Traffic (live load) surcharges on retaining walls 3

[hermosa]

I'm designing a 4'tall, 8" cmu retaining wall to provide support to a private residential driveway adjacent to it. I'm am looking at applying the AASHTO 250psf surcharge since the driveway might be called upon to host the occasional fire truck, concrete truck...

Since this is only an occasional load, is it still considered as a permanent dead load in conjunction with the backfill's equivalent fluid pressure. Any suggestions?

Thanks..

 

[PEinc]

250 psf is the minimum AASHTO surcharge. It should cover most vehicle loads provided they are not immediately up against the back face of the wall. Check AASHTO for different amounts of surcharge for different height walls. The surcharge can be 2 feet or more of earth surcharge, but 250 pdf minimum. I don't have my AASHTO manual right now (moving my office) so I can't look it up for you. I'd say the load is a permanent load on the wall. Also, as another recent thread described, fire trucks and some other vehicles can sometimes have outriggers which could place a high relatively concentrated load behind the wall.

 

[BigH]

Agree with you PEinc to a point, but for the transient loads of a passing firetruck or an outrigger load of a firetruck as you say, these would be concentrated loads and I would think the wall should be designed for the actual point loading case rather than putting 250psf (minimum) as an areal loading.

 

[PEinc]

Isn't that what I said? High concentrated loads.

 

[jdonville]

hermosa,

Traffic surcharge is a live load - as such you will need to account for it for bearing capacity and overall slope stability. However, the load is not included for sliding (translation) or overturning (eccentricity) analyses unless it produces limiting cases.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[fndn]

Sometimes the 250 psf vertical stress is converted to 80 psf equivalent fluid pressure and may be added to the equivalent active soil backfill pressure of 35 to 50 psf. This is the most practical way to for your situatuation. As for larger vehicles, they would be several feet away from the edge of the wall and wouldn't exceed your design equivalent fluid pressures.

 

[hermosa]

Thanks everyone, your input has been very helpful!

Jeff, thanks for that added input. You're saying that I can keep the surcharge load "80psf", as Fndn had mentioned, out of the sliding analysis.

Limiting cases?

 

[PEinc]

If you use the 80 psf equivalent fluid pressure traffic surcharge, you will not be following AASHTO (not that would have to). For a tall wall, 80 psf/vf would be enormous. I would not use that surcharge. You would have to keep the 80 psf out of the sliding analysis because you probably could never show a tall wall wouldn't slide under such a big surcharge!

 

[DRC1]

The 80 psf would be a rectangular, not efp. This assumes the traffic is applied ove a large area and begins adjacent to the wall. I usually include the surcharge in all my analysis and find all though it has some effect on the calculations, it is generally not terribly significant. For a very tall wall I might be more analytical, if it makes sense. The further the surcharge is from the wall, the less load is applied to the wall. Generally if you can keep the edge of road at least 5 feet from the face of the wall, the pressure will be managable. To analyize the pressure fast & dirty: assme a 250 psf strip surcharge at edge of road to edge of road. At top of ground the pressure equals 0. Max pressure will occur at about a depth about equal to your offset, so if the front edge of your road is 5 feet of the wall, compute the pressure at this point (NavFac, Plebuck, Boweles, et.al. show methods to do this. I often use LPRES by Civiltec)
next go about twice that distance (in this case 10 feet deeper or depth = 15 feet) and compute pressure. Yuor pressure should be significantly less than that at 5 feet. Straight line from 5 through 15 to zero pressure at the wall. This is somewhat different than the classical distribution, parabolic at the top and assumtotic tail at the bottom, But for trying to solve a problem, it will generally work fine.
An even faster soution is to simply assume you have 2 more feet of soil to retain.

 

[PEinc]

DRC1,

I agree with you. 80 psf, horizontal, rectangular, traffic pressure is more appropriate. There are many ways to analyze traffic surcharges but, for highway jobs, most engineers who review my designs insist on using P = q x Ka as a rectangular pressure.

 

[jdonville]

hermosa,

If the lateral pressure due to the live load surcharge produces a load requirement that tends to decrease the factor of safety for your geotechnical resistance analysis (sliding, bearing capacity, overturning, or global stability), then you should include it in that particular analysis.

There are cases where for, say, MSE walls supporting live loads, that the surcharge tends to increase the resisting forces. The surcharge is, therefore, excluded from that case in order to produce a more conservative result.

I would imagine that from a structural resistance perspective, a similar philosophy would apply.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[MSEMan]

Er - this may seem like a silly point - but if I were designing someone's driveway - assuming they were paying me for my professional judgement - I can't imagine basing my design on what AASHTO would tell me to do for a state highway structure (unless of course my customer wanted his driveway adopted).

 

[PEinc]

Agree. But the original question asked about an AASHTO surcharge. I just pointed out that 250 psf, although commonly used, may not be the proper surcharge to use for certain vehicles or loading conditions. Then, things just kinda deteriorated from there.

 

[robito]

I have a similar condition, except in my case I a front entrance to a 40 unit condo one way drive 20ft wide abutting the proposed wall. I am predicting that large trucks will not use this drive, just cars, small delivery trucks, and ambulances. The driveway will be marked "no trucks" with a sign at the one-way entrance.

I am a practicing civil site development engineer with about two years structural experience from about ten years ago. The project building engineer (who since has been replaced on the job) reviewed my design and told me to use the 250 psf surcharge. The wall height varies up to eight feet high(including three feet buried) because I assummed excavation of a water main 8 feet away may have to be excavated for repair and/or a break may occur.

Any tips on this surcharge appreciated.

 

[miecz]

robito-

I would include the surcharge, as truck drivers may not "see" your sign. But I would include it as a lateral load, 80 psf or q x Ka. I do not include a 250 psf vertical surcharge load to sliding and stability calculations.

 

[bushel3]

From the very practical side, I don't think bumping the surcharge up to 250 psf will change your design too much. Just do it and enjoy the little bit of extra saftey.

That being said, I have designed several such walls and I usually use 125 psf in residential areas as you have done. I'm willing to have a slightly lower F.S. for rare instances where a random large truck goes into the residential area.

 

[Boonie]

I agree, better safe than sorry, use the 250 psf. Have you considered an MSE structure? It may be more cost effective considering the small size of the wall.

 

[shockstressed]

The input then is for the maximum load to be encountered and worse, after that you can sleep at night instead of seeing walls slipping down hills in a mudwash with odd lorries floating after it. Yes

 

[DirtForBrains]

If the surcharge is out of the range of the failure slope, should it still be included?

I have a 7.5' retaining wall with a parking area 5 feet from the wall. The Ka for the soil is 0.25 which would put the rankine failure surface about 4' from the wall at the top, given that the backfill is not inclined.

 

[jdonville]

DirtforBrains,

Nice handle

I would probably assume a reduced live load close to the wall just to be safe - maybe 125 psf? Don't forget to account for the equipment used to construct the wall in the first place.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.