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Difference in Results for Load Combination and Repeat Load

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Difference in results for load combination and repeat load case in STAAD.Pro lies in the calculation. In load combination, STAAD.Pro algebraically combines the results of the previous primary loads by factoring them. However, in Repeat load case, the structure is analyzed for the combination of loads.

Difference in results for Load combination and Repeat load
Difference in results for Load combination and Repeat load

  • A cross braced portal frame with two tension only member is modelled. The tension only members will only carry axial tension.
  • The end moments are released at the two ends of the portal frame.
  • Two nodal loads of equal magnitude are added at node 2 and 3.

The same arrangement is simulated both for Load Combination as well as Repeat Load case.

Load Combination

In this case, two nodal loads are applied individually along X and Z direction. These primary load cases are combined using Load combination. The model is then analyzed.

The video below shows how the braces behave under Load Combination

Load combination result

Have you wondered what would be the real life scenario?

In real scenario when two nodal loads at opposite direction acts on the frame, the cross bracings would behave like ropes. It would slag due to compressive forces acting on them. Since the cross bracings are tension only members, it should not be considered in analysis when compressive forces are developed in them.

This scenario can be dealt with Repeat Load case.

Repeat Load Case

In this case, similarly two individual nodal load cases are applied along X and Z directions. But in this case Repeat load case is added instead of Load combination.

The video below shows the braces behave under Repeat Load case.

Repeat Load case result

Want more?

Please have a look at this Bentley Communities wiki mentioned for further information regarding this topic.

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