How is Ductility and toughness important in structures?

Ductility and toughness are the structural properties that show the ability of a structural element to sustain damage when overloaded while continuing to carry the load without failure. These are extremely important for structures designed to sustain damage without collapse. Most structural elements are designed to provide sufficient strength to support anticipated loads without failure and enough stiffness so that they will not deflect excessively under these loads. If such an element is subjected to a load substantially larger than it was designed to carry, it may fail in an abrupt manner, losing load-carrying capacity and allowing the structure to collapse. Masonry and concrete, for example, will crash when overloaded in compression and will crack and pull apart when placed in tension or shear. Wood will crush when overloaded in compression, will split when overloaded in shear, and will break when overloaded in tension. Steel will buckle if overloaded in compression and will twist when loaded in bending if not accurately braced but will yield when overloaded in stress. The property of the steel to stretch a great deal while continuing to carry the load, allows it to be used in structures of all types to provide them with ductility and toughness. The buildings, that have no steel reinforcement, are not very ductile or tough and frequently collapse in earthquakes. In concrete structures, steel is used in the form of reinforcing bars that are placed integrally with the masonry and concrete. When reinforced masonry and concrete elements are loaded in bending or shear, the steel reinforcing bars will yield in tension and continue to carry the load, thus protecting the masonry and concrete from failure. Inwood structures, steel fasteners (typically nails, bolts, and straps) bind the pieces of wood together. On loading the wood in shear or bending, these steel connectors yield and shield the wood from breaking and crushing. In steel structures, ductility is achieved by proportioning the structural members with sufficient thickness to prevent local buckling, by bracing the members to prevent them from twisting, and by joining the members together using connections that are stronger than the members themselves so the structure does not pull apart. In all structures, ductility and toughness are achieved by proportioning the structure so that some members can yield to protect the rest of the structure from damage. The measures applied to obtain ductility and toughness in structural components are individual to each building material and to each type of structural system. The building codes specify the measures to use to provide ductility and toughness to steel structures.