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size of footing

 
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Ali_Design
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Joined: 11 Sep 2009
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PostPosted: Mon Apr 08, 2013 1:31 pm    Post subject: size of footing Reply with quote

Why unfactored loads are considered while calculating the size of a footing or deciding the pile dia.. ????
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sagarhuddar
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Joined: 28 Jun 2011
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PostPosted: Mon Apr 08, 2013 1:50 pm    Post subject: Reply with quote

Dear Ali,
We take unfactored loads to calculate footing area, because factor of safety is applied to the SBC. i.e Bearing capacity value is divided by the factor of safety(Between 2 to 5 depending on the soil condition).
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rajancivildesign@gmail.co
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Joined: 26 Jan 2011
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PostPosted: Mon Apr 08, 2013 3:26 pm    Post subject: Footing size Reply with quote

The factor of safety for SBC  varies from 2.5 to 3 for soil. Similarly the ultimate capacity of pile varies from type of pile to be adopted and the formula adopted for arriving safe load capacity of pile. In limit state design the ultimate load is 1.5 times working load for vertical load . As the FOS varies , it is better to arrive the size of footing and the pile based on working load. After that we can design the footing and pile cap as per limit state design for design the reinforcement.
V.M.RAJAN
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Dr. N. Subramanian
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Joined: 21 Feb 2008
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Location: Gaithersburg, MD, U.S.A.

PostPosted: Tue Apr 09, 2013 1:14 pm    Post subject: Re: size of footing Reply with quote

Dear Er Ali,

It is a good question. Unfortunately limit states design has not been adopted for soil design in ACI 318 and IS 456. That is the reason we use working loads for checking SBC and after fixing dimensions of footing use limit states method for design.

But note that Eurocode and bridge codes in USA have adopted limit states design for soils also. See:


  • http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_651.pdf
  • http://www.wsdot.wa.gov/NR/rdonlyres/56610316-29DB-4E6F-B457-F137FDE15077/0/ResistanceFactorsFoundations.pdf
  • http://www.dot.state.oh.us/Divisions/Engineering/Geotechnical/Geotechnical_Documents/2012%20Workshop%20LRFD%20Implementation%20in%20Geotechnical%20Design.pdf

Best wishes,
NS


Ali_Design wrote:
Why unfactored loads are considered while calculating the size of a footing or deciding the pile dia.. ????
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