LL from Vertical Roadway Slopes on Bridges 1

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Does AASHTO provide any commentary or additional live loads for the case of a vertical roadway slope on a bridge?

I'm currently designing a bridge where the longitudinal forces on the substructure are the critical factor. The bridge is on a 6% vertical slope. I contend that AASHTO builds a certain amount of safety into the loads and load factors which allow longitudinal loads incurred from any vertical roadway slope to be neglected, but I can't find any reference to this in AASHTO. I believe this is so because I have designed bridges with 5% and lower slopes and have not given this a consideration.

Any help would be appreciated!

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[Qshake]

I assume that you're referring to the profile grade when you say "vertical roadway slope" The profile grade is the vertical control (vertical curves or tangents etc.) for the roadway and associative structures.

I am not aware of such a supplemental load. We typically design the superstructure assuming it is flat which is conservative for the purpose of vertical loading. As far as the longitudinal loading, from which I can see your question arising, I don't know what exactly is considered in this emipircal 5%. Personally, I have never taken this into consideration - only superelevation as noted in AASHTO.

I can tell you that there is a document (green book maybe?) that limits the slope of the roadway. As you might know, depending on the roadway profile extreme slopes are avoided as it is difficult for large vehicles to transverse them. They simply don't have the horsepower. Not to mention areas that may ice over. I don't remember the exact limit but 8% or so sounds familiar. And there are some other factors that play into this limit as well.

Anyone else?

 

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Thanks for your quick response, QShake!

I believe the green book you refer to is the AASHTO Roadway Design book, although I'm not sure of the precise name. The slop limit is about 7.5 to 8%, as you stated - I'm not sure of the exact limit, either.

And yes, I was referring to a 6% profile grade. Thanks for your input!