"Today bridge structures are usually fabricated with steel or steel reinforced concrete. These materials are subjected to stresses due to gravitational loads and due to live loads. The gravitational loads (weight of the structure are constant with respect to time. The live loads are due to vehicular traffic, which varies with time. To insure the safety of the structure, the strength of the materials (steel or concrete) must be adequate to support the applied stresses with a comfortabel but not excessive margin of safety. This premise seems simple, but on careful study, we find several strengths are needed to characterize the behavior of structural materials - yield, tensile or ultimate and fatigue. To perform design analysis, it is also necessary to describe elastic constants that define the stiffness of these materials. To judge the adequacy of these materials for large structures, we examine their ductility (i.e. elongation and percent reduction in area). In this chapter, we define these different strengths, and demonstrate methods for accounting for the strength of construction materials in the design of structural components." p7-1

DEFORMATION
A CHANGE IN SHAPE

ELASTIC DEFORMATION

ELASTIC DEFORMATION
A CHANGE IN SHAPE OF A BODY THAT IS NOT PERMANENT. THE DEFORMATION (DENT, STRETCH, COMPRESSION) LASTS ONLY FOR THE DURATION OF THE IMPACT.

FRACTURING

YIELD STRENGTH

YIELD STRENGTH

YIELDING