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[Siddhartha_Balakrishnan]

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 ??

[stharish]

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

[B.V.Harsoda]

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

[Zia]

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

[es_jayakumar]

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

[vikram.jeet]

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.


因此,我认为,李mit 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.