ON THE FAILURE STRENGTH OF WEB-FLANGE JUNCTION OF THIN-WALLED PULTRUDED I-PROFILES

Historically, off-the-shelf pultruded fibre-reinforced polymer (PFRP) composites were developed and designed by the pultrusion industry and were intended for low-stress applications. Recently, composites have been introduced as primary structural members to replace or complement other conventional materials, such as steel, concrete and wood, in critical applications such as bridge decks, pedestrian bridges, and recently in highway bridges and other infrastructural systems [1, 2]. In order to make pultruded fibre-reinforced members appealing to the construction industry, most manufacturers produce profiles that imitate standard structural steel members (e.g. I-, H-, C- and angles profiles); however, these steel-like profiles do not represent the optimum geometry for PFRP composites. Unlike isotropic, time-independent structural materials, all PFRP pultruded materials are anisotropic and are characterized as viscoelastic materials [3-5]. Their stiffness and strength have values that depend on the orientation of the fibres. Consequently, these materials, under ambient environments, behave differently from those isotropic time-independent structural materials such as steel, under service, ultimate loads and dynamic excitations. In order to ensure the structural reliability of load bearing pultruded composite members, the shape and fibre architecture of PFRP profiles must be optimized and designed properly. In recent years, relevant studies have been conducted and focused on the performance of PFRP frame structures and several research programs on optimizing pultruded composites have been initiated. For example Davalos et al. [6] presented an approach for flexural analysis and design of pultruded beams. This approach involved computational procedures for utilizing fibre volume fraction of the constituents, ply stiffness and panel laminate engineering constants. Over the past two decades or so, a number of studies focusing on the performance of PFRP connections and frame structures have also been reported. Some of the pioneering studies on PFRP frame structures were reported by Mosallam et al. [7-11] presenting the results of a comprehensive theoretical and experimental program to evaluate both the short-and long-term behavior of PFRP structures subjected to both quasi-static and sustained loading. A highly complex and delicate mechanical aspect unique to steel-like unidirectional PFRP profiles is associated with the strength and stiffness of web-flange junctions (WFJ) of such profiles due to the insufficient fibre continuity. This insufficient fibre continuity will lead to progressive degradations of both axial and rotational stiffnesses and strengths affecting the buckling, post-buckling and the overall short- and long-term structural integrity of the PFRP profiles [12-25]. This paper presents experimental and numerical results of the first phase of a multi-phase comprehensive joint research program between University of Salerno, Italy, and the University of California, Irvine, USA, on investigating one of the major structural issues that defines the strength limit-state of pultruded fibre-reinforced polymer (PFRP) profiles. Specifically, the strength and stiffness of the web-flange junction (WFJ) of the majority of commercially-produced pultruded composite profiles. A summary of experimental results for twenty-eight full-scale pull-out tests are presented and typical modes failure are identified. Moreover, the influence of the pull-out load distance (d) from the edge of the specimens on the failure strength of the web-flange junction has been investigated and a new definition for an “influence zone” is proposed that is found to be dependent on the loaded length, with a maximum value equal to approximately the PFRP member’s depth. This proposed zone was observed in all laboratory tests and its existence was confirmed by the results of FEM numerical analysis. 3-D finite element models were also developed to predict the behavior of these specimens. The results from the numerical models were compared to those obtained from the experimental program and found to be satisfactory.