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Traffic Surcharge Load
- Thread starter m8589
- Start date
AASHTO calls for 2 feet of earth surcharge but has a minimum traffic surcharge of 250 psf on retaining walls. PADOT calls for a 3 feet minimum earth surcharge on its retaining walls. I guess PA walls are 50% more important than walls in other states.
m8589,
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.
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.
On permanent tiedback and cantilevered sheet pile and soldier beam walls, where there is a concrete median barrier attached to the top of wall, I have had to design the wall for a 10 kip force, spread out over 5 linear feet of wall, acting at about 2'-8" above the road grade. Other than that, I am not aware of any impact requirements.
- Thread starter
- #8
jmiec,
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."
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."
bridgebuster
Active member
PEInc,
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%).
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%).
PEInc-
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.
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.
jmiec,
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.
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.
AASHT0 LRFD 2004 (clause 3.11.6.4.2)provides that for parallel traffic surcharges applied more than 300mm away from the wall (back face) the surcharge is equivalent to 600mm of soil wieght. LRFD 2004 also applies a load factor of up to 1.75.
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.
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.
MSEMan,
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.
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.
Am I understanding you folks correctly?
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
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
ctbailey,
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.
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.
PEInc,
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.
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.
AASHTO, 15th Edition (not the latest, but the one I have handy), Article 5.5.2 says, "When traffic can come within a horizontal distance from the top of the wall equal to one-half the wall height, the lateral earth pressure shall be increased by a minimum surcharge acting on the backslope equivalent to that applied by 2 feet of soil as described in Article 3.20.3. The surcharge will result in the application of an additional uniform pressure on the back of the wall having a resultant magnitude: Ps = (Hs)x gamma' x K acting at the mid-height of the wall where K is equal to Ka or Ko depending on wall restraint. If the surcharge is greater than that applied by 2 feet of soil, the design earth pressure shall be increased by the actual amount of the surcharge. Unless actual data regarding the magnitude of anticipated surcharge loads is available, assume a minimum soil unit weight of 0.125 kcf in determining the surcharge load. The effects of permanent point or line loads (other than normal traffic live loads) on backslopes shall also be considered in developing the design earth pressures. See Figure 5.5.2C to estimate the effects of permanent point and line surcharge loads."
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.
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.
PEInc,
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.
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.
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