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[hishamajmalpc]

Hi sefians,
If slab on grade at ground floor is connected to the plinth beams, do the slab act as a footing? ie do we need to design the slab for the soil reaction. if we assume slab also participate in load bearing, the upward pressure on slab will be total column loads/(footing area+slab area) which will give a huge value.
Can anyone please shed some light on the actual structural behaviour here

?

[vikram.jeet]

The soil under grade slab level is generally filled up, though little compacted, but has low bearing capacity compared to strata at foundation level.
Not a practice to take advantage of this in design,  but if required,, a small value of sbc be taken for filled strata and load carrying capacity ascertained,, This will require Rcc design of grade slab for base pressure  depending on sbc of filled soil taken.

[spsvasan]

[quote="hishamajmalpc"]Hi sefians,
If slab on grade at ground floor is connected to the plinth beams, do the slab act as a footing? ie do we need to design the slab for the soil reaction. if we assume slab also participate in load bearing, the upward pressure on slab will be total column loads/(footing area+slab area) which will give a huge value.
Can anyone please shed some light on the actual structural behaviour here



We occasionally see the flooring undergoing settlement. This can be due to poor quality of plinth filling or due to movement of natural soil below - especially if it is poor quality clay.


Hence it is not desirable to support the columns on the grade slab.


For the same reason I do not connect the grade slab to the plinth beams or the side walls. I provide a clean joint between grade slab and the side wall


S.P.Srinivasan

[JVCSNL]

Ideally, the grade slab shall not be connected to the plinth beams.  

The stiffness of soil at grade slab is much smaller (due to filled soil with compaction) than stiffness of naturally consolidated soil (in case of open foundation or pile foundation).  Moreover, the stiffness of grade slab (usually 100 to 200mm thick) is much smaller to the footing pads.

The actual bearing pressure depends on location of load points, stiffness of soil and stiffness of foundation elements and not total load divided by footing + slab area, as described by you.  To estimate realistic soil pressure distribution, proper FE model shall be developed having all structural elements including soil based on its stiffnesses.

Hence, even if the grade slab is connected with plinth beams (by the way, they are also supported by soil along beam length and will have bearing pressure below them), the actual bearing pressure below grade slab will be very small.  If you review the output from FE model, you will notice that bearing pressure will be mainly near the column footing elements and there may be some pressure seen at the junction of grade slab and plinth beam.  This region will be very small, and majority of grade slab area will be largely unstressed, meaning they don't participate in pressure distribution.

This pressure distribution varies with different loading conditions and since there is different pressure at footing and at grade slab, the settlement values will also be different.  As stiffness of grade slab is very small, even miniscule amount of settlement difference will crack the joint at plinth beam and grade slab, which is not a desirable scenario for a good structural engineer.  This is the reason the grade slab and plinth beams are not connected, and they can move independently.  

While on subject of grade slab, you may also note that where the grade slabs are provided in large areas, proper movement joints (expansion, contraction or control joints) within the grade slab regions are required to take care for thermal and shrinkage effects of the concrete.  If such joints are not provided properly, you will see cracks in grade slab even if they are not loaded.  

Hope this helps.

Best Regards,

Jignesh V Chokshi






Hi sefians,
If slab on grade at ground floor is connected to the plinth beams, do the slab act as a footing? ie do we need to design the slab for the soil reaction. if we assume slab also participate in load bearing, the upward pressure on slab will be total column loads/(footing area+slab area) which will give a huge value.
Can anyone please shed some light on the actual structural behaviour here