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Raft foundation design
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bkkk
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Joined: 25 Mar 2009
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Location: Tamil Nadu

PostPosted: Mon Dec 21, 2009 8:02 am    Post subject: Raft foundation design Reply with quote

Sir

How to consider base moments of column in the analysis for Raft foundation design.Please clarify.

Balaji Venkateswaran
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abhy_vivek
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PostPosted: Mon Dec 21, 2009 10:27 am    Post subject: Re: Raft foundation design Reply with quote

Dear Shri Venkateswaran ji,

   The columns mentioned in your 'RAFT-Problem' are resting on Slab Directly or There is a Grid work of Beams (Inv T-Beams) to house the lower end of coulmn. If yes then the end moments can be transfered by the conventional Frame action. If the Columns are resting directly on the Raft, then to withstand vertical Loads and moments and shears - is the slab provided with Local thickenning ??

The Arrangment which you shall select may depend on the following points
1) Architectural Demands
2) Loads on the Columns
3) SBC of soil


Regards
Vivek Abhyankar
Structural Engr
Mumbai
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thirumalaichettiar
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PostPosted: Mon Dec 21, 2009 2:34 pm    Post subject: Reply with quote

Er.Venkateswaran,
In addition to the information furnished by Er.Vivek Abhyankar you should be knowing that there are various types of RAFT which are:
1.Raft slab only carrying the vertical columns.
2.Raft with the column pedestal +column.
3.Raft with the slab + thickened at the slab bottom directly under column location.
4.Raft with inverted beams in both directions.
6.Raft with the above with a top slab.
7.raft designed with pile supports.

So in any one of them you have to consider the column footing (bottom) moment due to DL+LL  in addition to the DL+LL or the critical one you get from analysis.
What ever the forces there are two means to design- one is as a rigid and the another beams on elastic supports.

If you are interested to learn more the following reference books may be useful.


1.RAFT FOUNDATIONS DESIGN & ANALYSIS WITH A PRACTICAL APPROACH by SHARAT CHANDRA GUPTA
List Price:      Rs. 500.00

2.DESIGN APPLICATION OF RAFT FOUNDATION BY J.A.HEMSLEY.

T.Rangarajan.
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thirumalaichettiar
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PostPosted: Mon Dec 21, 2009 4:36 pm    Post subject: Reply with quote

Er.Venkateswaran,

In addition to the information I posted please find some attachments that is loaded under the Experts-Dedicated Forum-Er.T.Rangarajan on Raft topic.

You will have one pdf file of a chapter on mat from a good reference and some sketches.

This will help you to some extent to understand the Raft foundation design.

Hope the SAFE software from CSI is one of the good softwares for analysis and design for raft  foundation.

For the Modulus of subgrade reactions browse the  Experts-Dedicated Forum-Er.T.Rangarajan where you can get the values. and guidance.

T.Rangarajan.


Last edited by thirumalaichettiar on Thu Dec 24, 2009 5:53 pm; edited 1 time in total
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abhy_vivek
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PostPosted: Wed Dec 23, 2009 4:37 am    Post subject: Raft slab Reply with quote

Dear Shri Venkateswaran ji,

    You may also refer to IS2950-Part (1) for more details about the Design procedure of Raft slab.

Reg
Vivek Abhyankar
Mumbai
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thirumalaichettiar
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PostPosted: Wed Dec 23, 2009 7:15 pm    Post subject: Reply with quote

Er.Vivek Abhyankar ,

A went through the code IS 2950-part 1 after a long spell of time and I find a very good values for MODULUS OF SUBGRADE REACTIONS for various types of soil which is needed for the support input in software. But I do not find the value of Es for soil.

Is there any tentative values given for different type of soil from any reference?


T.Rangarajan.
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Dr. N. Subramanian
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PostPosted: Fri Dec 25, 2009 6:09 pm    Post subject: Reply with quote

Dear Er Rangarajan,

Please find enclosed Appendix C of my book on Design of Steel Structures, OUP, which contains the soil details requested by you as well as other soil particulars which may be useful to structural designers.

Regards,
Subramanian
thirumalaichettiar wrote:
Er.Vivek Abhyankar ,

A went through the code IS 2950-part 1 after a long spell of time and I find a very good values for MODULUS OF SUBGRADE REACTIONS for various types of soil which is needed for the support input in software. But I do not find the value of Es for soil.

Is there any tentative values given for different type of soil from any reference?

T.Rangarajan.



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APPC-Soil Properties.pdf
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thirumalaichettiar
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PostPosted: Sat Dec 26, 2009 12:36 am    Post subject: Reply with quote

Dear Dr.N.S Sir,

Thank you very much for the PDF file.

I need to know why seniors are not responding or sharing their knowledge    on SEISMIC DESIGN PROBLE topic ? Hope that our country India is free from  seismic effect?

I still expecting more from experienced engineers.

T.Rangarajan.
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ajayapc
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PostPosted: Sat Dec 26, 2009 4:33 pm    Post subject: RAFT FOUNDATION DESIGN. Reply with quote

I appreciate the discussion with experts on raft foundation design.  Thanks to seniors for sharing their valuable knowledge.  To extend further, i will be happy if some-one can share the worksheets or staad model comprising the design of such foundations, which will be helpful for total engineering fraternity to comment/modify so that mistakes can be minimized.  Ofcourse, such programs/models and their output should be cross-checkned/validated by the user before arriving at final conclusion.

Thanks in advance.
ajay
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bkkk
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PostPosted: Tue Dec 29, 2009 7:41 am    Post subject: Reply with quote

thirumalaichettiar wrote:
Er.Vivek Abhyankar ,

A went through the code IS 2950-part 1 after a long spell of time and I find a very good values for MODULUS OF SUBGRADE REACTIONS for various types of soil which is needed for the support input in software. But I do not find the value of Es for soil.

Is there any tentative values given for different type of soil from any reference?

T.Rangarajan.

Sir,
In IS 2950 - part 1 - values are given for Modulus of Subgarde. Do we neeed to apply correction for size , shape and depth.
Regards
Er. Balaji venkateswaran
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