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Old Timber Bridge Load Rating

Old Timber Bridge Load Rating

Old Timber Bridge Load Rating

Hey all,

I've been asked by a client to look at an old timber/concrete deck (?) bridge and give it a load rating. Apparently the fire dept. was concerned their trucks would not be able to go over this bridge and serve the 9-10 houses on the dead end it leads to. Full disclosure, all my experience is in building design, but it seems like an interesting problem. After looking at the pictures it seems much older than I originally thought and I haven't been able to make it out there to take a look myself yet. Pictures are below:


My questions before taking the job would be:

1. Is this even worth looking at? Based on the construction would an older wiser bridge engineer just say there is now way it works?
2. How does one strengthen a bridge like this? Is the only option total replacement?
3. Are there other issues beyond just the structural engineering that need to be considered?

Thanks for any help!

RE: Old Timber Bridge Load Rating

IMHO, the bridge is past saving; remove and replace.
The concrete deck (assume unreinforced, unless known otherwise) appears to have been placed on top of the original timber deck (now falling apart). The concrete could settle, crack, or break under load.
Just the dead load of the concrete is probably significant.

It would not be too hard to do the math that will likely support my opinion. Would need the following:
Diameter, span, and spacing of longitudinal "logs".
Three dimensions of the original timber deck.
Three dimensions of the concrete deck.

If the bridge passes the above "math" test, condition of the "logs" needs to be determined.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Old Timber Bridge Load Rating

That's kind of what I imagined. Thanks for the input. To replace - do you think some kind of premanuf. bridge or even a box culvert makes sense? I know nothing about the stacked stone abutments but they appear to be in good condition (I'll try to verify while on site).

RE: Old Timber Bridge Load Rating

I would look into a premanufactured bridge first. The span looks really short, maybe use new piers behind the existing if existing abutments are not satisfactory - but they "look" good in the photos.

For a culvert you will have to get into water flow details... and likely do a lot of demolition with that approach.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Old Timber Bridge Load Rating

Replacement is obviously the most direct solution, if you do have to rate it (which you most likely will, since that's what they hired you to do), at the very least you'll need a copy of AASHTO's Manual for Bridge Evaluation. It includes exclusions for decks based on the spacing of the supporting members. Yours may be close enough that the capacity of the deck can be ignored.

The distribution of the wheel loads across the logs will be based on the thickness of the timber planks and the concrete, but it's out of the AASHTO Standard Specifications for Highway Bridges, Table 3.23.1. If you don't have that spec, you can post the thicknesses of those layers and I'll help you with the Wheel Fraction.

We do load ratings on bridges in that kind of shape and worse, ones that have routinely carried loads heavier than a typical fire truck for years. I wouldn't be too quick to write it off as a loss. It will require a thorough inspection, particularly for transverse cracks in the logs.

Replacing the superstructure and placing it on or behind those existing abutments should be an option.

If the concrete is as thick as it looks (or the approach grading can be raised a few inches), there may be an opportunity to use the existing planks and logs as shoring for a new reinforced concrete slab bridge. If a bond breaker, such as a couple layers of roofing felt is laid on the planks, the logs and planks could be removed after the slab cures, which would increase the flow capacity under the bridge.

RE: Old Timber Bridge Load Rating

Your first bridge rating (technically it's a culvert; span <20'); looks like you'll have to get your feet wet figuratively and literally. It'll probably take you longer to inspect the bridge than it will to perform the load rating. The calculations aren't difficult; one for the planks and one for the logs. Even if the owner wants to replace the bridge, I imagine they would still want the rating because that is one way to justify replacement and it will likely take some time before a construction contract can be awarded. Make sure you bring an ice pick to probe the logs and the deck planks and a piece of rebar or similar to probe the base of the abutments for scour. You might be able to get a ruler between the plank to get the plank thickness; if not drill a small hole. Take several measurements of the logs to obtain an average section.

Ultimately, if the owner wants a new bridge, go to the Contech web site (no I'm not an employee). They have a variety of solutions for short spans.

RE: Old Timber Bridge Load Rating

Thanks for all the help. I'm going to advise the client that we can do an inspection and analysis but replacement may end up being a possibility. I'm wondering if the old timber was used as a form and the concrete deck is reinforced to span. Only way to know would be GPR I presume.

RE: Old Timber Bridge Load Rating

Looks like there was an extra pole on the RHS as you look at the bridge ( pictures 12 & 13 show the gap where the pole went).

Those curious finger like things look like concrete which escaped through the boards when it was first poured - odd looking things.

I would have thought a couple of steel channels or beans in between the poles would be sufficient - the thing only has to take one axle loading at time.

If its the right size you could always break it apart and drop a 20 foot container in after you cut the doors off and then concrete over it....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Old Timber Bridge Load Rating

If the "dirt dobber" nests (bee nests) are removed the bridge will fail. 😜😂

RE: Old Timber Bridge Load Rating

Thanks everyone - I'm wondering if there is a chance the wood framing was shoring to begin with and the concrete is reinforced? If that's a possibility it seems we would need to scan it to verify rebar. Seem reasonable?

RE: Old Timber Bridge Load Rating

Quote (T Bat)

I'm wondering if there is a chance the wood framing was shoring to begin with and the concrete is reinforced?

Quote (T Bat)

I've been asked by a client to look at an old timber/concrete deck (?) bridge and give it a load rating. Apparently the fire dept. was concerned their trucks would not be able to go over this bridge...

Does the client want a load rating to:

A) Justify "doing nothing"?
B) Justify spending what may be a endless stream of money on "investigations (like verifying if the slab contains rebar) and repairs"?
C) Justify solving the problem by removing and replacing?

Find out, any of these options could be the "real" reason.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Old Timber Bridge Load Rating

We had something similar on an old 22' span Township Bridge (1930s era CIP) that the State Bridge Office had down rated to 5T. The Township kept chip sealing over the bridge deck and had literally tons of additional dead load on top. I offered to mill it off to regain some capacity back, but they didn't even want a skid steer with grinder on the deck. This ignoring the fact that the farmer's semis were crossing this near daily during harvest season? The Bridge Office comp'd us a self-supporting slab design that we setup as an in-house day labor job. We striped the old seal coat off with a backhoe bucket from each side of the bank and used the old deck as the bottom form. The old abutments remained and we overlapped them with the new deck by a few feet for bearing. We cast embeds into the deck so that a code railing was also added. Turned out great and I have to thank the Illinois DOT Bridge office for helping us out.

RE: Old Timber Bridge Load Rating

Thanks for the replies - the client just wants to see if they need to reinforce or replace this thing so the fire department will drive over it. This bridge is owned by the homeowner, which my client is one of. They presumably don't want to caught SOL if there is some kind of fire or emergency.

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