Lateral-torsional buckling of steel I-beams retrofitted by bonded and un-bonded CFRP laminates with different pre-stress levels: Experimental and numerical study

• Pre-stressed CFRP laminates for strengthening of steel beams.
• Bonded and un-bonded retrofit systems.
• Comparisons between the static behaviour of the bonded and un-bonded systems.
• Effect of CFRP strengthening on lateral-torsional buckling capacity of steel beams.

Lateral-torsional buckling (LTB) of steel beams that are strengthened by normal modulus (NM) carbon fibre-reinforced polymer (CFRP) laminates was studied experimentally and numerically. A total of seven steel beams, which included a reference unstrengthened beam, were tested statically until failure. The steel beams were strengthened by bonded and un-bonded CFRP laminates. For each type, three steel beams that were strengthened by CFRP laminates with 0%, 20% and 40% pre-stress levels were prepared and tested. The effects of various strengthening schemes on the linear elastic behaviour, as well as the buckling strength of the retrofitted beams, were investigated. The test results showed that retrofitting the steel beams using CFRP laminates increases the elastic stiffness of the specimens almost the same for both bonded and un-bonded systems compared to the reference beam. The pre-stressing of CFRP laminate has no effect on the elastic stiffness of the retrofitted beams but substantially influences the buckling strength. Higher pre-stress levels do not necessarily lead to an increased buckling strength in the retrofitted beams. In most of the tests, the specimens that were strengthened with the un-bonded CFRP laminate showed slightly higher strength than the specimens that were strengthened by bonded CFRP laminate. Preparation of the bonded specimens took approximately twice as long as that of the un-bonded specimens. Finite element (FE) models of the retrofitted beams with pre-stressed bonded and un-bonded laminates were created. The results from the FE analysis were compared with the results obtained from the experiments.