Anchoring a Highway Plate
Anchoring a Highway Plate
(OP)
A client I work with sometimes has to do some repairs (on underground piping) on some paved roads at the plant. This means covering the pit with a (A36) plate. They've got a 3/4" plate that they use over and over......and they'll ask me if it's ok for a pit of [such and such dimensions in plan view]. Never any drawings associated with it.....just something I green light over the phone or via e-mail.
The guy (at the plant) I work with today asked me a question I've thought of before but haven't addressed: what about anchoring this thing? A braking vehicle sliding into it could potentially push it off the hole. I (in the past) have reasoned that the weight of the vehicle (with a conservative coefficient of friction) would prevent that from happening and haven't given it much thought. (That of course presupposes the vehicle is on the plate while pushing it. Not 100% sure that would happen either.) But that was when the braking/longitudinal force in the AASHTO manuals was 5% of the live loads. Now you see it called out as up to 25% of the axial load in many DOT codes. (The reason given that the braking systems on trucks are so much "better".) So with that being pretty close to a reasonable assumption of the coefficient of friction between asphalt and steel.....I'm not so cozy with it anymore. (Not sure I should have been in the first place.)
So my question is: what would be a good way (commonly done) to anchor the plate to insure it doesn't slide off a hole? About the only way I've ever seen them set up where it would avoid this is with the edges of the plate embedded in the asphalt. (Where the top of plate is the same as top of asphalt.)
I'm not a AASHTO/bridge design guy so any ideas are welcome.
The guy (at the plant) I work with today asked me a question I've thought of before but haven't addressed: what about anchoring this thing? A braking vehicle sliding into it could potentially push it off the hole. I (in the past) have reasoned that the weight of the vehicle (with a conservative coefficient of friction) would prevent that from happening and haven't given it much thought. (That of course presupposes the vehicle is on the plate while pushing it. Not 100% sure that would happen either.) But that was when the braking/longitudinal force in the AASHTO manuals was 5% of the live loads. Now you see it called out as up to 25% of the axial load in many DOT codes. (The reason given that the braking systems on trucks are so much "better".) So with that being pretty close to a reasonable assumption of the coefficient of friction between asphalt and steel.....I'm not so cozy with it anymore. (Not sure I should have been in the first place.)
So my question is: what would be a good way (commonly done) to anchor the plate to insure it doesn't slide off a hole? About the only way I've ever seen them set up where it would avoid this is with the edges of the plate embedded in the asphalt. (Where the top of plate is the same as top of asphalt.)
I'm not a AASHTO/bridge design guy so any ideas are welcome.






RE: Anchoring a Highway Plate
Is the pit lined with temporary cribbing to prevent the sides from caving inward? If this is an open pit, a potential problem is collapse of the pit walls. What are the plan dimensions of the plate? What is the maximum size of pit that could be covered? What is the depth of pit? What kind of soil could be present on site? What wheel loads are you considering?
I would be hesitant about green lighting such a practice without providing a drawing stipulating anchorage details and limitations on its use.
BA
RE: Anchoring a Highway Plate
I agree....but typically their pits are pretty shallow. And it is a concern I always ask about. (And typically have a disclaimer about when we communicate.)
I am considering AASHTO HS20-44 truck loading. It is understood that is the worst it will see. The rest of your questions are getting away from the point I want to address. I'm not the first person who has dealt with this......and I drive over unanchored plates every day (or at least ones not embedded). So it is a experienced AASHTO guy I am hoping to hear from. (I.e. as to their approach.)
RE: Anchoring a Highway Plate
RE: Anchoring a Highway Plate
RE: Anchoring a Highway Plate
Under 10 (iii) it states "...spiking, pinning,..." On bridge decks I've seen railroad spikes used, on city streets they're generally not pinned down. Contractors will put a lot of blacktop to keep them in place.
Also, I attached a Caltrans spec; similar to NYCDOT but a little more definitive.
RE: Anchoring a Highway Plate
This is conservative since we did not account for the resisting friction force between the plate and the soil (which actually could be quite substantial when you consider the weight of truck on top of the plate).
RE: Anchoring a Highway Plate