The document discusses trusses and steel sections. A truss is a structure comprising triangular units constructed with straight members connected at joints. Trusses distribute external forces through a framework of triangular elements and are used to span large distances. There are different types of trusses including Pratt trusses, Town's lattice trusses, and bowstring trusses. The document also discusses steel sections which are construction materials formed into specific shapes used in engineering structures. Steel is an alloy of iron and carbon.