CFRP strengthening and rehabilitation of degraded steel welded RHS beams under combined bending and bearing

To rehabilitate damaged or sub-standard box girders, techniques utilising the lightweight, high strength and corrosion resistance of carbon fibres reinforced polymers (CFRP) composites have been proposed. This paper presents experimental results for two series of CFRP strengthened and rehabilitated model box girders under quasi-static large deformation 3-point bending. The first series represents strengthening 12 un-degraded rectangular hollow section (RHS) beams from the manufacturer using externally wrapped CFRP sheets. The second series was for rehabilitation of 41 artificially degraded RHS beams strengthened using externally wrapped sheets or bonded plates. The main parameters examined in this paper were the section type, section and member slenderness and the type and number of the CFRP sheets. The flange and web slenderness examined in this paper was in the range of b/t=20 to 66.67 and d/tw=20 to 75. The CFRP sheets were wrapped around the section in the transverse direction with a sufficient overlap. The results show that the combined flexural and bearing strength of the steel box girder can be significantly increased by adhesively bonding CFRP. Expressions for the bearing strength and plastic moment of the composite section were obtained by means of an equivalent thickness approach. The newly derived interaction equations were compared against the present design rules in steel specifications. The average gain in strength due to bonding the CFRP laminate was 65% and 19.9% for the strengthening and rehabilitations series, respectively. The percent increase in strength was mostly affected by the section slenderness where the maximum gain was obtained for the slender section.