Column Unfactored Load 1

[dr. rifat]

Hello,
I have a query about foundation design.

In building foundation design why we use unfactored column force,why not factored load?

Thanks

RIFAT

 

[dik]

They have been using factored foundation loading for some time... also a serviceability loading. Partial, from my project notes for friction piles:

FOUNDATIONS (FRICTION PILES)

THE FOUNDATION DESIGN IS BASED ON USING DRILLED CAST-IN-PLACE CONC FRICTION PILES

FRICTION PILES HAVE BEEN DESIGNED ASSUMING A SLS SKIN FRICTION VALUE OF 235 PSF AND A ULS SKIN FRICTION VALUE OF 330 PSF

Dik

 

[JAE]

It also depends on what you are designing.

Typically we use the following:

Factored loads in building structures for concrete or steel design.
These factored loads correspond with the steel or concrete codes/standards that we use.

However, in our area, the geotechnical engineers supply us with allowable soil pressures for design of the footings.
So we need to take our factored column reactions and unfactor them to determine the size of the footing based on an allowable soil pressure.
The footing itself, however, being concrete, uses the factored loads for checking footing shear, flexure and bearing.



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[dr. rifat]

So we need to take our factored column reactions and unfactor them to determine the size of the footing based on an allowable soil pressure.

That means when we have to determine footing size as well as cast-in-place pile design we use unfactored column reaction.Then to design its thickness we have to use factored load,is it??

From some of other engineer I heard that,we use foundation design 2-3 F.S.,so that to make foundation economical we have to design foundation in according to column unfactored reaction,is it true??

RIFAT

 

[JAE]

Yes, the allowable soil pressure (for shallow footings). And the allowable end bearing or allowable skin friction for piling or piers is usually based on a safety factor of 2 to 3.

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[thegcarmo]

Short answer:

Because we usually use allowable stress design.

Detailed answer:

Regardless of what it is required to design (shallow or deep foundations), we can find in the literature methods based both on ASD and LRFD. However, most engineers I know prefer to use ASD -- which many call the conventional design method -- rather than LRFD. That is why you see unfactored loads when designing foundations. Thus, in order to consider the uncertainties involved in the analysis, the gross FS is applied on the capacity of the soil only (e. g. bearing capacity, or end bearing and skin friction). The gross FS is commonly taken greater than 2 because it is believed to limit the settlement to about an inch. Nevertheless, it does not waive anyone from estimating the settlements.