0.6D + 0.7E

[sme75]

Lets say that an analysis of a frame gives you an uplift force at the base of a column for the load combination of 0.6D + 0.7E. Now to size the footing are you required to multiply the weight of the footing and any soil above it by 0.6 to calculate the uplift resistance?

 

[Lion06]

Yes. I don't agree with it, but it is part of the load combination. I can see that for other things that you estimate dead weights for, but you can figure out the weight of a concrete footing pretty accurately. I think a 0.9 factor would be more appropriate.

 

[PMR06]

I agree StructuralEIT, 0.6 seems overly conservative at first, but I think the intent of 0.6 is to satisfy the old 1.5 factor of safety on overturning and uplift. (1/0.6 = 1.67)

 

[cessna98j]

So when using this load case to check sliding, would this be the case as well? Since the load case only uses .6D (*coeff. friction) to resist sliding, then you would no longer use the 1.5 safety factor?

 

[PMR06]

Yes, the 0.6D case already has the safety factor built in.

Here are some other forums discussing this:

thread726-119113

thread176-67397

thread507-183966

 

[archeng59]

When I calculate the uplift on the column for a moment frame or for a shear wall, the 0.6D does not include the weight of the footing. If there is net uplift at the base of the column, I need adequate dead load to resist the uplift calculated using 0.6D on the moment frame. In my opinion, the footing must be sized to provide a dead load equal to 1.5 times the uplift force. Did someone already make that statement and I just missed it?

 

[JKStruct]

archeng, I think you're saying close to the same as everyone else. You're calculating the net uplift on the foundation using the .6D load combination. Then, if you have net uplift, the resistance is provided by the weight of soil and concrete and you're using a 1.5 factor of safety for overturning. Essentially, they above posters are multiplying the foundation and soil weight by .6, whereas your 1.5 factor of safety correlates to a .66 coefficient.

 

[archeng59]

maybe I shouldn't try and read late in the day.

 

[sme75]

So is this a check to simply address overturning stability? Do we need to review the soil bearing pressures for a footing that is subjected to a vertical load and a moment? By reducing the dead load, the eccentricity is increased and will require a larger footing than would be required to simply provide a 1.5 FS against overturning.

 

[UcfSE]

sme, that combination is there to address situations when increasing the dead load reduces the load effect due to other sources.