BS 8110 Load Combination Factors
Depending on the impact of the load on the structure, we select safety factors for load combinations. The following table in BS 8110 shows the load factors for load combinations.
For example, we can create load combinations as follows.
Load combination
1.4 dead load + 1.6 useful load
Operational load combination
Dead load 1.0 + payload 1.0
Taking soil pressure and dead load into consideration, the following load combination results
1.4 dead weight + 1.2 earth pressure
This combination can also be used when applying water pressure or water and earth pressure.
If we need to find the greatest impact of the above load on the structure, we also change the above load combination as follows, depending on the approval of the structural engineer.
1.0 dead load + 1.2 earth load
In situations where permanent loads reduce forces (bending moments, shear forces, etc.), we use this combination to obtain maximum effect on the structure. A similar effect on loads is also taken into account for wind load.
1.4 dead load + 1.4 wind load
1.0 dead load + 1.4 wind load
For some structural elements, load combinations with a factor of 1.4 (for permanent load) result in greater forces, while other elements present greater forces with a factor of 1.0. Therefore, it is advisable to consider both combinations when analyzing a structure. Additionally, it has been observed that some engineers use a factor of 0.9 (for dead load) to ensure safety, especially when analyzing tall structures.
When internal pressure, useful pressure, wind or earth pressure act on a structure, the following load combinations may arise.
1.2 dead load + 1.2 live load + 1.2 wind load
After we create all the load combinations as per the analysis requirements, the envelope is created to find the overall effect of the load combinations. When we design shell structural elements, we need to be very careful with the analysis results.
For example, consider building a board.
Some programs display maximum or minimum values when displaying envelope results, depending on their default settings, without taking signal conversions into account. Maximum values can be displayed. The maximum negative value (absolute value) can be ignored and the smaller negative value is displayed because the maximum values on the positive side are taken into account.
Therefore, it is very important to determine the absolute maximum values or values of each load combination for design.
Furthermore, we must be very careful when analyzing columns with load combinations, as axial forces and bending moments in both directions affect the design.
For example, consider a column with greater axial load and small bending moments. For this support we can find some reinforcements. Let's consider another column with small axial load and larger bending moments. This can provide a greater amount of reinforcement than the support above. When we consider the envelope results and create the design, we consider the maximum axial load and maximum moments for the design. The amount of reinforcement found in the analysis is probably greater than the above two methods, and so we will provide a greater amount of reinforcement than the amount of reinforcement required. Therefore, we need to be very careful when working with load and envelope combinations.
This table is also used to analyze alternative loading effects on the structure. We can create BS 8110 load combinations to take into account the impact of alternative actions. For example, we can assign different load cases to an adjacent span. Then we can adjust the factor to take the alternative effect into account.
