Types of Structural Loads
Structures must resist permanent self-weight (dead loads) and variable loads from occupancy and environment (live, wind, snow loads). Design uses factored combinations for safety.
Dead Loads (Permanent)
Concrete slab (150mm): 3.6 kN/m²
Timber floor + finish: 1.0-1.5 kN/m²
Steel deck + concrete: 2.5-3.5 kN/m²
Roof (flat, membrane): 0.5-1.0 kN/m²Live Loads (Variable, by occupancy)
Residential: 1.5-2.0 kN/m²
Office: 2.5-3.0 kN/m²
Retail/shop: 4.0-5.0 kN/m²
Storage: 7.5+ kN/m²
Stairs: 3.0-4.0 kN/m²Factored Load Combinations (Eurocode/LRFD)
Ultimate: 1.35×DL + 1.5×LL (design strength check)
Service: 1.0×DL + 1.0×LL (deflection check)
Example: office floor
DL = 2.5 kN/m², LL = 3.0 kN/m²
Design load = 1.35×2.5 + 1.5×3.0 = 3.375 + 4.5 = 7.875 kN/m²Key Conversion
- 1 kN/m² = 20.9 psf (pounds per square foot)
- 1 psf = 47.9 Pa
- 1 kN = 0.2248 kip (US)
Calculate structural loads: Free Structural Load Calculator
Load Types in Structural Engineering
- Dead load (DL): Permanent weight of structure — self-weight of beams, slabs, cladding. Concrete: ~24 kN/m³. Steel: ~78 kN/m³.
- Live load (LL) / imposed load: Occupancy loads — people, furniture, stored goods. Office floors: 2.5–3.0 kN/m². Retail: 4–5 kN/m². Storage: 7.5+ kN/m².
- Wind load (WL): Dynamic pressure from wind. Depends on location, height, exposure, and building shape.
- Snow load (SL): Accumulation of snow on roofs. Varies by location and roof geometry.
- Seismic load (EL): Earthquake forces. Governed by local seismic zone and soil conditions.
Load Combinations
Structural design codes (Eurocode, ACI, ASCE 7) specify factored load combinations to account for the probability that maximum loads act simultaneously. A typical Eurocode ULS combination: 1.35 × DL + 1.5 × LL (ultimate limit state). Not all loads are at maximum simultaneously — pattern loading, load combinations, and load reduction factors reflect statistical probability. Service limit state checks (deflection, vibration) typically use unfactored loads. Safety factors embedded in load factors account for variability in load magnitudes and material properties.
Frequently Asked Questions
What is the difference between ULS and SLS?
Ultimate Limit State (ULS) checks that the structure will not collapse — member strengths must exceed factored loads. Serviceability Limit State (SLS) checks that the structure performs acceptably in use — limiting deflection, vibration, and cracking under unfactored (working) loads. A structure can pass ULS but fail SLS if beams are strong but too flexible. Both must be satisfied.
What is load path and why is it important?
Load path describes how forces travel through a structure from where they are applied to the foundations. Slab → beams → columns → foundations → soil is a typical load path. Every element must be capable of carrying the loads passed to it. Interrupting or misunderstanding the load path — such as removing a wall without providing an alternative load path — causes structural failure. This is why structural engineers must be consulted before any load-bearing element is altered.
How are foundations sized for structural loads?
Foundations spread column or wall loads over sufficient ground area that the bearing pressure (stress on soil) does not exceed the ground's allowable bearing capacity. Typical allowable bearing capacities: firm clay ~100–150 kPa, dense gravel ~200–400 kPa, rock >1,000 kPa. A column carrying 500 kN on 150 kPa soil needs a footing area of at least 500/150 = 3.33 m², i.e., approximately 1.85 m × 1.85 m pad.