stress under raft 2

[swelm]

hello

we have raft foundation it is has stress 25 kg/cm2 for 30% of the whole area the allowable stress under the raft as per soil report is 25kg/cm2 i need to increase the area of the raft
but the structural engineer for the contractor refuse and he ask to take average stress under the raft

he ask to take (max stress+ min stress /2) but i doubt on that

we use ACI and ubc

regards

 

[rapt]

swelm,

The average has nothing to do with it. For a flexible RAFT, the actual stress distribution shopuld be used.

 

[BigH]

Why are you using units of kg and cm? Location? Isn't it now kPa (kN/m2) or English (tsf, psf, ksf)?

 

[rapt]

BigH,

Some countries still use French Metric units which use kg/cm2 rather than SI units like kPa (kN/m2).

 

[swelm]

sorry BIGH why do you leave the main point (my question)

and look for the unit

where is your reply?

this is not order by expect form you valuable information

thank for all cases

 

[BigH]

Okay Swelm. I will give you my thoughts on this. I see that you give very little over the year – some 24 threads started (seeking answers) but only 7 replies (providing any thoughts). So please . . . let’s put this aside for now.

I see that you have a soils report – but yet you do not indicate the material on which you are seeking purchase of your foundation raft. Is it sand? – in this case, the edges of a rigid footing have higher stresses than in the centre. Is it clay? – in this case the edges are lower and the centre is higher. But your stresses quoted are very high – 25 kg/cm2. This is the same as 25 tsf or over 50,000 psf (2.5 MPa). Clearly, you are not founding on soil – but on rock. The stresses are far too high for soil.

With the rock – the concrete and the rock mass will be more or less of the same order of magnitude of modulus of deformation. While the raft may be judged flexible – this can be checked by determining the rigidity of the raft (see Poulos and Davis), the rock will keep the raft from being considered flexible in my view. I don’t see that you have a bearing problem and that the high modulus of deformation of the rock would give very little deformation under your load. I would posit that the concrete slab and founding base would act pretty well in keel. A bit of overstress here or there will be dissipated very rapidly in the rock and would not cause undue “deformation” in localized overstressed zones. The “soils” report probably was speaking of average bearing stress in any event for a rock material.

Anyway – those are my thoughts. What are yours?

 

[hokie66]

Well, now that BigH has converted the quirky units of kg/cm^2 to units most of us understand and use everyday, the question of the OP becomes clearer.

To my way of thinking, you don't have a raft foundation. A raft foundation is typically used on materials that deform, and are normally analysed as a slab on elastic foundation.

What you have is a rigid substrate of 2.5 MPa rock, so your loads want to go straight down.

Suggest a better approach is to thicken your "raft" where the loads occur and reinforce those sections as spread footings.

 

[swelm]

thanks BIGH and RAPT for your response but my question have mistake

becuase the alloawble stress as per soil report is 20kg/cm2 not 25kg/cm2

i appericate your avaluable informtion sir BIGH


regards

 

[BigH]

His raft is likely thick enough without having to thicken more. I posed the question of units because, yes 25 kg/cm2 seemed very high to be asking this question implying soil; we may have found out it was not the real value. 20 kg/cm2 -- not much different than 25 kg/cm2 - it is still rock, any settlements will be elastic. Hokie66 points out that at that bearing, why have a "raft" - use spread footings with a floor slab as ground floor. to Hokie66

 

[MarcbSE]

swelm,

I don't have much experience designing large raft foundations but just to clarify your problem - If I understand you have actual design pressures of 25 kg/cm2 under 30% of your raft. The allowable pressure is only 20 kg/cm2.

Thus the problem is your actual pressure exceeds the allowable by 5 kg/cm2. Is that correct?

 

[swelm]

yes this is my problem now

 

[hokie66]

So you have 2.5 MPa over 30% of the area, which equates to .75MPa or 750kPa over the entire area. So we are dealing with something like a 50 storey building.

You have 2.0 MPa allowable stress on the rock at the level of the bottom of your "raft". You just need to confirm that under each column or wall that you have adequate thickness and reinforcing to carry the load. You will have continuous top and bottom mats, and extra bars (and possibly extra depth) at the load points as required. For instance, if you have a column with 30000 kN load, you will need to design an area of 15 m^2 to take the load.

 

[apsix]

I suspect that the remainder 70% has greater than zero stress, so average stress would be greater than 750 kPa.

swelm, it would help if a link to the foundation/raft plan was posted (see Step 3 Attachment) to enable more specific advice.

 

[rapt]

swelm,

I would be checking the soil capacity to see if there is not a mistake by a factor of 10. You do not normally use a RAPT on rock that good, normally individual footings.

For a raft to work the soil has to compress under the loads to distribute load across the whole raft. With such a high soil bearing capacity, there will be very little compression of the soil so the load will basically transfer straight into the rock from the columns above with no dispersion sideways. Thus it is a very flexible raft. You cannot assume the load distributes evenly across the whole raft in this case.

 

[hokie66]

I suppose I could have misinterpreted the loading pattern swelm described, but 2.5 MPa equates to about 100 metres of concrete. Hard to imagine.

 

[hokie66]

Swelm,

Are you still there?

What type of structure are you supporting, anyway?

Your 25 kg/cm^2 is 250,000 kg/m^2. That is equivalent to 100 metres of solid concrete or 250 metres of water. VERY tall building!

 

[swelm]

the height of building is 50 meter above the ground floor with two basement

so the total hieght is 58 meter this office buidling with two floor for parking

many thanks for all above comments but noone give me lear answer for my question if that is urgent to increase the area of raft or not

thanks for all vaulabe post
,

 

[BigH]

There is always one other option than the forums. Call up the geotechnical engineer of record and ask him point blank. Explain your loadings to him, etc. and he will provide you the answer you need (hopefully).

 

[hokie66]

Swelm,

You are not reading the posts. Your 25 kg/cm^2 is incorrect. Read my previous post. Your 58 metre tall building would impose only 14.5 kg/cm^2 if it were a solid block of concrete.

 

[apsix]

Rapt's first reply post answered your original question, no you can use the average stress if 30% is over-stressed.

Subsequent posts questioned the validity of using a raft and the magnitude of stresses obtained.

Without you providing more, specific information, the general consensus appears to be that you should re-assess the problem before proceeding with your current design.