Upgrading the performance of steel box piers subjected to earthquakes

Abstract

Bridge pier failures during earthquakes usually start with the formation of plastic hinges. In order to enhance member efficiency in the post-buckling stage, the deterioration rate must be controlled and reduced. This paper presents experimental information on the cyclic behavior of steel box piers with added internal strengthening devices. Test results show that the strength and elastic stiffness of the upgraded members stay at levels equivalent to those of the original members, however, the energy dissipation capacities of such members are substantially increased and the rates of strength deterioration are significantly reduced. Relationships between performance enhancement of members and characteristics of strengthening devices are also reported.

Introduction

The use of steel box sections for bridge pier construction is common and is considered advantageous in urban areas and in areas of high seismic activity because these sections possess high strength/mass ratios and significant ductility, which greatly enhance their structural performance during strong earthquakes. Although steel box sections possess high flexural capacity, their cross-sections, composed of thin-walled plates, are susceptible to local buckling if the members are not adequately proportioned or detailed. The parameters governing the member behavior of members composed of thin-walled plates are the width/thickness ratios of the plates. In general, members' width/thickness ratios must not exceed specified limits for box-section design and is usually achieved by welding transverse and longitudinal stiffeners to the thin-walled plates [1]. This fabrication method requires extensive welding work to form the design sections and inevitably induces heavy welding stresses on the plates and the corners of the box sections. The welding-induced brittleness in the wall plates also reduces the inelastic deformation capacities of members. Terayama et al. [2] recently reported severe fracturing of welds on the corners of steel box columns subjected to repeated loading. These brittle fractures significantly influenced member performance under earthquakes and should be effectively remedied to improve member behavior.

The column bottoms of bridge piers subjected to earthquakes sustain maximum loading, therefore, the failure mechanisms usually start with formation of plastic hinges, as shown in Fig. 1, attributable to local earthquake-induced plate buckling in those regions. Repeated local buckling of member plates causing increasing relative deformation of the members' opposite wall plates then leads to plate fracture in the corners of the buckling zone, thus forming plastic hinges. Many research studies [3], [4], [5], [6] have shown that the propagation rates of members' plate deformations are the major parameter governing post-buckling performance, and that members' energy dissipation mechanisms are related to buckling-zone sizes. Therefore, development of methods to delay the occurrence of local buckling, to reduce the propagation rates of relative deformation in member plates, and to stabilize locally buckled plates during post-buckling stages to enhance member performance is essential. In a previous study [7], the author found that adding internal cross braces to tie opposing plates in box members could effectively reduce the rate of member performance deterioration. This type of strengthening provides only point support to the plates at the locations of plate–brace connections, therefore, further improvement to achieve higher performance gains is possible.

In this paper, the authors present a simple and effective device for further strengthening and helping to stabilize section integrity during post-buckling stages. This device consists of pairs of steel cross tie bars and steel ductile grids of various lengths, as shown in Fig. 2. These are placed inside possible buckling zones, located at the bottoms of piers. The tie bars provide additional stiffness to the wall plates when buckling-induced plate deformation occurs, and the ductile grids function as planar lateral supports for wall plates after they buckle. When local buckling occurs, the added mechanisms deform accordingly forming inelastic zones to help stabilize members and dissipate seismic energy. This paper presents the experimental information on the cyclic behavior of strengthened and unstrengthened steel box members. Relationships between member performance and the parameters of the added strengthening device, such as tie-bar stiffness and ductile-grid length are also discussed so that design guidelines can be established.