Structural Systems
Start with the load path
Every structural question is really a load-path question: deck or sheathing → joist or beam → girder → column → foundation → soil. Sketch the path before doing arithmetic; most wrong answers come from geometry mistakes, not math mistakes. Determine tributary area first. A member takes load from half the span on each side; interior members carry full tributary area, edge members half, corner members a quarter. Two-way slabs shed load on 45° lines from the corners.
Gravity members
Separate dead load (self-weight, fixed, not reducible) from live load (occupancy, reducible in code for large tributary areas), and add soil, fluid, rain and snow where they apply. ASD and LRFD combinations are not interchangeable - never pair a service-level load with a strength-level capacity. Continuous spans deflect far less than simple spans of the same length; a cantilever tip deflects much more than intuition suggests.
Lateral systems
Shear walls and braced frames are stiff and attract force; moment frames drift more but tolerate movement. Diaphragms must be continuous, chorded and positively tied to the lateral system.
Serviceability
Deflection often governs before stress does: L/360 for floors, L/240 for roofs, and vibration on long spans.
Traps: treating wind (pressure acting on area) like seismic (inertial force proportional to weight); forgetting live-load reduction; assuming a cantilever behaves like half a simple span.
Rule of thumb: steel beam depth ≈ span/20 (20 ft span → about 12 in); sawn joist nominal depth in inches ≈ 0.75 × span in feet (16 ft → 2x12).
Worked example: a 24 ft × 30 ft bay with an interior column gives 720 sf tributary; at 40 psf live + 60 psf dead the column carries 720 × 100 = 72,000 lb, or 72 kips.