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RFEM 5

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5.5 Load Combinations

General description
Difference between load and result combination

Load cases can be superimposed in a load combination (CO) and in a result combination (RC).

Whilst taking partial safety factors into account, a load combination combines the loads of the contained load cases into "one big load case", which is subsequently calculated. In a result combination (see Chapter 5.6), the included load cases are calculated first. Then these results are superimposed in consideration of the partial safety factors.

Check box in Model - General Data dialog box
Check box in Model - General Data dialog box

Load cases can be combined manually (see Chapter 5.5.1) or superimposed automatically by RFEM (see Chapter 5.5.2), depending on settings in the Model - General Data dialog box (see Figure 12.23). These Settings also affect the appearance of the Result Combinations dialog tab in the Edit Load Cases and Combinations dialog box.

If you want to calculate combined load cases according to the second-order or large deformation analysis, you generally have to create load combinations. The same applies to models with nonlinear elements. The following example is used to demonstrate the subject.

Two load cases act on a slab with elastic foundations: In load case 1, the surface load acts on the entire plate; in load case 2, it stresses only part of the surface. The self-weight is not taken into account. The elastic foundation of the slab is ineffective in case of tension. Hence, no lifting forces are absorbed.

Figure 5.26 Load and deformation in LC 1

The foundation in load case 1 is effective for the entire surface.

Figure 5.27 Load and deformation in LC 2

The foundation in load case 2 is only effective for the right part of the surface. The left part of the slab is lifting.

When combining both load cases in a result combination, a warning will appear, because adding up results would be unacceptable due to nonlinear effects: Deformations in both load cases are based on different structural systems. For a result combination, you would see the lifting in the left zone shown in the second case.

Therefore, it is correct to superimpose the two load cases in a load combination. The figure below shows that the elastic foundation is effective for the added loads without failure.

Figure 5.28 Load and deformation of the load combination

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