Date of Award

1-2013

Document Type

Thesis

Degree Name

Master of Science in Civil Engineering (MSCE)

Department

Civil and Environmental Engineering

First Advisor

Dr. Michael J. Tait

Second Advisor

Dr. Samir A. Emam

Third Advisor

Dr. Khaled EI-Sawy

Abstract

Retrofitting and strengthening of steel structures have gained significant importance due to the highly increasing number of deteriorated steel structures in many places around the globe. The conventional method of retrofitting or strengthening of steel structures by replacing steel members or attaching additional external steel plates are usually time-consuming, corrodible, and a cumbersome task. Many of the drawbacks of the conventional retrofitting systems can be overcome through the use of Fiber Reinforced Polymers (FRP) due to their high strength-to-weight ratio. Furthermore, FRP materials are corrosion resistant, which makes them more durable especially when environmental deterioration is a concern. In recent years, the application of FRP in the strengthening of existing structures has increased considerably. A significant amount of research studies have been conducted to explore the effectiveness of implementing externally bonded FRP to strengthen reinforced concrete (RC) structures. Following the successful introduction of FRP in the strengthening of RC beams and columns, researchers started to explore the concept of using the FRP in the strengthening of steel elements. Although this idea was initially rejected by many researchers because of the significantly low elastic modulus of the FRP relative to steel, the idea started to float to the surface again when high-modulus FRP were successfully produced. The elastic modulus of such FRP approaches and even, in some cases, exceeds the elastic modulus of steel. Similar to the case of RC, researchers initially focused on the application of externally bonded FRP (EB-FRP) for flexural strengthening of steel beams. The research outcomes revealed that steel beams strengthened with EB-FRP strips exhibit unfavorable brittle failure mechanism due to debonding of the FRP. More recently, research work on application of mechanically fastened FRP (MF-FRP) to RC elements has shown promising results in term of installation efficiency, level of strengthening achieved, and, more importantly, preventing FRP delamination prior to concrete crushing. As such, a high potential exists for achieving a successful and efficient strengthening scheme when utilizing the MF-FRP laminates to strengthen steel beams. A unique study on the application of MF-FRP to steel beams was conducted by Alhadid (2011). The study revealed that MF-FRP leads to ductile response of the strengthened system provided that adequate number and strength of anchoring fasteners are used. Insufficient FRP length-to-span ratio or insufficient number of steel fasteners will result in unfavorable brittle mode of failure by shear rupture of the fasteners or tensile rupture in the FRP laminate.

The driving force behind the current research study stems from the need to gain a better understanding of the mechanical behavior of the steel beams strengthened with MF-FRP laminates. The research is conducted numerically and analytically. Three-dimensional (3D) finite element (FE) analysis using the general purpose software package ANSYS is conducted in the numerical phase of the study. The 3D FE model developed in this study accounts for the effect of both material and geometrical nonlinearities in addition to the interfacial slip between the FRP laminates and the steel beam. The FE model is validated against the experimental results reported by Alhadid (2011), and excellent agreement is found. The validated FE model is then used to study the behavior of the composite steel-FRP beam parameters including the force distribution in anchoring steel fasteners, the stress distribution and spread of yielding in the steel section and the corresponding stress distribution in the FRP laminates. Furthermore, the FE model is utilized to investigate the effect of different parameters on the mechanical behavior of the strengthened beams namely: the steel section height; length, thickness and stiffness of FRP laminates; and distribution and configuration of the steel fasteners. For the analytical analysis, a closed-form analytical model is derived to predict the elastic behavior of the steel-FRP composite beams taking into consideration the slip at the steel-FRP interface. The analytical model is then utilized to evaluate the deflection, the first yielding load of the steel-FRP system and the distribution of shear forces induced in the steel fasteners.

The current study concludes that the contribution of the FRP in reducing mid-span deflection and load-carrying capacity in the elastic stage (i.e., when all materials are still elastic) increases if the elastic modulus of FRP is close to or higher than the steel section. As the length of the FRP increases, the index of elastic composite action increases indicating higher efficiency of the FRP laminate, especially at low fastener stiffness. After yielding in the extreme fibers of the bottom steel flange, the FRP laminates contributes significantly in carrying the mid-span loads because the FRP laminate remain elastic and contributes significantly in carrying the tensile stresses.

The study also shows that the steel beam with deeper cross-section and strengthened with MF-FRP at the bottom flange exhibits higher improvement in its flexural capacity relative to the beam with shallow section with almost the same stiffness. This is because the shear forces carried by the steel fasteners cause a bending moment in the steel beam that is proportional to the section height, and counteracts the bending moment due to the applied mid-span load.

Increasing the thickness of the FRP laminate significantly improves the load-carrying capacity of composite steel-FRP beams. Provided that a sufficient number of fasteners is provided to avoid shear failure at the interface, increasing the number of steel fasteners, or reducing the pitch distance does not increase the load-carrying capacity significantly. However, it will ensure a ductile failure mode of the composite steel-FRP beams.

The analytical solution used in the current study provides a convenient, but accurate, tool that can be used to calculate the deflection of the composite beam while considering interfacial slip. The solution can also be used to estimate the load that initiates yielding in the steel component of the composite beam and finding the distribution of the shear forces induced in the steel fasteners.

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