Activation
Preliminary design is activated in the Ultimate Configuration. You can find the corresponding checkbox in the “General” section. Preliminary design is performed for all joints that have been assigned an ultimate configuration with preliminary design enabled.
Reduction of Load Combinations
The computational effort is reduced by limiting the number of load combinations to be designed. A complete design requires that all load configurations resulting from the applied load combinations be analyzed at all nodes of the joint. However, the set of governing load configurations can be limited to a significantly smaller number, provided the following assumptions hold:
- The load combinations that result in the largest shear force components at the connected member ends are also governing for the joint.
- For shear forces in the same direction, the largest loads in magnitude are governing.
- If an extreme shear force component (with a tolerance of 0.1% of the magnitude) occurs under two load positions, both are to be assumed as governing.
- Shear forces of the order of +/- 1 N or 1 Nm are negligible.
- Only the member ends that are not supported in the submodel are included.
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 may be neglected due to an unusual load configuration. Therefore, the method is only suitable for preliminary design.
Application Example
This simple example of a two-sided column connection illustrates its efficiency and how it works. The live, wind, and snow loads in action on a steel frame structure in 21 load combinations result in the following section forces at the connection node in question.
The table highlights all rows that are identified as governing load combinations based on the assumptions explained above. The column base is assumed to be supported, which means that the loads in the lower part of the column are not taken into account. Furthermore, CO 11 in Member 4 has a maximum shear force in the z-direction of 28.243 kN, which lies within the 0.1% tolerance limit of the maximum shear force in the z-direction for the design situation in CO 13 (28.268 kN). This results in the reduced, likely governing load combinations 6, 7, 8, 9, 11, and 13, which are also listed in the “Summary of Preliminary Design” table. This reduces the number of combinations to be analyzed from 21 to 6.
A comparison of the design ratios of the joint components with the full design shows that the components actually subjected to critical loads can already be correctly dimensioned based on the preliminary design. In contrast, the design ratio for a bolt in the end plate connection is underestimated by 0.8%. The governing load combination CO 3 was not identified by the preliminary design. This highlights the method’s sensitivity to specific load configurations, although the error in this case—as in most other typical applications—is negligible.
