Peak Axial Stress Research Articles

Axial stresses in pressure pipe liners spanning joints with initial gap, opening as a result of differential ground movements

The mechanical behavior of a pressure pipe liner is studied where it stretches across a ring fracture or joint. Peak axial stresses are calculated for the case where there is an initial gap between the ends of the host pipe, and/or when the gap extends as a result of ground movements. A finite element model is established and evaluated against results from available solutions. The minimum difference between the finite element and the existing closed form solution is 0.15%, and the maximum difference is 6.79%. A parametric investigation is presented considering various values of initial gap width, liner diameter, internal fluid pressure, friction coefficient between the liner and host pipe, and the liner thickness. Finally, a simplified equation for the maximum axial stress in the liner is fitted from 524 data sets. The results show that when there is an initial gap, longitudinal bending leads to development of a tensile stress on the outer face of the liner, which peaks at the midpoint of the gap, and compressive stress occurs on the outside of the liner at the edges of the gap. The initial gap can significantly reduce the ability of the liner to tolerate ground movements that subsequently lead to gap extension. The new equation for maximum axial stress can be used to predict the liner stress when a host pipe with an initial gap (which previous existing solutions do not address) experiences gap extension.

Confinement-dependent failure criterion for high-strength concrete in multiaxial stress states

High-strength concrete is becoming progressively wide applications in modern civil engineering structures because of its excellent mechanical properties and workability. However, most of the commonly adopted failure criteria were developed based on the low-strength concrete (cylinder compressive strength less than 40 MPa), and they may not accurately reflect the behavior of high-strength concrete due to the changes in its failure modes from intergranular failure to transgranular failure. The systematic analysis of databases consisting of 2120 experimental results of concrete multiaxial test collected from a wide range of existing literature has been carried out. It was found that the ratios of concrete ultimate strengths under uniaxial tension and equibiaxial compression to its uniaxial compressive strength (fc0) were negatively correlated with the uniaxial compressive strength. While under triaxial compression, the ultimate strength enhancement was affected by the uniaxial compressive strength and the degree of confinement (σm/fc0) simultaneously. On this basis, through the careful discussion of the morphological characteristics of concrete cracks after multiaxial failure, the concepts of the weakly and heavily confined states of concrete under triaxial compression were developed quantitatively, and the critical degree of confinement σm/fc0 = −0.8 was proposed. Under the heavily confined state, the ultimate strength enhancement of the high-strength concrete was less than that of the low-strength concrete, whereas there was no difference between them under the weakly confined state. A confinement-dependent failure criterion was formulated that utilized the piecewise compressive meridian to achieve a more rational description of the multiaxial ultimate strength for high-strength concrete. With the suggested failure criterion, a peak axial stress model for confined concrete was proposed. As verified by the experimental data of concrete confined by fiber-reinforced polymer (FRP) wraps from existing literature, which contains both high-strength and low-strength concrete, the proposed model showed better prediction accuracies with less scatters than the existing counterparts. This study can also be a reference for the research of the multiaxial failure criterion of other concrete-type materials.

Size effect of circular concrete‐filled stainless steel tubular short columns under axial compression

SummaryConcrete‐filled stainless steel tube (CFSST) members combine the advantages of stainless steel materials and concrete‐filled steel tube (CFST) members. Therefore, it has a broad range of applications than CFST members in the marine environment and other scenarios requiring great durability and corrosion resistance. However, there are limited researches on the large‐sized CFSST members. In this paper, 30 circular CFSST members with varying steel ratios (3.7% ≤ α ≤ 10.3%), diameters (500 mm ≤ D ≤ 900 mm), and strength of concrete ( = 40 MPa, 50 MPa) are studied on the size effect under axial compression. For peak axial stress, peak axial strain, and composite elastic modulus, size effects are investigated. According to the results, the peak axial stress and peak axial strain of the members increase with the increase in diameter. The modulus of composite elasticity essentially stays constant as the diameter increases, showing that there is no obvious size effect on the composite elastic modulus. The size effect of peak axial stress and peak axial strain is influenced by the steel ratio. Increasing the steel ratio tended to decrease the size effect. According to the generated data, it was found that the current codes of Chinese and European underestimate the ultimate bearing capacity of CFSST short columns significantly. To this end, the resistances of the large‐sized austenitic CFSST columns with a low steel ratio are well predicted by the proposed design model after being modified, based on GB 50936‐2014 and EN 1994‐1‐1 design codes.

Size effect of square concrete-filled stainless steel tubular short columns under axial compression

Concrete-filled stainless steel tube (CFSST) members combine the advantages of stainless steel materials and concrete-filled steel tube (CFST) members. Therefore, it has a broad range of applications than CFST members in the marine environment and other scenarios requiring great durability and corrosion resistance. However, there are limited researches on the large-size CFSST members. In this paper, 40 square CFSST members with varying steel ratios (4 % ≤α≤ 10 %), widths (500 mm ≤B≤ 900 mm) and strength of concrete (fcu = 40 MPa, 50 MPa) are studied on the size effect under axial compression. The size effect on the peak axial stress, peak axial strain, composite elastic modulus and ductility coefficient are investigated. Then this paper adopted the two-way analysis of variance test (ANOVA) to analyze the data. According to the results, the peak axial strain, composite elastic modulus and ductility coefficient have size effect, but the size effect in peak axial stress does not significant. It was found that the steel ratio will also affect the size effect of CFSST members, and the size effect decreases with the increase of steel ratio. Compared with the current CFST codes, the size effect coefficient included steel ratio is introduced can better predict the ultimate bearing capacity of square CFSST short columns with low steel ratio of range of 4–10 %.

Graphene nanoplatelets reinforced cement as a solution to leaky wellbores reinforcing weak points in hydrated Portland cement with graphene nanoparticles improves mechanical and chemical durability of wellbore cements

To improve the performance of wellbore cement under subsurface environments, graphene nanoplatelets (GNPs) were added in various percentages to Class-H cement slurry. Microstructural characterization of cement slurries cured at 90 °C and 95% RH indicates that GNP modifies the microstructure of hydrated cement by reinforcing pore spaces. As a result, the mechanical properties, such as Young's modulus and axial peak stress, obtained from high temperature triaxial compression tests are significantly improved based on different percentages of added GNPs. Furthermore, the hydrated 1 × 2inch GNP- Portland cement cores tested under simulated deep wellbore conditions of high-temperature and high pressure (HTHP), appear to have a ductile-like behavior, when compared to a typical brittle nature of Portland cement pastes. From our observations and published data on graphene resistance to fracture, GNP addition to cement is evidently enhancing the flexibility of cement. These improvements would reduce the risks associated with wellbore cement deterioration and a consequent leakage in fossil fuel production, geothermal energy production, CO2 storage as well as long-term sealing materials in plugging and abandonment of all wellbores at the end of their service life.

Behavior of BFRP strips confined PVC tubes with internal fillers under axial compressive load

Poly Vinyl Chloride (PVC) material, which exhibits high tensile capacity, economical cost, and excellent corrosion resistance, has attracted the interest of many researchers for using in construction engineering. Existing studies reveal that fiber-reinforced polymer (FRP) confinement is a high-efficiency method for strengthening concrete columns, which can be divided into FRP sheets fully and partially wrapped columns. In recent years, the combination of PVC tubes and FRP as a hybrid confinement system can effectively improve the mechanical properties of concrete columns. Rubberized concrete, which incorporates rubber waste as a partial replacement for conventional aggregates, can improve the ductility, and energy absorption, while lowering other qualities such as compressive strength, tensile strength, and elastic modulus. In order to overcome these shortages, this paper proposes a new type of composite column aimed for new construction, that rubberized concrete is filled into PVC tubes externally wrapped by basalt fiber-reinforced polymer (BFRP) strips. Its compression performance was investigated, and the test variables included the replacement percentage of rubber waste (i.e. 10% and 60%) and the net-spacing of FRP strips (i.e. 25 mm and 50 mm). The test results demonstrated that the PVC-FRP system considerably improved the load capacity and deformation capacity of rubberized concrete columns. The peak axial stress and axial strain increased obviously with the shortened net distance of FRP strips. Besides, the promotional effect of PVC-FRP confinement became more apparent when the mixture of rubber particles in concrete was enhanced. A modified efficiency coefficient was given at the end of this paper which can provide a comparative accurate prediction of the compressive strength for PVC-FRP confined rubberized concrete.

Mechanical properties and failure characteristics of granite treated with a combined water–air cooling cycle

Granite is the main material used to enhance geothermal reservoirs, and its mechanical properties play a key role in the safe and efficient development of geothermal materials. High temperature and cold shock issues are often encountered during the process of geothermal development. For this reason, numerous studies have been conducted on the mechanical properties of granite treated at different temperatures and with various cooling methods. In addition, granite is often affected by combined water-air cooling, which affects the development direction and stability of reservoir heat transfer channels. However, few studies on the mechanical properties of granite treated with combined water-air cooling have been conducted thus far. In this study, uniaxial compressive tests were conducted on granite to study the effect of localized water cooling at different temperatures and numbers of heating-cooling cycles. The results show that the thermal cracks in granite subjected to combined cooling treatments exhibited regional distribution characteristics with temperature and cycle times; there were no apparent thermal cracks at 200 °C, thermal cracks were mainly distributed in the water-cooling zone between 300 and 400 °C, and thermal cracks were distributed throughout the granite specimens above 500 °C as the number of cycles increased. Within 300 °C, the peak axial stress and Young's modulus of the granite exhibited slow linear decreasing trends with temperature and cycle times, but the peak axial stress and Young's modulus exhibited nonlinear decreasing characteristics when the temperature exceeded 400 °C. The failure mode gradually changed from brittleness to ductility. Above 500 °C, thermal cracks near the interface of the water-cooling and air-cooling zones became denser than those in the other two areas (the water-cooling and air-cooling zones) as the number of cycles increased, and the splitting failure during loading failure shifted from the water-cooling zone to the interface of water cooling and air cooling. The temperature condition for fracture and peeling between the air-cooling and water-cooling zones of the granite was 600 °C.

Numerical investigation of residual stresses in welded joints of cylindrical shell

Welded cylinder structures such as pressure vessels and pipes for transportation have been applied in power stations, aerospace, and shipping industries. The study of weld-induced residual stress is vital in predicting the life of welded cylinder vessels. Depending upon the required diameter and length, they are welded by either circumferential welding or longitudinal welding. In the present work, a sequentially coupled thermal, structural analysis is carried out on circumferential and longitudinal butt weld joints of AH-36 cylinder components. The thermal field distribution and subsequent residual stresses during Gas Tungsten Arc Welding (GTAW) are studied. The moving heat source considered for analysis is based upon Goldak’s double ellipsoidal model. The weld-induced axial and hoop stresses are evaluated on both the outer and inner surfaces of the cylinder. The results for circumferential and longitudinal butt weld joints are compared. The magnitude of peak hoop and axial stresses in longitudinal butt weld joints are 45% and 95% higher than in circumferential butt weld joints. The developed analysis model, used to evaluate the thermal histories and residual stresses, is validated with experimental measurements.

Size effect on compressive behaviour of stirrup-confined concrete columns

Numerous studies have indicated the existence of a size effect on the axial compressive behaviour of stirrup-confined concrete columns. However, most of these studies have focused on nominal compressive strength. Investigations of the size effect on the axial strain (at peak load) and of the descending branch are limited. In this study, the size effect behaviour of square stirrup-confined concrete columns under axial compression was explored, using a three-dimensional mesoscale simulation method. Based on the numerical and available experimental results, the influence of specimen size on peak axial stress (i.e. compressive strength), corresponding strain and softening rate was explored. Moreover, quantitative relationships between specimen size and peak axial stress, corresponding strain and softening rate for circular and square stirrup-confined concrete columns were derived. Finally, considering the size effect on the peak axial stress, corresponding strain and softening rate, a novel stress–strain model describing the axial compression behaviour of stirrup-confined concrete was developed. The proposed model was verified by comparison with the available experimental results and existing models.

Experimental investigation on the triaxial behavior of lightweight concrete

In this research, active true triaxial tests with different stress ratios were conducted on cubic specimens made from structural lightweight aggregate concrete (LWAC) to study their mechanical behavior. Different combinations of normal stresses were applied to the specimens to obtain various load paths and load angles. Force and displacement values were recorded during the tests, and stress–strain curves were plotted at each principal direction. Furthermore, the obtained peak stresses at failure points were used to draw the compressive and tensile meridians and failure surfaces in the deviatoric plane. Results show that different combinations of lateral confining stresses have a remarkable effect on the strength and deformation of LWAC under triaxial stresses. Results of the tests intended to capture the compressive meridian show that the variation of lateral confining stresses in a range of (0 – 0.98)fcu yields in peak axial stresses, which vary in a range of (1.00–4.38)fcu. In contrast, for the tests aimed to obtain the tensile meridian, the change in the lateral confining stress in a range of (0–0.25)fcu leads to the peak axial stress variations in the range of (1.14–1.92)fcu. Comparison of different types of LWAC shows that increasing the uniaxial strength of LWAC leads to improvement in their triaxial compressive characteristics. Finally, the model parameters of the well-known Willam-Warnke five-parameter failure criterion for normal concrete were modified for LWAC based on the test results. The experimental test data of the current study can be used for the development of other constitutive models for LWAC.

Probabilistic calibration of strength and strain enhancement models for FRP-confined concrete in circular section

Lateral confinement with fiber-reinforced polymer (FRP) jackets can effectively improve the axial compressive behavior of concrete but with considerable variability in outcomes. Therefore, that there is a need for calibrations of deterministic models for strength enhancement (SEE) and strain enhancement (SAE) based on proposed probabilistic prediction models and comprehensive available databases. The probabilistic models that incorporate the essential factors identified from the previous study were updated based on the Bayesian theory, and Markov Chain Monte Carlo (MCMC). Moreover, nine representative deterministic SEE models and six SAE models were evaluated by credible interval (CI) and confidence level (CL) under different conditions of the axial strain of unconfined concrete in the literature, the hoop rupture strain, the peak axial compressive stress of unconfined concrete, and the lateral confinement stiffness. Analysis of different FRP types was also conducted individually for more critical results. The proposed probabilistic models are capable of predicting the characteristics of ultimate axial stress and corresponding strain and providing an efficient approach to calibrate the confidence level and computational accuracy of deterministic models in literatures.

Experimental investigation of rigid confinement effects of radial strain on dynamic mechanical properties and failure modes of concrete

In this study, to confirm the effect of confining pressure on dynamic mechanical behavior and failure modes of concrete, a split Hopkinson pressure bar dynamic loading device was utilized to perform dynamic compressive experiments under confined and unconfined conditions. The confining pressure was achieved by applying a lateral metal sleeve on the testing specimen which was loaded in the axial direction. The experimental results prove that dynamic peak axial stress, dynamic peak lateral stress, and peak axial strain of concrete are strongly sensitive to the strain rate under confined conditions. Moreover, the failure patterns are significantly affected by the stress-loading rate and confining pressure. Concrete shows stronger strain rate effects under an unconfined condition than that under a confined condition. More cracks are created in concrete subjected to uniaxial dynamic compression at a higher strain rate, which can be explained by a thermal-activated mechanism. By contrast, crack generation is prevented by confinement. Fitting formulas of the dynamic peak stress and dynamic peak axial strain are established by considering strain rate effects (50–250 s−1) as well as the dynamic confining increase factor (DIFc).

AN EXPERIMENTAL STUDY ON FRACTAL PORE SIZE DISTRIBUTION AND HYDRO-MECHANICAL PROPERTIES OF GRANITES AFTER HIGH TEMPERATURE TREATMENT

This study experimentally estimated the fractal pore size distribution of granites, which is then linked to the hydro-mechanical properties of rocks treated by temperatures that range from 25[Formula: see text]C to 900[Formula: see text]C. The mercury intrusion experiment was carried out to characterize the pore size distributions and the MTS815.02 triaxial testing system was used to investigate hydro-mechanical properties of rocks. Finally, the micro-CT scanning system and scanning electron microscope system were employed to exhibit the evolutions of microstructures of cracks that were then linked to the macroscopic hydro-mechanical properties. The results show that the pore size distribution of granites follows the fractal scaling law and the fractal dimension ranges from 2.45 to 2.94. The fractal dimension decreases significantly when the temperature increases from 25[Formula: see text]C to 100[Formula: see text]C and then holds a constant with continuously increasing the temperature to 400[Formula: see text]C. The fractal dimension slightly increases as the temperature increases from 400[Formula: see text]C to 500[Formula: see text]C and decreases following a linear relationship until the temperature of 900[Formula: see text]C. The porosity obtained by the mercury intrusion experiment shows an exponential relationship with the fractal dimension. Both the axial stain and peak total stain, as well as the initial permeability and the permeability at the peak axial stress, have quadratic functions with the fractal dimension. The mean aperture of fractures increases from 56.75[Formula: see text][Formula: see text]m to 76.10[Formula: see text][Formula: see text]m with increasing the temperature from 100[Formula: see text]C to 900[Formula: see text]C through micro-CT scanning. The scanning electron microscope test clearly shows that the fractures are generated when the temperature exceeds 300[Formula: see text]C, which agrees well with the micro-CT scanning results. With increasing the temperature from 400[Formula: see text]C to 900[Formula: see text]C, both the number and aperture of fractures increase, which well interprets that both the strain and permeability increase as indicated by the triaxial stress-flow tests.

Decrypting healed fault zones: how gouge production reduces the influence of fault roughness

SUMMARY Two key parameters control the localization of deformation and seismicity along and surrounding crustal faults: the strength and roughness of the pre-existing fault surface. Using 3-D discrete element method simulations, we investigate how the anisotropy and amplitude of roughness control the mechanical behaviour of healed faults within granite blocks during quasi-static triaxial compression. We focus on models in which the uniaxial compressive strength of the healed faults is about 25 per cent of that strength of the surrounding host rock. These models provide insights into the evolution of fracture network localization, fault roughness, gouge production, fault slip and stress concentrations along initially healed faults of varying roughness. In contrast to expectations, the uniaxial compressive strengths of models that host faults with root-mean-squared roughness amplitudes of 0.2–1.4 mm do not vary more than the change produced by variations in particle packing. To assess if this lack of influence arises from the evolving roughness of the faults, we track the roughness amplitudes parallel and perpendicular to the downdip direction throughout fault failure and slip. The de facto roughness does not provide an explanation for the lack of influence of roughness on compressive strength because the roughness of the faults does not evolve to similar values with slip. Rather, smoother faults remain smoother than rougher faults throughout the simulation. However, the rougher faults produce larger volumes of gouge than the smoother faults. The gouge lubricates the fault and thereby reduces the influence of roughness on compressive strength. These observations suggest that fault topography and the asperities that build this topography do not exert a significant impact on deformation. To quantify the influence of asperities on slip, we calculate correlation coefficients between the fault surface topography and components of the slip vectors. The observed negative correlation coefficients between the fault topography and fault-plane parallel slip quantify the degree to which asperities slow slip in the downdip direction. The observed positive correlation coefficients between the topography and fault-plane perpendicular movement quantify the degree to which asperities promote opening. Thus, this analysis shows how asperities control slip by acting as speed bumps that hinder fault-plane parallel slip and promote fault-plane normal opening as the healed faults slide. The asperities do not significantly control fault movement during the unlocking and failure of the healed faults, but only following the peak axial stress as the faults slide and damage zones develop. These models thus provide unparalleled access to the dynamics of reactivated healed faults.

Experimental Study on the Effect of Freeze‐Thaw Cycles on Axial Tension and Compression Performance of Concrete after Complete Carbonization

The effect of freeze‐thaw cycles on the axial tension and axial compression properties of completely carbonized concrete are investigated in this study. Three grade concrete specimens (C30, C40, and C45) were fabricated. The freeze‐thaw cycle test was carried out on the completely carbonized specimens, followed by axial tension and axial compression tests. The results show that completed carbonization increases the axial tensile peak stress of C30, C40, and C45 concrete specimens by 8.7%, 9.7%, and, 12.1%, respectively. The peak axial tension strain increased by 1.9%, 7.2%, and 9.6%, respectively. The peak axial compressive stress increased by 10.5%, 19.1%, and 24.1%, respectively. The peak axial compressive strain decreased by 13.7%, 14.1%, and 14.3%, respectively. With the increase of freeze‐thaw cycles, the peak stress of tensile stress, peak strain, and compressive stress of concrete decrease continuously. The peak strain of compressive strain increases. The lower the strength grade of concrete, the faster the decline rate of stress and strain. According to the data changes of peak stress and peak strain at different times of freeze thaw after carbonization, the stress‐strain curve fitting formula for concrete under freeze‐thaw cycles after complete carbonization is put forward, which has a good coincidence with the experimental result.

Study on Mechanical Properties and Cracking Mode of Coal Samples under Compression–Shear Coupled Load Considering the Effect of Loading Rate

Under coupled compression–shear loading, the failure and instability behavior of inclined pillars is different from that of horizontal pillars. To enhance the reliability and accuracy of pillar strength design, the influence of different inclination angles and loading rates on mechanical property and the failure behavior of inclined pillar should be studied. In this paper, the combined compression and shear test (C-CAST) system was developed, and mechanical properties and macro failure behavior of coal samples under different inclination angles and loading rates were studied, and acoustic emission (AE) technology was used to determine the internal cracking mode of the sample. The results show that with the increase of inclination angle, the peak shear stress of coal sample increases gradually, while the peak axial stress and elastic modulus slightly increase first and then decrease, and reach the maximum value at an inclination angle of 5°. Within the inclination angle range of 0°–15°, with the increase of loading rate, the peak axial stress and elastic modulus of coal samples first increase and then decrease, while the loading rate corresponding to peak axial stress and elastic modulus decreases. Within the inclination angle range of 20°–25°, the peak axial stress and elastic modulus of the sample gradually decrease with the increase of loading rate. The failure mode of coal samples changes from tension-splitting failure (0°–5°), tension–shear composite failure (10°) to single shear failure (15°–25°). Meanwhile, the loading rate has little effect on the failure mode of coal samples, but has a significant effect on the failure degree. When the loading rate is 1.0 and 10 mm/min and the inclination angle ranges from 0°–5°, the proportion of tensile crack is significantly greater than that of the shear crack, and tensile failure is the main failure mode; when the inclination angle ranges from 10°–25°, the proportion of shear crack is more than 50% and increases gradually with the increase of inclination angle, and shear failure is the main failure mode. This law is consistent with the macroscopic failure mode of the sample.

Experimental investigation on residual stress distribution in an engineering-scale pipe girth weld

The hoop and axial residual stress maps in an engineering-scale narrow-gap austenitic stainless pipe girth weld were obtained with the two-cut contour method and the stress distribution characteristics are discussed. Results show that the through-thickness hoop stress at the weld centreline demonstrates a bending-type distribution; the cap beads and the root weld have great effects on the axial stress distribution pattern at the weld centreline; the weld start/end cause stress fluctuation in the cap weld; the locations of peak hoop stress and axial stress do not change significantly within a large range of R/t and heat inputs for stainless girth welds.

Hysteretic Behavior of Geopolymer Concrete with Active Confinement Subjected to Monotonic and Cyclic Axial Compression: An Experimental Study.

This paper investigated the performance of actively confined geopolymer concrete (GPC) through experiments. The mechanical properties of GPC under triaxial stress states were analyzed and discussed from the prospects of failure modes, axial peak stress and strain, monotonic and cyclic constitutive relationships. The experimental results demonstrated that the loading modes (monotonic loading and cyclic loading) had little effect on the failure mode and axial peak stress and strain. The improvement of the strength and ductility of GPC with the increase in confinement level was consistent with that of the conventional cement concrete while the strain enhancement of confined GPC was lower than that of confined conventional cement concrete at the same confinement level. The curves of the monotonic stress–strain and the envelop of cyclic compression were predicted through Mander’s model with good accuracy. The unloading/reloading models proposed by Lokuge were modified and the predicted cyclic hysteresis curves for actively confined GPC were in good agreement with the cyclic compression results. Findings from this study provide references for the application of geopolymer concrete.

Influence of chord compactness and slenderness on axial compression behavior of built-up battened CFS columns

This study presents the behavior of built-up cold-formed steel (CFS) columns under axial compression loading. Two CFS unstiffened channel sections arranged in the toe-to-toe configuration are used as the chord members interconnected by the batten plates. An extensive parametric study has been conducted on 230 numbers of built-up battened CFS columns using a finite element software ABAQUS. The numerical models developed have been calibrated against the test results available on CFS battened columns with plain channels in the back-to-back arrangement reported in the literature. The main parameters varied in the numerical models are the width-to-thickness ratios of chords, overall column slenderness ratios, and the toe-to-toe spacing of chords. The behavior of built-up columns is studied in terms of peak axial compressive strengths, mode of failure, post-peak resistance, and lateral drift. The chord width-to-thickness ratios have a predominant effect on the peak axial stresses of short columns, and a meager effect on long columns. Also, large lateral displacements can be avoided by restricting the overall column slenderness to 75. Numerical results are compared with the design strengths predicted by North American Standards (NAS) and Eurocode (EC3). Both these standards are found to be un-conservative in predicting the axial strengths of short built-up columns in the range of 15–30%. Finally, the design recommendations are proposed for built-up battened CFS columns with channel chords in toe-to-toe configuration, which are verified through reliability analysis as well.

Thermal effect of load platen stiffness during high-temperature rock-mechanical tests

The accuracy of rock property measurements in real-time heating loading tests has been considered as questionable due to the thermal effect on loading platens. In this study, the impact of steel platen performance at high temperatures (up to 1000 °C) on granite property measurements is analysed by numerical simulations. Simulation results show that a smaller stiffness of the loading platen can result in specific stress redistributions, increasing micro cracks, and peak axial stress reduction of the sample during uniaxial compression. This can subsequently lead to an underestimation of failure stress and strain. However, the decreasing platen stiffness with rising temperature does not produce any remarkable impact on the temperature-dependent property measurements of the selected Eibenstock granite (EG) heated up to 1000 °C. This is related to the progressively decreased strength of EG and the consequently negligible deformation of the softened platen at elevated temperatures. Therefore, the loading platen can still be treated as ideal stiff. Considering the big variations in strengths and thermal response of different rock types, the suggested procedure to obtain reliable rock properties at high temperatures (e.g. above 800 °C) is to perform an evaluation by replicating the lab tests via numerical simulations.