Stability of steel columns with bolted strengthening under preload: Experimental study

Strengthening of steel columns is usually conducted with existing loads, so there is a need to study the effect of the preload (the existing load during strengthening) on the stability of strengthened columns. This study presents an experimental work, in which H-shaped steel columns were strengthened with bolted steel angles. Specimens strengthened under different preload levels were tested. The test results showed that the preload had an influence on column behaviors including deformations, strains, and stresses. In particular, the preload had an effect on the ultimate load capacity. The (normalized) ultimate load capacities of the specimens slightly increased and then decreased with the increasing preload. The effect of the preload on the ultimate load capacities was generally insignificant (no more than 8% in the study), and the effect could be either favorable or unfavorable depending on the case. This varying effect of the preload could be interpreted by the combined effects of the equivalent initial deformation and initial stresses. In addition, specimens with different steel angle properties, strengthening length ratios, and slenderness ratios were tested. The test results indicated that all these variables affected the behaviors and ultimate load capacities of the strengthened specimens.