Question
I have a model containing an Engineered Element. I'm checking the results from LimitState:GEO against a solution I have found using a different method and the values of forces and moments that I see after solve are different to what I was expecting. Is something wrong?
Solution
This is to be expected. With LimitState:GEO, bending moments and forces are likely to be a little higher than would be predicted by a conventional hand calculation. The reason for this is that Limit Analysis methods, such as DLO, give unique solutions only in the area of failure. In areas outside of the identified failure mechanism the software only has to identify a stress state that does not violate yield in order to generate a valid solution. It is therefore viable for it to identify an at rest earth pressure distribution or anything between active or passive that doesn’t cause yield of the Engineered Element. Since the bending strength of the Engineered Element is specified, the given solution is valid – LimitState:GEO has identified that the element will not be part of the critical (Ultimate Limit State) ULS mechanism in this scenario.
If it is required to identify the lowest strength of Element that will just be critical, it is necessary to find the strength that results in a mechanism that includes the Element. In general if you are designing against a specific failure mode then if the limiting conditions are required, failure in that mode needs to be identified by the software. Note that for a cantilever wall the LimitState:GEO analysis (with one plastic hinge in the wall identified) should give an answer similar to that from a limit equilibrium analysis assuming active earth pressures. For a propped/anchored wall, LimitState:GEO will also model the arching effects with wall bending so it may predict lower critical bending strengths than a conventional limit equilibrium analysis that does not include arching. Note also that in general an ULS analysis of a retaining wall will predict a smaller critical bending moment than a Serviceability Limit State (SLS) analysis, since it is mobilising the full strength of the system.