monolithic slab design 2

[Simlac450]

Hey guys,

Hi have a monolithic slab to design for a costumer, 2 storeys, residential building, 75 kPa soil capacity and 30ftx30ft. My question is how should I tackle this ? Should I use a strip of 1 m for the inside part and design it like a strip footing (a mix of one way slab design and footing design)? And use the same idea for the perimeter beam ? What technic you guys uses

 

[Greenalleycat]

I'm going to be honest, I do not understand what you're trying to design at all. By monolithic slab do you mean that the foundation is a consistent thickness raft (rather than piles or strip footings)?
Can you provide some pictures showing what is going on

 

[CovertShear]

Hi have a monolithic slab to design for a costumer, 2 storeys, residential building, 75 kPa soil capacity and 30ftx30ft

75kPa soil capacity is quite bad. Is that really so?
I would assume there is no underground storey, otherwise it couldn't be 75 kPa.

Should I use a strip of 1 m for the inside part and design it like a strip footing

I would probably use FEM 2D slab on subsoil as a design method, or you could use the strip method.
You should sketch a floor-plan for us, the vertical load members matter in the raft design you know.

 

[Simlac450]

This is what I mean by monolithic slab.

 

[Simlac450]

@CovertShear, if I use the strip method what kind of support should I use ? fixed on the edge (assuming the footing act like a rigid beam?) it’s going to give me an higher Mf then the reality though

 

[mronlinetutor]

No, the foundation footing has to be below the frost line. You better design the building with conventional footing. I came across a job where the engineer uses a foundation slab for a two-story house on collapsable soil where the load of the house has to be spread evenly over the soil. But still, the foundation slab has to be below the frost line. I am not sure about other data in your job.

disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.

 

[Simlac450]

I cannot, the costumer want this kind of foundation, the architect designed something with styrofoam to protect it against the freeze and thaw

 

[Greenalleycat]

So, if I understand your problem correctly, you have a foundation that consists of a min 12" wide strip footing cast integrally with a min 6" reinforced slab-on-grade.
This foundation supports a two-storey residential building above of 30ft x 30ft plan dimension.
You have not given a picture of the building to know how the structural system works - is it a frame building? Timber walls? Are there internal load-bearing lines?

You have soil with a capacity of 75kPa (working stress? ultimate? huge difference, you have not given enough information to fully resolve this problem).
Your question is: how to approach the design of the strip footing to keep bearing demands within your allowable stress of 75kPa?
I think you're asking whether it is appropriate to consider some portion of the monolithically-cast slab as contributing to the bearing area resisting your gravity loads?

In my opinion, I would not consider a flange benefit from the slab for this purpose.
I would design my foundation so that all bearing demands could be satisfied within my designated strip footing.
In reality, we know that there is some flange effect, but it is convention in my area to ignore that benefit and I think that it is appropriate to do so.
This is particularly true when you have what seem to be soft soils - why rely on a very difficult to quantify and prove load path to eek your numbers in?
Much better to bite the bullet on ground improvement, deeper foundations, or a wider strip footing, and keep the flange effect as a 'sleep at night' factor.

Relying only on the strip footing provides clarity of load path and is a much more dependable system to prove
If you try to include the slab then you have to consider relative stiffness of the slab vs beam, and the relative stiffness of the soil vs these elements yadda yadda.
Add to that you might have differing soil properties beneath the slab and beam (as they are at differing depths)...

I know that, when designs get tough due to factors out of our control (e.g. soft soils) the temptation is there to start quantifying the 'extra factors' to get the design to work with what looks reasonable
However, if you take a step back and think about it, this makes no sense.
For a design on firm (low risk) soils we are able to satisfy our checks without using these factors, so there remains the additional 'sleep at night' benefit of the flange effect (etc)
Why then, for a design on soft (high risk) soils would we double down on adding risk by incorporating all these extra factors that wouldn't normally be quantified, removing our factors of safety?

 

[Simlac450]

Ok… not what I asked! Simple question here, monolithic slab on ground uses as foundation like the picture I shared, what technic/method you guys uses to find the required thickness and the As required that’s it lol that’s my question

 

[Greenalleycat]

If that was your question then why not ask that

All good anyway.
I'm assuming that your soils are too soft for standard code foundation solutions?

Trying to estimate steel area/foundation depth is obviously dependent on the demands that you put on your foundation]
The hard part is estimating those demands obviously

You have two load cases
1) Gravity load case caused by varying loads along the length of the beam
Build a foundation beam model, assume a depth of concrete (as this affects stiffness) and put it on soil springs
The deflection of the soil springs will induce moment into the foundation which you can then design for

2) Lateral loads from winds/EQ
You will need to allow for your gravity loads and applied up/down loads from your bracing elements onto the foundation beam

Given that you have a soft site you could also choose to consider a 'loss of support' case whereby you assume that (as an example) 2m of foundation is unsupported
Then you can design for sufficient top/bottom steel to handle the sagging or cantilever moments that result

These designs are a tad iterative as you should try multiple soil stiffnesses and beam depths for sensitivity analysis
Pretty quick to do though - this is maybe an hour's work at most, assuming you have your gravity and lateral load paths clear

 

[CovertShear]

@Simlac450 ( if I use the strip method what kind of support should I use ? fixed on the edge (assuming the footing act like a rigid beam?) it’s going to give me an higher Mf then the reality though

If you are hell bent on using strip method instead of FEM consider edge support pinned for slab's top reinforcement (affects slab thickness).
Consider fixed when designing bottom reinforcement (stirrup maybe) in the "perimeter beam".
Also check shear in the "perimeter beam" when designing its thickness.

The edge support probably wont be near fixed, especially if the vertical load bearing members are wooden posts like in that picture you posted (low rotational stiffness).
But if you are not willing to give us a floor-plan, this is the advice I have.

 

[mronlinetutor]

The code says the foundation has to be below the frost line. Insulating the bottom of the foundation is not equivalent to the foundation below the frost line. You have to build up the basement to ground level.

You have to thicken the perimeter beams but also you have to watch out for an unbalanced load on the foundation pad which might cause a turning moment on the foundation pad. There are two chances, either the load on the foundation is trapezoidal or triangular under the foundation. The foundation pad has to be reinforced in the xy direction with more reinforcement on the perimeter beams. Draw a free body diagram and see if the foundation pad is ok.

disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.

 

[Simlac450]

Well in the NBCC 2015 part 4, clause 4.2.4.4 (Im living in Canada), we can build a foundation above frost line if its designed against frost action.