AASHTO requirement for wheel setback from wall

[countyreviewer]

Sorry to ask the question but I am not a structual. However the county is required to review plans submitted by engineers. It is required that for this design it should be a registered structual however in Washington laws allow a civil to perform if they feel competent. My question is that there is a creek crossing, 14 foot culvert, that a retaining wall is proposed for a new road crossing, and the wall is proposed to set 1 foot above existing steel culvert (thought had to be minimum 2 feet). The engineer states he can use a uniform load for meeting AASHTO for H20-40 by allowing the wheel up against the wall (road is narrow) and assumed uniform load of 2 feet equivalent height or 250 psi. I look at the AASHTO and it looks like for 2 feet eq. height the wheel is assume 5 foot offset? Can you help? I am worried the wall is going to slide toward creek.

 

[ztengguy]

If the surcharge on the wall is 2', then 250 is a good pressure to use. If the truck is about 1/2 the wall height away from the wall, then yo can ignore the surcharge.

If the wall has impact, then it should be at 1.75' above the grade to the impact location. Either a parapet of guide rail will be needed if impact is required.

Just check what pressure he used for his surcharge. If he used 250psi, that seems high. It should be 250psf I think.

 

[countyreviewer]

What does table 3.11.6.4-2 mean when a wall is 5 feet, it says 5 and then 2 in the columns? Sorry you are right it is 250 psf. Thanks for quick response. Would you have concerns that it is sitting just a foot above steel culvert?

 

[countyreviewer]

The table out of AASHTO is what I am asking what does it mean, does it mean 2 foot eq. if 5 feet set back?

 

[ztengguy]

I think those tables are set up for calculating Heq in the formula. So if you have a 5' wall, use 5' soil equivialnt pressure for a 20K load. As the wall gets taller, less pressure will be on the total wall. If your 0' away, use column 1, if your 1 foot or further, use column 2.

Why is he using a HS-20 load with LRFD surcharge design? It should be HL93 truck with LRFD, or 2' surcharge with the HS-20 vehicle.

 

[countyreviewer]

Thanks for explaining. I am not sure of the differences of the H20 versus HS20 and the HL 93. The road crosses a creek that is a 90 foot span, and about 14 feet down from top of pavement. Since it is a private road servicing a public storage facilities (rents storage space). He claims he doesn't have to meet any standards. We have been arguing for him to please get a structual so at least the county will feel comfortable with a professionals recommendation. Just looking at the wall spanning the creek I was concerned that he was using the weight of the truck stating that is what keeps his wall from sliding, but thought this could only be assumed if it was always there. The truck load is used to size the rebar, not to verify the wall won't slide? Right? He is using 2 feet of additional soil weight to determine the pressure exerted on the wall to show his calculations for his rebar. Our inspector stopped the work due to the size of the bars he was using and was concerned it didn't meet a standard. We asked him to provide calculations from a structural that our small county just verifies it's a professional engineer and rubber stamps it. The problem started when his first set of calculations showed steel yielding strength of 40k and our spec's say 60k. Next set of calc's showed old IBC standards, something anyone in building dept could catch. The wall calc's just don't seem to jive, so I was nervous about approving and we don't have good standards to require him to get a structual

 

[RFreund]

Do you have a Table number or section for the AASHTO reference? I don't use AASHTO extensively, however...
250 psf is usually used for highway traffic, given that it is a reinforced concrete wall it should be able to distribute the wheel point load fairly well.
The live load is not generally used to help resit sliding and overturning. I thought AASHTO stated this somewhere.
However, I'm pretty sure it can be used in impact design though.

EIT

http://www.HowToEngineer.com

 

[ztengguy]

Have it signed and sealed by a PE. Usually if the vehicle can easily get within half the height of the wall, you need surcharge per AASHTO. usually any headwall that crosses a narrow bridge we add surcharge. If its wide, and some sort of guard rail between the road and headwall, we sometimes let it go.

If he signs and seals, and the county signs off, and the owner is ok, its on him then to design correct. usually the wall would be at least 2'-0" tall to meet any impact requirements.