Concrete Retaining Wall Design
Concrete Retaining Wall Design
(OP)
I routinely design cantilever concrete retaining walls. I am often faced with the dilemma of figuring out what, if any, surcharge load I should place behind the wall. I feel that not including any surcharge load is irresponsible because at some point some load will be placed on the high side of the wall. I often include a 100psf or 200psf load depending on what I figure might be placed behind the wall. Recently I have been criticized for this practice by another engineer who says that unless there is to be a large permanent load they use 0psf.
Is this practice of using a surcharge load too conservative? I have always figured that there will be equipment used for back filling the wall or equipment used during routine maintenance of the landscape etc...
Is this practice of using a surcharge load too conservative? I have always figured that there will be equipment used for back filling the wall or equipment used during routine maintenance of the landscape etc...






RE: Concrete Retaining Wall Design
RE: Concrete Retaining Wall Design
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Concrete Retaining Wall Design
Late in my career we had a facility on a sloping grade where one side of the building would be a "retaining wall" for about a year before the floor would be completed and it would be a basement wall (i.e. fixed base; pinned top). The Contractor defined locations where their crane would be positioned for the steel erection and we had to review the crane's surcharge loading and revise the wall reinforcing accordingly.
gjc
RE: Concrete Retaining Wall Design
If you assume the average soil weights 125pcf, then the 250psf loading is equivalent to a 1ft wide strip 2ft in depth.
RE: Concrete Retaining Wall Design
RE: Concrete Retaining Wall Design
Hmm - large permanent load. So load combos for soil retaining systems should have a 0.0 factor next to the L? No.
The minimum code substantiated "surcharge" is 250 psf or, if vehicles can be excluded, 100 psf as a live load per table 4-1 "Minimum Uniform Live Loads...".
Don't forget to add the barrier load as well.
RE: Concrete Retaining Wall Design
My thought is that you are always going to have the opportunity for tractors, lawnmowers, or other maintenance vehicles to be above the wall. During construction, you could have much higher loads.
If you have to design for seismic, then maybe an argument could be made that the surcharge could be removed for that design. We are in Texas though, so we don't have a ton of seismic experience.
RE: Concrete Retaining Wall Design
RE: Concrete Retaining Wall Design
It's pretty common in the areas that I've practiced (AB/WI) for retaining walls to be designed without surcharge load on light frame residential projects. It seems to work historically although I don't love that form of proof.
Some interaction with the local fire departments has calmed me down a bit with regard to the emergency vehicle loads. I've worked on a number of larger developments where there was a defined fire access plan based on the rules that firemen themselves follow in the course of their duties. And the fire department tells me that they take that pretty seriously. Depending on how far one chooses to take things, a fire truck outrigger load will be the death of many an 8" residential basement wall, even without the cantilever.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Concrete Retaining Wall Design
The one note I'll add is that, in my opinion, unless your surcharge is a permanent or quasi-permanent load, then you wouldn't need to include it with other short-term loads on your wall such as seismic soil load (if that load is a consideration in your area). The thought is that because seismic is so rare, you are extremely unlikely to have a seismic event at the exact same time a large truck drives immediately adjacent to your wall.
RE: Concrete Retaining Wall Design
I cannot find where ASCE 7-10 discusses the 250psf surcharge loads. Is it on table 4-1? Or a footnote?
I wasn't aware this was mentioned in ASCE 7 and would like to add it to my notes/citations for the next wall we do.
Thank you.
RE: Concrete Retaining Wall Design
I don't recall it being in ASCE. It is (as far as I know) an old AASHTO standard when vehicular traffic is close to a retaining wall.
RE: Concrete Retaining Wall Design
ASCE 7-10, pg 18 - ALL the way at the bottom
paraphrasing -
subscript a - no live load reduction
subscript p - other uniform loading methods can be considered - see KootKs 2 ft soil surcharge above
subscript q - point load really is a point load - don't be silly and distribute to the length of the wall (if running the numbers over a 4 1/2" square gets it to work, you probably spent too much time on a spreadsheet you'll never use again).
RE: Concrete Retaining Wall Design
This brings up another point. When looking at the CRSI, they have a case with no surcharge load applied to the back side of the wall. If it is good practice to put a minimum surcharge load on the back side of the wall (and it appears that is the consensus) then why even have this as an option?
RE: Concrete Retaining Wall Design
RE: Concrete Retaining Wall Design
I also am pretty conservative with my designs.
RE: Concrete Retaining Wall Design
RE: Concrete Retaining Wall Design
I agree.
RE: Concrete Retaining Wall Design
RE: Concrete Retaining Wall Design