<p>To investigate the effect of preload on jacketed members, a method has been proposed to evaluate the strength of these members by considering the preload. The strain distribution resulting from the preloading was determined using the root-finding technique. This distribution was then applied to the core section, and an optimization technique was employed to derive the interaction diagram of the jacketed section. The proposed method is validated through experiments on columns and beams subjected to preloading and jacketing. A parametric study examined the effects of various preload levels as well as the geometry and characteristics of the jacketed section. The analysis results indicated that the effect of the preload resulted in a variation of 86–106% of the original strength in a typical square jacket cross-section. Notably, there was no significant difference in the effect of the preload across various other jacket cross-sections.</p>