Simultaneous topological and element sizing optimization of a braced tall steel frameworks subject to lateral drift constraints is considered in two examples. Cross diagonal bracing elements are allowed to be added to the central bay of a 40-story, 3-bay planar building frame as shown in Fig. 3. The effectiveness of the hybrid OC-GA method on performing the optimal form searching as well as element sizing for the building framework was assessed by varying the different values of the probability of the OC operator and population sizes. The results produced by the hybrid method are compared with that generated by the GA method alone.