Compact Sections Research Articles

Modified plastic stress distribution method incorporating size effect for strength prediction of CFST members

Large-size concrete-filled steel tubular (CFST) members are widely applied in long-span and heavily-loaded structures due to increasing engineering demands. However, due to the size effect, applying the existing design methods to large-size CFST members may overestimate the load capacity, leading to unsafe designs. The plastic stress distribution (PSD) method is widely used as the primary method in typical design codes for calculating the strength of CFST members that are not susceptible to local buckling, while the PSD method was mainly used previously to evaluate the strength of CFST members with relatively small size. In this work, the experimental database of short CFST members with compact sections was complied, and the applicability of the PSD method was evaluated. The results showed that the test-to-calculated ratios decreased with increasing size of CFST members, and unsafe designs emerged for large-size CFST members. Based on the observations, the mechanics-based modified plastic stress distribution (MPSD) method was proposed, which not only considered the size effect at the material level similar to the way treated in most of the previous calculation methods, but also incorporated the size effect of the interaction action between the steel tube and concrete at the member level. Finally, the MPSD method was evaluated against the experimental database, and good agreement was achieved. These results provide strong justification for changes to the PSD method in design standards.

A new observable for holographic cosmology

The double-cone geometry is a saddle of the gravitational path integral, which explains the chaotic statistics of the spectrum of black hole microstates. This geometry is the usual AdS-Schwarzschild black hole, but with a periodic identification of the time coordinate; the resulting singularity at the black hole horizon is regulated by making the geometry slightly complex. Here, we consider generalizations of the double-cone geometry which include the Lorentzian cosmology that sits between the event horizon and the black hole singularity. We analyze this in two and three dimensions, where the cosmology has compact spatial sections and big bang/crunch singularities. These singularities are regulated in the same way by slightly complexifying the metric. We show that this is possible while satisfying the Kontsevich-Segal criterion, implying that these geometries can be interpreted as perturbatively stable saddle points in general relativity. This procedure leads to a novel description of the cosmology in terms of standard observables in the dual boundary CFT. In three dimensions, the cosmological solution gives a new contribution to the two-point function of the density of states in the boundary CFT. Unlike the usual double cone, it describes correlations between black hole microstates with different masses, and in a limit describes correlations between the statistics of heavy states and states near the BTZ threshold.

Seismic Design and Ductility Evaluation of Thin-Walled Stiffened Steel Square Box Columns

This paper investigates the seismic performance of thin-walled stiffened steel square box columns, modeling bridge piers subjected to unidirectional cyclic lateral loading with a constant axial load, focusing on local, global, and local-global interactive buckling phenomena. Initially, the finite element model was validated against existing experimental results. The study further explored the degradation in strength and ductility of both thin-walled and compact columns under cyclic loading. Thin-walled, stiffened steel square box columns exhibited buckling near the base, forming a half-sine wave shape. The research also addresses discrepancies from different material models used to analyze steel tubular bridge piers. Analysis using a modified two-surface plasticity model (2SM) yielded results closer to experimental data than a multi-linear kinematic hardening model, particularly for compact sections. The 2SM, which accounts for cycling within the yield plateau and strain hardening regime, demonstrated enhanced accuracy over the multi-linear kinematic hardening model. Additionally, a parametric study was conducted to assess the impact of key design parameters—such as width-to-thickness ratio (Rf), column slenderness ratio (λ), and magnitude of axial load (P/Py)—on the performance of thin-walled stiffened steel square box columns. Design equations were then developed to predict the strength and ductility of bridge piers. These equations closely matched experimental results, achieving an accuracy of 95% for ultimate strength and 97% for ductility.

Analyzing the Combined Effects of Axial Loads and Bending Moments on Unequal-Angle Steel Members

Angle steel is commonly used in inclined bracing components such as transmission towers. Due to its inherent section characteristics, unequal angle steel undergoes angular rotation when subjected to eccentric axial pressure, resulting in a change in the direction of action along the X and Y axes. In comparison to equal angle steel, the analysis of unequal angle steel is more complex. In design, particular attention should be given to whether the angle steel section is compact or noncompact, with a significant emphasis on the impact of the width-to-thickness ratio on strength. In study analyzed nine different angle steel sections, revealing that two of them exhibit a phenomenon known as control point transfer. This is especially prone to occur with shorter lengths of angle steel components, leading to Leg Local Buckling. In contrast, longer components are more susceptible to Lateral-Torsional Buckling. Summarizing the research findings, it is recommended to exercise caution when using non-compact section angle steel in design, and a preference for compact sections is advised to ensure the safety of the structure. Furthermore, it is advised to pay attention to the parameter βw in the AISC unequal angle steel calculation formula and adhere to relevant specifications to enhance the accuracy and reliability of structural design.

Behavior of UHPC filled high-strength square steel tubular stub columns under combined flexural and axial loads

This study experimentally and numerically investigated the behaviors of UHPC-filled high-strength square steel tube stub columns with compact, noncompact and slender sections under combined flexural and axial loads. Eighteen tests were conducted, addressing the gap of limited tests on concrete-filled square steel tubular stub columns with noncompact or slender sections. Additionally, finite element analyses were developed and benchmarked for parametric studies. The results of tests and parametric studies show that (1) steel tube outward buckling and concrete crushing occurred at 1/4–1/2L of the compression zone; (2) the load-axial displacement curves organized into three phases: elastic, elastic-plastic, and recession phase; (3) the compressive strength increased with increasing tube thickness, concrete strength, and steel yield strength. The compressive strength decreased with increasing eccentric ratio and slenderness coefficient. Test and numerical results are utilized to assess the suitability of current design specifications for predicting compressive strength. The evaluation showed that neither GB 50936–2014 nor Eurocode 4 provided reasonable predictions of strength, while AISC 360–16 provided conservative predictions.

Post-fire behavior and residual resistances of S890 ultra-high strength steel circular hollow sections under combined compression and bending

This paper presents experimental and numerical investigations into the post-fire local buckling behavior and residual resistances of S890 ultra-high strength steel (UHSS) circular hollow section (CHS) under combined compression and bending. The physical testing was firstly conducted and encompassed heating, soaking and cooling of specimens, as well as post-fire tensile coupon tests, measurements of initial local geometric imperfections and eccentrically loaded stub column tests. Then, finite element models were developed and validated with reference to the experimental observations and then adopted for parametric analyses to derive additional numerical data, considering various cross-section dimensions and loading combinations. As a result of the lack of design provisions for ultra-high strength steel structures after exposure to elevated temperatures, the applicability of the relevant ambient temperature design interactive curves given in the current European, American and Australian standards to post-fire S890 UHSS CHS under combined compression and bending was assessed, with post-fire material properties used. The assessment results showed that all design standards exhibited conservative and scattered predictions of residual resistances for S890 UHSS CHS under combined compression and bending at ambient temperature and after exposure to elevated temperatures. The American specification provided more precise predictions of residual failure loads for non-compact and slender (Class 3 and 4) sections than the Eurocode and Australian standard, due to the accurate compression and bending endpoints. In comparison to the American and Australian specifications, the Eurocode resulted in more precise results in predicting residual failure loads of compact (Class 1 and 2) sections, owing to the accurate compression and bending endpoints as well as the nonlinear design interaction curve.

Numerical and experimental studies on bending behavior of circular aluminium-concrete-steel double skin tubular cross-sections and bending capacity design approach

Considering the excellent corrosion resistance of aluminium alloy, a new-type aluminium-concrete-steel double skin tubular (ACSDST) member is proposed by replacing the outer steel tube of concrete-filled double skin steel tubular members with an aluminium alloy tube to improve structures' corrosion resistance and durability. Until now, no literature has been available on ACSDST cross-section in bending. This paper presents experimental and numerical investigations on the flexural behaviour of circular ACSDST cross-sections in bending. Broad parameters are examined in this research, including varying diameter-to-thickness ratios of the outer tube (20–120), hollow ratio (0.2–0.75), concrete compressive strength (30–70 MPa) and steel yield strength (235–355 MPa). The experimental results show that the final failure is controlled by the tensile rupture of aluminium alloy tubes, but it generally happens after the maximum moment, and the composite section behaves in good curvature ductility. The parametric analysis based on the developed finite element (FE) model discloses the different influences of diameter-to-thickness ratios of the outer tube, hollow ratio, and concrete and steel strengths on the bending moment capacity. Among them, the diameter-to-thickness ratio of the outer tube and hollow ratio significantly affect the bending moment capacity. The design equations for bending moment capacity are induced based on the plastic stress distribution method. Those equations are safe and accurate for the maximum moment (a widely used definition of bending moment capacity) within a certain range of parameters. However, a reduction factor 0.9 is proposed for the equations to predict the moment at a longitudinal flexural tensile strain of 1% (another widely used definition of bending moment capacity). Moreover, the cross-section slenderness limit for the compact ACSDST section is recommended based on the analysis of the experimental and FE results.

Design and beam dynamics simulation of an 8 MeV compact accelerator-driven neutron source

A compact accelerator-driven neutron source is proposed at Sino-French Institute of Nuclear Engineering and Technology, Sun Yat-Sen University, called Sun Yat-Sen University Proton Accelerator Facility (SYSU-PAFA). The proton accelerator is composed of a proton electron cyclotron resonance source, a four-vane radio frequency quadrupole (RFQ), and an alternative phase focusing drift tube linac (APF-DTL). It can accelerate 10 mA proton beam to 8 MeV. Due to the high current, beam matching is particularly important. In order to achieve beam matching between various components, beam transport sections are needed. The beam transport line is divided into three segments. The Low Energy Beam Transport (LEBT) ensures that the beam parameters are matched before entering the RFQ. The Medium Energy Beam Transport (MEBT) segment efficiently transfers the beam between the RFQ and DTL. The High Energy Beam Transport (HEBT) focuses on transporting the beam to the targets. The design goal of beam transport line is as short as possible while ensuring high efficiency of beam transportation. SYSU-PAFA has an overall transmission efficiency of 99%, with optimal transverse matching conditions between beam transport and RFQ or DTL accelerators. The efficient use of solenoids and magnets allows for a compact transmission section, resulting in a total length of 13.6 meters, shorter than most accelerators at the same beam energy. This paper will provide the detailed beam dynamics of the compact accelerator.

Alternating phase focusing beam dynamics for drift tube linacs

In contrast to conventional E-mode resonance accelerators, H-mode DTLs provide for compact linac sections and have been established as highly efficient resonators during the last decades. Thus, H-mode structures are widely applied for heavy-ion acceleration with medium beam energies because of their outstanding capability to provide high acceleration gradients with relatively low energy consumption. To build upon those advantages, an alternating phase focusing beam dynamics layout has been applied to provide for a resonance accelerator design without internal lenses, which allows for eased commissioning, routine operation, maintenance, and potential future upgrades. The features of such a channel are going to be demonstrated on the example of two interdigital H-mode cavities, separated by an external quadrupole triplet. This setup provides for heavy ion (mass-to-charge ratio A/z≤6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$A/z\\le 6$\\end{document}) acceleration from 300 keV/u to 1400 keV/u and is used as an injector part of the superconducting continuous wave accelerator HELIAC. Hence, this promising approach generally enables effective and compact routine operation for various applications, such as super heavy ion research, material science, and radio biological applications such as heavy-ion tumor therapy.

Vacua in locally de Sitter cosmologies, and how to distinguish them

(1+1)-dimensional locally de Sitter–Friedmann-Robertson-Walker cosmologies with compact spatial sections allow cosh, sinh, and exponential evolution laws, each with a freely specifiable spatial circumference parameter, and the value of this parameter has an invariant geometric meaning for the cosh and sinh evolution laws. We identify geometrically preferred states for a quantized massive scalar field on these cosmologies, some singled out by adiabatic criteria in the distant past, with an ambiguity remaining due to a massive zero mode, and others induced from the Euclidean vacuum on standard (1+1)-dimensional de Sitter space by a quotient construction. We show that a comoving quantum observer, modelled as an Unruh-DeWitt detector, can distinguish these states from the Euclidean vacuum on standard de Sitter space. Numerical plots are given in selected parameter regimes. We also evaluate the field’s stress-energy tensor expectation value for those states that are induced from the Euclidean vacuum by a quotient construction. Published by the American Physical Society 2024

Uma contribuição ao estudo da confiabilidade à flexão de vigas mistas de aço e concreto projetadas segundo a NBR 8800:2008

Abstract This work assesses the flexural reliability of compact sections and full-interaction steel-concrete composite beams designed according to NBR 8800:2008. Reliability indexes are obtained for different grades of concrete, slab heights, and load combination. The results show a lower level of safety in situations where live loads are predominant in the load combination. A partial safety factors calibration procedure is also proposed based on a target reliability index βT = 3.5. It is shown that the calibrated partial safety factors reduce the variability of reliability indexes in the proposed design situations.

Dynamic evolution of gas hydrate systems affected by magmatism and Quaternary mass-transport deposits in the Qiongdongnan Basin, South China Sea, and implications for the global carbon cycle

Methane release from hydrate systems has long been considered as one of the important mechanisms for global carbon cycle perturbations and ocean acidification. Here, a Quaternary active gas hydrate system in Qiongdongnan Basin, South China Sea (SCS), has been analyzed by using seismic, logging while drilling (LWD), and core data. The results show that post-rift magmatism led to the reactivation of a magma diapir, and the faults and fractures related to its emplacement created efficient pathways for deep gas migration. The increase in overburden temperature and circulation of deep thermal fluids, especially in the center of the diapir area, thinned the gas hydrate stability zone (GHSZ). The accumulation of natural gas at the base of GHSZ and the thinning of GHSZ decreased the ability of hydrate bearing sediments to seal free gas and lead to hydraulic fracturing. The free gas entering the GHSZ was buried by mass transport deposits (MTDs) and then formed hydrates due to the overlying compaction and so further sealed the free gas under the GHSZ. This study indicates that two contrasting mechanisms are acting: (1) tectonic activity and magmatism favor the release of deep natural gas which may reach the ocean, (2) MTDs and the overlying compacted sections act as seals, inhibiting the release of such gas to the ocean. This study shows how the seal capacity of MTDs may reduce methane release from deep-seated reservoirs into the ocean. Given their widespread distribution along continental margins, we suggest that the impact of MTDs on vertical fluid flow, and potential consequences for ocean chemistry and the global carbon cycle, might be underestimated.

Parametric Study of the Effect of Diameter-to-Thickness Ratio Against Bending and Shear Load on the Behavior of Round Hollow Structural Section Beam

The steel-constructed buildings in Indonesia are on the rise, indicating a growing preference for their durability and versatility in construction projects. This led to the general application of Round Hollow Structural Section (Round HSS) as column and beam elements. Therefore, this study aimed to conduct parametric analysis of the effect of Diameter-to-Thickness Ratio (D t-1) on the three-point and four-point flexural analysis of Round HSS beam using MSC Marc/Mentat software. Nonlinear materials and geometries were employed, along with the application of contact analysis, with contacted and contacting bodies. Moreover, the load and boundary condition were set to be similar to the experiment. The results showed that a greater D t-1 led to the possibility of withholding a smaller load and causing smaller displacement in Group A with fixed diameter and different thickness values. Meanwhile, in Group B with fixed thickness but different diameters, a greater D t-1 led to the potential of restraining more load and experiencing smaller displacement. All specimens from both groups were observed to have failed due to a combination of global and local buckling at the right location under the load applied. AISC bending moment calculated was found to be greater than the values obtained from the finite element analysis for all sections under three-point loads. It was also discovered in the four-point flexural analysis that the bending moment of noncompact section was greater than for AISC while those for the compact section were lower. Furthermore, shear strength (Vn) calculated was observed to be greater thanshear force from finite element analysis (Pmax/2).

Ultra-low cycle fatigue evaluation method for unstiffened steel piers using fiber model

The safety of steel bridge piers is threatened by ultra-low cycle fatigue (ULCF) damage, which dominates the failure mode in those with relatively compact section under strong seismic action. Existing methods for calculating such ULCF damage face the dilemma between accuracy and efficiency. To this end, this paper proposes a simplified evaluation method based on fiber model to expediently evaluate the ULCF damage of unstiffened steel piers, evenly with sound precision. Specifically, the average stress triaxiality is assumed as 0.63 for steel piers, and it is validated by cyclic loading experiments with the modified Coffin-Masson formula and solid element sub-model. The disparity between local plastic strain range in fiber and solid models is compensated by introducing a magnified coefficient. The analyzed results indicate the slenderness ratio, axial compressive ratio, especially the flange thickness and the width-to-thickness ratio, strongly influence the strain concentration effect occurring in the pier bottom. In addition, the proposed close equation for the magnified coefficient fits well with the discrete calculated points. The 3.05% mean absolute error is realized on the predicting half cycles when applying this method in random cyclic loading. Its favorable applicability in practical engineering is also verified with considering different ground motion types.

Nonlinear finite element analysis of partially encased composite columns under non-uniform moments

This paper presents a numerical investigation to evaluate the behavior of partially encased composite (PEC) columns with compact sections of three different heights submitted to non-uniform moment distribution. A 3D finite element model developed using the software ABAQUS was calibrated based on results from previous experimental research. The influence of concrete constitutive model parameters, column slenderness, load eccentricities, and moment distribution along height was investigated. The parametric study showed that constant moment distribution (case 1) is the most critical compared to linear moment variation (cases 2 and 3). This pattern is a direct consequence of the larger displacements at the mid-height section that causes higher second-order moments in the case 1 column. Long columns activated more confinement when a single curvature bending moment was applied at a minor axis. An increase in slenderness decreased the load capacity for non-uniform bending moments through the major axis.

Testing, numerical modelling and design of S890 and S960 ultra-high strength steel circular hollow sections under combined loading

The present paper reports the experimental and numerical investigations into the structural behavior and resistances of S890 and S960 ultra-high strength steel circular hollow sections (CHS) under combined loading. The experimental investigation was performed firstly, comprising the tensile coupon tests on S890 and S960 ultra-high strength steel materials, precise measurements of initial local geometric imperfections and eccentrically loaded tests on 12 stub columns, with each 6 respectively conducted on S890 and S960 ultra-high strength steel CHS. The numerical investigation was subsequently conducted, where numerical models were created and verified against the experimental responses and adopted to conduct parametric studies to produce more numerical data over an extensive variety of loading combinations and cross-section geometries. The design interaction curves, as codified in the European, Australian and American standards, were assessed for their applicability, based on the numerical and test data, to S890 and S960 ultra-high strength steel CHS under combined loading. The evaluation findings demonstrated that (i) the European code leads to the slightly conservative failure load predictions for Class 1 (and 2) S890 and S960 ultra-high strength steel CHS under combined loading, but overly conservative predictions for Class 3 and 4 counterparts, (ii) the predicted failure loads by American specification for non-compact (i.e. Class 3 and 4) are more accurate due to the more precise compression and bending end points and (iii) the resistance predictions by Australian standard for compact (i.e. Class 1 and 2) sections are rather conservative due mainly to its inaccurate design interaction curve.

Load introduction and transfer mechanism of K-type CFST circular section connections

This paper investigates the mechanism of load introduction and transfers for K-type concrete-filled steel tubular (CFST) circular section connections experimentally and numerically. Six K-type CFST connections were tested. Three-dimensional finite element (FE) models were then developed and validated against the test results, where the degradation and failure of the direct bond interaction were considered explicitly. The longitudinal strain distribution along the circumferential direction of chord-wall demonstrated that the non-uniform force transfer in the chord was caused by the one side load introduction through braces. The effects of the chord length, cross-sectional slenderness and interfacial interactions on the force transfer of tube-concrete interface were evaluated: 1) the chord length above the connecting region has a positive influence on the force transfer; 2) for the K-type CFST connections in this study, the material strength of concrete in the chord with non-compact and slender sections could not be fully utilized due to the insufficient force transfer; 3) the direct shear interaction dominated the force-transferring process from chord-wall to concrete for the compact section chord with reinforcing plates. Furthermore, the test and FE result confirmed that the load introduction length of the CFST chord with braces included the chord above the connecting region and the full connecting region. In addition, the equation of effective load introduction length for the CFST chord of the K-type connections was proposed.

An evaluation of slenderness limitations of H-beams for special moment connections

This paper presents the results of seismic connection testing and finite element analyses (FEA) on the slenderness limitations of steel H-beams in special moment-frame (SMF) connections. The effectiveness of FEA models was first validated by the full-scale testing of a reduced-web-section (RWS) connection, and a post-earthquake bolted-web-welded-flange (BWWF) connection. FEA simulations were then performed to study the beam section properties and seismic connection performance. The result shows that if a H-beam has a compact section, the adopted connection will develop strength and ductility meeting the AISC seismic requirements, even though the plastic section modulus ratio between the beam flanges and whole section is less than 70%. The result also suggests the effectiveness of adding a web opening to a BWWF connection. These findings call for a re-examination of the requirements of highly ductile members in SMF connections.

Lateral distortional buckling of continuous composite beams in fire situation: Experimental and numerical analyses

In continuous steel-concrete composite beams, hogging moments may occur near the internal supports. This type of bending causes compression in the bottom flange, which may buckle laterally in an ultimate limit state known as lateral distortional buckling. Despite the consideration that continuity may result in an economic design, no research had yet been conducted on the possibility of lateral distortional buckling in continuous composite beams in fire situation. This paper presents an experimental and numerical study on the lateral distortional buckling of steel concrete composite beams. Four real scale fire tests were conducted and proved that it may indeed occur. A parametric study was also developed. It has been shown that this buckling mode may lead to a reduction in the strength of composite beams of up to 50% when compared to the plastic moment. It was also noted that this buckling mode is usually accompanied by local buckling, which may occur even in compact sections, leading to a decrease in bearing capacity. These results indicate a need of reviewing the design codes to consider the possibility of lateral distortional buckling at high temperatures, as it may be more than at ambient temperature.

Finite element models with node-dependent kinematics based on Legendre polynomials for the global–local analysis of compact and thin walled beams

In the present work, enhanced one-dimensional finite elements with node-dependent kinematics are proposed for the static analysis. Finite element governing equations are derived by applying the Carrera Unified Formulation. This framework subdivides the three-dimensional displacement field into a cross-section domain and an axis domain. The dimension along the beam is discretized by using Lagrange-based shape functions. At each node of the element, an independent structural theory can be imposed, thus obtaining a node-dependent kinematic model. This method permits to focus on the finite element node. In this way, several combinations of kinematics can be used together. In particular, Taylor-based and Legendre-based expansions have been adopted in this paper to create global–local models without using special coupling methods. The results have been compared with well-established benchmarks from the literature. Compact section and thin-walled beams have been taken into account. Results have been given in terms of displacements and stresses. It is shown that the present model provides high accuracy with a reduced number of degrees of freedom.