Stepped foundation of 7 meters 1

[structural87]

Hello
I would like to take your advise regarding a stepped foundation I am facing.
2 different thick raft are being designed each one located at a different level.
The level difference is 7 meters between the top and bottom mat. At columns location a thick pedestal is being considered.
The mat foundation is 1.5 meter thick (quite thick because of large uplift forces and big loads from the superstructure).
The 2 rafts are connected with a wall foundation of 2 meters thick ensuring the reinforcement continuity and supporting columns from the above (please see my sketch)
Some engineers are proposing to deal with this foundation system as a one unit. I have a dount since 7 meters is quite huge despite the big thick wall.
My questions are the following:
1- is there any height limitation which if we exceed we can't treat 2 foundations as one entity?
2- if we will design each foundation separately how will you divide the columns reactions supported on the wall between the top and bottom raft ?
3- if 2 foundations are to be treated as one unit, how will u asess the forces inside the connecting wall ? Is it simply the moment from the above raft added to the moment coming from the surcharge and the soil / eater horizontal pressure ?
Thank you

 

[emmgjld]

Before this can be properly approached, it MUST be determined what the foundation will be founded upon.
Is there any hope the foundation foundings will behave in similar manner under the existing and applied loads or not?

 

[structural87]

The foundation at both levels will be founded on the same soil type. Hence, The geotechnical reduced the sibgrade modulus for the upper part.

 

[oldestguy]

This is a combination of two slabs and a retaining wall. It is a single unit and individual designs is not logical. Backfill to the connecting wall has to be designed to also carry the upper slab loads causing horizontal loads on that wall that may be very high.

 

[KootK]

1- is there any height limitation which if we exceed we can't treat 2 foundations as one entity?

Not that I'm aware of.

If you're going to try to move bending moments across the step, I favor a detail arranged closer to what I've shown in the first sketch below. I realize that's a ton of concrete when your step is 7m though.

Without the joint thickening, I don't feel that it's feasible to attempt to move serious moments around the 90 degree bends. Additionally, you'd need to account for the 7m as, effectively, that much extra slab length for the purpose of slab stiffness. Instead, I'd recommend a model similar to that shown in my markup of your sketch below.

Complex foundation work rarely affords one the luxury of knowing load distribution with much accuracy. Keep it simple and keep it moving forward is usually the order of the day.

 

[structural87]

Thank u oldestguy and kootk for your input.
Kootk I was thinking of using Safe software to design the foundation and while the wall is modeled as a drop area having the wall heigh, I use the vertical offset option and enter the horizontal forces rising from the soil pressure.
As such I will get the correct moment at the base of the foundation wall.
Did u mean in your sketch to model the foundation as a 3d model or do u think a 2d analysis as I am planning to model will do the job ?
As for increasing the width of the wall as shown in your first sketch, I always do it for small steps but for a 7m step and since the wall has a length of around 150m I will not take the chance and will see what the reviewer has to say regarding this issue.

 

[oldestguy]

KootK's idea might be useful if that tons of excess concrete instead would be some spaced webs. Still a lot of work and very costly.

I'd look at a piled foundation for the upper slab so as to take the major horizontal loading off the vertical wall.

Leave this post on and likely some more economical schemes will come up here. This is a complicated problem, not something that two individual foundation designs can handle. Probably not suitable for a computer program either.

 

[structural87]

Anymore ideas or suggestions?