Load and resistance factor design (LRFD) approach for reinforced-plastic channel beam buckling

Abstract

Pultruded fiber reinforced plastic (PFRP) structural sections are rapidly gaining impetus in civil engineering applications. Thin-walled open beams with channel, I-shaped, and other types of sections are of practical importance to designers. In this paper, a load and resistance factor design (LRFD) approach for lateraltorsional buckling is presented based on an experimental and theoretical study of the behavior of PFRP channel section beams under the influence of gradually increasing static loads. Some experimental results for combined bending and torsion are also presented. Single span members with unrestrained end warping are considered with concentrated vertical loads passing through (a) the shear center, (b) the geometric centroid, and (c) a location which is neither at the shear center nor at the centroid. A pair of concentrated loads are applied symmetrically about the beam midspan, through a system of loading plates and tie rods in order to allow an unrestrained deformation of the beam. The loads are increased gradually and the resulting midspan vertical, lateral, and torsional deflections are recorded. The loads are generated with hydraulic jacks and recorded by means of calibrated load cells. Strains are also recorded at key locations using electrical resistance gages. Relationships between the applied load and the resulting deflections and strains are plotted and compared for the three load cases. The magnitude and significance of the warping stresses in comparison to the flexural stresses are identified. For the case of shear center loading, the deflections are monitored for various clear spans of the beam. This information is then utilized to generate the relationship between the lateral torsional instability load versus the minor axis slenderness ratio. An approximate theoretical formula is also developed to predict the lateral-torsional buckling load as possible prelude to the evolution of a formal construction specification formula for use by design engineers. The predicted buckling loads found using the formula are in excellent agreement with the experimental results. A comparison of the results from the formula applied to the case of compression flange loading is also made to those using the current AISC-LRFD specification. Lastly, the use of a proposed LRFD approach is demonstrated by means of analysis and design examples. The effect of the applied load height on the buckling load capacity is explained using the buckling formula.