stress under raf

huangyhg

stress under raft
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
swelm,
the average has nothing to do with it. for a flexible raft, the actual stress distribution shopuld be used.
why are you using units of kg and cm? location? isn't it now kpa (kn/m2) or english (tsf, psf, ksf)?
bigh,
some countries still use french metric units which use kg/cm2 rather than si units like kpa (kn/m2).
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
okay swelm.  i will give you my thoughts on this.  i see that you give very little over the year 鈥?some 24 threads started (seeking answers) but only 7 replies (providing any thoughts). so please . . .  let鈥檚 put this aside for now.
i see that you have a soils report 鈥?but yet you do not indicate the material on which you are seeking purchase of your foundation raft.  is it sand? 鈥?in this case, the edges of a rigid footing have higher stresses than in the centre.  is it clay? 鈥?in this case the edges are lower and the centre is higher.  but your stresses quoted are very high 鈥?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 鈥?but on rock.  the stresses are far too high for soil.
with the rock 鈥?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 鈥?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鈥檛 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 鈥渄eformation鈥?in localized overstressed zones.  the 鈥渟oils鈥?report probably was speaking of average bearing stress in any event for a rock material.
anyway 鈥?those are my thoughts.  what are yours?
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.
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
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
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?
yes this is my problem now