When modeling a foundation slab with stiffening ribs, a question that pops up is it is necessary to model both the soil layer springs under the slab and under the foundation beams?
The beams are deeper and possibly on a different layer. So how to model this correctly?
Answer
The bottom side of the beams is indeed deeper than the bottom side of the foundation slab. In most cases however, we are only taking about 50cm, which usually does not make an enormous difference in the soil layers to be used. This effect may be neglected.
But, how the interaction between plate and beams is modeled, can make a difference:
- for beams that are centrically aligned (so no eccentric beams in the model) it does not matter whether you support the beams separately with soil layers or not. The beam shares mesh nodes with the plate and will receive the spring support in these common mesh nodes.
- for beams that are eccentrically placed – it will make a difference. For eccentric beams the mesh nodes are duplicated (1 node on plate and 1 node in the beam, connected with rigid link:
- With only springs under plate
- The ‘plate’ mesh node has the soil spring support
- The ‘beam’ mesh node has no support
- With soil springs under plate & beam
- The ‘plate’ mesh node has the soil spring support
- The ‘beam’ mesh node has also soil spring support. As the beam element is physically positioned at a lower level than the plate, the soil layers to be used will be adjusted accordingly.
- With only springs under plate
