The Dead/Live Distinction Affects Every Structural Decision
Dead and live loads are the two fundamental categories of gravity load that every structural system must resist. While both loads press downward, they behave differently: dead load is always present and unchanging, while live load comes and goes with building use. This distinction matters for structural design because members sized for dead load alone are reliable - the load does not change. Members sized for live load must account for the maximum possible live load at the worst moment, which could be much higher than the average condition. The ARE PPD exam tests both the conceptual distinction and the calculation of each load type.
Dead Load: Permanent and Constant
Dead load includes everything permanently attached to or incorporated into the structure. Accurately estimating dead load is critical because underestimating it means the structure is undersized; overestimating it wastes material and cost. Common dead load values architects and engineers use:
| Element | Typical Weight |
|---|---|
| Normal weight concrete (per inch thickness) | 12.5 psf |
| 6-inch concrete slab | 75 psf |
| Steel deck + concrete topping (3.5 in.) | 40–50 psf |
| Typical floor finish (tile + mortar) | 15–25 psf |
| Mechanical/electrical/plumbing allowance | 5–10 psf |
| Suspended ceiling + lighting | 5–10 psf |
In preliminary design, total dead load for a typical office floor (structure + finishes + MEP + ceiling) is often estimated at 80–120 psf, depending on structural system and finish quality.
Live Load: Variable and Occupancy-Based
Live load reflects the variable load from people, furniture, equipment, and stored materials. ASCE 7 Table 4.3-1 tabulates minimum uniformly distributed live loads by occupancy type. Key values to know for the ARE:
| Occupancy | Minimum Live Load |
|---|---|
| Residential floors | 40 psf |
| Office space | 50 psf |
| Corridors (above first floor) | 80 psf |
| Assembly (fixed seats) | 60 psf |
| Assembly (movable seating) | 100 psf |
| Retail / mercantile (first floor) | 100 psf |
| Storage (light) | 125 psf |
| Storage (heavy) | 250 psf |
| Roofs (accessible) | 100 psf |
| Roofs (non-accessible) | 20 psf min |
Live Load Reduction
ASCE 7 permits reduction of design live loads for members with large tributary areas because it is statistically unlikely that the entire large area will be fully loaded simultaneously. The maximum reduction is 50% for members supporting one floor and 40% for members supporting two or more floors. Live load reduction is NOT permitted for assembly occupancies (crowds could fill the entire area simultaneously), storage areas, or roof live loads.
Key Exam Points
- Dead load = permanent; does not change after construction.
- Live load = variable; depends on occupancy and use.
- Office live load: 50 psf. Assembly (movable): 100 psf. Storage: 125–250 psf.
- Live load reduction permitted for large tributary areas - not assembly, not storage.
- Total design load = dead load + live load (plus other applicable loads).
Study PPD on AREprep
AREprep’s PPD flashcards cover every concept on this exam with spaced repetition, and the practice exams mirror the real question formats so the actual test feels familiar.