Factor of Safety for Uplift -Tension Piles

[ahmedhegazi]

Hi All ,

According to ASCE , Service Load Combination for uplift check =0.6D+U , this would produce factor of safety 1.67

When using tension Piles to resist uplift , normaly factor of safety for skin friction =3.0 .

As a result of this , we end up getting global factor of safety =1.67*3.0 = 5.0 which is extremely high.

Would be appreaciated to hear your input in this Gentlemen.


Regards

 

[ron9876]

Yes that is correct. Crazy isn't it. That load case has the effect of taking the old safety factor of 1.5 against overturning/uplift and now requiring an added safety factor for elements that already have safety factors built in. The academics rule the code process. Drives me nuts.

 

[spats]

Would it help you to use strength design? At least strength design lets you use 90% of the dead load!

One thing that kills me about current ASD... I'm supposed to pretend that 40% of my dead load is not even there, when it is. How could this ever represent a true loading condition?

Which brings me to another point... What do engineers out there use for dead load? Actual dead load , or the dead load allowance? Strictly speaking, codes define dead load as "the weight of materials of construction incorporated into the building". Because of the definition, I think most engineers use actual dead load only when checking against uplift. It is my opinion that the 0.6 multiplier was originally intended to be the fraction of dead load allowance that was likely to actually be there. Now it's morphed into pretending somehow that real dead load, like dead weight of a structure, dead weight of soil, dead weight of a footing or pile cap, etc. is not there. Dead weight of soil especially kills me. Is this really a "construction material"? I'm interested to know what others think.

ahmedhegazi - I'm also interested to know where you came up with the safety factor of three against uplift. I thought it was normally two.

 

[SteelPE]

Spats, my old boss used to do uplift calculations the exact way you describe for single story structures. In this case, we would design a roof for 15-20psf dead load depending on what was going on with the roof. When it came time to design the joists against uplift we used to use 5psf for the weight of the joists and deck to resist the uplift and then that was it. The same theory was used for design of the footings against uplift. Our 15-20psf used to include things like lights, sprinklers, ceilings and a misc. that might not necessarily be applied to the whole structure. The process seemed reasonable to me and I still use it to this day.

He also made some arguments against using the 0.6D + U calculation bu that mostly revolved around the self weight of the footing and soil.

 

[shobroco]

I have always (okay, maybe not always, but usually) used an allowance for dead load for design instead of trying to figure out an actual, just as SteelPE says: X psf DL for whatever the area is, and X live load, and do your design. Then when it comes to uplift, figure out the real weight of everything and use the .6 factor, & the question is why?? It would make more sense as Spats says to use the theoretical DL for uplift just as you do for bending or bearing or whatever, since it is typically going to be more than the actual and result in a larger resistance. Why penalize ourselves?

 

[slickdeals]

My understanding for the 0.6 factor is to include a 1.5 factor of safety against uplift and overturning. If the loads were 1.0D or 0.9D with a safety factor of 1.5, I think few people would have as much heartache as they do when they see a 0.6 factor and say, "Wait a minute, why do I have only 60% of my dead load?".

 

[shobroco]

Okay, except that DL is 1.25, not 1.5, so we're still cheated.

 

[slickdeals]

thread507-283074

There is an interesting discussion in this thread, which pertains mainly to metal buildings. Most engineers, when designing for uplift, design the base connection for 0.6D + W. In this case, the "D" only includes the superstructure and not weight of the footings or soil.

ash060's discussion talks about this. What's the point in providing all the weight in the footing preventing uplift, when the connections aren't designed for it.