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

 

[hokie66]

swelm,

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

 

[apsix]

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.

 

[swelm]

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

 

[hokie66]

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

 

[msquared48]

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

 

[hokie66]

Mike,

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

 

[msquared48]

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

 

[hokie66]

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

 

[csd72]

hokie66,

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

csd

 

[msquared48]

From my standpoint, I suggest abandoning the Metric System!

Mike McCann
McCann Engineering

 

[hokie66]

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.

 

[BigH]

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

 

[rapt]

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.

 

[hokie66]

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.

 

[swelm]

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

 

[hokie66]

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?

 

[BigH]

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).

 

[hokie66]

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.

 

[BigH]

True - MPa is important except for soil bearing pressures!

 

[hokie66]

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.