Safety Factor for Spread Footing

[hello12345hello]

I am trying to design the spread footing for uplift. I have ASD Earthquake load A kips (No factor) on Footing and DL B (from Column) kips (No Factor), and Self weight of Footing (C Kips) So now do i have to satisfy the Factor of Safety Requirement of 1.5 [DL = (B+C) / EQ = (A)]. Or IBC allows to use 1.0 for Factor of Safety.
Can i use FS = 1.0 for {[(0.6B + C)]/[0.7A]}.?
And where can i find the information in IBC.

 

[Lion06]

You don't use a factor of safety with nominal loads, you just make sure that it's stable under the load combinations per IBC.

 

[JAE]

Use the IBC load combination 0.6D +/- 1.0E for uplift check.

E includes the horizontal effects as well as the vertical effects The vertical effect is equal to (0.2 x SDS x D)

The D includes the footing and all Dead Loads.

So 0.6D >= 1.0E for uplift.

0.6D - 0.2(SDS)D >= 1.0 x Eh

 

[hello12345hello]

so i have to check for Factor of safety = 1.0 but Is there any reference in IBC for factor of safety for Spread footing uplift check?

 

[Lion06]

The reference is the load combinations. You have to make sure, that under the load combinations, you do not have a net uplift.

 

[csd72]

by using only 60% of the deadload to resist uplift you effectively get a factor of safety over 1.5.

As far as the maximum downward pressure, you are working with an allowable bearing pressure which already has a factor of safety built in.

 

[JoshPlumSE]

2009 IBC:
1604.9 Counteracting Structural Actions: This basically just says that you need to design the structures to resist overturning, sliding and uplift. To me that implies an overturning safety factor of 1.0.

1605.3.2 Alternative basic load combinations: “For load combinations that include the counteracting effects of dead and wind loads, only two-thirds of the minimum dead load likely to be in place during a design wind event shall be used.“ To me, this implies a 1.5 Overturing, sliding and uplift safety factor whenever these LC's re being used. Whenever the other ASD load combinations are being used (the ones with 0.6DL) then you would be free to use the 1.0 stability safety factor implied by 1605.9.

FWIW: The two provisions are equivalent in terms of factor of safety..... 0.6 DL = 0.9DL / 1.5 safety factor.