Foundation elasticity uplift

[Anonymous]

I have a raft slab that along one entire edge has an approx 3.5m cantilever to avoid loading onto a zone of influence of a tunnel. I am satisfied with my elastic foundation coefficient of 6000kN/m3.

In the soil reactions I am getting tension which I am not happy about. Given that we will not have anything to 'hold down' that edge of the slab I need the analysis to provide no resistance to uplift. 

The soil reactions results display a positive reaction near the start of the cantilver as expected. But at the back end it is displaying a negative value indicating that a force is providing a resistance to the uplift. 

What is confusing me is that the soil reactions results remain identical whether I choose none, Uz- or Uz+ under the uplift tab. 

How can I ensure the raft analysis does not provide a resistance to uplift?

[Rafacascudo]

Uz+ or Uz- will be according to the panel Uz local direction .In your case if thepanel local Z (red)axis is pointing upwards , then you have to use Uz+ to have uplift considered

[Anonymous]

If the analysis is altered so it is not to resist uplift (as I desire) then I would expect the soil reaction results for the region where I was getting negative results to be zero and a higher compression (positive) load elsewhere. Except my results are the same for all 3 types of analysis. 

[Rafacascudo · Accepted solution]

only now I could seethe model.

Your slab has no uplift assigned

Set Uz+ and it will work

[Anonymous]

Thanks. 

I realised while i thought I had ran the analysis using the 3 different uplift options for comparison I was not saving the change so it was just running using 'none'. 

[Rafacascudo]

it happens

You should take extra care and evalute very well your loads for this situation because the slab area that is really effective is narrow and any bad evaluation of the loads could make the structure unstable in real life.

If this happens ,it can be be detected by the program as the analysis would not converge

[Anonymous]

If Uplift is not selected and there are negative pressures (i.e. tension in the slab), are the pressures redistributed and increased correspondingly on Robot to account for the slab's smaller bearing surface , as effectively shown by the Meyerhoff's effective area methods?

[Rafacascudo]

Yes, it should, so global z translation stability can be achieved