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Anchoring a Highway Plate
2

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.

RE: Anchoring a Highway Plate

Embedding the plate in asphalt seems like an adequate measure to anchor against sideways movement.

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

(OP)

Quote:

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.

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.)

Quote:

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 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

I have faced this issue once before. While I cannot point to any references, I can tell you what we did that seemed to satisfy everyone (assuming you cannot temporarily anchor the road plate to the pit walls below.) We drove steel rods into the ground through holes in the plate, around the perimeter of the plate. The number and cross section of the rods were sized to resist the braking force. If I recall, the rods were 3' to 4' long and had a cap plate welded to the top of them. The rods were then driven into the ground until the cap plate was in full contact with the road plate.

RE: Anchoring a Highway Plate

(OP)
Thanks MotorCity. Do you remember how you arrived at the lateral capacity of the rods?

RE: Anchoring a Highway Plate

Here's a blurb from NYCDOT Highway Rules Section 2:

.

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

It was a rather crude way of calculating it, but we started with a trial rod depth and diameter. After we deducted the passive soil resistance in front of each rod, the remaining braking force (equally distributed among all rods) was used to size the rod as a cantilever. The deflection at the tip of the "cantilever" was then checked against the overlap of the plate over the hole to verify it would not slide off the hole. The allowable deflection can be quite large, say 12" (assuming the plate overlaps the hole by at least a foot on each side).

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

(OP)
Thanks again MotorCity.

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