THE OBJECTIVE in the structural design of large complex structures, such as bridges, ships, pressure vessels, buildings, etc., is to optimize the desired performance, safety requirements, and cost (i.e., the overall cost of materials, design, fabrication, operation, and maintenance). In other words, the purpose of engineering design is to produce a structure that will perform the operating function efficiently, safely, and economically. To achieve these objectives, engineers make predictions of service loads and conditions, calculate stresses in various structural members resulting from these loads and service conditions, and compare these stresses with the critical stresses for the particular failure modes that may lead to failure of the structure. Members are then proportioned and materials specified so that failure does not occur by any of the possible failure modes. Because the response to loading can be a function of the member geometry, an iterative process may be necessary.