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stress under raft
2

stress under raft

stress under raft

(OP)
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

RE: stress under raft

swelm,

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

RE: stress under raft

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

RE: stress under raft

BigH,

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

RE: stress under raft

(OP)
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

RE: stress under raft

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?

RE: stress under raft

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.

RE: stress under raft

(OP)
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

RE: stress under raft

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. cook to Hokie66

RE: stress under raft

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?

RE: stress under raft

(OP)
yes this is my problem now

RE: stress under raft

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.

RE: stress under raft

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.

RE: stress under raft

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.

RE: stress under raft

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.

RE: stress under raft

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!  

RE: stress under raft

(OP)
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
,

RE: stress under raft

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).

RE: stress under raft

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.

RE: stress under raft

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.

RE: stress under raft

swelm,

Would appreciate feedback on what you decided.  Your problem should have gone away on reconsideration of the loading.

RE: stress under raft

hokie66

Re your point of concern, swelm could possibly be designing a grillage of beams, with bearing on the underside of the beams only.
Nonetheless, feedback would still be good.

RE: stress under raft

(OP)
many thanks for all answer

we take decision to approved this raft the Geotechnical engineer
inform us we can increase the bearing ability up to 22 kg/cm2

we make reduction for the live load so the stress under raft became 23 kg/cm2

for the answer for MR hokie66
this is 23 kg/cm2 UNDER CORE AREA

RE: stress under raft

Yes, I thought so.  But it impossible to have that much uniform stress for a 58 metre tall building core.

RE: stress under raft

23 kg per sq cm is about 1130 psf for a 190 foot tall building.  Seems like a real low bearing pressure for a building that size.  You are removing two stories - say 20 feet of earth at 115 pcf = 2300 psf.  Balance is off by about 1200 psf.

If my figures are correct, you are liable to get some ground swelling here.  Might want to consider balancing the building weight with the depth of cut for the foundation.

Mike McCann
McCann Engineering

RE: stress under raft

Mike,

Unless I am sadly mistaken, 23 kg/cm^2 is about 47000 psf.  Thus my argument with swelm about his loading.

RE: stress under raft

OK...Units... duh.  I screwed up on the 2.54 cm/in and maqde it 2.54 in/cm.  Big difference.  Thanks.  Comments retreacted.

Mike McCann
McCann Engineering

RE: stress under raft

Centimetres should be outlawed.  There should be no reason for engineers to use other than metres and millimetres.

RE: stress under raft

hokie66,

I agree. There are also other wierdo european measurements such as dL (deci Litres) that should also be banned.

csd

RE: stress under raft

From my standpoint, I suggest abandoning the Metric System!  bigsmile

Mike McCann
McCann Engineering

RE: stress under raft

I thought so as well, but that was before I used it for a time; now wouldn't want to go back to feet, inches, and 1/16 an inch.  So nice to have everything in decimal form.  But as csd72 says, there are some poor usages of the system.

RE: stress under raft

Indonesia likes to use cm, not mm.  Why?  .... ?

RE: stress under raft

BigH,

Most countries now use the SI system in which distances tend to be measured in m or mm and loads in N or KN and stresses in N/mm2 or MPa. All of the other units are still available if you want. I do not like cm because they introdcue decimal points (or commas in Europe) in a lot of dimensions so tend to use mm for most dimensions for engineering.

But some countries still use the old French metric system where cm was used a lot and kg rather then KN for force (like lb compared to lbf) eg Thailand, Indonesia and France and probably many other early adopters of metric.


Hokie66,
I agree entirly. You would never go back to ft, inches and fractions of an inch.

RE: stress under raft

But we have gotten off the topic.  Swelm still hasn't acknowledged that his 23 kg/cm^2 stress under the core is impossible for a 58 metre high building.  I think his answer may support my aversion to centimetres.  

RE: stress under raft

(OP)
I am wondering form all that discussion  we leave the main point (my question) and discuss major point (the unit)

put let me join your discussion

in my country in pervious time we use metric unit
ton for force and meter and ton /square meter like that

but now we use KN for force and MPA for stress

RE: stress under raft

Yes, Swelm, good to get back to the main point, and glad you like SI units.  So you have 2.2 MPa rock as your founding material.  And you say you have 23 kg/cm^2, which is 2.3 MPa or 2300 kPa stress under the mat foundation slab.  2300 kPa represent a column of water 230 metres high, or a column of concrete almost 100 metres high.  But you say your building is 58 metres high.  How can it be?

RE: stress under raft

rapt - I have practiced in Canada (the SI system using kN and kPa when many were still in kg) - and have worked overseas for 12 years in SI system.  I understand SI system - my point, was that given dimensions and measurements are almost wholly in m and mm, why does Indonesia seem to be the only country (that I am familiar with) where cm is still common - and they like to use, as added fodder, MN for bearing pressures on soils.  SI is good - what I hate is when, instead of using kPa (or kN/m2), we see N/mm2, t/m2 (not SI), etc.  Indonesia was NEVER French - Dutch, British for a while, then Dutch and INDEPENDENCE.  I also grew up with the Imperial system (USA) - I liked it - not fractions of inch but if all in decimal form.  It was a bit easier to have a feel if you were out some decimal points (i.e., 2.5 tsf vs 25 tsf).  But, I'll live with SI; now if we could only get Microsoft to use ISO dates (yyyy-mm-dd).

RE: stress under raft

BigH,

I think you and rapt Rapt are on the same page with this.  But one unit which is an integral part of the SI system is MPa (N/mm^2), so don't leave that one out for us structural folk.

RE: stress under raft

True - MPa is important except for soil bearing pressures! wink

RE: stress under raft

We tend to use Mpa whenever the number in kPa exceeds 1000.  So it is for stress in concrete, steel, etc.  Just saves a lot of zeros.  Even for bearing pressure, same applies for me--if weathered rock has allowable bearing of 1200 kPa, I prefer 1.2 MPa.  Sort of distinguishes soil from rock.

RE: stress under raft

(OP)
DEAR ENG'R HOKIE66;

sorry for being so late in respond this is the resualt which i get form safe programm.we check the load more than
10 times.

i think the stress is very high because the core at the edge of footing and we have settelment joint beside the core so we can not incease the raft dimenion

thanks for your interesting.

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