Traffic Surcharge Load
Traffic Surcharge Load
(OP)
What a surcharge should be used modeling traffic load next to the cantiliver RW? I use 2.5 ft of soil or 300 psft.
Are there any code recomendations? What about dynamic impact?
Are there any code recomendations? What about dynamic impact?





RE: Traffic Surcharge Load
RE: Traffic Surcharge Load
I would guess that vehicle impact would be considered an extreme event (a la LRFD approach) and would be modeled separately from the standard live load requirements.
The ASD AASHTO HB-17 doesn't, as far as I know, address vehicle impact loading for external analysis of retaining walls, but the local DOT may have requirements. I suggest you contact them or look up their bridge design specifications.
Most railroads and I am certain structures adjacent to or over navigable waters have impact load requirements.
Hope this is helpful.
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
RE: Traffic Surcharge Load
RE: Traffic Surcharge Load
RE: Traffic Surcharge Load
RE: Traffic Surcharge Load
Where does AASHTO call for the 10 kip requirement for walls other than MSE?
RE: Traffic Surcharge Load
So, I have to add 2kips/ft. Is there example on the web how to design RW for a seismic load.
RE: Traffic Surcharge Load
AASHTO, 17th Edition, pages 13 & 14, Figure 2.7.4B shows a traffic design load (P = 10 kips, see "Nomenclature" at end of Figure 2.7.4B on page 14) acting on vehicular traffic railings and Jersey barriers. Section 2.7.1.3.6 on page 11 indicates that "The transverse force on concrete parapet and barrier walls shall be spread over a longitudinal length of 5 feet."
RE: Traffic Surcharge Load
As a resident of PA perhaps I can shed some light on your theory. Our walls are not 50% more important rather PENNDoT is 50% more inefficient than other DoT's (better make that 500%).
RE: Traffic Surcharge Load
RE: Traffic Surcharge Load
OK, but Article 2.7.4 is directed toward traffic railings and barriers, parapets and such. Section 5 deals specifically with retaining walls. While 5.8.12.2 calls for the lateral force in the design of MSE walls, I cannot find any provision for lateral forces for other types of retaining walls.
RE: Traffic Surcharge Load
The original m8589 post addressed a retaining wall next to traffic. m8589 asked about traffic surcharges that should be applied to the wall, including traffic impact. I stated that I have designed walls which supported traffic and which needed traffic barriers mounted on top of the wall. I then told m8589 about the 10 kip traffic load that would be applied to the barrier and therefore to the wall.
Whether the wall is a cantilevered wall or an MSE wall, the mounted barrier would have the same design requirements, wouldn't it? AASHTO Section 5 addresses retaining walls in general while Section 5.8 addresses MSE walls only. Section 5.8 is the only place in Section 5 that addresses the 10 kips load. Does that mean that only MSE walls have the 10 kip requirement? No. As you stated above, you also use the 10 kip load for concrete walls with parapets. What AASHTO spec told you to do that? Your initial response only repeated what I had already stated. We seem to be beating a dead horse.
RE: Traffic Surcharge Load
Where live load surchagre is applied within 300mm of the wall back face (for what reason God only knows) then the surcharge depends on the wall height (to account for stiffness). If the wall height is less than 3000mm then the equivalent surhcarge is 1050mm and if the wall heihgt is less than 1500mm then the equivalent surcharge is 1500mm.
The above is compeltely non-sensical and I have no idea why AASHTO doesn't just specify an equivalent HL-93 blanket surcharge for retaining wall design and let the designer work out the pressure coefficinets from the wall stiffness and deflection analysis.
Anyway - hope this helps.
RE: Traffic Surcharge Load
You are correct. It is crazy. I don't know how they come up with this stuff. They keep trying to fix things that are not broken. I have never had to use the higher surcharges. Never had a DOT even ask about higher surcharges. Usually the 2' (3' for PADOT) surcharge is acceptable.
RE: Traffic Surcharge Load
If a retaining structure is expected to support a traffic load, a designer can neglect the traffic surcharge in the wall design calculations, but instead increase the soil overburden by 2-3 feet?
2 feet of soil overburden (assuming 120 pcf) would give a 240 psf surcharge DEAD LOAD.
Shouldn't the conservative designer include a traffic surcharge as a LIVE LOAD?
A live load does not help in the overall "weight" of the retained soil, which would help with sliding, and overturning.
In fact, a live load would only act against the wall. (and the reinforcement in MSE walls.)
___
Craig T. Bailey, PE
www.bailey-associates.com
RE: Traffic Surcharge Load
The 2 (or 3) foot soil surcharge permits modelling of the live load effects in software or calculation methods that do not differentiate between soil loads and external loads.
Naturally, care is required to determine when to use the surcharge in the analysis at hand.
Please see the FHWA reference for MSE walls that I posted in the other thread for recommendations on how to apply live load surcharges for MSE structures, although similar provisions can be adopted for many types of retaining structures.
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
RE: Traffic Surcharge Load
Does AASHTO or anybody else provide a "commentary" indicating where some of these formulae come from (if not obvious) such as a citation of the papers or studies where they were developed?
Just curious.
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com
RE: Traffic Surcharge Load
AASHTO Article 3.20.3 says, "When highway traffic can come within a horizontal distance from the top of the structure equal to one-half its height, the pressure shall have added to it a live load surcharge pressure equal to not less than 2 feet of earth."
I am not aware of any other references or commentary.
RE: Traffic Surcharge Load
Thanks for the expansion on the AASHTO stuff.
I was musing more than anything else. I was thinking that AASHTO might include footnotes to a citation of the source material so that 1) a check could be made to the source material to confirm the correctness of the transcription of formulae, figures, etc. and 2) information on the derivation (and possible limitations) of the formulae, figures, etc. could also be more easily checked by the practitioner. I know that this information is already included in some, or possibly many, individual Articles - I just wish that it were more complete.
However, I am getting off-topic for this thread.
Jeff
Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com