Retaining wall design for ASCE7 load combo's

[ron9876]

Want to check to see how everyone else designs retaining walls for soil pressures per ASCE7 load combinations.

D+H+L is obvious.

0.6D+H is my question. H is caused by live loads behind the wall plus lateral soil pressures. Do you use 0.6 x soil weight plus live load (which isn't in this load case).

Since the safety factor for overturning in ASCE7 is accomplished by meeting the load cases it seems that you should use 0.6 x weight of soil for the resisting portion of the calculation and full soil weight to calculate H.

Seems irrational to me but I think that is what the code says. For the case that I am looking at the old resisting moment/overturning moment = 1.92.

 

[JoshPlumSE]

I do use the the 0.6DL + H load combination for the design of retaining walls. With the understanding that this LC already covers overturning / stability requirements (i.e. the 1.5 safety factor).

The interesting difference is that this can larger soil bearing stresses than we got using the 0.9 DL + HL load combinations of the past. That's the main drawback. I'm okay with that extra bit of conservatism. But, if I'm desperate to get an existing design to work (for soil bearing) then I'll use a variation of the alternate IBC load combinations instead. Essentially just doing the design the old way.... i.e. enforcing a 1.5 overturning safety factor rather than factoring the dead load down so far.

 

[HeavyCivil]

Minimum lateral earth pressure for good granular fill are 30psf/f for the "active" condition and 60 psf/f for the at rest condition.

Cantilever wall is "active" so an easy way to design is to design it just like a water tank (use triangular pressure dist.) but instead of saying your water is 62.4 just say it's 30.

You wont have to worry about two way action if its straight. Then just use weights and summation of moments to ck overturning and sliding.

Like you said- you do have to add surcharge loads to whatever forces you get by using a light-weight fluid approximation.

 

[ron9876]

Thanks for the input. I am familiar with retaining wall design. My question is mostly about the use of the 0.6D+H load case.

 

[JoshPlumSE]

Ron -

How do you arrive at an OTM safety factor of 1.92?

The way I see is is that the old 0.9 factor vs. the new 0.6 factor provides an increase in the OTM safety factor of = 0.9/0.6 = 1.5. Therefore, we just use and OTM factor of 1.0 when using the new equations.

To me the main different is that the design of soil pressures may be more conservative for these types of footings. But, for OTM they should be virtually the same.

 

[ron9876]

What I meant by the 1.92 was that I sized the footing using 0.6D+H with applied soil pressures from full soil and live load resisting with 0.6D. Then I solved for resisting moment divided by overturning moment=1.92.

That was the source of the question. If you use full soil pressure for the overturning but resist it with 60% dead that doesn't make any sense to me but that is my understanding of the ASCE7 load cases. I am looking for opinions.

 

[JoshPlumSE]

I get it, you're saying that you sized the footing for the 0.6D + 1.0H. Soil capacity controlled which required a larger footing. Then you back checked what the OTM saftey factor would be based on a 1.0D + 1.0H load combination. And, it ended up being 1.92!

That's something of a wake up call. I've known for awhile that for highly eccentric footings the 0.6 multiplier would have a conservative effect on my soil bearing stress calculations. But, I didn't expect that much of a different.