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
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
The average has nothing to do with it. For a flexible RAFT, the actual stress distribution shopuld be used.
RE: stress under raft
RE: stress under raft
Some countries still use French Metric units which use kg/cm2 rather than SI units like kPa (kN/m2).
RE: stress under raft
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
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
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
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
RE: stress under raft
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
RE: stress under raft
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
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
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
RE: stress under raft
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
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
RE: stress under raft
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
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
Would appreciate feedback on what you decided. Your problem should have gone away on reconsideration of the loading.
RE: stress under raft
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
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
RE: stress under raft
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
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
Mike McCann
McCann Engineering
RE: stress under raft
RE: stress under raft
I agree. There are also other wierdo european measurements such as dL (deci Litres) that should also be banned.
csd
RE: stress under raft
Mike McCann
McCann Engineering
RE: stress under raft
RE: stress under raft
RE: stress under raft
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
RE: stress under raft
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
RE: stress under raft
RE: stress under raft
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
RE: stress under raft
RE: stress under raft
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.