Design Of Steel Structures

A: Option A ties together the clean in-plane bending with out-of-plane twist and the timing under service load. Grating often looks like a brace but doesn't deliver torsional stiffness unless detailed. Option B would show local plate waves before global twist, and you'd see it at higher stress zones. Option C explains rotation but not the classic lateral displacement of the compression flange. Option D is real physics, but residual stress alone doesn't create the sudden service-load sensitivity without a restraint issue.

A: Option A reflects why the rule exists: soil springs, joint can crushing, and installation tolerances all eat into fixity. Option B sounds materials-based but flips the logic; variability would argue the opposite. Option C mixes design philosophies but doesn't explain effective length. Option D confuses tolerance with imperfection sensitivity; tighter tolerance doesn't drive conservatism.

A: Option A drops out when you run δ ≈ 5wL⁴/(384EI) with consistent units; you land in the tens of millimetres. Option B underestimates L⁴ sensitivity. Option C overreaches by nearly a factor of four, confusing ultimate capacity with service response. Option D sneaks in fixity that wasn't stated, a common shortcut that breaks the estimate.

A: Redundancy helps A, B, and D by sharing load and damping redistribution. Option C sits outside that envelope; redundancy doesn't stop steel losing strength when heated. That's a safeguard problem, not a load-path one.