Useful Application of Function "Reduce Number of Generated Combinations"
Tips & Tricks
This post describes two practical examples based on the Eurocodes where the reduction of combinations is reasonable. There is a large number of various National Annexes as well as several material standards (EC 2 to EC 9) that are not in compliance with the rules for structural design (EC 0).
Example 1: Selecting leading variable actions
According to DIN EN 1990, fire resistance design should be performed in the accidental design situation. It is necessary to check the national application document to decide whether to use the frequent value (ψ1,1 × Qk,1) or the quasi-permanent value (ψ2,1 × Qk,1).
In this case, the information can be found in the National Annex to DIN EN 1991‑1‑2 under the Nationally Determined Parameters (NDP) to 4.3.1(2). It says that for components whose main action is the wind, the frequent value (ψ1,1 × Qk,1) is to be used. The other cases apply the quasi‑permanent value (ψ2,1 × Qk,1).
In RFEM/RSTAB, it is possible to choose between both values in the combination expressions. For the fire situation, you can select the combination expression “Accidental - psi 1,1 – Eq. 6.11c” and only define Wind as the leading action under “Select Leading Variable Actions.” Thus, only the wind with the frequent value and the remaining variable actions with the quasi‑permanent value are considered in the combinations.
Example 2: Reducing number of load cases
For serviceability limit state design, the British National Annex to BS EN 1993‑1‑1 (steel construction) requires the characteristic combination without ratio of permanent loads (see BS EN 1993‑1‑1/NA.2.23). It means, the deflection components wg,inst due to self‑weight are to be reduced.
In this case, you can select the option “Reducing number of load cases.” In the load cases to be considered, all load cases are selected except the self‑weight. Click [Yes] to confirm the message that appears. Then, the combinations are displayed without considering the self‑weight.
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