Semester

Spring

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Civil and Environmental Engineering

Committee Chair

Hota GangaRao

Committee Co-Chair

Ruifeng Liang

Committee Member

Rakesh Gupta

Committee Member

Udaya Halabe

Committee Member

Nithi Sivaneri

Abstract

For infrastructure system implementation, high strength/stiffness to self-weight ratio, corrosion resistance and ease of fabrication are just a few of the many advantages of pultruded fiber reinforced polymer (PFRP) composites over traditional construction materials. However, challenges of engineered FRP materials are addressed via continuous updating of design codes. For example, the anisotropic nature of FRP composite materials includes different mechanical properties in different directions (pull or longitudinal direction vs. transverse to pull direction) of a structural member, resulting in design complexities over isotropic materials. Typically, infrastructure systems are built by bolting FRP structural members because of certain advantages of bolted joints including higher joint efficiency over adhesive joints. Advantages such as ease of field installation and inspection for structural integrity, and excellent clamping capability at a joint excel high stress concentrations around joint, differing thermal coefficients and susceptibility to corrosion with steel bolts. In general, connections of pultruded FRPs are designed and constructed as simple framing joints that are pin connected. Stress concentration is induced around a bolted location due to sudden changes in stiffnesses of gusset plate(s) connecting the FRP members and the loss of FRP material to create connector holes. Orthotropic nature of FRPs leads to different failure modes.

This study is mainly intended to evaluate doubler and tube-in-tube concepts for their feasibility and effectiveness of multi-bolted joints in composite under different static loading to ensure load/moment transfer and minimize the stress concentration in joints. Experimental and theoretical evaluations of this study will provide fundamental understanding about failure modes and joint efficiency of bolted joints in FRPs.

Moment transfer efficiency of splice joint specimens with tube-in-tube concept under bending was investigated. These specimens consisted of tight fit square tube sections in a way that a smaller profile with a specific length was connected to a telescopic two-equal-length-component beam member. A 2.5×2.5 in. section overlap was multi-bolted (two bolts at each side of the joint) at mid-span of the beam made up of a 3×3 in. profile. Joint efficiency of the specimens was evaluated under 41-in. span in three-point bending tests. Average joint efficiency in bearing failure mode was evaluated ~23%, over the 3×3 in. beam member with no discontinuity (i.e., no joint). Joint efficiency in bearing of the same configuration was investigated by two methods with (a) adding a bottom plate to, and (b) wrapping of the splice joint. Bottom plate redirected the joint to have a controlled failure in compression zone with an average joint efficiency of ~37%, whereas, wrapping of the splice joint, reduced stress concentrations at the corners of the joint sections and had an average joint efficiency of ~30%.

In channel splice testing, the connection underwent tensile loading and it failed due to shear-out of bolt holes in the channel splice. The channel sections had the least bearing area when compared to the combined area of the plate and spacer. Flanges of the channel section at the failure expanded outward. This was attributed to increase in section depth.

To evaluate the efficiency of increasing thickness perpendicular to bolt axis in bolted joints, doubler from the same FRP material was bolted to channel joints. The channel connections consist of a doubler showed an average increase in bearing strength by ~91% with initiating of bearing failure mode, hence resulting in shear failure mode with increasing of the static axial loading for all the experimented specimens. Increase in bearing load was directly proportional to increase in bearing area. In the specimens with 8 bolts where, (bearing) loading is perpendicular to pull direction, strain data showed that the bolt farther to line of loading resisted substantially higher load than the bolt closer to loading by ~60% of applied load. This is due to flange boundary condition which increased the stiffness of the bolt hole. Also, slip load due to torquing of bolts would delay (increase) failure load of an FRP connection by increasing the friction force between FRP and bolt washer.

The tube test specimens with tube-in-tube concept followed pattern of bearing to shear and shear out failure mode for the specimens in static axial compression and tension loading, respectively. However, in the tube specimens reinforced with solid square bars under static axial tension loading, cracks initiated in the solid bar and propagated to the adjacent tube resulting in increasing the depth of test specimens due to Poisson’s effect.

Embargo Reason

Publication Pending

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