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Raft foundation design
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SUCHA SINGH
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PostPosted: Wed Jan 13, 2010 4:03 am    Post subject: Reply with quote

Dear Dr. T. Rangarajan,
There is no minimum thickness specified in IS 2950 (Part I).The terms thickness of raft slab/depth of raft slab or beam have been used in this code in Clause 4 and Appendix C respectively. The thickness of raft slab / beam has to be designed based on structural analysis and design.

In my earlier post I have only attempted to clarify terms depth of foundation and thickness / depth of raft slab as per BIS Codes.
Thanks n regards
SUCHA SINGH
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bkkk
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PostPosted: Wed Jan 13, 2010 7:31 am    Post subject: correction facotrs for Modulus of subgrade-NPTEL notes Reply with quote

Rangarajan sir,

Here are the lecture notes on Bearing capacity by NPTEL.I think this also hold good for modulus of sub grade reation.


"Presumptive bearing capacity : Building codes of various organizations in different countries gives the allowable bearing capacity that can be used for proportioning footings. These are “Presumptive bearing capacity values based on experience with other structures already built. As presumptive values are based only on visual classification of surface soils, they are not reliable. These values don't consider important factors affecting the bearing capacity such as the shape, width, depth of footing, location of water table, strength and compressibility of the soil. Generally these values are conservative and can be used for preliminary design or even for final design of small unimportant structure. IS1904-1978 recommends that the safe bearing capacity should be calculated on the basis of the soil test data. But, in absence of such data, the values of safe bearing capacity can be taken equal to the presumptive bearing capacity values given in table 4.1, for different types of soils and rocks. It is further recommended that for non-cohesive soils, the values should be reduced by 50% if the water table is above or near base of footing"

Er.C.balaji venkateswaran
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spsvasan
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PostPosted: Thu Jan 14, 2010 3:05 am    Post subject: Minimum depth of raft foundation Reply with quote

Dear Er.Rangarajan

I went through the design example on Raft foundation from IIT Bombay.

The document does state that the minimum thickness of raft is 1m. I still think it is a mis-interpretation of the IS 2950 clause.

There seems to be no satisfactory reason for specifying minimum thickness of raft as 1m. I hope more SEFIans will throw some light on this.

I have come across much thinner raft foundations and they are performing satisfactorily.

Apart from clause 4.3, IS:2950 uses the term "depth" at seven locations, and in all these seven cases "depth" clearly refers to the depth of a soil layer. It uses the term "thickness" at two locations and in these two cases "thickness" refers to the thickness of the raft.

With best wishes
S.P.Srinivasan
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thirumalaichettiar
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PostPosted: Thu Jan 14, 2010 10:28 am    Post subject: Reply with quote

Dear Er.Srinivasn,

Thanks for your prompt reply post . As you informed that many sefians may respond to this.

T.Rangarajan.
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Dr. N. Subramanian
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PostPosted: Thu Jan 14, 2010 6:38 pm    Post subject: Re: Minimum depth of raft foundation Reply with quote

Hi Er SPS,

I do not have IS 2950 with me now. Thanks for analysing it. I also feel 1m is on the higher side for min thickness of raft. I feel that there is no necessity of specifying minimum thickness for the raft.

Regards,
Subramanian
spsvasan wrote:
Dear Er.Rangarajan

I went through the design example on Raft foundation from IIT Bombay.

The document does state that the minimum thickness of raft is 1m. I still think it is a mis-interpretation of the IS 2950 clause.

There seems to be no satisfactory reason for specifying minimum thickness of raft as 1m. I hope more SEFIans will throw some light on this.

I have come across much thinner raft foundations and they are performing satisfactorily.

Apart from clause 4.3, IS:2950 uses the term "depth" at seven locations, and in all these seven cases "depth" clearly refers to the depth of a soil layer. It uses the term "thickness" at two locations and in these two cases "thickness" refers to the thickness of the raft.

With best wishes
S.P.Srinivasan
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ibarua
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PostPosted: Fri Jan 15, 2010 4:32 am    Post subject: Raft foundation design Reply with quote

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thirumalaichettiar
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PostPosted: Fri Jan 15, 2010 12:08 pm    Post subject: Re: Raft foundation design Reply with quote

ibarua wrote:
Posted via Email


Respected IB ji,

Nothing is seen. Can you post it again.

T.Rangarajan.
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PostPosted: Tue Jan 19, 2010 10:25 am    Post subject: Depth of foundation in IS 2950 - reg Reply with quote

Dear sefians

Depth of foundation as mentioned in IS 2950 -part I is a depth from ground level to the bottom of foundation.The figure given in IS 2950 is self explanatory.Hence depth of founadtion is not the depth of Raft itself.

Er.Balaji Venkateswaran



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Deja vu
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PostPosted: Mon Aug 09, 2010 9:32 am    Post subject: Modulas of subgrade reaction Reply with quote

Sir,

Can we use the emperical formulas as given by E. Bowles using bearing capacity of soil to calculate modulas of subgrade reaction? if yes, To what extent the values should be assumed to be appropriate ?

Regards.
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PostPosted: Mon Aug 09, 2010 9:41 am    Post subject: Re: Minimum depth of raft foundation Reply with quote

Sir,

In my previous projects where we designed raft foundations for diesel power plants (the machine foundation was saparate), only the super structure load amounted to thinner raft of 0.3 metre on a relatively loose soil.

Regards.

drnsmani wrote:
Hi Er SPS,

I do not have IS 2950 with me now. Thanks for analysing it. I also feel 1m is on the higher side for min thickness of raft. I feel that there is no necessity of specifying minimum thickness for the raft.

Regards,
Subramanian
spsvasan wrote:
Dear Er.Rangarajan

I went through the design example on Raft foundation from IIT Bombay.

The document does state that the minimum thickness of raft is 1m. I still think it is a mis-interpretation of the IS 2950 clause.

There seems to be no satisfactory reason for specifying minimum thickness of raft as 1m. I hope more SEFIans will throw some light on this.

I have come across much thinner raft foundations and they are performing satisfactorily.

Apart from clause 4.3, IS:2950 uses the term "depth" at seven locations, and in all these seven cases "depth" clearly refers to the depth of a soil layer. It uses the term "thickness" at two locations and in these two cases "thickness" refers to the thickness of the raft.

With best wishes
S.P.Srinivasan
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