stress state, it is necessary to develop an appropriate criterion for the strength of concrete. Conclusions. As a result, the proposed limit curves and the surface (material strength criterion) on the plane of main stresses in the form of a catenoid accurately reflect the behavior of concrete under conditions of uniform and uneven flat stress state, and the equation of the surface in the form of a catenoid is a generalization of the equations of limit curves for each of the three types of flat stress state. At the same time, there is no overestimation of strength in the "compression – compression" area in this case.

Behavior of randomly pitted Q690 high-strength steel H-section members under eccentric compression

Six Q690 high-strength steel H-section specimens with pits were tested under eccentric compression loads. The results showed that the pits changed the failure modes of the members under eccentric compression by inducing local buckling, which resulted in a significant reduction in the ultimate resistance, reaching 31.7 % when the volume loss ratio was 15 %. A numerical study was then conducted, which revealed that the ultimate resistance loss ratio varied with the volume loss ratio and the eccentricity ratio, but not with the slenderness ratio. Finally, the existing rules for determining the ultimate resistance were modified by using the equivalent residual thickness, and the applicability of the modification was assessed.

Experimental studies on fully demountable P-DBSCs shear connectors in sustainable reinforced concrete beam-slab

To improve the increasingly serious environmental pollution and waste of resources in the construction of traditional buildings, a novel type of shear connector P-DBSCs (Demountable Bolted Shear Connectors with connection Plate) was designed. The shear performance of P-DBSCs in RC beam and slabs were comprehensively evaluated by seven push-out tests and secondary loading. The failure mode, load-slip curve, ultimate shear capacity and ductility of the specimens were carefully discussed. As illustrated by the experimental results, the failure mode of the 7 push-out specimens is bolt fracture with no obvious crack in the beam and slabs, and the demount-ability is excellent after failure. The ultimate slip of all specimens is greater than 6 mm, which meets the requirements of Eurocode 4. The ultimate strength of P-DBSCs shear connectors increased with the increase of steel box thickness and bolt diameter, it decreased with the increase of channel spacing. The ductility of P-DBSCs shear connectors increased with the reducing of channel spacing and bolt diameter. Additionally, the shear performance of P-DBSCs is insensitive to the strength of concrete of slabs. After the first loading, the ultimate shear resistance; ultimate displacement and initial stiffness of the push-out specimen decrease slightly in secondary loading, but the influence code of each parameter remain constant. Finally, a comparison of ultimate strength between the calculated value and the experimental results showed that the theoretical values given by the Eurocode 3, Eurocode 4 and code AISC360–16 were higher than the experimental values, which indicated that the calculations overestimated the shear capacity of connection. Based on this, two formulae were proposed to predict the friction-type shear capacity and pressure-type shear capacity of P-DBSCs and the efficacy of the proposed design formula was proved by the comparison with the test results, which provides a research basis for such shear connectors.

Ultimate internal pressure bearing capacity of unconstrained and constrained X80 oil and gas pipelines with three typical dents

Three typical dents, denoted as types I, II, and III, encapsulate the prevailing manifestations of mechanical impairment encountered by oil and gas pipelines. Presently, scholarly attention is predominantly directed towards the scrutiny of load-bearing capabilities in pipelines afflicted by a solitary type of dent. To unravel the intricate impact of three typical dents on the performances of dented pipelines, three distinct models of pipeline dents under different constraints were studied in this work. These findings reveal that unconstrained dents consistently exhibit greater plastic strain and deformation zone dimensions compared to their constrained counterparts. The spring-back and rebound performances in unconstrained pipelines were fortified and the external load resistance in constrained pipelines was enhanced by augmenting the indenter diameter and introducing internal pressure. Both unconstrained dents and type I constrained dents failed consistently in the non-dented region, with their ultimate internal pressure resistance unaffected by the constraining factors. In contrast, type II and III constrained dents, featuring smaller indenters, greater dent depths, and reduced diameter-to-thickness ratios, failed within the dented region, which results in a weakened ultimate internal pressure-bearing capacity. Failure occurred in the non-dented region under opposite conditions, leaving the internal pressure resistance unaffected. Finally, dimensionless predictive formulas for the ultimate internal pressures of the type II constrained and type III X80 dented pipelines were obtained through nonlinear fitting. This comprehensive exploration revealed the variations in the ultimate internal pressure-bearing capacities induced by dents, thereby providing valuable insights for pipeline design and safety considerations.

Simulating lunar highlands regolith profiles on Earth to inform infrastructure development and ISRU activities on the Moon

Lunar exploration and infrastructure development near the Moon's south pole, and at other future lunar settlements, demand a detailed understanding of the geotechnical properties, structure, composition, and stratigraphy of the near-surface lunar regolith. Long term exploration and development activities will require the use of regolith as a construction material for multiple structures, and as unrefined material for resource extraction during in situ resource utilization (ISRU) activities. Upcoming robotic and human exploration will be concentrated near the lunar south pole, which has a dominantly feldspathic composition typical of the lunar highlands. Past missions to lunar highlands terrain (Apollo 16, Luna 20) provide valuable “ground truth” data regarding the geomechanical properties of highlands regolith that serve as a guide for future exploration at the lunar south pole. Lunar highlands simulant LHS-1 has well-characterized geotechnical properties and is an appropriate analogue for highlands lunar regolith in terrestrial engineering studies. Cone penetrometer testing (CPT) measurements of LHS-1 reveal a strong exponential correlation between the G slope parameter and bulk density, which allows information to be obtained regarding the density profile of simulated regolith columns directly from CPT stress versus penetration curves. We show that LHS-1 can be used to replicate the near-surface regolith stratigraphy at the Apollo 16 highland landing region in Earth-based laboratories. The ultimate CPT penetration resistance measured in the laboratory under terrestrial gravity is not directly comparable to that measured under lunar gravity, and here we derive an average reduction factor of Rf = 0.29 to scale penetration resistance values to those measured in situ on the lunar surface. Geotechnical measurements of other lunar simulants combined with experiments similar to those described herein, tailored to the terrain type of interest, will provide crucial information to guide future regolith excavation, construction, and ISRU activities on the lunar surface.

Novel pre-pressure friction spring RC rocking column with enhanced segment: Practical design and numerical investigation

A novel pre-pressure friction spring rocking column (PFSRC) with enhanced segment is proposed to improve the seismic resilient of RC frames. The construction of the PFSRC was introduced, and parameters of pre-pressure friction spring device (PFSD), and limit state of PFSRD were analyzed. Based on seismic performance objectives, a practical design method for PFSRC was proposed. Utilizing the developed self-centering constitutive model and rocking interface three-dimensional simulation method, a macroscopic numerical model of PFSRC was established using OpenSEES, and numerical simulations of designed PFSRC under cyclic loading were conducted. Subsequently, further parameter investigations were conducted on the seismic performance of PFSRC. The results indicate that the designed PFSRC demonstrates strength and stiffness equivalent to the fixed-base column under drift limit of 1/550, and, at a drift limit of 1/50, there is no stiffness degradation, and the ultimate resistance is comparable to the fixed-base column. The steel tube effectively avoid the damage of the concrete at the foot of column. Increasing the friction coefficient of friction spring most effectively enhanced the energy dissipation of the PFSRC, and the taper angle has the most significant effect on the post-yield stiffness of the PFSRC, but increasing this parameter leads to decrease the energy dissipation. While increasing the pre-pressure effectively improves the yield strength and resistance of PFSRC. The damage extent of the PFSRC is affected by both axial compression ratio and steel tube diameter-to-thickness ration, and the low damageability of PFSRC can be achieved by adjusting the diameter-to-thickness ratio. For the buildings with unidirectional seismic demands (such as retrofit buildings), it is recommended to placed PFSD at 30° along that direction, for the general building with bidirectional seismic demands, PFSD should be arranged at 45° along that direction.

Flexural behaviour of shallow-embedded steel column bases

This paper investigated the flexural behaviour (the initial rotational stiffness and ultimate moment resistance, in particular) of the shallow-embedded column bases through experimental and numerical approaches. Three full-scale tests with different embedded depth ratios (i.e., 0.5, 1.0 and 1.5) were carried out, from which the moment-resisting mechanisms were identified and the effects of the embedded depth ratio on the initial stiffness, moment resistance and ductility were evaluated. The test results were further used to evaluate the accuracy of the available resistance and stiffness models for the shallow-embedded column bases. The resistance models codified in design codes GB 50017-2017 and AISC 341-16 are highly conservative, with an average prediction accuracy of 0.268 and 0.451, respectively; by contrast, the resistance models recently reported in the published articles show better accuracy. The stiffness models are fewer in number and are also inaccurate for the shallow-embedded column bases. Complementary finite-element models were developed and validated. Subsequently, systematic parametric finite-element analyses were performed to investigate the effects of axial force and shear force in the steel column, as well as those of the structural details such as the presence of base plate, arrangement of anchor bolts and width of concrete foundation. Shear force has an adverse effect on both the initial stiffness and moment resistance, and this effect becomes more significant with the increase in the embedded depth ratio; axial compression force leads to an increase in both the initial stiffness and moment resistance, which is more pronounced for embedded column bases with smaller η-values. Among all structural details, the presence of the base plate has the most notable influence on the flexural behaviour of the shallow-embedded column base, increasing both its initial stiffness and moment resistance; comparatively, the effects of the other structural details are minor or even could be ignored.

Modelling the ultimate pullout resistance of geogrids

This paper provides a review of the modelling of geogrid ultimate pullout resistance. Several analytical models were found to disregard the effect of the geogrid stiffness as well as the particle size distribution of the soil. A sensitivity analysis was done on 289 pullout tests to evaluate the significance of these omitted variables as well as other key variables contributing to pullout resistance. It was found that ultimate pullout resistance was the most sensitive to normal stress followed by geogrid length, geogrid stiffness and the friction angle of the soil. By considering geogrid geometry, stiffness and soil grading descriptors, in addition to the standard variables of length, stress and friction angle, regression models of ultimate pullout capacity improved by 15%. In addition, an alternative model to the simplified linear models of FHWA and EBGEO which maintains dimensional consistency while incorporating geogrid stiffness and non-linearity in the model, was proposed. This alternative model performed 32% better than the simplified linear models.

Behaviour of steel-fibre-reinforced geopolymer concrete-filled square steel tubular stub column under axial compression

Resource utilisation of industrial waste has become a crucial issue in construction aiming for sustainable development. Research on geopolymer concrete-filled square steel tubular (GCFST) structures is limited. Moreover, methods for calculating the bearing capacity of steel-fibre-reinforced GCFST (SFRGCFST) structures require further investigation. This study presents an experimental and analytical investigation of the axial compression performance of SFRGCFST stub columns. Twenty-four specimens were tested to assess the effect of steel tube thickness, steel fibre (SF) volume fraction, SF aspect ratio, and concrete strength grade on failure modes, load–deformation behaviour, load–strain response, ultimate resistance, and ductility of SFRGCFST columns. The results indicated that the failure modes of SFRGCFST columns are influenced by the coefficient index, which varies with the test parameters. When the coefficient index exceeded 0.88, the failure mode of the specimens transitioned from shearing failure to waist-drum-shaped failure. The average ultimate bearing capacity of the SFRGCFST columns increased by 35.13 % compared with that of the plain concrete-filled steel tubes (CFSTs). When the SF volume fraction reached 0.9 %, the ultimate bearing capacity of the SFRGCFST columns increased by 21.85 % compared with that of steel-fibre-reinforced concrete-filled steel tube (SFRCFST) columns. The incorporation of SF significantly improved the deformation capacity and ductility of the SFRGCFST columns and moderated the slope of the post-peak load–displacement and load–strain curves, slowing the descent of the curve. Utilising the multi-axial strength criterion and mechanical equilibrium equation for square CFSTs, a formula for calculating the ultimate bearing capacity of SFRGCFST stub columns was developed. This formula enables accurate prediction of the axial compressive load capacity of SFRGCFST stub columns and provides a basis for revising relevant standards.

Cold-formed Q1200 ultra-high strength steel angle- and channel-section stub columns

This paper presents the experimental and numerical analysis of cold-formed Q1200 ultra-high-strength steel (UHSS) angle- and channel-section stub columns to elucidate their resistance performance. Initially, the mechanical property test including the flat and corner coupon specimens was conducted to quantify the effect of the cold-forming process on Q1200 UHSS. Then, the stub column test was conducted on 16 cold-formed Q1200 UHSS angle- and channel-section stub columns. Based on the experimental results, a finite element model was established to simulate the resistance performance of the cold-formed Q1200 UHSS angle- and channel-section stub columns. Subsequently, a parametric study involving 133 numerical models was performed. Then, the suitability of the design approaches in EN 1993-1-12, AISI S100, AS/NZS 4600, and the direct strength method (DSM) was evaluated. For angle-section stub columns, the design approach outlined in EN 1993-1-12 was deemed suitable and the effective cross-sectional area design approach and DSM in AISI S100 were found to be safe. However, these approaches were excessively conservative for slender cross-sections in directly predicting the ultimate resistance of angle sections. Because of unsafe predictions, the design approaches in EN 1993-1-12, AISI S100, and DSM were deemed inappropriate for channel-section stub columns. The modification suggestions were proposed for the current design approaches, where the beneficial effects from the strain hardening were included. Based on the comparisons, the revised design approaches in EN 1993-1-12, AISI S100, and DSM could be used to accurately predict the ultimate resistance of the cold-formed Q1200 UHSS angle- and channel-section stub columns.