The performance of plate girders reinforced with CFRP plates of various lengths

Highlights

• Strengthening of plate girders with CFRP plates raises the resistance and stiffness of the beams.

• Reinforcing of plate girders by CFRP plate change the failure mode of the beams.

• Increment in length of laminates results in the development of several plastic hinges.

Abstract

This paper presents an experimental study into the behavior of reinforced steel plate girders strengthened by carbon fiber reinforced polymer (CFRP) laminates subjected to three-point bending. The tests were conducted on six specimens, which utilized different lengths of CFRP plates, and of different ratios of top to bottom flange thickness (t_r = t_tf/t_bf). It was concluded that the CFRP plates enhance the stiffness and resistance and change failure mechanism of the plate girders. Additionally, Finite element (FE) analyses were employed to verify the experimental results, and close correlation between the two was found.

Introduction

For a variety of reasons, civil structures can become inadequate during their design lifetime. These reasons include the deterioration of materials, an increase in the actual loads acting on the structure, design errors, amendments of the design standards [1]. There are a great number of aged structures such as bridges, building floors, and cranes, requiring reconstruction or retrofitting [2]. Plate girders care designed to support heavy loads over long spans, and they are widely employed where standard rolled sections or compound girders are inadequate for such a task. For plate girders that require rehabilitation, standard techniques consist mainly of attaching steel plates to the existing system by using welding, bolting or adhesive bonding methods. Nevertheless, problems such as steel corrosion, difficulty in fitting complex profiles, the augmentation of member self-weight or dead load, probability of an increase in the sensitivity of members to fatigue failure due to stress concentrations produced by drilling or welding, and the cost of retrofitting are still recognized as major concerns [1], [2], [3].

Because of their high tensile strength, stiffness, and fatigue resistance of CFRP plates, as well as their low self-weight, and the ease of shaping them, the employment of CFRP laminates to repair and rehabilitate structures has seen a steady increase [1], [3], [4]. Several studies have been conducted on the use of CFRP plates in girders. In 2001, Sen et al. [5] investigated experimentally the feasibility of using CFRP laminates to repair composite steel bridge members. The obtained results revealed a significant increase in ultimate strength and modest improvement in the elastic response of reinforced specimens. Colombi and Poggi [3] conducted an experimental and numerical program to characterize the static behavior of steel H shaped beams. They gave an insight into the influence of two layers of CFRP that were bonded over the tensile flange of beams. It was shown that a significant increase in elastic stiffness of the beams was possible if the reinforcement consisted of two layers of CFRP.

In a longitudinal research study, Deng and Lee [6] carried out experiments on steel beams bonded with a CFRP plates under static loading. It was concluded that the longer the CFRP laminate length is, the greater the strength enhancement. In addition, it was identified that it's primarily the thickness, and not the length, of the CFRP plates that influences the stiffness of the retrofitted beams. Al-Saidy et al. [7] used two types of CFRP laminates of different elastic moduli and surveyed the resistance of the beams under four-point bending. They delved deeply into the influence of CFRP laminates in the elastic and post-elastic range. It was concluded that, even though the CFRP laminates have minor influence on the behavior of beams while elastically responding, its impact during the post-elastic response is remarkable. Through the use of CFRP plates, up to 45% enhancement in bending resistance was reported. In 2008, Patnaik et al. [8] carried out a laboratory study into the influence of CFRP laminates on both the bending and shear resistance of steel beams. They found that the failure of CFRP flexure strengthened steel beams was sudden and brittle, while the failure of shear strengthened steel beams was observed to be gradual. Linghoff et al. [4] conducted an experimental and analytical study on increasing the resistance of strengthened steel beams against four-point bending. They concluded that an enhancement in stiffness of beams is obtainable if a large amount of CFRP laminates are used. However, this technique may result in earlier failure and brittle behavior of the beams. Salama and Abd-el-Maguid [9] undertook research into the flexural behavior of steel girders retrofitted with CFRP laminates under four-point bending. Confirming the positive effect of CFRP laminates on enhancing the resistance of the girders, they concluded that the CFRP laminate length will not be an important parameter for consideration if the maximum moment zone along the span is covered by CFRP laminate. Based upon this analytical work, it was also recommended that the CFRP laminate length should not be less than 75% of the girder length. Furthermore, they recommended that the CFRP laminate width should cover the whole beam soffit. Recently, Ghafoori and Motavalli [10], [11], [12] experimentally and numerically studied the response of steel beams strengthened with CFRP laminates under four-point bending. The laminates covered all the length of the tensile flange. They concluded that the application of CFRP laminates increased the lateral buckling strength and more significantly the in-plane bending strength of the beams.

Taking an overview of the past investigations, it is seen that CFRP reinforcement of the beams has been studied extensively. Nonetheless, there is the need to investigate the influence of CFRP in plate girders. Therefore, the behavior of strengthened plate girders with an attached CFRP plate on the tensile flange under the three-point bending will be surveyed in the current study.