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White Oak vs GC treated lumber for bridge deck

Started by therman, December 29, 2021, 12:40:24 AM

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therman

Hello,

I have a 12 ft wide locust log residential vehicular bridge in need of new decking boards, span is around 27 ft over a fairly large cascading creek in western NC.   Currently it has 8/4x6" locust decking that has deteriorated pretty severely in spots and I'm thinking I will replace all the decking and salvage the locust that is in good shape for other projects.

What are thoughts on the longevity of rough sawn 8/4 white oak vs GC rated pressure treated lumber?  I am thinking 6" wide boards with 1" gaps for decent air flow around them, but open to any and all suggestions.

The internet has not been very helpful with information about this type of construction, and this forum seems to be the right place for answers.

Thanks in advance!


Don P

That is a long span for a light bridge. We recently built one here of about that width, about half that span. We used 5 stringers and 3" thick white oak decking. I had proposed culvert which would have cost about the same there.
Neither the white oak or the locust is treated so they are relying on the very variable natural decay resistance of those woods. We tarred the top of the stringers hoping to help keep some water out of the tops. You could flash them with a rubberized roofing membrane as well.

2x is not thick enough for a safe bridge deck IMO. Rather than specifying "ground contact". Look up and review the treated lumber UC classifications. Somewhere on the little tag on each board you will see a UC-xx number. That is a better clue as to how thoroughly the wood is treated. For foundations, which is another demanding use, the correct spec is to look for UC-4 or UC-4b on the tags. I was actually glad I could find a tag to show the inspector a time or two and one time I had to go back to the supplier and get a letter and copy of my sales ticket when I didn't have any tags showing. Some of those guys are sticklers. Actually it was kind of funny. The salesman said "that's all I carry, this is not our first rodeo with him  :D". But do be aware, I've seen the big box selling interior treated lumber as deck framing. Their young helpers are anything but.

There's a pretty good graphic explaining the UC's and tags here;
Microsoft Word - Open Task Group Letter on AWPA Above Ground Contact Changes_rev081219.docx

kantuckid

In my area of E KY, small private bridges are really common and even though sawmills and rough lumber are commonplace here, so too is the many oil & gas wells. Most everyone welds up a bridge from used pipe, not rough sawed. Often a landowner who has well/s or a gas line through their property gets to keep pipe pulled up in repair jobs. Not far from me Oliver Trucking, Winchester, KY, used to have a yard of maybe 10 acres of pipe of all sizes they'd moved from pipeline jobs to storage. Seems to be kaput place now? A farmer down the road from me who owns lots of land near a gas compression station has had pipe stacks for sale for years, but not something I keep up with. My basketball goal pipe came from him.  Some weld up pipe frames & use wood decking. I'd look at both PT & used steel. Some salvage yards will set aside steel that they think people might buy back. FB Marketplace & CL would be my 1st look/see. 
Kan=Kansas;tuck=Kentucky;kid=what I'm not

therman

I guess I should clarify the construction of the bridge.  Though currently for residential use (planning to build a cabin nearby along the creek it crosses), the bridge was built in the 1980s for logging, and according to the neighbors held full logging trucks without issue.  It has 6 locust stringers 16"+ in diameter, larger than any locust trees I can remember seeing anywhere.  The previous owner took a track hoe across it regularly, which I suspect is why the decking deteriorated badly, but the stringers look solid with no obvious deterioration.  I think any one of them could hold up a vehicle alone, so I am not keen on ripping out the whole bridge to replace it.

btulloh

If you have a sawmill and white oak logs then saw some 2 1/2 inch oak planks.  Try not to keep any sapwood if possible. Treat them with motor oil/diesel mixture once a year. They'll last a pretty good while.

If you're buying stuff, might as well buy the good PT and use that. 
HM126

kantuckid

Yesterday, when I logged onto FB marketplace a few minutes after I'd left this spot- a picture full of 28' steel I-beams popped up in WI. Used and by the LF for sale. I wasn't looking either- I guess the web wizards saw my post here as I have never, ever, searched for steel I-beams. 
A 1/2 mile from my house the school board had a school & gym torn down this fall-lots of bridge parts loaded on those trucks. 

IMO, it's more about how deep your pockets are and what goes over that bridge, than WO vs. Locust, etc..
Kan=Kansas;tuck=Kentucky;kid=what I'm not

scsmith42

Black or yellow locust should outlast 2nd growth white oak by several years.  If you go with white oak, after the planks have aged for a year try treating them with some copper napthenate (CuNap) to improve their decay resistance.

Don't leave any sapwood in the boards; it will rot quickly.
Peterson 10" WPF with 65' of track
Smith - Gallagher dedicated slabber
Tom's 3638D Baker band mill
and a mix of log handling heavy equipment.

therman

Seems like 4B PT 4x6s would cost about the same as white oak, and will last longer, so I guess I will probably go that route. 

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