Raft with stand down beams

[oloksy]

Good day, I am trying to model this type of raft slab and stiffened beam on staad pro.



My question goes thus
[ul]
[li][/li]
[/ul]from my few years of experience on raft foundation, the slab is designed with the net upward pressure. so now I have a beam and slab. does it mean some of my net pressure would be transferred to the beam



when I modelled my raft slab on staad pro with elastic mat Foundation. I got the same results using my manual method. but when I decided to add beams to split the slab I noticed my plate moments reduced. now my superior colleague is saying something is wrong with staad, as all the pressure is supposed to be on the raft slab.

 

[BAretired]

Your sketch is a little hard to read (see below), but if you have perimeter beams on all sides, then the beams you show in cross section will help to distribute the load. Even the slab thickening will help, but not as much as the deeper beams. As a result, the plate moments will be reduced. I don't think there is anything wrong with staad.

BA

 

[WARose]

I can't read your sketch either.

[blue](OP)[/blue]

from my few years of experience on raft foundation, the slab is designed with the net upward pressure. so now I have a beam and slab. does it mean some of my net pressure would be transferred to the beam

Upward pressure from what ?

 

[MSUK90]

Upward pressure from what ?

Most probably from SOIL?

 

[WARose]

Most probably from SOIL?

Yeah but I need him to answer that. Mainly because if we are talking uplift from (for example) swelling clays....trying to hold that down with the set up I am seeing isn't going to work. Swelling clays can generate thousands of pounds (per square foot) of uplift. If (on the other hand) we are talking a few feet of head (of water) during a flooding event (from a elevated water table level).....that's a different ballgame.

So yeah: the what and how much are big questions here.

 

[oloksy]

@WARose let me try to explain.

if we have a load of 1000kN coming from the super-structure and the base area of 10m x 10m
it means the total pressure on the soil is 1000kN/100m[sup]2[/sup] that is 10kN/m[sup]2[/sup] so it means the load coming on the raft is 10kN/m[sup]2[/sup]. that was what I meant by the net upward pressure

 

[WARose]

f we have a load of 1000kN coming from the super-structure and the base area of 10m x 10m
it means the total pressure on the soil is 1000kN/100m2 that is 10kN/m2 so it means the load coming on the raft is 10kN/m2. that was what I meant by the net upward pressure

Ok but that isn't a "net upward pressure". If it was, your foundation would be unstable. But anyway, now that I get the question.....the answer is: yes, some of that will wind up in the (perimeter) grade beam if the connection [slab to beam] can transfer it.

 

[MSUK90]

Yeah but I need him to answer that. Mainly because if we are talking uplift from (for example) swelling clays....trying to hold that down with the set up I am seeing isn't going to work. Swelling clays can generate thousands of pounds (per square foot) of uplift. If (on the other hand) we are talking a few feet of head (of water) during a flooding event (from a elevated water table level).....that's a different ballgame.

So yeah: the what and how much are big questions here.

Spot on!
Also, regarding the uplift from swelling clays, I have never encountered that situation until now in my career. Would Love to design with its consideration once.