Strengthening steel beams using bonded Carbon-Fibre-Reinforced-Polymers laminates

Other conference contribution, 2006

In recent years, carbon-fibre-reinforced-polymer (CFRP) laminates have been used in the repair and/or strengthening of existing steel structures. One advantage of using CFRP is its high tensile strength and stiffness compared with its low self-weight. This paper describes the research of strengthening steel structures with CFRP laminates under static conditions and comprises both laboratory tests and simplified analytical methods for predicting the capacity of steel girders strengthened with bonded CFRP laminates. The analytical solutions consist of calculation of bending capacity, normal stress distribution over the cross-section and interfacial shear stresses. By bonding CFRP laminates to the tensioned flange of a steel girder, it is possible to increase the moment capacity. The forces which are acting over the strengthened section, will cause interfacial stresses in the bond line. These interfacial stresses may cause debonding between the steel substrate and the laminate why these stresses must be considered in the design process. Simplified analyses were performed using Matlab to calculate the difference in stiffness and moment capacity depending on quantity and stiffness of the applied laminates. These solutions are based upon equilibrium statements over the section. The simplified analyses show that it is possible to increase the moment capacity by using bonded CFRP laminates to steel girders to a level of 20%, see Figure 1. By using developed simplified solution described above, it is also possible to calculate to which extend the steel section has reached yielding. The solution shows for studied cross-section that a maximum increase in terms of the moment capacity is about 20%, and, as a result, nearly the whole cross-section will be in compression. Tension will almost only be transmit by the CFRP laminate. To increase the moment capacity further, strengthening has to be made on the compressed area. Also, interfacial shear stresses in the bond line for the studied beams were calculated using an existing solution for calculation of interfacial stresses for arbitrary substrates. (Graph Presented) With results from the analytical solutions laboratory tests were conducted. The test specimens consist of CFRP laminates and epoxies of different material properties, which were attached to the tensioned flange of undistorted steel beams wifh H-shaped cross-sections. All beams were sand blasted cleaned and coated with a primer directly afterward. When the primer had cured, the epoxy was applied to the CFRP laminate and then bonded to the steel substrate by a hard roller. At the ends, the redundant epoxy was tapered shaped to reduce the stresses. The strengthened beams were tested in static four points bending where load-deformation of the beam and strain both in the steel section and the laminate was recorded. [...]