Philosophy of Engineering Design of Structures

Design of structures is usually based on prior estimation of member sizes and subsequent analysis. The analysis must be for a simplified structure, rather than for the real one. This simplification is made on the designer's estimate of the basic statical actions by which the structure carries its loads. If the designer's estimate is correct, even a drastically simplified analysis can give adequate results, and such results usually can be improved by additional corrections. The designer's basic problem is to picture the geometry of the deformed structure well enough to cope with the statics, so that the important internal actions can be calculated. The data are the equations of statics, the equations of geometrical compatibility, and the load-deformation relations. This convenient and natural separation leads to the two alternative methods of analysis; the force method and the deformation method. Energy methods use either the force approach or the deformation approach. Complementary energy furnishes a powerful tool for approximate analysis because its use is based directly on an understanding of the statical action of a structure. Limit design can assure safety against collapse for the same reason.