Reducing Considered Load Cases for Combinations 2

Tips & Tricks

In addition to the reduction method “Reduce number of load cases”, you can also use the method “Examine results”. This option is also aimed at reducing the number of mathematically possible combinations by certain criteria.

As a reduction criterion, this option does not use the sign attribute as in the "Reduce Number of Load Cases ..." method, but instead internally computes a linear result combination RC from the individual load cases on the basis of the selected superposition standard and examines the resulting extreme values and their according load cases LC at each point. Thus, we obtain a ‘max’ and a ‘min’ load case combination at each point. These ‘extreme’ load case combinations are the new results and are used by the automatic generation of load combinations. In most cases, only certain load combinations yield distinctive results, so this method is very well suited to reduce the possible load case combinations.

We can imagine, for example, the following load cases for a symmetrical two-hinged frame:
Load Case | Description | Effect | Axial Force - column left | Axial Force - column right
1 | self-weight | permanent | compression (-10 kN) | compression (-10 kN)
2 | wind to the right | alternative | tension (5 kN) | compression (-5 kN)
3 | lifting wind | alternative | tension (3 kN) | tension (3 kN)
4 | snow | alternative | compression (-12 kN) | compression (-12 kN)

Thus, the algorithm computes the following result combination RC according to the standard criterion:
RC: LC1/permanent + LC2 + LC3 + LC4

Considering the axial forces of the columns, the superposition yields the following resulting extreme values:
Left Column
maximum N = -2 kN (LC1, LC2, LC3)
minimum N = -22 kN (LC1, LC4)
Right Column
maximum N = -7 kN (LC1, LC3)
minimum N = -27 kN (LC1, LC2, LC4)

If we take into account the load case combinations yielding the extreme value results, we obtain:
CO1: LC1 + LC2 + LC3
CO2: LC1 + LC4
CO3: LC1 + LC3
CO4: LC1 + LC2 + LC4

If we sum up the groups, we obtain just four load case combinations instead of only eight possible ones.

The detail settings of the ‘Examine results’ method allow you to generate load combinations for certain structural elements based on certain types of results. Furthermore, this evaluation method allows you to freely specify the basic result combination for the determination of the distinctive load case combinations. This option is the actual advantage of the method and allows you to include your experience in the generation of the combinations.

Keywords

Combinatorics Reduction method Load combination Load case Number

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  • Updated 7 December 2020

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