Calculating Sliding Resistance for Equipment Foundation Partially Supported by Soil

[mqr7]

Hello. I am seeking opinions here as I am second guessing myself. I have a piece of equipment that is being supported by the top slab of a precast vault. See snapshot for a very rudimentary sketch. My question is regarding calculating the sliding resistance since the top slab is supported partially by the walls of the vault, and partially by the soil along the edges of the slab, and there is a large portion of the slab that is unsupported, i.e. there is essentially a large void under the slab. The top slab rests on the vault walls, there is no mechanical connection of the top slab to the vault. Since sliding resistance is calculated as a function of weight and friction, do you discount the unsupported slab weight from the weight calculation effective in resisting sliding, since that area is not in contact with the soil or the walls of the vault? Or does the contact area not matter since the weight will be distributed to the edge supports anyway, and as long as the slab has the flexural and shear resistance to support itself and the equipment and transfer that load to the supported edges, the full weight of the top slab can be used in the friction calculation. In other words, does contact area matter?

Additionally, since the top slab is supported partially by the walls and partially by the soil, would you use a combined/average coefficient of friction?

Edited to add the snapshot as I forgot to attach it to the original post.

 

[NOLAscience]

We usually neglect any sliding resistance of the soil, as it could be eroded or settled and not in contact.

I admit I only spent approximately 90 seconds thinking about this.

 

[mqr7]

Hi @NOLAscience I'm not sure what you mean. Typically how I have done it is when a foundation is uniformly supported on the soil, sliding is resisted through friction between the foundation bottom that is in contact with the soil, and the soil itself. That is the case here, except there are "discontinuities" due to the presence of the vault, which minimizes the contact area. Unless I am misunderstanding what you wrote, I don't see how the soil can be neglected when resisting sliding ? What would provide the resistance from the foundation sliding?

 

[CarlB]

The contact area is not a factor in sliding resistance, the resistance=weight x coefficient of friction.
As you recognized, all the weight is transferred to the walls, the void has no 'bearing' on the analysis.
There may be a small portion of the slab weight supported on the soil, but as mentioned this is likely minimal or zero due to compaction/settlement to below the bottom of the slab. And the coefficient off friction on soil is probably similar to on the concrete wall, so I suggest you can neglect it. You'd likely be using a conservative coefficient anyway.

 

[HTURKAK]

For sure , the sliding resistance will be calculated as weight *friction and the weight here is sum of weight of equipment and slab . However ,It is hard to say which part will resist to sliding either vault or perimeter soil or both.
If you are looking for safety against sliding , you may assume two alternatives ;
- Assume that , the slab deflected and soil settled around the vault so the slab supported by vault and find the sliding resistance using friction factor btw conc. surfaces,
- Assume that the vault settled and the slab supported by perimeter soil and find the sliding resistance using friction factor btw conc and soil. You nay add some passive resistance at side , remember that , the sliding resistance of soil can not be higher than shear resistance.
Use the smaller and look for a reasonable FS say 1.5
- The real case probably be , the wt will be supported by both soil and vault.

 

[mqr7]

@CarlB and @HTURKAK yes I agree regarding the contact area as not a factor, but rather the weight. I think the discontinuity was throwing me for a loop. Thank you both for your thoughtful replies.

 

[NOLAscience]

- Assume that the vault settled and the slab supported by perimeter soil and find the sliding resistance using friction factor btw conc and soil. You nay add some passive resistance at side , remember that , the sliding resistance of soil can not be higher than shear resistance.

I doubt that the vault will settle more than the soil, as it bears on a lower, possibly stronger strata that was probably pre-loaded by the overlying soil that was removed for the vault. However, I guess if the applied load on the vault is very high that is a possibility. It's worth it to try that case as well as the "soil is settled and not in contact" case.