Steel and Other Structural Materials
Steel behavior and design
Structural steel design uses the load and resistance factor design or allowable strength design. Key concepts include yielding, local buckling, lateral-torsional buckling, and the plastic section modulus. Members are classified as compact, noncompact, or slender based on width-to-thickness ratios. Traps include ignoring unbraced length when selecting a beam capacity, confusing the elastic and plastic section moduli, and forgetting that a slender element reduces the effective capacity.
Other materials
Aluminum, cold-formed steel, and composites each have distinct design rules. Cold-formed steel members are thin-walled and prone to local and distortional buckling. Candidates should know that composite construction relies on shear connectors to develop the interaction between steel and concrete.
Worked rule of thumb
For a compact steel beam with plastic section modulus Z = 100 in³ and Fy = 50 ksi, plastic moment Mp = Fy Z = 5,000 kip-in ≈ 417 kip-ft. If the unbraced length is long, the actual capacity may be governed by lateral-torsional buckling and could be much lower. Always check the unbraced length against the limiting lengths before using Mp.