Question
Why do I get many warnings when I use compression-only springs below the footing?
Answer
As the name suggests, a compression-only Spring is a non-linear spring type that can only take compression. So, whenever we apply a lateral load in a particular load case, like wind and static seismic cases, an uplift is generated, which causes these warnings.
As an example, let's consider Static Load Case 3 in the image below. It basically has a nodal load which is applied in the lateral direction on the structure.
In linear analysis, all the load cases are analyzed individually and then the results are combined as required. Practically, we know all of these (dead load, live load, and lateral loads) loads would be acting simultaneously on the structure. The dead load will prevent the uplift in the structure that may happen due to lateral load application. So, to avoid the warnings, we have to generate the load cases where the combination of these loads can act on the footings simultaneously.
To do this we can use the “using load function” of GEN. It directly converts the load combination selected into new load cases having the same intensity. Kindly refer to the image below for using this feature.
If you wish to retain the original load cases in the model, you can simply ignore the warning for all relevant load cases. Alternatively, you may choose to delete the original load cases. If you choose to delete the original load cases, there will be no warning in the model.
Note: Please make sure that you have provided sufficient restraint in the lateral directions as well.