Grade Beam to slab connection 1

[Atooz]

Any ideas on how to connect the concrete slab to grade beam (supported on drilled shaft). Have two options as below:

1. Can slab be casted monolithic to the grade beams? OR
2. It would be a good idea to pour them separately with sealant at the interface

What is a good practice.

 

[BAretired]

Grade beams are usually cast first. Supported on drilled concrete shafts (belled or friction piles) grade beams do not settle significantly. The slab, on the other hand, can move as a result of swelling or shrinking soil, so my practice has always been to keep it separate from the grade beam.

Some engineers use asphalt impregnated fiber board (AIFB) between slab and beam. At one time I used it but came to the conclusion that AIFB serves no useful purpose, so I don't use it anymore. The use of a sealant is optional. Personally I don't recommend it.

BA

 

[dik]

BART: gives you a bit of a bond breaker...

Dik

 

[MotorCity]

If your slab on grade is to function as a structural slab (i.e. span across poor soil), then I'd say the grade beams should be cast first, then the slab. Also, if your grade beam extends above your slab to serve as a curb around the slab, the grade beam should be cast first, then the slab. Depending on size and thickness of your slab, you could have significant shrinking and curling forces as it sets. These forces could transfer to your gradebeam and cause cracking issues. Ideally, you don't want the grade beam to form a restraint mechanism in the slab.

 

[BAretired]

gives you a bit of a bond breaker...

Polythene does the same. The slab will shrink away from the grade beam.

BA

 

[dik]

BART... tell me... when I first started, a contractor on a project used plywood pour strips. I remember distinctly telling him that the concrete would 'shrink away' and he's be able to remove the strips... betcha they're still there if the building is still around.

Dik

 

[BAretired]

Yes dik, but that is understandable. There seems to be no problem in omitting AIFB around grade slabs.

BA

 

[Atooz]

Great replies everyone. How about extending dowels going into the slabs, will that be a good idea

 

[dik]

OP: use a 2x? ledge on top of the beam with 'z' bars cast into the beam to be straightened into the slab... done that too.

Dik

 

[hokie66]

This all depends on the purpose of the ground beams. Are they to support the slab or not? If not, what do they support, or do they just tie the piles together?

 

[dik]

BART: I've added your poly detail to my typical details as attached.

https://files.engineering.com/getfi...4953-87e8-271fae77632a&file=Beam_and_Slab.pdf

Dik

 

[BAretired]

I use grade beams to support walls. Usually, they are supported on concrete piles which are spaced a few meters apart. To protect against frost heave, exterior grade beams are underlain by void former.

Grade slabs are poured to the face of grade beam. There are no dowels between beam and slab and grade slabs are never recessed into grade beams. Grade slabs in Alberta have a tendency to rise as a result of swelling clay soil which gradually becomes wetter until the soil reaches an equilibrium moisture content some years after the building is constructed. The slab must be able to move freely relative to the beams which are prevented from moving by the piles.

This practice is typical in Alberta and other prairie provinces but may not be common in areas which have not been exposed to glaciation where the soil has been compressed by huge amounts of ice and snow for eons.

There are exceptions to the above. Sometimes, as a result of soil conditions, a structural slab on grade may be required. In that case, void former is used under the entire slab as well as the beams and grade slabs are tied rigidly to beams.

BA

 

[hokie66]

BA, thanks, and I understand your practice and exceptions. My question was to Atooz, who just said he had grade beam and slab, without further explanation.

Where I am, we have swelling clays in places, but not freezing. In some cases, void formers are used under the a structural slab and ground beams. In other cases, particularly houses, stiffened raft systems are employed in the clay, with the ground beams and slab cast monolithically.

 

[dik]

BART: Only place for dowels to slab is at door openings, else generally run slab over beam and add some bot bars... Nearly always use void form between piles... highly plastic clay here and also frost issues. turned off the voidform layer to not confuse... Manitoba is still 'springing up' from glaciation. Until your poly solution, have used flexcell...

Dik

 

[BAretired]

Yes dik, Flexcell (bitumen impregnated wood fiber) is often used, particularly at expansion joints but interior grade slabs do not usually expand.

BA

 

[BAretired]

One note of caution...failure to tie grade beams and grade slabs together may present a potential seismic problem. Piles, according to code, should be tied in two directions, preferably orthogonal directions. Corner piles are tied in two directions, but in the absence of dowels to slab, other piles are tied in only one direction by the grade beam. Some engineers provide dowels between beam and slab in order to meet code seismic requirements. This is of greater concern in jurisdictions where seismic events are more prevalent than they are here.

BA

 

[Atooz]

If you tie the slab to grade beam using dowel bar then my understanding is that slab weight will transfer to the grade beam. This will make the grade beam size larger as it has to carry DL and LL of the slab for the design. Please correct me if wrong. I visited the construction site recently and they have dowel the bar from grade beam into the slab.

 

[Atooz]

If you tie the slab to grade beam using dowel bar then my understanding is that slab weight will transfer to the grade beam. This will make the grade beam size larger as it has to carry DL and LL of the slab for the design. Please correct me if wrong. I visited a construction site recently and they have dowel the bar from grade beam into the slab. Can I consider only part of the weight of slab DL and LL (say about 25%) transferring to grade beams

 

[BAretired]

If tied together and the soil settles after casting the slab, the grade beam will receive some load from the slab. The amount depends on the strength and stiffness of the slab. If the subsoil heaves, the grade beam will be lifted by the slab. I have seen cases where the engineer tied them together and one grade beam actually deflected upward, cracking the masonry wall it supported.

In my locale, I believe it is better not to tie them together, but in some locations it may be better to tie them. It is an engineering decision whether to tie or not to tie, then to decide how much load is carried by the beam based on good engineering practice and to design the grade beams accordingly.

If the beams are to be tied to the slab, a strip of void form can be used adjacent to the beam under the slab to reduce or eliminate potential upward load on the beam.

BA