REPEAT LOAD is an instruction to the program to analyze a set of loading conditions as though they act concurrently. It "always" provides a technically correct analysis of a structure, because it considers all of the specified forces acting together, as they do in the real structure. For this reason, it will provide technically correct results in situations that incorporate any non-linearities such as tension-only or compression-only members, compression-only foundations, etc. It is also ESSENTIAL to combine load effects with the REPEAT LOAD specification if a PDELTA analysis is being performed, or else the analysis won't "see" the P at the same time that it predicts the Delta. So it will be a meaningless analysis.
By comparison, LOAD COMB does not instruct the program to analyze the model as if the loads acted concurrently. Instead, it is just an instruction to combine (in a variety of possible ways) the results that were obtained from the independent analysis of the constituent load cases. It is important to note that there are instances where LOAD COMB is a perfectly legitimate method of obtaining combined results. If the model has no non-linearities, and if an elastic (stiffness) analysis is being performed, and if the superposition is valid, then LOAD COMB provides an analytically efficient way to obtain results without performing any unnecessary analysis.
Having said all of that, what is the downside to REPEAT LOADS, and why not use them all the time. Well, the downside is that they require some additional analysis efforts compared to LOAD COMB, so run times may be somewhat longer. But depending on the complexity of your typical models, that may be a negligible effect. Personally, I promote the idea of using REPEAT LOADS as the general rule, and considering LOAD COMB as the exception to the rule...to be taken advantage of only when technically permissible.
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 the Repeat load case, the structure is analyzed for the combination of loads.
- 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.
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.
No comments:
Post a Comment