An introduction to empirical design

A structural design method is called ‘empirical’ when the sizing and configuration of the designed elements (beams, columns, girders, joists) are based on judgments that result from experience with similar structures. For instance, wood joists designed based on tables of allowable loading and allowable stresses could equally well have been designed by a framing carpenter with experience in choosing joists for residential floors, based on the required span and the perceived intensity of the anticipated loads. The former is a form of analytical design, while the latter is characterised as empirical design.

Much of empirical design can be reduced to rules, either rules of proportioning, or rules about the response of a structure to loads. In this book we will look at various rules of proportioning for commonly occurring materials and construction types. For instance, using a standard 40 lb/ft² live load, 10 lb/ft² dead load, the application of an analytical design procedure gives the following product selections (WWPA, 2008) for a 10, 12 and 16 ft span joist.