17.8 : Design Consideration

Designing a structure involves a series of considerations, primarily the material's ultimate strength, calculated through tests that measure changes under increased force until the material reaches its breaking point or limit. The ultimate load, where the material breaks, is divided by its original cross-sectional area, resulting in the ultimate normal stress or strength. The ultimate shearing stress is another significant factor taken into account.

The factor of safety is another key aspect that relates the ultimate load to the allowable load. It ensures a buffer in the load-carrying capacity for safe operation. Selecting the factor of safety requires consideration of variations in material properties, different types of potential failure, uncertainties in analytical methods, future deterioration, structural importance, and risk to life and property. A factor of safety that is too small risks failure, whereas one that's too large can result in uneconomical or nonfunctional designs.

Traditionally, the allowable-stress method is used, which consolidates all uncertainties into a single safety factor. However, an alternative method, the Load and Resistance Factor Design method, differentiates between uncertainties related to a structure and its load and distinguishes between live and dead loads.