How do you choose the appropriate safety factor for different load-bearing structures?

2 Reliability of material

Material is the substance or mixture of substances that make up the structure, such as steel, concrete, wood, or composite. Material has different properties, such as strength, stiffness, ductility, and durability, that affect its ability to resist load and deformation. Material also has different sources of uncertainty, such as variability, defects, deterioration, or corrosion, that affect its quality and performance over time. The reliability of material is a measure of how confident you are that the material will behave as expected under load and not fail prematurely. The lower the reliability of material, the higher the safety factor you need to account for the possible flaws and weaknesses.

3 Consequences of failure

Failure is the loss of function or integrity of a structure due to excessive load or deformation. Failure can result in damage, collapse, injury, or death. The consequences of failure depend on the importance, location, and usage of the structure, as well as the potential impact on the environment and society. The consequences of failure can be classified into four categories: negligible, moderate, considerable, and catastrophic. The higher the consequences of failure, the higher the safety factor you need to ensure the safety and serviceability of the structure and to avoid the legal and ethical implications of failure.

5 Safety factor calculation

Safety factor is calculated by dividing the ultimate load capacity of a structure by the design load. Ultimate load capacity is the maximum load that a structure can withstand before failure. Design load is the expected load that a structure will experience during its service life. Safety factor is usually expressed as a ratio or a percentage. For example, a safety factor of 2 means that the structure can resist twice as much load as it is designed for, or that it has a 100% margin of safety. A typical safety factor range is between 1.2 and 4, depending on the type of structure and load.

6 Safety factor scenarios

To illustrate how to choose the appropriate safety factor for different load-bearing structures, let's consider some examples. A bridge that carries heavy traffic and is exposed to wind and seismic load needs a high safety factor, as it has a high live load, a low reliability of material, a catastrophic consequence of failure, and a strict design standard. A safety factor of 3 or 4 might be suitable for this case. A roof that supports snow and rain load needs a moderate safety factor, as it has a medium environmental load, a medium reliability of material, a considerable consequence of failure, and a moderate design standard. A safety factor of 1.5 or 2 might be appropriate for this case. A shelf that holds books and ornaments needs a low safety factor, as it has a low dead load, a high reliability of material, a negligible consequence of failure, and a simple design standard. A safety factor of 1.2 or 1.3 might be enough for this case.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?