||Size determination of the main structural elements is an objective of foremost importance in preliminary structural design of tall buildings. After the structural system is determined, all the remaining design activities are focused on size determination. The procedure goes something like this: assignment of initial sizes; building a structural model; analysis of the model; revision of the model; reanalysis of the model; and so on and so forth until a satisfactory result is yielded. This time consuming "trial-and error" procedure depends heavily on intuition and rules of thumb. Various software applications have been developed to assist with the design activities in this cycle, but most of them only focus on a specific activity rather than on the whole process. Design activity is still done in a piecemeal manner. This thesis introduces an automated size synthesis technique with a software implementation for tall building design under both vertical and lateral loads. Given little structural typological information, the system can automatically carry out most of the tasks in the preliminary design stage, including slab layout, vertical load transfer and run-down analysis, member size optimization under both vertical and lateral loads, code compliance checking and design, detailed design, coordination of stiffness design and strength design, and design documentation and reporting. Engineering practice and code requirements are carefully considered and integrated into the algorithm. Economic parameters such as reinforcement ratios for different types of elements can be controlled by users at the very beginning of the design process through a convenient graphic user interface. Thus, the final sizes can be directly used in the detailed design stage. The system has been employed in several real projects to assist with the preliminary structural design. It has proven to be an efficient and practical computer aid for size determination in the preliminary structural design of tall buildings.