Reducing the Considered Load Cases for Combinations - Part 3
Tips & Tricks
In addition to the reduction method ‘Reduce number of load cases’ and ‘Examine results’, RFEM and RSTAB also offer the method ‘Select leading variable actions’. This method also allows you to reduce the number of mathematically possibly combinations by using certain criteria.
As reduction criterion, this option uses neither the signs of the internal forces nor the extreme superposition results, but the leading property of an action.
Generally, the superposition standards help to define realistic load superposition states for the design of the construction. To satisfy all requirements from safety and reality, most regulations do not superimpose all actions (permanent load, wind, snow, etc.) with a 100 percent contribution, but they specify one respective leading main action and the corresponding accompanying actions.
The advantage here is that only the leading action must be considered with a higher percentage than the accompanying action in a combination. However, this approach has also the disadvantage that we do not know the leading action and therefore each action must be assumed as leading action.
Since every action is considered as leading action independent of the load magnitude, there is a hidden potential to reduce the considered load cases for the combinations.
For example, we can imagine the following load cases for a symmetrical two-hinged frame:
Load Case | Description | Action | Action group
1 | self-weight | permanent | 1 (permanent load)
2 | wind to the right | alternative | 2 (wind)
3 | lifting wind | alternative | 2 (wind)
4 | snow | alternative | 3 (snow)
Thus, we obtain the following action combinations (L = leading action, A = accompanying action):
AC1: 1 (permanent load)
AC2: 1 (permanent load) + 2 (wind)
AC3: 1 (permanent load) + 3 (snow)
AC4: 1 (permanent load) + 2L (wind) + 3A (snow)
AC5: 1 (permanent load) + 3L (snow) + 2A (wind)
If we assume that the two wind loads 2 and 3 do not always occur simultaneously, we obtain 11 possible load combinations:
AC1 -> 1
AC2 -> 3
AC3 -> 1
AC4 -> 3
AC5 -> 3
If when entering the snow load you already realize that the surface with the snow layer is very small and the load is not governing, you can reduce three of the eleven load combinations by deactivating the leading property of the snow action.
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