Dead, Live, and Snow Loads
Dead load
Dead load is the self-weight of structure and permanent finishes. It is computed from material densities and thicknesses. Traps include forgetting superimposed mechanical and ceiling loads, and double-counting slab weight already included in a member's self-weight.
Live load
Live load is occupancy-dependent and specified in code tables by use. Key concepts are uniform versus concentrated live loads, reducible live loads based on tributary area (with limits for storage and public assembly), and the distinction between floor live load and roof live load. A common trap is reducing a live load when the code forbids it for that occupancy.
Snow load
Snow load derives from ground snow load modified by exposure, thermal, and importance factors, plus a rain-on-snow surcharge in some regions. Unbalanced snow (drifts, sliding snow, and leeward drift on lower roofs) frequently governs local members even when balanced snow governs the main framing. Candidates should understand the difference between ground and roof snow load and the role of roof slope and eave overhangs.
Worked rule of thumb
For a flat roof with ground snow pg = 40 psf, exposure Ce = 1.0, thermal Ct = 1.0, importance I = 1.0, and a 0.7 conversion factor, roof snow is 0.7 × 1.0 × 1.0 × 1.0 × 40 = 28 psf. If a higher adjacent roof can drift snow onto this roof, add the drift surcharge before sizing the members near the step.