Timber deck strong enough?
Timber deck strong enough?
(OP)
I have bought and had erected a used/discarded county bridge onto my private property. It is a steel pony truss 76' long and (now) 14' wide. There should be no issues with the bridge's structural strength but I need to know about what kind of deck to use. Timber planks seem the cheapest way to go but how strong can I get this? HS-20? The tranverse stringers are 18' 10" apart and the longitudal girders are 4' 3" apart.





RE: Timber deck strong enough?
With this kind of framing I think that nail-laminated 2x4 or 2x6 decking could get you to HS-20 capacity. This is simple to build but you will need to hire an engineer to verify capacity and recommend the correct grade of timber.
Nail laminated decking consists of boards set on edge and joined by closely spaced nails. The USDA Forest Service has published "Timber Bridges - Design, Construction, Inspection, and Maintenance," which is a good resource.
Good Luck!
RE: Timber deck strong enough?
RE: Timber deck strong enough?
RE: Timber deck strong enough?
I doubt you can safely accomodate a H20 live load with either 2"x4" or 2"x6" laminated decking with stringers greater than 2'-10" spacing. You will also need a wearing surface of either 2" timber, or asphalt pavement. I think you will find that steel grid deck will be significantly more expensive than an equivalent timber deck.
I'm guessing these trusses are Warren type trusses, probably with vertical web elements??? There are thousands of these and many were designed for 16'0" clear (inside) width between trusses. If all this is true, then I'd be very surprised (read shocked) to find this bridge was designed for an AASHTO H20-44. I'm guessing closer to H15 sight unseen. Do you know if the floor beams were shortened an equal amount at each end before remounting to your trusses?
Given that you will no doubt want to accommodate services trucks, construction trucks and equipment and furniature moving trucks, then you might consider having your bridge evaluated by a structural engineer. You need to be concerned about the deterioration of the steel, particularly the connections. Armed with such a report, you should be able to fix the weak points and develop a maintenance schedule for your new Farmerfield Bridge.
As a final point, assuming you have freedom of information legislation in your state, you should be able to recover documents from your county about the maintenance history of the bridge, as well as all the design documentation. They probably also have standard details for how they modify these bridges to upgrade to H20 live loads. I recommend you commission an engineer to inspect / design your new bridge.
Regards,
RE: Timber deck strong enough?
Nail-laminated decks are capable of spanning up to 6 feet under highway loads, as shown in my previous reference.
PM's suggestion of a wearing surface is a good one, as wheel traffic will cause deck wear and reduce its strength. A lightweight alternative for rural or private bridges which has been used in the past is longituninal running boards, which are placed only on the wheel tracks.
I believe that steel grating will be more costly, unless you can find a salvage supply. Even so, attachment of steel grating requires a good deal of know-how.
Concrete, properly constructed, will be low maintenance, but will also be your heaviest option, which will lower the available truck load cpapcity.
Inquiring about the availability of design/inspection/maintenance history is a solid idea.
I reiterate: Get an engineer to help you with this project. I remember my dad telling me about pulling a fuel oil truck out of the creek at his farm.....
RE: Timber deck strong enough?
RE: Timber deck strong enough?
HAPPY NEW YEAR
RE: Timber deck strong enough?
RE: Timber deck strong enough?
http://www.contech-cpi.com
Contech can furnish you with span tables. I think timber panels will be cheaper in initial cost.
As the other posts have stated the USDA is great resource. They have an entire group to help apply timber to transportation.
http://www.fs.fed.us/na/wit/
Lots of pdf online. There several other great publications beside the timber bridge book. I emailed them and they sent me an entire box of research reports.
RE: Timber deck strong enough?
plesae go for steel decking due its longer life if proprly painted.sound can be arrested after proper tieing.
r k dhiman
RE: Timber deck strong enough?
If your vehicles are not HS-20, a nail laminated deck might still work for you. even for the "wide" spacing of 4ft.
RE: Timber deck strong enough?
- Building codes usually call for minimum levels of capacity regardless of whether structures are on private land. You may be unable to get a building permit unless the design conforms with local norms.
- Even private facilities require public access, . . .fire trucks, ambulances, fuel deliveries, snow plows (?), furniature deliveries, etc. etc.
- You may not be insurable for liability if your bridge is sub-standard.
- 1, 2 and 3 not-with-standing, it would be prudent to assess the maximum probable load expected on the bridge during its lifetime.
Regards,RE: Timber deck strong enough?