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Design of Isolated Fooring....

 
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Ramesh Babu G
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PostPosted: Sat Nov 02, 2019 11:31 am    Post subject: Design of Isolated Fooring.... Reply with quote

In design of an Isolated Footing(at a depth 1.5 m below GL) - for SBC of the Soil

General consideration for the Self weight of Footing is 10 % of the Axial Load from the Column. But, when I calculate the actual Self Weight of Footing by considering its dimension and density and include the weight of the Earth above the Footing-the load exceeds 10 % of the column load

Axial Load - 710 kN, 10 % of axial Load - 71 kN
Footing - 3.5 m x 3.5 m x 450 mm (150 mm depth at the edge)
Actual weight of Footing - 110 kN
Weight of Earthfill above the Footing - 365 kN

Actual Load - 475 kN
10 % Load - 71 kN

In this condition, why is 10 % load adopted for Design when the actual load exceeds it.
Kindly help me with this....
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Dr. N. Subramanian
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PostPosted: Sat Nov 02, 2019 7:48 pm    Post subject: Reply with quote

Dear Er Ramesh Babu

Design is an iterative process. The size of footing will not be known before the design. Hence 10% of the column load is taken in the initial design. If after the design, the weight is more you need to redesign. This process is needed in all the structural elements.

Best wishes
Subramanian
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Ramesh Babu G
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PostPosted: Sun Nov 03, 2019 2:15 am    Post subject: Reply with quote

Thanks for the reply Sir...
The weight of footing is calculated...
But as in my case...the load from the Earth fill is almost 60 % of the Axial load from the column...
Is this weight to be considered durimg the SBC check for the footing ?
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Dr. N. Subramanian
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PostPosted: Sun Nov 03, 2019 4:10 am    Post subject: Reply with quote

Dear Er.Ramesh Babu

See section 15.3.5 of my RC design book for more details.

Best wishes
NS
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es_jayakumar
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PostPosted: Sun Nov 03, 2019 4:54 am    Post subject: Reply with quote

Dear Er Ramesh Babu,
Suppose, your footing was to be placed at 3.00m depth below the GL, instead of 1.50m. In that case, what would have been the weight of overburden soil to be considered ? It is 365+365 = 730 kN. This exceeds  your axial load of column. Is it rational ?
IS 6403 : 1981 (CODE OF PRACTICE FOR DETERMINATION OF BEARING CAPACITY OF SHALLOW FOUNDATIONS) at Cl.2.1.5 defines  Allowable Bearing Capacity as “The net intensity of loading which the foundation will carry without undergoing settlement in excess of the permissible value for the structure under consideration but not exceeding net safe bearing capacity”. In this definition, the SBC part considered is the “Net safe bearing capacity”.  Net safe bearing capacity is the safe load in excess of the weight of the overburden soil at the particular depth that the footing can carry, per unit area. That means, if the SBC (or Allowable bearing capacity) of the soil prescribed is based on Net safe bearing capacity, you need not consider the soil overburden load. You have to take into consideration the additional extra load due to foundation concrete only, ie., [volume of sub structure below GL *(density of concrete – density of soil)]. It is always advisable, in my opinion, that the SBC be prescribed in consideration of the Net safe bearing capacity. If so, the effective weight of foundation can be taken even 5% of the column load.
The 10% concept is only a “Rule of thumb”, which is perhaps adopted in the case of a medium to dense soil. SBC in your case seems to be low (you have not mentioned the value), and hence the foundation size and weight increases. Hence, the same created confusion for you. As mentioned by our mentor, Subramanian sir, you have to adopt iteration process for accurately fixing the size of the foundation. Hope this helps a bit.

E S Jayakumar


Last edited by es_jayakumar on Sun Nov 03, 2019 7:55 am; edited 3 times in total
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Ramesh Babu G
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PostPosted: Sun Nov 03, 2019 5:59 am    Post subject: Reply with quote

Thank you for your replies...
They gave me the answers i was looking for...
Thank You
IS 6403:1981 - safe bearing capacity of soil
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es_jayakumar
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PostPosted: Wed Nov 06, 2019 3:37 pm    Post subject: Reply with quote

For further clarification, take the example of a square footing of size 2.00m at a depth of 1.50m  in granular soil with N = 30, density = 18 kN/m3. The ground water is at greater depths. The (gross) Safe Bearing Capacity (SBC)  = 752 kN/m2, The Net Safe Bearing Capacity (NSBC)  = 725 kN/m2 (worked out using a Spreadsheet-for general shear failure). The gross SBC  is on the higher side, because the effect of  surcharge of soil is subsumed in it. In other words, if you adopt gross SBC, then work out the size of the footing based on the gross load, including the weight of backfilled soil and if you use net SBC, then use the net load  ie., the column load plus the net load from the volume of RCC that has replaced soil. Adding 2% of the column load may very well take care of this, I hope. However, in most of the cases of Allowable Bearing Capacity (ABC) in granular soils, the settlement criterion governs. In this case, for 25mm permissible settlement, it is 309 kN/m2 for net load. [Worksheet / Fig 9 of IS :8009 (Part I) :1976 :- This is based on the formula by Terzaghi & Peck (1948). Teng modified this in 1962 by introducing a Depth Factor (1+Df/B). But IS Code has overlooked it, to be on conservative side]
Assume the column load is 1000kN in the above soil profile. Add 2% for the net increase in the load due to the substructure weight. We are considering only the net load and net SBC here. The attached excerpt from 2 different books emphasizes this. Now, the net load is 1020 kN. Assume the SBC (or ABC) is 250kN/m2. The size of the square footing is sqrt(1020/250) = 2.02m. Inputting this value in the worksheet, ABC = 308 kN/m2. With this ABC, the size of the footing is 1.82m. With this width, the ABC =  317 kN/m2. Then, the size required with this ABC = 1.79m. So, finally provide 1.80m x 1.80m footing. It’s ABC = 318 kN/m2. The load carrying capacity of the footing now is 1.80 x1.80 x 318 = 1030 kN. This is the iteration process to work out the exact size of the footing required. Let’s assume the depth of the pad footing adopted is 450mm. The net increase of load at foundation (neglecting the column base) = 1.80 x 1.80 x 0.45 x(25-18 ) = 10 kN. So, the 2% extra load assumed is more than adequate, in this case of fairly dense soil.
Please check the correctness of the attached Excel worksheet to calculate SBC.

E S Jayakumar



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