Reducing Considered Load Cases for Combinations 1

Tips & Tricks

To carry out a structural analysis for a structural system according to the current standards, it is necessary not only to deal with actions and resistances of structural components but also with the combinations of the actions. Some of the most common actions in structural analysis are, for example, the permanently acting load case self‑weight, the suddenly acting load case wind, and snow.

The combination of actions should serve the purpose to find for each portion of the structure the respectively most unfavorable load combination for the design. To do this, you can:

  • Determine all mathematically possible combinations and use them for the design or
  • Try to use the logical relationships of the combinations of actions and reduce the number of the possible combinations

To illustrate this, we can, for example, imagine a symmetrical two‑hinged frame with the following load cases:

Load Case Description Effect Axial Force - Left Column Axial Force - Right Column
LC1 Self-weight Permanent Compression Compression
LC2 Wind to the right Alternative Tension Compression
LC3 Lifting wind Alternative Tension Tension
LC4 Snow Alternative Compression Compression

Because LC1 is always acting and the other three load cases can occur alternatively, the following eight possible combinations result mathematically:
CO1: LC1 + LC2 + LC3 + LC4
CO2: LC1
CO3: LC1 + LC2
CO4: LC1 + LC3
CO5: LC1 + LC4
CO6: LC1 + LC2 + LC3
CO7: LC1 + LC3 + LC4
CO8: LC1 + LC2 + LC4

To reduce them, you can, for example, only search for the combinations with the most extreme axial forces. For each column, create one group with load cases acting positively, and one with load cases acting negatively:

Group Left Column Right Column
Positive LC1, LC2, LC3 LC1, LC3
Negative LC1, LC4 LC1, LC2, LC4

Now you obtain only four instead of eight load combinations:
CO1: LC1 + LC2 + LC3
CO2: LC1 + LC4
CO3: LC1 + LC3
CO4: LC1 + LC2 + LC4

To carry out this reduction in RFEM 5 and RSTAB 8, select the ‘Combination Expressions’ tab in the ‘Edit Load Cases and Combinations’ dialog box, and then select the ‘Reduce number of load cases’ check box, creating the respective groups for the column members only for the axial force. The program then considers this information for the determination of the necessary combinations.

Furthermore, you have the possibility to exclude the load cases of the groups by using a relevance criterion. This additional option allows you to exclude load cases with lesser relevance.

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RFEM Main Program
RFEM 5.xx

Main Program

Structural engineering software for finite element analysis (FEA) of planar and spatial structural systems consisting of plates, walls, shells, members (beams), solids and contact elements

RSTAB Main Program
RSTAB 8.xx

Main Program

The structural engineering software for design of frame, beam and truss structures, performing linear and nonlinear calculations of internal forces, deformations, and support reactions