Traffic (live load) surcharges on retaining walls
Traffic (live load) surcharges on retaining walls
(OP)
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..
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..





RE: Traffic (live load) surcharges on retaining walls
RE: Traffic (live load) surcharges on retaining walls
RE: Traffic (live load) surcharges on retaining walls
RE: Traffic (live load) surcharges on retaining walls
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.
www.ttlassoc.com
RE: Traffic (live load) surcharges on retaining walls
RE: Traffic (live load) surcharges on retaining walls
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?
RE: Traffic (live load) surcharges on retaining walls
RE: Traffic (live load) surcharges on retaining walls
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.
RE: Traffic (live load) surcharges on retaining walls
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.
RE: Traffic (live load) surcharges on retaining walls
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.
www.ttlassoc.com
RE: Traffic (live load) surcharges on retaining walls
RE: Traffic (live load) surcharges on retaining walls
RE: Traffic (live load) surcharges on retaining walls
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.
RE: Traffic (live load) surcharges on retaining walls
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.
RE: Traffic (live load) surcharges on retaining walls
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.
RE: Traffic (live load) surcharges on retaining walls
RE: Traffic (live load) surcharges on retaining walls
RE: Traffic (live load) surcharges on retaining walls
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.
RE: Traffic (live load) surcharges on retaining walls
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.
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.