Note
Please refer back to A3.2 on the an introduction to Safety Factor before progressing onto this section.
Safety Factor Calculations
Definition
Safety Factor (SF)
The ratio of the maximum load a structure can bear to its expected load.
Note
- Safety factors are not just about adding extra material or making structures heavier.
- They are about strategically designing for unpredictable conditions.
The Formula for Safety Factor
Note
$$\text{Safety Factor (SF)} = \frac{\text{Ultimate Load}}{\text{Working Load}}$$
- Ultimate Load: The maximum force a structure or material can withstand before failure.
- Working Load (also called Allowable Load): The expected everyday load the structure is designed to carry safely.
Tip
When calculating safety factors, always ensure that the units for ultimate load and allowable load are consistent.
Calculating Maximum Intended Loads
- Identifying the Expected Load: Consider all forces acting on the structure, including:
- Dead Loads: Permanent forces such as the weight of the structure itself.
- Live Loads: Temporary forces such as people, furniture, or vehicles.
- Environmental Loads: Forces from wind, snow, earthquakes, etc.
- Applying the Safety Factor: Multiply the expected load by the safety factor to determine the design load.
$$\text{Design Load} = \text{Expected Load} \times \text{Safety Factor}$$
Example
- A steel beam has a maximum tensile strength of 600 MPa.
- It's expected to carry a load causing 200 MPa of stress in normal use.
- $\text{SF} = \frac{600}{200} = 3$
- Interpretation: The beam can handle 3 times the expected load before failing.
Choosing a Safety Factor
| Application | Typical SF |
|---|---|
| Non-critical (benches, furniture) | 1.5–2 |
| Buildings and bridges | 2–3 |
| Aircraft or medical devices | 3–5+ |
Note
The higher the risk or uncertainty, the larger the safety factor required.
Common Mistake
- Don't assume that a higher safety factor always means a better design.
- Overdesign can lead to unnecessary costs and resource use.
