Concrete Retaining Wall Design

[SteelPE]

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...

 

[Guest262653]

I use the same surcharge loads that you do. 5kPa or 10kPa depending on if it's a private or public property, heavy vehicle access, etc. 5kPa is only 25 to 30 cm or soil and it's the load we use for general surcharge for public pedestrian access areas so I'd rather have one of those wonderful and brilliant engineers criticize my work than have it fall down with a couple of people behind it.

 

[KootK]

I believe that it's been a relatively common, long standing practice in the US to design for at least a backfill equivalent to 2' of soil. That'll be close to the low end of what you're doing. Compaction loads can be substantial so many engineers spec the use of hand operated equipment within a certain distance behind the wall. In urban situations, some consideration for emergency access vehicles may be prudent.

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.

 

[mtu1972]

I typically did what you describe. As most of my career was with industrial facilities I tended to be in the 200 to 250 psf range. Back when I worked on commercial facilities I used 100 psf.

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

 

[JoelTXCive]

We do a lot of cantilevered retaining wall designs, and if we do not have specific instructions; then we use a 250psf surcharge load standard. I am sure there have been special circumstances where we designed for a lower surcharge, but I can't think of one off the top of my head.

If you assume the average soil weights 125pcf, then the 250psf loading is equivalent to a 1ft wide strip 2ft in depth.

 

[SteelPE]

Most of my work is in the design of commercial buildings and it appears as if I am at least doing it correctly. However, what if the application were residential vs commercial, would you still design for the surcharge?

 

[Teguci]

Using ASCE 7:

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.

 

[JoelTXCive]

We always design for a surcharge unless there are specific reasons to exclude it. Generally speaking, if it is possible for a vehicle or surcharge load to be above the wall, then we design for the surcharge. This is true for residential, commercial, and TxDOT (highway projects).

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.

 

[MotorCity]

Construction surcharge loads are usually higher than in service surcharge loads like maintenance vehicles or mowers. We allow for a construction surcharge load of 200 to 300 psf. You obviously cannot design for every scenario of surcharge loading, but I think to not include SOME allowance for it is a disservice to the owner and/or public (unless you have some physical barrier or other way of preventing the potential for surcharge)

 

[KootK]

However, what if the application were residential vs commercial, would you still design for the surcharge?

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.

 

[Nor Cal SE]

I agree with all the input here, and the practice of applying at least some surcharge load, the amount as determined by the various factors discussed.

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.

 

[JoelTXCive]

"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...".

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.

 

[WARose]

I cannot find where ASCE 7-10 discusses the 250psf surcharge loads. Is it on table 4-1? Or a footnote?

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.

 

[Teguci]

"Sidewalks, vehicular driveways, and yards subject to trucking - 250 psf - 8,000 lb"

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).

 

[SteelPE]

Usually where I would have trucks I would apply a 2' surcharge to the area. This argument is for areas where the grade is different with no sidewalks etc, so it would not fall under the criteria above. I have made the argument of construction loads being placed on the back of the wall to my colleague but he still doesn't buy it.

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?

 

[darthsoilsguy2]

the greater of 2' or 250 pcf surcharge criteria is AASHTO. i use this surcharge as design basis for handling construction traffic.

 

[XR250]

I do mainly residential and never add a surcharge load. As KootK mentioned, historically, they seem to work fine.
I also am pretty conservative with my designs.

 

[dhengr]

We seem to spend and inordinate amount of time and effort trying to design to allow someone to use a 60ton vibrating compacting roller and a D10 Cat to backfill a basement wall. This adds time and cost to designing and building the bsmt. walls or retaining walls, which we are then invariable blamed for and damned for. Obviously, all real loads should be considered in our designs, but we are not responsible for any and every stupid eventuality. Others have some responsibility and obligation to pay some attention to what they are doing to their surroundings, we aren’t responsible for covering their stupidity. We are constantly asked to make it cheaper, lighter, quicker to build, etc., etc. and then we are also asked, at the same time, in the same breath can we park a 100ton crane right there?

 

[WARose]

Others have some responsibility and obligation to pay some attention to what they are doing to their surroundings, we aren’t responsible for covering their stupidity. We are constantly asked to make it cheaper, lighter, quicker to build, etc., etc. and then we are also asked, at the same time, in the same breath can we park a 100ton crane right there?

I agree.

 

[oldestguy]

dhengr: Just in time. My new neighbor building next to me had the forms stripped today. His contractor is one of those "know it all"guys. My copying your great post ans sending it to the new owner hopefully will do some good. I've sen a lot of what is warned about.