Load factor design is a structural engineering approach that uses load factors to ensure structures can safely withstand anticipated loads throughout their lifespan. At its core, it's about accounting for uncertainties in actual loads and material strengths by applying factors to the expected service loads.
Understanding Load Factor
Central to load factor design is the concept of the load factor. According to the provided reference, the load factor in plastic design is defined as:
"the ratio of ultimate collapse load to the working load that can be applied on the structure."
This can be represented by the formula:
Load Factor = Ultimate Collapse Load / Working Load
The reference also provides another relationship:
Load Factor = Factor of safety x Shape factor
The working load (or service load) is the load expected under normal use conditions. The ultimate collapse load is the theoretical load at which the structure would fail or collapse. The load factor indicates how many times the working load the structure can theoretically withstand before collapse.
How Load Factor Is Used in Design
Instead of designing based directly on working loads and a single factor of safety applied to material strength (as in Allowable Stress Design), load factor design (commonly part of Load and Resistance Factor Design - LRFD, or Ultimate Strength Design) applies factors directly to the loads.
Here's how it generally works:
- Identify Service Loads: Determine the expected loads the structure will experience (e.g., dead load, live load, wind load, seismic load).
- Apply Load Factors: Each type of load is multiplied by a specific load factor. These factors are typically greater than 1.0 and vary depending on the load type and combination (e.g., dead load might have a factor of 1.2, while live load might have a factor of 1.6). This results in "factored loads" or "ultimate loads".
- Combine Factored Loads: Various combinations of these factored loads are considered to find the most critical design scenario.
- Design for Factored Loads: The structural elements are then designed to have a strength (resistance) that is at least equal to these combined factored loads, often after applying a separate "resistance factor" (which is typically less than 1.0) to the material strength.
This approach accounts for:
- Variability in the actual magnitude of loads compared to their nominal values.
- Uncertainty in load combinations occurring simultaneously.
- The desire for a consistent level of safety across different load types and combinations.
Key Aspects of Load Factor Design
- Focus on Ultimate Limit State: Load factor design primarily focuses on preventing structural collapse under extreme, factored loads (the ultimate limit state). Serviceability (like deflection or vibration under working loads) is checked separately.
- Load Combinations: Design codes specify various load combinations with different load factors to cover realistic scenarios the structure might face.
- Factors Reflect Uncertainty: Loads with higher uncertainty (like live load or wind) typically have higher load factors than loads with lower uncertainty (like the structure's own weight or dead load).
Example: Comparing Working Load vs. Factored Load
Let's say a beam is subjected to a dead load (D) of 10 kN/m and a live load (L) of 20 kN/m.
| Load Type | Working Load (kN/m) | Typical Load Factor | Factored Load (kN/m) |
|---|---|---|---|
| Dead Load | 10 | 1.2 | 1.2 * 10 = 12 |
| Live Load | 20 | 1.6 | 1.6 * 20 = 32 |
A common load combination for design is 1.2D + 1.6L.
- Combined Working Load: 10 + 20 = 30 kN/m
- Combined Factored Load: 12 + 32 = 44 kN/m
The structure would be designed to safely resist the factored load of 44 kN/m (after considering material strength reduction factors). This factored load is significantly higher than the expected working load, providing a margin of safety against potential overloads and variations.
Load factor design is a fundamental concept in modern structural codes (like those used in LRFD), providing a more rational and probabilistic approach to achieving structural safety compared to older methods.
