Date of Graduation


Document Type

Problem/Project Report

Degree Type



Statler College of Engineering and Mineral Resources


Civil and Environmental Engineering

Committee Chair

Hota GangaRao

Committee Co-Chair

Roger Chen

Committee Member

Udaya Halabe


Fiber Reinforced Polymer (FRP) composites continue to gain popularity in civil and mechanical infrastructure due to a high strength-to-weight ratio, corrosion-resistance, and low maintenance requirements. FRP can also fulfill niche roles requiring non-conductivity and magnetic transparency. The longitudinal forming of pultruded FRP makes it a natural choice for lightweight beams. Although FRP composites have a high strength for their weight, the elastic and shear moduli for glass FRP may only be 1/7 and 1/30 that of steel, respectively. These low stiffnesses make FRP composite beams particularly susceptible to lateral-torsional buckling (LTB). In addition, the low shear to elastic stiffness amplifies shear deformation which can cause deviations from the LTB critical moment expression originally derived for steel beams.

The objective of this study is to better characterize the lateral-torsional buckling response of pultruded FRP beams through finite element analysis (FEA). A parametric study with a wide range of beam sizes, shapes, material properties, and boundary conditions is the primary focus of the report. In addition, the effect of asymmetrical axial response to stress on LTB is briefly discussed.

Orthotropic beams were modeled with general purpose shell element in ABAQUS to account for the shear deformation in the transverse direction and the lateral shear deformation in the flanges. Python scripts were developed to automate the modeling of wide flange, channel, and narrow rectangular beams of arbitrary dimensions and to apply all material properties, loadings, and boundary conditions in the parameter space. Linear eigen analyses and non-linear geometric Riks analyses were performed to determine critical LTB loads. Results from this study indicate that the moment gradient and load height factors for orthotropic wide flange and narrow rectangular section beams may use the same values as those for steel design. The channel sections exhibited significantly larger error when compared to the closed form expression.

While GFRP is often assumed to be symmetrical and have the same longitudinal elastic modulus for tensile and compressive stresses, this is not true for all FRP composites. To study this asymmetry on LTB, a WF was split at mid-height and separate longitudinal moduli applied to the halves. Despite the bending and twisting involved, the critical LTB moments is found to depend almost entirely on the compressive modulus. Thus, using only the tensile modulus to predict critical LTB will lead to errors proportional to the asymmetry.