Abstract: Ten scaled models of hollow reinforced concrete piers were designed, fabricated, and strengthened by adhering steel plates in lab. A number of important parameters, including initial damage degree, steel plate thickness, axial compression ratio and slenderness ratio etc., were considered in these models. The multi-dimensional hysteretic behaviors of the strengthened models were explored by using the bi-directional quasi-static testing method. Based on the section M-#x003a6;relations, the bi-directional load-displacement skeleton curves and three-segments-lines skeleton curves of the piers were derived, and the unloading stiffness was obtained from the theoretical analysis and the experimental data. It is shown that the theoretical restoring force curves, which agree well with the experimental results, can reflect the influence of the main parameters with good versatility. Finally, according to the theoretical model the parametric analysis on the maximum load and the ultimate displacement were conducted.