CFRP strengthened butt-welded very high strength (VHS) circular steel tubes

This paper investigates the behavior of carbon fibre reinforced plastics (CFRP) strengthened butt-welded very high strength (VHS) circular steel tubes. The VHS steel has a yield stress of 1350 MPa and an ultimate strength of 1500 MPa. Three types of epoxy resins with different lap shear strength were used. Tests were conducted to determine the lap shear strength between CFRP and VHS steel tubes. A total of 21 butt-welded VHS tubes strengthened with CFRP were tested in axial tension. Three kinds of failure modes, i.e. adhesive failure, fiber tear and mixed failure were observed. The suitable epoxy adhesive for strengthening VHS tubes was recommended. A significant strength increase was achieved using CFRP–epoxy strengthening technique. A theoretical model was developed to estimate the load carrying capacity of butt-welded VHS tubes strengthened using CFRP.     

Section slenderness limits of very high strength circular steel tubes in bending

This paper investigates the bending behavior of very high strength (VHS) circular steel tubes. The tested VHS tube has a normal yield stress of 1350 MPa and an ultimate strength of 1500 MPa. A total of 12 specimens, with the normal diameter ranging from 31.8 to 75 mm and the tube thickness between 1.6 and 2.0 mm, were tested. Large bending deflection was achieved for VHS tubes with the diameter to thickness ratio up to 24. Full plastic moment was reached for tubes with the section slenderness of 197 or less. New plastic slenderness limits of 117 and 153 were proposed based on the beam plastic rotation capacity of 4 and 3. A yield slenderness limit of 264 was proposed for VHS tubes.     

Section capacity of very high strength (VHS) circular tubes under compression

This paper investigates the section capacity of very high strength (VHS®) circular tubes under compression. Full-section tensile tests were performed to determine the Young's modulus of elasticity, tensile yield stress and ultimate tensile strength. The modulus of elasticity was found around 200,000 MPa, the tensile yield stress (0.2% proof stress) was around 1350 MPa and the ultimate tensile strength was around 1500 MPa. Twelve stub column tests were carried out. The plate element slenderness varies from 86 to 130. It has been demonstrated that the limits on local buckling in most existing design standards are conservative when applied to VHS circular tubes. The reliability of section capacity based on the full effective section model has been verified using a FOSM (first order second moment) approach.     

Rational design analysis of stub columns fabricated using very high strength circular steel tubes

The mechanics of the compressional behaviour of stub columns fabricated from Grade 350 steel plates welded to very high strength (VHS) circular tubes are investigated in this paper. In the investigations comparisons of experimental load-axial shortening curves are made with the results of a postulated design analysis set up to examine the complete load-shortening behaviour of such members. Ramberg-Osgood type stress–strain equations are set up for the plate and tube materials. A procedure is described to evaluate the loads in individual plate and tube elements as the axial shortening is increased until the 0.2% proof strain for the VHS tube material is attained. Upon reaching this strain it is assumed that failure ensues as the result of a simple plastic mechanism occurring in the tubes. Comparisons of the load-axial shortening curves given by this analysis and experimental results indicate good agreement.     

Stability and ductility of thin high strength G550 steel members and connections

High strength cold-reduced steel is typically of stress grade G550 (550 MPa nominal yield and tensile strength) and less than 1 mm thick. The steel has been used for many years for sheeting and decking but is now being used for structural members such as roof trusses and stud walls of steel framed houses. The paper summarises a major research program on the stability and ductility of this steel which has been proceeding for several years at the University of Sydney. The paper relates the Sydney research to the work of others being undertaken around the world.     

Investigation of high strength steel connections with several bolts in double shear

A series of 26 tests on tension splices with three or four bolts positioned in the direction of bearing force are presented. Steel grade S690 was used for the fabrication of connection plates. The tests were also numerically simulated to obtain the distribution of forces between bolts. Tests on similar connections made of high strength steel were gathered from the literature. These tests were also numerically simulated. The numerical results agreed with experimental data very well, and were used to evaluate the bearing resistance formula according to EN 1993-1-8.     

Strength and ductility of high strength concrete-filled steel tubular beam-columns

The ultimate strength and ductility of high strength thin-walled concrete-filled steel tubular (CFST) beam-columns with local buckling effects, are investigated in this paper, using a performance-based analysis (PBA) technique. The PBA technique accounts for the effects of geometric imperfections, residual stresses, strain hardening, local buckling and concrete confinement on the behavior of high strength thin-walled CFST beam-columns. The accuracy of the PBA technique is further examined by comparisons with experimental results. The PBA program is employed to study the effects of depth-to-thickness ratio, concrete compressive strengths, steel yield strengths and axial load levels on the stiffness, strength and ductility of high strength thin-walled CFST beam-columns under combined axial load and biaxial bending. The results obtained indicate that increasing the depth-to-thickness ratio and axial load levels significantly reduces the stiffness, strength and ductility of CFST beam-columns. Increasing concrete compressive strengths increases the stiffness and strength, but reduces the axial ductility and section performance of CFST beam-columns. Moreover, the steel yield strength has a significant effect on the section and strength performance of CFST beam-columns but does not have a significant effect on their axial and curvature ductility.     

Performance and calculations of concrete filled steel tubes (CFST) under axial tension

This paper reports the behavior of concrete filled steel tubes (CFST) under axial tension. A total of 18 specimens were tested. The main parameters were steel ratio, concrete type and bond or unbonded between the steel tube and its core concrete. A finite element model (FEM) was developed to perform mechanism analysis and parametric studies for CFST under axial tension. It was found that the tensile strength of steel tube can be increased due to the existence of the core concrete in CFST. Finally, a simplified formula that can predict the tensile strength was proposed.     

Flexural strength analysis of non-post-tensioned and post-tensioned concrete-filled circular steel tubes

Concrete-filled circular steel tubes (CFT) have recently gained significant attention for their enhanced strength and ductility over the conventional steel and reinforced concrete construction. The concrete compressive strength is significantly increased by the lateral confinement provided by the steel tube and local buckling of the steel tube is restrained by the concrete infill. This paper investigates the further enhancement of these composite actions due to post-tensioning the concrete cores inside circular steel tubes. The flexural behavior of a post-tensioned CFT as well as a non-post-tensioned CFT was studied experimentally and analytically. A numerical algorithm for predicting the moment capacities of circular CFT, with or without the post-tensioning effects, has been validated against the test results by the authors as well as those published in the literature.     

Stability and ductility of high performance steel sections with concrete infill

This paper aims to investigate the stability and ductility characteristics of concrete filled columns using high performance steels (HPS). HPS includes both high strength steel and stainless steel which exhibit improved strength, as well as corrosion resistance, hardness etc. When such steel sections are filled with concrete the stability characteristics for both short and slender columns’ behaviour need to be modified to take account of the potential advantages for both local and overall bucklings. In addition, the concrete infill will also play a role in improving the post-peak behaviour of both short and slender columns and this needs to be considered. This paper will provide both an experimental and a theoretical treatment of these issues and will provide comparisons with existing design procedures, to illustrate the significant advantages which the juxtaposition of concrete provides to high performance steel sections.     

Field of application of high strength steel circular tubes for steel and composite columns from an economic point of view

This paper presents the results of a global comparison between high strength steel and normal steel circular tube used to build steel and composite columns submitted to static loading, as regards the economic aspects. The comparison is based on an optimum design taking into account the strength, stability and stiffness conditions of Eurocode 3 and 4. The automatic implementation of the algorithms allows achieving a high amount of case studies, covering the realistic possibilities of building columns. The investigations are realized on simple columns, columns included in braced or un-braced frames and whole frames. The field of application of high strength steel (vs normal steel), regarding the total cost of the member, is provided in a chart clearly indicating where the use of high strength steel becomes economic.     

Net cross-section design resistance and local ductility of elements made of high strength steel

Extensive experimental research on net cross-section failures of plates with holes (23 tests) and bolted connections (20 tests) made of high strength steel S690 is presented. The aim was to determine their local ductility and resistance. The verification of various strength functions and the determination of an appropriate partial factor γ_M was done by means of statistical analysis according to EN 1990, Annex D. Emphasis is given to the reliability of Eurocode provisions for the design resistance of net cross-sections.     

The suitability of round clinching tools for high strength structural steel

The use of light gauge steel framing is increasing rapidly. New methods for joining frames are being sought. One of the most promising methods is clinching. Clinching has been used for almost 20 years. There has been much research into the method, the tools, suitable materials and applications. Few articles have been published concerning clinching of high strength structural steels, which are currently the most relevant materials for house construction.This study included 11 different clinching methods. In total, 469 test pieces were produced. Despite the high strength of the materials, all the clinching methods that were examined could be used satisfactorily. The most significant result was that a round tool is appropriate for all the materials tested. In fact, in comparison with results obtained using other types of tool, when the methods were ranked by maximum shear load capacity, the round design appeared in the top three classes seven times out of nine.     

Influence of steel fibres on strength and ductility of normal and lightweight high strength concrete

This paper presents the results of a series of experiments conducted to investigate the effectiveness of fibre inclusion in the improvement of mechanical performance of concrete with regard to concrete type and specimen size. Lightweight aggregate concrete and limestone aggregate concrete with and without steel fibres were used in the study. The compressive strength of the concrete mixes varied between 90 and 115 MPa and the fibre content was 1% by volume. Splitting tests on prisms and three-point bending test on notched beams were carried out on specimens of varying sizes to examine the size effect on splitting strength, flexural strength and toughness.

The experimental findings indicate that the low volume of fibre has little effect on compressive strength but improve remarkably splitting tensile strength, flexural strength and toughness. The increase in splitting tensile strength, flexural strength and toughness index for lightweight concrete seems much higher than that of normal aggregate concrete.

The size effect on prism splitting tensile strength is not significant beyond a critical (transition) size. There are apparent size effects on flexural strength and toughness index. As the specimen size increases, splitting and flexural strengths appear to decrease, and fracture behaviour tends to be more brittle.     

钢纤维高强混凝土轴拉性能试验研究

完成了22组共110个钢纤维高强混凝土试件的轴拉试验。分析研究了钢纤维高强混凝土的轴拉强度和劈拉强度的关系,钢纤维高强混凝土轴拉性能随钢纤维体积掺量、基体强度及钢纤维类型的变化规律。给出了钢纤维高强混凝土轴拉应力-应变全曲线的数学模型,根据试验数据的回归分析确定了曲线相关的参数。研究成果对钢纤维高强混凝土在结构中的应用提供了依据。     

Prediction of the strength of bolted cold-formed channel sections in tension

This study is concentrated on the investigation of the shear lag effect on bolted cold-formed steel tension members. Channel sections with different dimensions tested by using bolted connections were discussed in this study. The comparisons were made between the test results and predictions computed based on several specifications. In order to study the stress distribution at the various locations of the cross-section of specimen, the finite-element software ANSYS was also utilized in this research. Based on the experimental results, it was found that the tension strengths of test specimens predicted by the AISC-Code [Load and resistance factor design specification for structural steel buildings, Chicago, IL, 1999], which takes into account the shear lag effect, provide good correlation with the test results for most specimens. The predictions according to AISI-Code [2001 Edition of the specification for the design of cold-formed steel structural members. Washington DC, 2001] and AS/NZS 4600 Code [Cold-formed steel structures, AS/NZS 4600:1996, Australia, 1996] seem to be overestimated as comparing to the test results. It is also noted that there is a quite discrepancy between the test results and the values predicted by both British Standard [British Standard: structural use of steelwork in building—part 5, Code of practice for design of cold-formed thin gauge sections, London, 1998] and Holcomb Recommendation [Tensile and bearing capacities of bolted connections, Second Summary Report, Civil Engineering Study 95-1. University of Missouri-Rolla, 1995]. The equation proposed by this study provides good agreement with test results.     

High strength circular concrete-filled steel tubular slender beam-columns, Part I: Numerical analysis

High strength circular concrete-filled steel tubular (CFST) slender beam-columns are frequently used in high-rise composite buildings because they possess higher strength and stiffness than normal strength ones. Most nonlinear inelastic methods of analysis for circular CFST slender beam-columns have not considered the effects of high strength materials and concrete confinement that significantly increases the strength and ductility of the concrete core. As a result, these methods produce computational solutions that deviate considerably from experimental results. This paper presents a new numerical model for predicting the nonlinear inelastic behavior of high strength circular CFST slender beam-columns under axial load and bending. The numerical model developed not only accounts for confinement effects on the concrete core and circular steel tubes but also incorporates initial geometric imperfections of beam-columns. Axial load-moment-curvature relationships obtained from the fiber element analysis of column cross-sections are utilized to determine the equilibrium states in the inelastic stability analysis of slender beam-columns. Computational algorithms are developed for determining the axial load-deflection and axial load-moment interaction curves for slender beam-columns. The numerical model is implemented in a computer program, which is shown to be an efficient and accurate simulation tool that can be used to investigate the fundamental behavior of high strength circular CFST slender beam-columns. The verification and applications of the numerical model are given in a companion paper.     

High strength circular concrete-filled steel tubular slender beam-columns, Part II: Fundamental behavior

There is relatively little experimental and numerical research on the fundamental behavior of high strength circular concrete-filled steel tubular (CFST) slender beam-columns. In a companion paper, a new numerical model for predicting the nonlinear inelastic behavior of high strength circular CFST slender beam-columns under axial load and bending was presented. The numerical model developed accounts for confinement effects on the strength and ductility of the concrete core and on circular steel tubes as well as initial geometric imperfections of beam-columns. This paper presents the verification of the numerical model and extensive parametric studies on the fundamental behavior of high strength circular CFST slender beam-columns. The ultimate strengths and axial load-deflection responses of circular CFST slender beam-columns under eccentric loading predicted by the numerical model are verified by corresponding experimental results. The computer program implementing the numerical model is used to investigate the fundamental behavior of high strength circular CFST slender beam-columns in terms of load-deflection responses, ultimate strengths, axial load-moment interaction diagrams, and strength increase due to concrete confinement. Parameters examined include column slenderness ratio, eccentricity ratio, concrete compressive strengths, steel yield strengths, steel ratio and concrete confinement. It is demonstrated that the numerical model developed is an efficient computer simulation and design tool for high strength circular CFST slender beam-columns. Benchmark numerical results presented in this paper are valuable in the development of composite design codes for high strength circular CFST slender beam-columns.     

Design of high strength steel columns at elevated temperatures

The main objective of this paper is to study the behaviour and design of high strength steel columns at elevated temperatures using finite element analysis. In this study, equations predicting the yield strength and elastic modulus of high strength steel and mild steel at elevated temperatures are proposed. In addition, stress-strain curve model for high strength steel and mild steel materials at elevated temperatures is also proposed. The numerical analysis was performed on high strength steel columns over a range of column lengths for various temperatures. The nonlinear finite element model was verified against experimental results of columns at normal room and elevated temperatures. The effects of initial local and overall geometrical imperfections have been taken into consideration in the analysis. The material properties and stress-strain curves at elevated temperatures used in the finite element model were obtained from the proposed equations based on the material tests. Two series of box and I-section columns were studied using the finite element analysis to investigate the strength and behaviour of high strength steel columns at elevated temperatures. Both fixed-ended stub columns and pin-ended slender columns were considered. The column strengths predicted from the finite element analysis were compared with the design strengths predicted using the American, European and Australian specifications for hot-rolled steel columns at elevated temperatures by substituting the reduced material properties. In addition, the direct strength method, which was developed for the design of cold-formed steel columns at normal room temperature, was also used in this study to predict the high strength steel column strengths at elevated temperatures. The suitability of these design rules for high strength steel columns at elevated temperatures is assessed. Generally, it is shown that the American and European specifications as well as the direct strength method conservatively predicted the column strengths of high strength steel at elevated temperatures. The European Code predictions are slightly more conservative than the American Specification and the direct strength method predictions.