Utilisation factor

Stability effects are influenced by the properties along the entire length of a member. Therefore, the highest section class along the member is currently used as the class for the stability checks. This approach is a very conservative one, as the position of this highest class is not always relevant. Therefore, in version 21 the implementation of the valorisation project SEMI-COMP+ n° RFS2-CT-2010-00023 has been extended with the introduction of the utilisation factor η. It allows the use of less conservative section classes for the EC3 stability checks, which can lead to considerable material savings.

The utilisation factor is a measure for how close a section is to using its maximal section capacity. It is determined by proportionally increasing the loading until the design resistance of the cross-section is reached. The utilisation factor is then obtained by the ratio between the applied loads and the loads at the design resistance.

Instead of simply using the highest class along the member, the SEMI-COMP+ project proposes to use the class of the section with the highest utilisation factor. Or, in other words, to use the class of the most utilised section along the beam.

For example, in this column the highest class is found at the level of the hinge, where the column is in full compression. According to the old method the stability checks would be executed with class 4 section properties.

When we plot out the utilisation factor, we observe that the forces at the level of the hinges are very small compared to the maximum resistance. The class 4 sections are not relevant sections in the behaviour of this column.

Using the utilisation factor method, this column will be checked using class 1 section properties. In this particular case, it leads to around 10% material savings.

The preferred method for the stability classification can be selected in the steel settings.

When choosing the utilisation factor, the detailed output of the steel code check shows the decisive utilisation factor and its position along the member.

A table with the utilisation factor in each section can be printed as well by activating the option 'Print decision table' in the output settings of the steel ULS check.

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