Identifier

etd-06292016-100109

Degree

Doctor of Philosophy (PhD)

Department

Civil and Environmental Engineering

Document Type

Dissertation

Abstract

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.

Date

2016

Document Availability at the Time of Submission

Secure the entire work for patent and/or proprietary purposes for a period of one year. Student has submitted appropriate documentation which states: During this period the copyright owner also agrees not to exercise her/his ownership rights, including public use in works, without prior authorization from LSU. At the end of the one year period, either we or LSU may request an automatic extension for one additional year. At the end of the one year secure period (or its extension, if such is requested), the work will be released for access worldwide.

Committee Chair

Okeil, Ayman M

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