Live Loads
Part of Structural Engineering
Estimating the variable loads from people, goods, animals, snow, and wind that a structure must carry in addition to its own weight.
Why This Matters
Dead load — the weight of the structure itself — is constant and predictable. Live load is everything else: the people who use the building, the goods stored inside, the grain piled in a granary, the snow on the roof, the wind pressing against the walls. Live loads change over time and vary with use. They can be much larger than the dead load for some building types, or relatively small for others.
Getting live loads wrong is dangerous in both directions. Underestimating live load leads to structural failure when the building is put to its intended use. Overestimating live load wastes materials in the foundation and structure unnecessarily. For a rebuilding community operating with limited materials, this balance matters.
Live loads also include dynamic effects — loads that change rapidly, like the impact when a heavy cart rolls over a bridge, or the vibration of machinery on a floor. Dynamic loads can be several times larger than the equivalent static load and must be accounted for separately.
Occupancy Live Loads
These are the loads from people and their activities. Use these values for floors, stairs, and platforms.
Floor live loads by occupancy type:
| Occupancy | Live load (lb per square foot) |
|---|---|
| Residential rooms | 40 |
| Residential corridors | 40 |
| Assembly hall, church | 100 |
| Classrooms | 40 |
| Storage (general light) | 50–75 |
| Storage (heavy goods) | 125–250 |
| Market, trading floor | 75–100 |
| Granary (grain piled) | 100–200 |
| Workshop, light industry | 50–100 |
| Workshop, heavy equipment | 100–200 |
| Roof access (no use) | 20 |
| Roof with congregation | 50–100 |
How these values are derived: Standard values represent the maximum realistic density of people plus furnishings. A full assembly hall at maximum occupancy has roughly 1 person per 4 square feet; a 150 lb person on 4 square feet = 37.5 lb/sq ft, which rounds to 40–50 lb/sq ft as a standard value.
Storage live loads: These can be enormous. A floor loaded with stacked grain (density ~50 lb/cu ft) to 3 feet depth exerts 150 lb/sq ft live load. A floor storing iron products or stone blocks can easily reach 500+ lb/sq ft. Always calculate the actual weight of planned storage loads rather than assuming a standard value will cover it.
Snow Loads
Snow accumulates on roofs and must be carried by the structure. Snow load depends on location, roof slope, and wind patterns.
Fresh snow density: 5–15 lb per cubic foot (light, dry snow is lighter; wet snow is heavier) Old compacted snow: 15–25 lb per cubic foot Wet spring snow: 25–40 lb per cubic foot
Typical design snow loads (on horizontal roof projection):
| Climate | Snow load (lb per sq ft) |
|---|---|
| No snow zone | 0 |
| Light snow zone | 10–20 |
| Moderate snow zone | 20–40 |
| Heavy snow zone | 40–70 |
| Very heavy (mountain) | 70–150+ |
Effect of roof slope: Snow slides off steep roofs. For roof slopes over 30°, reduce calculated snow load by 50%. For slopes over 60°, snow load is negligible (snow slides off before it accumulates significantly). Flat or nearly flat roofs retain full snow load — and can accumulate drifts even deeper than the baseline load if surrounded by higher obstructions.
Drift loading: Snow drifts against vertical obstructions (higher roof sections, walls above the roof, equipment housing). A roof adjacent to a taller wall can accumulate drifts 2–3× the normal design snow load immediately adjacent to the wall. Design these areas for extra load.
Calculating roof snow load: Load on roof structure = (snow load on plan area) / (cos² of roof slope)
For a 30° pitched roof in a moderate snow zone (30 lb/sq ft flat): Load = 30 / cos²(30°) = 30 / 0.75 = 40 lb/sq ft along the rafter
Wind Loads
Wind exerts pressure on surfaces facing into the wind (windward) and suction (negative pressure) on surfaces facing away from the wind (leeward and side walls, roof leeward slope).
Basic wind pressure: Pressure = 0.003 × V² (lb per sq ft), where V = wind speed in mph
- 50 mph wind: 0.003 × 2500 = 7.5 lb/sq ft
- 80 mph wind: 0.003 × 6400 = 19 lb/sq ft
- 100 mph wind: 0.003 × 10000 = 30 lb/sq ft
Modify for exposure: Open flat terrain amplifies wind speed; forests and buildings shelter downwind structures. For an exposed site, use the full calculated pressure. For a sheltered site, use 70–80% of calculated pressure.
Roof uplift: Wind creates suction over a roof surface. For typical building geometry, this uplift force is approximately equal to the windward wall pressure (positive). It effectively reduces the dead load holding the roof down. If the roof dead load is less than the wind uplift, the roof will be lifted off — anchor roofing and framing accordingly.
Overturning: Wind on the windward wall of a building creates an overturning moment — the building wants to rotate about its downwind foundation edge. Ensure foundations are heavy enough or anchored to resist this overturning.
Dynamic and Impact Loads
Impact loads: A load dropped or moving rapidly exerts a larger effective force than its static weight. For design purposes, apply these factors to the static load to get the effective design load:
| Impact source | Multiply static load by |
|---|---|
| Light vibrating machinery | 1.25 |
| Moderate vibrating machinery | 1.5 |
| Heavy machinery (hammers, presses) | 2.0–3.0 |
| People jumping or running | 1.5–2.0 |
| Cart or wagon on smooth floor | 1.2 |
| Cart or wagon on rough surface | 1.5–2.0 |
A working example: A blacksmith hammer mounted on a floor weighs 800 lb. With a dynamic factor of 2.5 (heavy hammer): design load = 800 × 2.5 = 2,000 lb concentrated load. The floor must be designed for this, not merely the 800 lb static weight.
Resonance danger: If machinery vibrates at a frequency near the natural frequency of the floor or structure, resonance can amplify vibrations to dangerous levels — small vibration forces produce large structural oscillations. Test: walk across the floor and feel for excessive springiness. If the floor bounces noticeably with each step, it may be near resonance. Stiffen the floor by reducing span or adding intermediate supports.
Load Combinations
Structures carry multiple loads simultaneously. Use load combinations to find the worst case:
Normal case: Dead load + full live load Snow case: Dead load + partial live load + full snow load Wind case: Dead load + partial live load + full wind load
Do not add maximum values of all loads together — maximum snow and maximum occupancy load are very unlikely to occur simultaneously (the building is unlikely to be fully occupied during the worst snowstorm, though it can happen). The code approach: use 100% of the dominant variable load and 75% of secondary variable loads.
For a building: 100% dead + 100% live + 75% snow is a reasonable worst-case combination for floor design. For roof design: 100% dead + 100% snow + 75% wind.