Beam Raft 2

[AnaAnt]

Hi to all.
Im looking for some tips on beam-raft design process.
Basicly I have few question and would love to get some assistence from more experienced people of this forum.
1) Should downstead beams and slab be poured at once, or is it ok to first cast beams, detail reinforcement overlaping from beam to slab, and then cast a slab.

2.)When defining a perimetar ground beam on top of wich is a bearing wall. Should the width of the beam be determined as it was a strip footing so that bearing capacity under the perimetar beam is always less then actual ground capacity. I dont know if this is maybe conservative because we are delaing with a raft foundation and not with a strip footing.

I hope you guys can spread some light. I thank you very much.
Ana

 

[hokie66]

1) It is preferable to cast the slab monolitically with the ribs. That way, longitudinal shear at the interface need not be considered, and less slab shrinkage cracking will result.
2) Rarely is bearing capacity an issue in this type slab foundation, but if used for buildings more than one or two storeys, the beams should be sized based on the direct loads supported.

 

[AnaAnt]

Sir hokie66,
thank you very much for your answers.
1.) You mean the shear that would appears along a casting joint between rib and slab? Can you plase explain how would this influence less shrinkage cracking of the slab? Maybe if cast together with ribs, slab would be stiffer wich would prevent shrinkage cracking?

2.) Yes, you are right. Raft has a big contact area wich results in load dispersal and finaly smaller pressure contacts. But this is contradicory to sizing raft ribs to line loads generated from load bearing walls located directly on top of them?!
Maybe these should be looked as localized pressures that should be designed with adequate width so that contact pressure is always under allowable bearing pressure?

I thank you again.

 

[hokie66]

1) If the ribs are cast first, they then restrain the later cast slab shrinkage. Drying shrinkage cracks occur due to restraint which places the slab in tension. This is the fundamental reason why drying shrinkage cracks occur.

2) I agree with designing the width of ribs for localized pressures due to direct loading.

 

[AnaAnt]

Dear hokie66,

1.) Shouldnt this cracking be prevented even with this casting joints? Bending reinforcement would be provided, wich almost always is greater then needed for controling shringing cracks.
Ribs would have longitudinal bending reinforcement + links wich would be placed at the bottom of the ribs all the way thru the rib, to the top face of the slab. Basicly a T or L beam that is encountered in floor slabs only cast in two phases.

2.) Agree

 

[hokie66]

Maybe the shrinkage cracks will be controlled adequately in your design. I was just pointing out that non-monolithic construction does result in tension stresses in the element cast second.

 

[mar2805]

Hi!
What if you have an reversed situation.
You are designing a strip footing.
You defined its width wich results in allowable bearing pressure and settlements under defined limits.

What if you "connect" strip footing with ground floor slab?

Please look at the picture attached.
Will this result in "raft action" if any differential settlement tries to take place?
This results is like a more stiffer solution?

 

[aeoliantexan]

mar2805, I'm a geotech, not structural, and I have practiced in the midwest where the footings and floor slabs are routinely separated by a 1/2" joint. Now I practice in Texas, where the footings or pier-supported grade beams are often connected to the floor slabs. Obviously, either practice works if the footing settlement is small and the floor does not heave significantly. I have heard of conditions where the floor heaved and caused the footing to rotate, tilting the wall outward. The isolation joint could have prevented the wall movement, or at least reduced it. I believe the same could happen if the footing settled significantly, say one-half to one inch. The floor can't follow, so it will crack two or three feet from the footing, and the footing may rotate.

My own sentiment is, if you want a raft, design one. Make it strong enough to act pretty much as a unit, because the behavior is complex. Otherwise, don't let the floor slab affect the footing behavior.

 

[lexim]

Hi AnaAnt,

I agree with Hokie66 that the bearing capacity will not be an issue as the entire slab (including beams) is spreading the loads. I have also designed several buildings with raft foundation and they have performed optimally for over twenty years.

First to deal with your question #1, YES, you cast the ground beams (ext and int) first and then remove your forms and backfill to required compaction. You have to detail the slab as shown in the sketch by mar2805 but I'd rather use a candy-cane rebar at ends face of slab.

Usually underneath of slab would have a mud slab so we have even thickness as required in yu or design. This detail usually take care of any shear stresses that may be within the junction of beam and slab.

Secondly, the behavior of the beam raft as I understand is that the column loads which are transferred to area pressure through the beams and slab matrix. These ground pressures which are not uniform (but can be idealised) is localized around the surrounding slabs and beams, and each panel pressure is computed separately, these panels and beams are designed for the appropriate required reinforcement as you envisaged.

Hope this is helpful.

 

[AnaAnt]

Hi lexim.
Thank you very much for your input.

1.)YOu designed rafts by casting beams first. Then after you compacted the soil for the slab wich was cast on top of the beams whos reinforcement was detailed like shown in the sketch?

2.) Can oyu plase explain whats a candy cain rebar? Can you provide some sketch? I thought of using links in beams that will span into slab. (plase look at the sketch)

3.)What is a mud slab?

Lexim thank you very much

 

[mar2805]

I think lexim is refering to edge bent-bars or so called "U bars" wich then overlapp top and bottom reinforcement in slabs.
These should be used if you dont have enough anchorange lenght of your top and bottom bars. Overlaping lenght should be adjusted to tennsion forces!
If however you do have enough anchorage lenght I think they shouldnt be use couse of the extra costs.
So maybe you might wanna adjust your edge beam width to enable full anchorage lenght and avoid unecesary cost of U reinforcement.
Any other opinions are welcome.