Doctor of Philosophy (PhD)


Civil and Environmental Engineering

Document Type



Rehabilitation and retrofitting methods offer economical and feasible alternatives for upgrading aged and deficient structures. Structural strengthening using Fiber Reinforced Polymer (FRP) composites have been widely investigated by researchers and used in field applications. The main advantage of FRP composites is the superior mechanical properties they offer over traditional structural materials. One novel alternative of these retrofitting methods was developed at Louisiana State University and called “Strengthening-by-Stiffening” (SBS). In SBS, the external strengthening of shear deficient thin-walled steel structures is achieved by bonding pultruded FRP sections to buckling prone web panels. Contrary to the commonly used uniaxial tension resistance of fibers, here, the geometric properties of pultruded FRP sections play the most important role in stiffening vulnerable thin plates. The research started by testing a series of full size steel beams before and after introducing SBS. The first web panel between the bearing and transverse steel stiffeners was selected as a control panel, and a point load was chosen in an asymmetric three-point loading setup. The experimental investigation was conducted considering different web panel aspect ratios (1.0:1.0; 1.5:1.0), web thicknesses (1/8; 5/32 inch), epoxy types (brittle; ductile), Glass FRP (GFRP) configuration (geometry and orientation). For comparison purposes, one conventionally strengthened beam (by welding additional steel stiffeners) and one beam strengthened by bonding Carbon FRP (CFRP) sheets to the critical web panel were also tested. The experimental tests showed that the global failure mechanism was mainly controlled by the debonding of adhesive layer. Therefore, failure modes and phase angles were investigated for the GFRP/steel interface. Local traction-separation laws for Mode I and Mode II failure modes were determined by conducting single leg bending (SLB) tests, in which digital image capturing and processing techniques were used to determine crack tip displacement fields. Delamination failure within the pultruded GFRP stiffeners was also simulated following Hashin’s failure criteria. Finally, effective SBS design parameters were investigated using an FE model that takes the adhesive’s mixed mode fracture into account using a cohesive zone model (CZM), which was validated using experimental results. Possible extension of SBS to new construction was studied to explore creating beams free from transverse steel stiffeners by fully bonding the GFRP stiffeners as a substitute for welding of transverse and bearing steel stiffeners as a means for improving the fatigue.



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Committee Chair

Okeil, Ayman M