AASHTO traffic surcharge on MSE Wall 2

[TehMightyEngineer]

Per AASHTO LRFD Bridge Design Spec. 5th ed. (2010) article 3.11.6.4 the live load surcharge can be ignored if it is applied on the backfill greater than 1/2 the wall height behind the back face of the wall. Makes sense.

Now, for a traditional wall this is easy and obvious. However, for a MSE wall what is the back face actually defined as? I think it should be the back of the reinforced earth area but my boss and P.E. thinks it should be the back face of the block wall in front of the reinforced earth. This makes sense to me but I'm not sure if it's the intent of the code. What is the opinion here?

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[TehMightyEngineer]

Well, that was an interesting discussion to read. Way over my head as a structural with very basic geotechnical experience but still interesting.

The rule of thumb I've always followed is soil design is always +/- 50% anyway so maybe that's where the 2x factor came from?

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[fattdad]

Just for the record, my posts have had nothing to do with geotechnical engineering, just elastic theory applied to geotechnical engineering. Rankine earth pressures have nothing to do with the point I was striving to make. Loading conditions that act on sheeting and shoring are also unrelated as well as "being conservative."


In elastic theory the calculation of horizontal loads at some distance removed from a vertical point, strip or limited-extent areal load assumes that there is elastic media on the opposite side of the point load. If there is not (i.e., it's the other side of the retaining wall, non-free-field elastic theory would say to double the horizontal loads obtained from free-field theory.

When asking the question how will a point load, strip load or limited-extent areal load affect my calculated retaining wall loads (i.e., as calculated by geotechnical engineering earth pressure equations) you are using elastic theory. Sure, you can adapt it for various values of Poisson's ratio, but friction angle is not in the equation.

I'm sorry to belabor my point, but if folks are extrapolating this free-field v. non-free-field distinction to "being conservative," safety factor or other such explanation, it's just not the point I was striving to make.

f-d

¡papá gordo ain’t no madre flaca!

 

[TehMightyEngineer]

Disclaimer: I'm way over my head here.

That said, wouldn't the soil under the wall footing/MSE section be the "other side" of your elastic theory? It strikes me as an elastic medium which resists the overturning load. Or does your elastic theory require something to be directly opposite of your lateral force?

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in April, wish me luck!

 

[fattdad]

but we're just talking about horizontal loads. It's just what's to the left and the right of the retaining wall and one side has air. That's what makes the free-field elastic theory for horizontal loads off by a factor of two.

Or so I think. . .

I don't want to sound like some expert and we are off the OP.

f-d

¡papá gordo ain’t no madre flaca!