Timber deck strong enough? 1

[farmerfield]

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.

 

[curvbridger]

The strength of conventional planking will not likely be adequate with a span of over four feet.

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!

 

[jheidt2543]

You might consider a steel grated deck. The suppliers have load tables that will help you determine the size. Also, snow and rain pass right through the deck. It might not be that much more expensive than laminated decking.

 

[farmerfield]

Thanks so much for the help! I am starting to read about laminated bridge decking. Very interesting. I am afraid that a grated deck won't work in my case. I need to walk horses over the bridge and they freak when they don't have solid looking ground to walk on. BTW, this bridge used to have 8" of concrete on it and I have not yet crossed concrete off the list. I'll have to see what the cost comparisons look like in my area (OK).

 

[PM]

Right off the bat you seem to have an inconsistency in dimensioning your floor beems and possibly the stringers. If the bridge is 76' long c.t.c. of bearings, then the floor beam c.t.c. spacing is likely to be 18'-0", not 18'-10". You note that the width is now 14'. I assume you mean the clear (inside) width between the main trusses. If this is the case, I assume your deck would cantilever over the outside stringers (ie. 4 stringers @ 4'-3" = 12'-9" leaving an overhang for your deck of 71/2".) The stringer spacing you indicate is more consistent with concrete decks than timber. If your stringers are web connected to the floor beams, you might suspect the original deck was concrete.

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,

 

[curvbridger]

Some follow-up:

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.....

 

[jheidt2543]

I can't help but wonder, does H20 loading mean 20 horses?

 

[bridgesdesign]

PLEASE TRY STEEL DECKING WHICH HAVE LONG LIFE AND COMPATABILTY WITH TRUSS ALSO.


HAPPY NEW YEAR

 

[justtodd]

An excellent resource if you've not already tried it is "Timber Bridges" published by the USDA Forest Service June 1990. It has a very helpful section on decking design, complete with several examples.

 

[erikpe]

Here is another suggestion. Check out using corrugated metal bridge panels. These panels are bolted or welded and then paved with asphalt. I have seen several local towns and counties use them to rehab the decks on old trusses. Contech sells them.

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.

 

[bridgesdesign]

plesae go for steel decking due its longer life if proprly painted.sound can be arrested after proper tieing.


r k dhiman

 

[AndyZ]

If this bridge is not open to the public, I suggest getting an engineer to confirm that your vehicles are really equivalent to an HS20 loading. Your vehicles may not approach HS20. In addition, if this is a private structure with low volumes you may want to design a deck for use under an "operating" level.

If your vehicles are not HS-20, a nail laminated deck might still work for you. even for the "wide" spacing of 4ft.

 

[PM]

I'd think very seriously about not designing to HS-20 minimum requirements.[ol][li]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.[/li][li]Even private facilities require public access, . . .fire trucks, ambulances, fuel deliveries, snow plows (?), furniature deliveries, etc. etc.[/li][li]You may not be insurable for liability if your bridge is sub-standard.[/li][li]1, 2 and 3 not-with-standing, it would be prudent to assess the maximum probable load expected on the bridge during its lifetime.[/li][/ol]Regards,

 

[farmerfield]

Thank you all for your thoughtful suggestions. As for the expected load, we will be occassionally driving a hay truck over the bridge. In addition, the horse van that comes to pick up is a semi-tractor trailer rig. The firetruck is also a possibility (my neighbor set the countryside on fire last year while welding on a windy day) and the fire trucks drove into my pasture. I am currently looking at using Contech panels with asphalt. Seems lighter than concrete, less troublesome than wood, and more 'horse friendly' than steel decking.