In high-rise buildings and heavy loaded structures where RC columns are subjected to heavy loads, the use of High Strength Concrete (HSC) in columns construction is essential for the purpose of reducing column size and increasing column capacity. However, from the economical standpoint, combination of high and normal strength concrete (NSC) in building construction is becoming common practice, where HSC is used for columns and NSC is used for the surrounding beams/slabs floor system. This creates a situation where concrete strength of the column portion at the beam/slab floor level is lower than concrete strength used for rest of the column. Previous studies indicated that such variation in concrete strength affects the load carrying capacity of the RC columns. This paper presents a theoretical study on the effect of using concrete with different strengths for columns and floor beams on the structural behavior of integrated RC building frames under static, lateral pushover and earthquake loading cases. Four-story frame was analyzed employing a ready package program for the inelastic structural analysis of buildings (IDARC-5). The obtained results indicated that under static loading, variation in the concrete strength of the transition zone has a negligible effect on the behavior of the studied frame. However, under lateral pushover and earthquake loading the behavior of the studied frame is influenced by the ratio of column concrete strength to the transition zone concrete strength. For a ratio of 1.4 or less, no real influence was noted. On exceeding this ratio, the frame response was adversely influenced. This agrees with the recommendations of the ACI 318 Building Code.