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Why do we oly consider service loads while designing footing

 
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Siddhartha_Balakrishnan
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PostPosted: Wed Nov 12, 2014 4:19 pm    Post subject: Why do we oly consider service loads while designing footing Reply with quote

While designing footings we consider the service load combinations whereas while designing a beam or column we take factored load combinations ?? I am also confused about why we must consider service load combinations ( limit state of servicibility ex: 1.2D.L +0.9L.L) when we are applying bigger factors like 1.5D.L + 1.5L.L ( limit state of collapse ) ?? It is our choice between how safe and how economic design we want  ??
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stharish
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PostPosted: Sun Aug 09, 2015 1:24 am    Post subject: Service loads used to determine bearing capacity Reply with quote

Factored loads need to be considered for the design of footings, base plates, punching shears and for providing reinforcement.

Only for bearing stress you consider allowable loads. This is because...

Geotechnical engineer provides you allowable bearing capacity. In USA, a safety factor of 4 is provided for soil bearing stresses i.e. Ultimate bearing stress / allowable bearing stress = 4.


Regards,
Harish


Last edited by stharish on Sun Aug 09, 2015 12:47 pm; edited 1 time in total
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B.V.Harsoda
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PostPosted: Sun Aug 09, 2015 4:17 am    Post subject: Re: Service loads used to determine bearing capacity Reply with quote

Your explanation is correct, I agree with you. In India factor of safety 3 is taken to calculate the  S.B.C. of soil.


stharish wrote:
Factored loads need to be considered for the design of footings, base plates, punching shears, and providing reinforcement.

Only for bearing stress you consider allowable loads. This is because...

Geotechnical engineer provides you allowable bearing capacity. In USA, a safety factor of 4 is provided for soil bearing stresses i.e. Ultimate bearing stress / allowable bearing stress = 4.


Regards,
Harish
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Zia
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PostPosted: Mon Aug 10, 2015 6:45 am    Post subject: Reply with quote

Hi Siddhartha,

You can also use limit stage design for the geotechnical design of footings.  

In the strength limit state design, the geotechnical design strength taken as a geotechnical strength reduction factor times the ultimate geotechnical design strength shall be greater than the ultimate loads (i.e. design action effect).  The geotechnical strength reduction factor depends on a number of factors and can range from 0.4 to 0.8.

Similarly in the serviceability limit state design, a suitable modulus reduction factor can be used in calculating settlements of footings for serviceability loads.

Regards,
Zia
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es_jayakumar
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PostPosted: Sun Oct 10, 2021 2:04 am    Post subject: Reply with quote

In continuation to the comments by Er.Zia, an article of me on the "Geotechnical Design of Foundations by LSD" published in the current CE&CR Magazine (excerpt attached) is expected to throw more light on this topic.

E S Jayakumar



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vikram.jeet
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PostPosted: Sun Oct 10, 2021 2:56 am    Post subject: Reply with quote

The FOS for bearing strata (soil/rock) is higher  since :
Testing is done at  few locations , it is not done at each foundation location.
Heterogeneous nature of soil /rocks
Water table assessment is at time of testing
In small projects SBC is either taken from nearby sites OR even assumed
Pressure bulb beneath footing depends upon nature of  bearing (strata type /rock )as also foundation system. But in design uniform
pressure is taken under foundation ,for working out bearing area.


Therefore , I think , in terms of Limit stage , The load factor for foundation bearing area requirement is 3.0 or  2.5  as per soil. Whereas Load Factor for structure design of structural components is about half since  heterogeneity and other  factors are not imperative in structure work.
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