Yongjiu Shi

Residual Stress Tests of High-Strength Steel Equal Angles

AbstractResidual stress is one of the most important imperfections of steel structure members because of its significant effects on buckling behavior. To quantify the residual stresses in 420-MPa high-strength steel hot-rolled equal angle sections, an experimental study was conducted by using the sectioning method. The residual stress magnitudes and distributions for 15 sections were obtained, and the effects of the width-thickness ratios were clarified. Based on the test results, it was found that the ratio between the residual stress and the steel yield strength for 420-MPa steel equal angles was much smaller than that of normal strength steel angles; however, the distribution was analogous. The residual magnitudes significantly correlated with the width-thickness ratios of the legs. In addition, calculation formulas for the residual stress magnitudes were proposed in which the width-thickness ratios of the angle legs were taken into account, and three distribution models were established to be incorpor...

Finite Element Analysis and Tests on Bolted End-Plate Connections in Steel Portal Frames

Bolted end-plate connections are being widely used in steel portal frames. In this paper, some tests on beam-to-column and beam-to-beam end-plate connections are reported and the corresponding finite element analysis (FEA) is carried out to analyze their loading resistance and rotational stiffness characteristics. The design method about extended end-plate connections in current design code is discussed, and the necessary improvements are proposed. Especially, some precautions about the classification criterion of the joint stiffness in steel structures are recommended. From the results of comparison, one new model of the bolt tension force distribution and the corresponding design method are presented. It has been verified by the tests that this method can calculate the actual stresses of the bolts and the end-plates more accurately and mobilize their loading capacities more efficiently with sufficient safety and reliability.

Recent Developments on High Performance Steel for Buildings

With structural steel becoming the main construction material in China, more high performance steel (HPS) is going to be applied in landmark building projects. Innovative structural systems and structural members need to be envisaged to take the advantages of HPS. The excellent properties of HPS with higher strength, moderate ductility, higher fracture toughness, better weldability, efficient machinability, can improve the structural performance significantly. The Q460 grade of structural steel has been successfully applied in the National Stadium and CCTV tower in China. It is demonstrated that the fabrication and construction efficiency of steel buildings can be improved remarkably, and economical solutions are provided by using HPS. This paper reviewed the availability of HPS in China and summarized the recent development to promote HPS in tall and large span buildings. The requirements on research to support the application of HPS in buildings are discussed.

FEM Analysis and Experimental Study on Monolayer Cable Net for Glass Facades: Static Performance

Monolayer cable nets have been widely applied in point-supported glass facades in recent years because of transparency, light weight and flexibility of construction. In this paper, work on the static load bearing performance of this type of cable net is described through the use of finite element analysis (FEA) and tests on a 1/10–scale model of a real project in P.R. China under construction. Firstly a method of analysis inverse to the sequence of erection and much different from conventional methods, is used to model the pre-stretching of cables with the aid of FEA to provide guidelines for the tests. Cable pre-stretching proceeds by three stages, then load bearing behaviour tests on the cable net are carried out both before and after the installation of glass panels. The agreement between the pre-stretching tests and the FEA analysis indicates that the inverse analysis method is convenient in calculation, and provides useful references for the pre-stretching of similar types of cable net in practical engineering. The nonlinear relationship found between loads and displacements just indicates the nonlinear properties of flexible cable nets. Before the installation of glass panels, the structural stiffness actually increases slowly with increasing external loads, and greater initial pre-tension is more beneficial to load bearing capacity of the structure. But too large an initial pre-tension may overload the supporting structures of the cable net. The increase in cable force due to external loads is small. Nevertheless, the stiffness of the glass panels themselves make a significant contribution to the overall structural stiffness, the contribution which becomes less obvious as the stiffness of the cable net itself increases.

Mechanical properties and fracture toughness of rail steels and thermite welds at low temperature

Brittle fracture occurs frequently in rails and thermite welded joints, which intimidates the security and reliability of railway service. Railways in cold regions, such as Qinghai-Tibet Railway, make the problem of brittle fracture in rails even worse. A series of tests such as uniaxial tensile tests, Charpy impact tests, and three-point bending tests were carried out at low temperature to investigate the mechanical properties and fracture toughness of U71Mn and U75V rail steels and their thermite welds. Fracture micromechanisms were analyzed by scanning electron microscopy (SEM) on the fracture surfaces of the tested specimens. The ductility indices (percentage elongation after fracture and percentage reduction of area) and the toughness indices (Charpy impact energy Ak and plane-strain fracture toughness KIC) of the two kinds of rail steels and the corresponding thermite welds all decrease as the temperature decreases. The thermite welds are more critical to fracture than the rail steel base metals, as indicated by a higher yield-to-ultimate ratio and a much lower Charpy impact energy. U71Mn rail steel is relatively higher in toughness than U75V, as demonstrated by larger Ak and KIC values. Therefore, U71Mn rail steel and the corresponding thermite weld are recommended in railway construction and maintenance in cold regions.

Fracture behavior analyses of welded beam-to-column connections based on elastic and inelastic fracture mechanics

Detailed two-dimensional finite element models are applied to investigate the fracture behavior of welded beam-to-column connections which are widely used in steel structures in China. The elastic stress intensity factor (KI) and inelastic J-integral (JI) around the crack tip are calculated to provide quantitative estimates of fracture toughness requirements in the connections with different configurations. Four types of weld access holes are modeled after the full-scale connection tests. Subsequently, several connection details including weld flaw size and location, weld backing bar, fillet reinforcement and other modifications, are examined in the effectiveness of reducing the toughness demands on the weld. The analyses have confirmed the observations from connection cyclic tests that the fracture is more likely to occur at the beam flange weld heat affected zone (HAZ) which sustains extensive yielding and exhibits relatively low fracture toughness. The weld flaw size and location have shown significant effects on the toughness requirements both by elastic and inelastic analyses. The modification of removing backing bar and reinforcing a fillet weld shows the most effective approach to reduce the toughness demands. By inelastic analyses, the initial weld defect at HAZ (10 mm away from the column face) shows more toughness demands than that at the interface of the column flange and weld. The evaluated moment at column face when crack predicted to propagate, agrees well with the ultimate moment obtained from the connection tests.

Monotonic Loading Tests on Semi-Rigid End-Plate Connections with Welded I-Shaped Columns and Beams

8 specimens of beam-to-column bolted end-plate connections with various details and welded I-shaped section columns and beams were tested under monotonic loads. Based on the specification for the end-plate connection rotation proposed, the contribution of the panel zone and the gap between the end-plate and column flange to the joint rotation have been obtained. The influences of both flush and extended types, column stiffeners, end-plate rib stiffeners, bolt sizes and end-plate thicknesses etc on the joint behaviour have been compared. The test results indicate that all these connections tested are typically semi-rigid. For the flush and extended end-plate connection, the joint rotation mainly derives from the relative deformation between the end plate and the column flange and the shearing deformation of the panel zone respectively. Using a special method to measure bolt strain, the bolt tension forces and their development and distribution have been depicted and studied, and two bolt tension force distribution models for end-plate connections using pretensioned bolts are recommended.

Experimental study on seismic behavior of 460MPa high strength steel box-section columns

In order to study the seismic behavior of 460MPa high strength steel (HSS) box-section columns, five specimens with equal-width box-section were examined under constant axial load and cyclic horizontal load simultaneously. The axial load ratio and width-to-thickness ratio were designed as major parameters in this investigation. Failure mode, hysteretic curve, energy dissipation capacity and ductility were analyzed and compared. The experimental results showed that local buckling initiated at flange and followed by web. Under the same axial load ratio, as width-to-thickness ratio increased, local buckling occurred at a lower displacement level and the displacement levels at both peak load and failure load decreased. On the other hand, with the increase of width-to-thickness ratio, the range of local buckling wave became smaller, and the position of buckling center became closer to fixed end. It is also evidenced that the full extent of hysteric curve, energy dissipation capacity, ductility of specimens and ultimate story drift angle decreased with the increase of width-to-thickness ratio and axial-load ratio. According to the experimental results and comparative study, limiting values of width-to-thickness ratio for flange plate of Q460 box-section may be suggested as 25 for applications with large inelastic deformation (i.e. large earthquake action) and 32 for applications with limited inelastic deformation (i.e. moderate earthquake action) in the case that axial-load ratios are no more than 0.2.

FEM Analysis and Experimental Study on Monolayer Cable Net for Glass Facades: Dynamic Properties

Study of the dynamic properties of a monolayer cable net is described in this part II of the paper. The linear vibration properties of a cable net are analyzed in the form of a lumped-mass system based on the assumption of small-amplitude vibration. The respective influence of two vital factors: geometric shape and cable pretension on the dynamic properties of cable nets at different deflected positions under load is especially focused upon. Results show that when the geometric shape has a small displacement, the displacement influence on frequencies of vibration is almost the same as that of pre-tension, otherwise, the influence of geometric shape is far greater than that of pre-tension. Dynamic tests on the cable net model mentioned in Part I were also carried out at each pre-stretching stage. The results of experiments agree well with those of finite element analysis (FEA). In addition, a modified method called “MML” based on Rayleighs approach for the calculation of fundamental frequency is put forward in this paper, since it was found that Rayleighs approach may sometimes introduce large errors. The nonlinear dynamic properties of the cable net were also analyzed. Different from the linear vibration case, nonlinear cases are to some extent amplitude-dependent. Thus explicitly approximate formulae for the first and second nonlinear modal frequencies are derived from continuous theory assuming the cable net to act as a continuous membrane. Transient dynamic analysis using the finite element software ANSYS was carried out to validate the formulae. The results given by the formulae and those via numerical analysis agree with each other well.

A New Method for System Fatigue Reliability Analysis of Offshore Steel Jacket

Fatigue Failure is a common failure mode for offshore steel jacket and the system fatigue reliability analysis for this type of structure has been developed greatly in recent year. Nevertheless none of those methods gives satisfactory results and there are some problems such as long calculating time, low accuracy, etc. To deal with these problems, a Synthesis Searching Method (SSM) and corresponding program for the system fatigue reliability analysis of structure system is proposed in this paper which can identify the dominant fatigue failure paths of the structures more efficiently and much faster. The analytical probability of system failure through these paths has been evaluated. A numerical example is given to illustrate the efficiency of the proposed method. It is demonstrated that this method has great advantage in analyzing the fatigue reliability of large and complex offshore steel jackets.