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Can we use allowable gross bearing capacity for checking pressure under mat foundation of tall building!

 
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HMA7
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PostPosted: Sun Aug 25, 2019 10:28 am    Post subject: Can we use allowable gross bearing capacity for checking pressure under mat foundation of tall building! Reply with quote

Hi
In our region, In soil report we only get the Net allowable bearing capacity and we use that Net allowable bearing capacity which is usually around 200 kN/m2 at depth of 7-9m (soil type is very stiff to hard light brown to grayish brown fat silty CLAY), which is larger enough for building around 12 stories, for building more than 20 stories we most of time we needed piles. However, they just designed 35 stories building without pile, when I asked about how it is possible, they said we used the

Gross allowable bearing capacity = Net allowable bearing capacity + ϒ df (over burden pressure)
Gross allowable bearing capacity = 200 + 17* 10 = 370 kN/m2

Even the pressure under the mat foundation was around 450-500 kN/m2 !! they said we just reduced factor of safety to 2.1 and that the gross bearing capacity become large enough for checking the pressure under the mat foundation!!!

Can we do this kind of method to increase the allowable bearing capacity or they’ve done some thing ridicules?

               

PS : i believe laboratory choose the value of Net allowable bearing capacity between minimum of (SPT test and Terzagi equations using CU,...)
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es_jayakumar
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PostPosted: Sun Aug 25, 2019 3:49 pm    Post subject: Reply with quote

Does the raft rest at a depth of 10m below the GL ? Does the structure have basement floors up to that depth ?

E S Jayakumar
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HMA7
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PostPosted: Sun Aug 25, 2019 8:11 pm    Post subject: Reply with quote

es_jayakumar wrote:
Does the raft rest at a depth of 10m below the GL ? Does the structure have basement floors up to that depth ?


E S Jayakumar


The building is 35 stories tall building and  has 2 basements with height of 3.4m and mat thickness 2.4m ,so the foundation rest at 9.2m below the natural ground.
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rojanmathewtsy
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PostPosted: Mon Aug 26, 2019 7:22 am    Post subject: Reply with quote

HMA7 wrote:
es_jayakumar wrote:
Does the raft rest at a depth of 10m below the GL ? Does the structure have basement floors up to that depth ?


E S Jayakumar


The building is 35 stories tall building and  has 2 basements with height of 3.4m and mat thickness 2.4m ,so the foundation rest at 9.2m below the natural ground.

Dear SEFIan,
  
As per my knowledge, for general footing design, we will consider the self weight and other loads from super structure. We Neglect the overburden soil above the foundation which in implicitly considered as we are using Net Bearing Capacity.

But in the cases of buildings with basements ,the overburden pressure of soil will not come in picture. So we can consider Gross Bearing capacity for foundation design in buildings with basements. If you go for net bearing capacity, then it is equivalent to consider an extra load of ϒ*depth of basement*Area of foundation.


Rojan Mathew
mathew.rojan@gmail.com
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es_jayakumar
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PostPosted: Mon Aug 26, 2019 9:09 am    Post subject: Reply with quote

Er.HMA7,

I put the problem in another way:
The structure is a 37 storied one, including the 2 basement floors. Using the thumb-rule that the load transmitted per unit area per floor is approximately 12.50 kN/m2, the approximate Gross Load Intensity (gli) below the footing level = 37 x 12.50 = 463 kN/m2. This tallies with your assessment of 500 kN/m2, when the self weight of raft slab is also considered. Now, the Net Load Intensity at footing level = gli-γDf = 500-18x9.20 = 334 kN/m2. Your Net safe bearing capacity of soil  should be more than this value. The net SBC is reported to be usually around 200kN/m2 only. It is highly unprofessional to manipulate the Factor of Safety value considered  in the SBC to make it suit your load intensity.
It is much advisable to use  pile foundation or piled-raft in this case. Piles under raft will help reduce the settlement also, in addition to taking up portion of load. Broader (within limits) apron can also be provided all round the raft, which will increase the bearing area of it, thereby reducing the pressure on soil.

E S Jayakumar
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HMA7
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PostPosted: Mon Aug 26, 2019 9:57 am    Post subject: Reply with quote

es_jayakumar wrote:
Er.HMA7,

I put the problem in another way:
The structure is a 37 storied one, including the 2 basement floors. Using the thumb-rule that the load transmitted per unit area per floor is approximately 12.50 kN/m2, the approximate Gross Load Intensity (gli) below the footing level = 37 x 12.50 = 463 kN/m2. This tallies with your assessment of 500 kN/m2, when the self weight of raft slab is also considered. Now, the Net Load Intensity at footing level = gli-γDf = 500-18x9.20 = 334 kN/m2. Your Net safe bearing capacity of soil  should be more than this value. The net SBC is reported to be usually around 200kN/m2 only. It is highly unprofessional to manipulate the Factor of Safety value considered  in the SBC to make it suit your load intensity.
It is much advisable to use  pile foundation or piled-raft in this case. Piles under raft will help reduce the settlement also, in addition to taking up portion of load. Broader (within limits) apron can also be provided all round the raft, which will increase the bearing area of it, thereby reducing the pressure on soil.


E S Jayakumar


thanks mr. jayakumar
This exactly what my opinion about this situation
As I remember from barja Das.
Q Net ultimate = Q gross ultimate - q (over burden pressure (ϒ df)) so
Q net allowable = (Q gross ultimate - q ) /F.O.S(3)
Q net allowable = Q gross allowable - q /3
so if we have Net allowable bearing capacity in our case and to get the Gross allowable bearing capacity we will have
Q gross allowable = Q net allowable + q /3 so we should add 1/3 of overburden pressure even in case that, if it is ok to compare the applied pressure to the Gross allowable bearing capacity!!

second, it is not recommended to just reduce the F.O.S of bearing capacity just to avoid casting piles !! as we know most of the soil equation are approximate or practical. that is why we have such a great factor of safety, beside if something happened to the foundation the whole structure will be effected.

thay already build the building, i have more concern about primary settlement after consolidation, even the building is stand still right now.
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es_jayakumar
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PostPosted: Tue Aug 27, 2019 3:16 am    Post subject: Reply with quote

Er. HMA7,
May we know your name and where you practice ?
Your 2nd equation above reads like this :
Q net allowable = (Q gross ultimate - q ) /F.O.S(3)
Sometimes, it is a practice to apply the FoS to the Qgu only, sparing the surcharge load, which is not probabilistic.
In other words, Qgross safe = Qnet safe +γDf
Please go through Cl.2.1.2(6) of the attached article.

E S Jayakumar



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rojanmathewtsy
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PostPosted: Tue Aug 27, 2019 7:00 am    Post subject: Re: Can we use allowable gross bearing capacity for checking pressure under mat foundation of tall building! Reply with quote

HMA7 wrote:
Hi
In our region, In soil report we only get the Net allowable bearing capacity and we use that Net allowable bearing capacity which is usually around 200 kN/m2 at depth of 7-9m (soil type is very stiff to hard light brown to grayish brown fat silty CLAY), which is larger enough for building around 12 stories, for building more than 20 stories we most of time we needed piles. However, they just designed 35 stories building without pile, when I asked about how it is possible, they said we used the

Gross allowable bearing capacity = Net allowable bearing capacity + ϒ df (over burden pressure)
Gross allowable bearing capacity = 200 + 17* 10 = 370 kN/m2

Even the pressure under the mat foundation was around 450-500 kN/m2 !! they said we just reduced factor of safety to 2.1 and that the gross bearing capacity become large enough for checking the pressure under the mat foundation!!!

Can we do this kind of method to increase the allowable bearing capacity or they’ve done some thing ridicules?

               

PS : i believe laboratory choose the value of Net allowable bearing capacity between minimum of (SPT test and Terzagi equations using CU,...)


Please find the notes on consideration of net sbc and gross sbc.

Rojan Mathew



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PostPosted: Wed Aug 28, 2019 3:46 pm    Post subject: Reply with quote

IS.6403.1981 defines the following :

6. ALLOWABLE BEARING CAPACITY
6.1 The allowable bearing capacity shall be taken as either of the following, whichever is less:
a) Net ultimate bearing capacity divided by suitable factor of safety, that is, net safe bearing capacity.
b) The net soil pressure that can be imposed on the base without the settlement exceeding the permissible values to be determined for each structure and type of soil, that is, safe bearing pressure.
As per the above definition, the Allowable Bearing Capacity is derived in consideration of the net safe bearing capacity, and not the gross SBC. It is imperative that in soil reports, it should be made clear as to whether net SBC of gross SBC is prescribed. Then only one can decide if one has to calculate the net load intensity or gross load intensity for comparison.

E S Jayakumar
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