Activation
The activation of the preliminary design is done in the capacity configuration. The corresponding checkbox can be found in the "General" section. A preliminary design is performed for all connections assigned with a capacity configuration with activated preliminary design.
Reduction of Load Combinations
The reduction of computational effort is achieved by reducing the load combinations to be designed. A complete design requires that all load cases from the acting load combinations are examined at all nodes of the connection. However, the number of probably governing load cases can be considerably reduced, assuming the following:
- The load combinations that cause the largest internal force components in the adjoining member ends are governing for the connection as well
- For aligned internal forces, the largest loads in magnitude are governing
- If an extreme internal force component (with a tolerance of 0.1% of the magnitude) occurs under two load cases, both are assumed to be governing
- Internal forces in the range of +/- 1 N or 1 Nm are negligible
- Only the member ends that are not supported in the submodel are considered
The load combinations identified under these assumptions actually include all governing ones in most cases. However, it cannot be ruled out that individual significant combinations are neglected due to an unusual load case. Therefore, the method is only suitable for preliminary design.
Application Example
This simple example of a two-sided column connection illustrates the effectiveness and functionality. The live, wind, and snow loads acting on a steel frame structure in 21 load combinations result in the following internal forces at the considered connection node.
In the table, all rows identified as governing load combinations according to the assumptions explained above are marked. The column base is assumed to be supported, so the loads in the lower part of the column are not considered. Additionally, the case where LC 11 in member 4 results in a maximum shear force in z of 28.243 kN, which is within the 0.1% tolerance limit of the maximum shear force in z of the design situation in LC 13 (28.268 kN), is considered. This results in the reduced, probably governing load combinations 6, 7, 8, 9, 11, and 13, which are also listed in the "Preliminary Design Summary" table. Thus, the combinations to be examined are reduced from 21 to 6.
A comparison of the utilization of the connection components with the complete design shows that the actually critically stressed components can already be correctly dimensioned based on the preliminary design. The utilization of a bolt in the end plate connection, on the other hand, is underestimated by 0.8%. The governing load combination LC 3 was not identified by the preliminary design. This highlights the sensitivity of the method to specific load cases, although the error is negligible in this and most other typical application cases.
