allowable vs ultimate coefficient of friction
allowable vs ultimate coefficient of friction
(OP)
In my soils report, the geotechnical engineer has given me an ALLOWABLE coefficient of friction of 0.3. When using retaining wall software, I input this value. The software calculates all of the dead load of the wall, D, multiplies this value by the inputted coefficient of friction, and gives you a warning when the ratio of 0.3*D / F (where F is the total horizontal lateral force from the soil) is below 1.5. Is this correct? What would be the difference in calculation if I input the ULTIMATE coefficient of friction?






RE: allowable vs ultimate coefficient of friction
RE: allowable vs ultimate coefficient of friction
There is only an allowable and ultimate dead load (your normal force to be multiplied by the coefficient to get the friction force).
RE: allowable vs ultimate coefficient of friction
If I use the allowable coefficient of friction value (and use the ASD load combo 0.6D + H), I just need to satisfy the inequality 0.3*(0.6D) >= H.
If I use the ultimate coefficient of friction value (and use the LRFD load combo 0.9D + 1.6H), I just need to satisfy the inequality 0.45*(0.9D) >= 1.6H.
Am I seeing this correctly?
RE: allowable vs ultimate coefficient of friction
Sounds to me more like a factor of safety of 1.5 at the .3 level than a load factor. Depends how you look at it I guess.
Mike McCann, PE, SE (WA)
RE: allowable vs ultimate coefficient of friction
RE: allowable vs ultimate coefficient of friction
Are you talking about 0.3*D/H >= 1.5? That's how I learned it in my geotech class.
RE: allowable vs ultimate coefficient of friction
So I would be careful about believing this statement, without doing some research. Certainly, you are working with a Geotech who feels comfortable providing both. Maybe you can pick his/her brain?
mathcadboy - where does it say that there is not one, or that it is not good practice, etc. etc. ?
Smvk3: you might not have them, but don't forget any other load effects which act with the primary effect H. But, in general, I think your on the right page based on the info your geotech has provided,
I am curious now, because I have only ever been provided allowable coefficients. And that is super conservative when used with LRFD loads. :/
"It is imperative Cunth doesn't get his hands on those codes."
RE: allowable vs ultimate coefficient of friction
RE: allowable vs ultimate coefficient of friction
"It is imperative Cunth doesn't get his hands on those codes."
RE: allowable vs ultimate coefficient of friction
RE: allowable vs ultimate coefficient of friction
The 0.3 coefficient of friction factor does include a safety factor of 1.5.
So essentially instead of designing for a factory of safety of 1.5 for sliding, you are designing with a safety factor of 1.5*(1/0.6) = 2.5 for allowable stress design. This is what has confused me. In the past, I always thought that you were just given one coefficient of friction value (with no safety factor) and then make sure the ratio of the friction force to total lateral soil load is more than 1.5.
RE: allowable vs ultimate coefficient of friction
If required safety factor for sliding = 1.5, N = normal force, 0.45 = ult. friction factor, 0.3 = allowable friction factor, SF = 0.45/0.3 = 1.5, then
Service Load Driving Force / (0.3 x N) > 1.0
or Service Load Driving Force / (0.45 x N) > 1.5
If required safety factor is > 1.5, then Service Load Driving Force / (0.45 x N) > Required SF
www.PeirceEngineering.com
RE: allowable vs ultimate coefficient of friction
RE: allowable vs ultimate coefficient of friction
Why would you want the driving force larger than your frictional resisting force? Shouldn't your equations be:
Service Load Driving Force / (0.3 x N) <= 1.0
(0.45 x N) / Service Load Driving Force >= 1.5
Am I seeing this correctly?
RE: allowable vs ultimate coefficient of friction
Mike McCann, PE, SE (WA)