Tsutomu Usami

Uniaxial stress–strain relationship of concrete confined by various shaped steel tubes

Abstract A method is presented to predict the complete stress–strain curve of concrete subjected to triaxial compressive stresses caused by axial load plus lateral pressure due to the confinement action in circular, box and octagonal shaped concrete-filled steel tubes. Available empirical formulas are adopted to determine the lateral pressure exerted on concrete in circular concrete-filled steel columns. To evaluate the lateral pressure exerted on the concrete in box and octagonal shaped columns, FEM analysis is adopted with the help of a concrete–steel interaction model. Subsequently, an extensive parametric study is conducted to propose an empirical equation for the maximum average lateral pressure, which depends on the material and geometric properties of the columns. Lateral pressure so calculated is correlated to confined concrete strength through a well known empirical formula. For determination of the post-peak stress–strain relation, available experimental results are used. Based on the test results, approximated expressions to predict the slope of the descending branch and the strain at sustained concrete strength are derived for the confined concrete in columns having each type of sectional shapes. The predicted concrete strength and post-peak behavior are found to exhibit good agreement with the test results within the accepted limits. The proposed model is intended to be used in fiber analysis involving beam–column elements in order to establish an ultimate state prediction criterion for concrete-filled steel columns designed as earthquake resisting structures.

Strength analysis of concrete-filled thin-walled steel box columns

Abstract An elasto-plastic finite displacement analysis of concrete-filled thin-walled steel stub-columns of box shape is presented. In the analysis, a hardening-softening model is used to describe rationally the elasto-plastic behavior of concrete. A contact element for the interface combined with a bilinear constrained shell element for the plate and a cubic element for the concrete is employed. Both initial geometrical imperfections and residual stresses are also considered in the plate elements. Analytical results are then compared with previous experimental results. In addition, a parmetric study is conducted to investigate the effects of various parameters such as the plate aspect ratio, the plate width-thickness ratio and the concrete strength on the column strength. Finally, a design formula for concrete-filled stub-columns in compression is proposed.

Stiffened steel box columns. Part 1: Cyclic behaviour

This paper aims to study the cyclic inelastic behaviour of stiffened steel box columns failed by local and overall interaction instability under a constant compressive axial force and cyclic lateral loading. Such columns find broad application in steel bridge piers. The columns are of box sections with longitudinal stiffeners. In the analysis, a modified two-surface plasticity model developed at Nagoya University is employed to model material non-linearity. For comparison, analyses using classical isotropic- and kinematic-hardening models are also carried out. Hysteretic curves and buckling modes obtained from analysis using the two-surface model and classical models are compared with experimental results. Moreover, the progression of deformation from occurrence of local buckling to structural failure is discussed in detail. The comparisons show that the use of an accurate plasticity model is quite important in the prediction of both the cyclic inelastic behaviour and failure characteristic of steel box columns failed by coupled local and overall instability. It is found that the modified two-surface model is a satisfactory model in predicting the cyclic hysteretic behaviour of both the thin- and thick-walled steel box columns. Copyright

Stiffened steel box columns. Part 2: Ductility evaluation

The purpose of this study is to evaluate the ultimate strength and ductility capacity of stiffened steel box columns failed by local and overall interaction instability under a constant compressive axial force and cyclic lateral loading. In a companion paper, a finite element formulation accounting for both geometrical and material non-linearity was developed to obtain cyclic hysteretic behaviour of such columns. In this paper, the effect of loading patterns on the cyclic inelastic behaviour is first studied; then, a parametric study is carried out to investigate the effects of flange plate width–thickness ratio parameter, column slenderness ratio parameter, stiffeners equivalent slenderness ratio parameter, magnitude of axial load, and material type of stiffeners on the strength and ductility of the columns. Last but not least, empirical formulae of both the ultimate strength and ductility capacities are proposed for stiffened steel box columns, and the limit values of various parameters for the required ductility demand are also discussed. Copyright

Inelastic large deflection analysis of structural steel members under cyclic loading

The present paper is concerned with the cyclic inelastic large deflection analysis of structural steel members, such as pin-ended columns and fixed-ended tubular beam-columns of strut type. An elastoplastic finite element formulation for beam-columns, accounting for both the material and geometrical nonlinearities, was developed and implemented in the computer program FEAP used in the analysis. The geometrical nonlinearity is considered using the modified approximate updated Lagrangian description of motion. The two-surface plasticity model, recently developed by the authors, is employed for material nonlinearity. The model accounts for the important cyclic characteristics of structural steel, even within the yield plateau, such as, the decrease and disappearance of the yield plateau, reduction of the elastic range and cyclic strain hardening. The cyclic elastoplastic performance of the formulation was found to be good when compared with the experimental results as well as the results obtained from other material models.

A PERFORMANCE-BASED SEISMIC DESIGN METHODOLOGY FOR STEEL BRIDGE SYSTEMS

This paper outlines a performance-based seismic design methodology for steel bridge systems. Two kinds of menus are proposed. The first is the displacement-based evaluation method which compares the response displacement with the ultimate displacement, and the second is the strain-based evaluation method which compares the response strain and the ultimate strain. Moreover, the seismic required performance matrix for bridge structures and soundness matrix for members are presented and discussed.

A CAPACITY PREDICTION PROCEDURE FOR CONCRETE-FILLED STEEL COLUMNS

A seismic design procedure for partially concrete-filled box-shaped steel columns is presented in this paper. To determine the ultimate state of such columns, concrete and steel segments are modelled using beam-column elements and a pushover analysis procedure is adopted. This is done by means of a new failure criterion based on the average strain of concrete and steel at critical regions. The proposed procedure is applicable to columns having thin- and thick-walled sections, which are longitudinally stiffened or not. An uniaxial constitutive relation recently developed is employed for concrete filled in the thick-walled unstiffened section columns. Modifications are introduced to this model for other types of columns. Subsequently, the strength and ductility predictions obtained using the present and previous procedures are compared with the corresponding experimental results. Comparisons show that the present procedure yields better predictions. It is revealed that the inclusion of the confinement effects and softening behaviour of concrete is important in the present kind of prediction procedures. Furthermore, an extensive parametric study is carried out to examine the effects of procedures and geometrical and material properties on capacity predictions.

Behaviour of partially concrete-filled steel bridge piers under cyclic and dynamic loading

Abstract In the present paper, experimental results of concrete-filled steel box columns under cyclic and dynamic loading are reported. In the experimental work of cyclic tests, the effects of main parameters, such as the width-thickness ratio, number of loading cycles, length of filled-in concrete and a diaphragm above the filled-in concrete on the column behavior, were studied. To investigate the dynamic behavior of concrete-filled steel box columns, three specimens with different natural periods were tested using the pseudodynamic test method. For comparison, a hollow steel column specimen with the same dimension was also tested. Test results showed that concrete-filled steel box columns can be effectively used as bridge substructures to withstand severe earthquakes.

Improving Low-Cycle Fatigue Performance of High-Performance Buckling-Restrained Braces by Toe-Finished Method

The purpose of this research is to improve the low-cycle fatigue life of the buckling-restrained brace (BRB) for the development of the high-performance buckling-restrained brace (HPBRB) used in bridge engineering. The HPBRB is expected to suffer three strong earthquakes without severe damage during the lifecycle of a bridge. This article employs a low-cycle fatigue experiment, including two series of BRB specimens, all of which had been tested to failure. In the second batch, the weld toes of the rib had been ground to improve the welds low-cycle fatigue life. According to test results under constant and variable strain amplitudes, the specimens have a great low-cycle fatigue performance. The weld of the rib clearly affects the performance of the as-welded specimens tested with the relatively small strain amplitude. The toe-finished method effectively improves the BRBs low-cycle fatigue performance. Besides, the in-plane gap width of the BRB slightly affects the low-cycle fatigue life. Finally, the low-cycle fatigue curves are compared between the BRB and the materials and the low-cycle fatigue damage evaluation formulae of the as-welded and toe-finished BRBs are recommended for the strain-based evaluation of the low-cycle fatigue damage.

Elastoplastic analysis of steel members and frames subjected to cyclic loading

The present paper addresses the cyclic elastoplastic large displacement analysis of steel columns and portal frames. In the analysis, a modified two-surface plasticity model developed at Nagoya University is employed to model material nonlinearity. The developed finite element formulation takes into account the important cyclic characteristics of structural steel, even within the yield plateau, such as the decrease and disappearance of the yield plateau, reduction of the elastic range and cyclic strain hardening, as well as the spread of plasticity across the section. Comparisons between the analytical and experimental results show that the modified two-surface model can be used to predict the inelastic cyclic behavior of steel structures with good accuracy. Based on the results of parametric studies, empirical formulas are proposed to predict the strength and ductility capacity of pipe-section steel bridge piers.