Sizing bridge beams

[2manyrocks]

I built a similar bridge 10 foot span, out of two 8x8 hemlock beams and 2x8 hemlock decking about 10 years ago. at the time I had my b7800 and it worked OK but over the years one beam failed due to rot.

I just rebuilt it with steel beams doubled up so i used two steel beams per side so four beams total with the same size 2x8 decking. they are six inch steel beams I think W15 (not sure) I doubled them up to match tire tread widths of all my stuff ranging from ATV to an old manure spreader used as a wood trailer that is wider than the tractor.

I also more than doubled my tractor weight in those ten years. b7800 was approx 3000 with a loader and my L4740 is atleast close to 8000lbs with the loader and filled tires. I get what feels like zero deflection when I cross the steel frame bridge with my L4740.

I hate doing stuff twice. my time is worth something, I vote for steel especially for your longer span.

Traditionally, creek crossings got built from whatever decay resistant timber was locally at hand. Then rot took its toll and the crossing had to be reworked. That is why over time as people around here could afford it, they switched to I beams and concrete for its durability and lack of maintenance.

Whatever an I beam costs today, it will probably cost a lot more 10 - 15 years from now after wood has rotted out and needs to be replaced.

 

[LD1]

I disagree, although I agree with the wet location beam, that's a very good point... the wood grains of any planks are longitudinal, the plywood keep the grains bonded uniformly together, so it doesn't separate under stress.

You are entitled to your opinion. But for the record.... I did not say plywood doesnt add strength.

The fact is, plywood is not considered a structural material for beams and headers. And the fact that I am giving it at least and "equal to" rating for dimensional lumber of equivalent size (IE.....3 pcs oc 1/2" plywood = one piece of 2x dimesional lumber) is MORE than any table, graph, formula, or structural engineer would give.

In the world of sizing beams and headers for construction....the plywood is not accounted for and adds nothing to the equation for most circumstances.

Look at the basement carrier beam in most houses sitting atop the jack posts. Not saying it doesnt exist, but I have NEVER seen plywood laminated in those beams unless it was done by a DIY'er with no engineering knowledge. IF they arent steel, they are 3 or 4 plys of 2x12 or 2x10....or LVL's. But never a mix of dimensional lumber and plywood. Why.....because the plywood doesnt add anything more than dimensional lumber does of equal size.

As I said, the reason it is used on headers is to match wall thicknesses. Nothing more. But with LVL's coming in at 1.75" wide instead of 1.5" like dimensional lumber.....the trend now is to just use a double LVL for a 2x4 wall header

 

[deezler]

I mean, it might be prudent to ignore the plywood or OSB that gets sandwiched with 2-bys for structural calculations, but using real 1/2 or 3/4" plywood obviously adds strength. You could build a glue-lam beam out of ripped plywood strips and it would be strong as hell. Stronger than dimensional? probably not. Maybe a little stiffer but with more brittle fracture.

 

[LD1]

I mean, it might be prudent to ignore the plywood or OSB that gets sandwiched with 2-bys for structural calculations, but using real 1/2 or 3/4" plywood obviously adds strength. You could build a glue-lam beam out of ripped plywood strips and it would be strong as hell. Stronger than dimensional? probably not. Maybe a little stiffer but with more brittle fracture.

Yes, it obviously adds strength.....and I never claimed it didnt.

But I swear, some people thing plywood is like some rare unobtanium level strength of material that when sandwiched in a beam magically makes it indestructible. And thats just simply not the case.

3 layers of 1/2" plywood totaling 1-1/2" thick will do NO MORE than simply adding another 2x dimensional board. Thats the reality.

Like I said, the ONLY reason using the plywood was to get a beam or header to match wall thickness. YES, it obviously adds strength....BUT.....for the engineering calculations that strength is irrelevant and omitted. As the 2 or 3 plies of the dimensional lumber is sufficient for the intended load, and the plywood acts as nothing more than filler/shims. And not to mention the fact that openings over 8' typically require multiple pieces of plywood. That seam in the middle......ZERO strength.

 

[2manyrocks]

From a carpentry perspective, every time one board is nailed to another that will be exposed to the elements, there's an opportunity for water to enter the seam and cause rot. Built up beams protected inside a house are one thing, but another thing when proposed for use in constructing a bridge.

Whatever the span tables say today, I'm not aware of any span tables that will tell us the weight carrying capability of a built up wooden beam exposed to the elements for 5, 6, 7, 10+ years.

 

[LD1]

From a carpentry perspective, every time one board is nailed to another that will be exposed to the elements, there's an opportunity for water to enter the seam and cause rot. Built up beams protected inside a house are one thing, but another thing when proposed for use in constructing a bridge.

Whatever the span tables say today, I'm not aware of any span tables that will tell us the weight carrying capability of a built up wooden beam exposed to the elements for 5, 6, 7, 10+ years.

They just require regular inspection and one must understand they arent made to last forever.

After all, treated lumber used to build decks....with joists, posts, and beams use similar calculations for for anticipated loading.

Certainly the lumber after 5, 6, or 10+ years isnt as strong.....but that doesnt necessarily mean failure.

Steel is actually the same way if not maintained and allowed to corrode and rust away.

 

[Sawyer Rob]

SR

 

[/pine]

Not germane to build up beams...but an example of the strength of plywood etc...is in the form of engineered wood joists...i.e., TJIs by Weyerhaeuser...
They basically consist of a piece of plywood or OSB etc. typically ripped to 8", 10" or 12" (sized to match common lumber dimensions) with a solid wood rail at the top and bottom...the load bearing capabilities typically surpass the strength of solid lumber joist etc...

 

[Slackdaddy]

Thanks for all the responses.
The creek bottom is only accessible descending a 6' wide path through the woods down a long "finger" , I am not getting and semi trailer beds down there or other large equipment.

The creek is 6-8 feet wide, typ the water is about 1-2' deep, and the top of the water is down about 2' from the surrounding ground,, so typ from the surrounding ground to the creek bed is 3-4'. Any fall could be easily fatal if you are pinned face down in 16" of water.

Steel beams from suppliers is a hard "no go",, they are outrageously priced. Last project (axle jig),, they wanted $300 for a 5-6' cut off of 8-10" high I beam.
I'll check the scrap yards.

Typ about once a year, the creek will flood in a heavy rain,, but the surrounding area is pool table flat for about 100 yards wide.
So when it does flood it is 6-18" of water across the whole bottom.
That is the reason I want to raise the beams up and sitting on the poured concrete "blocks" that are anchored into the ground.

So if I do go with steel beams,, what size to span 12' (I can find an area that the trench of the creek is 8' or less)
Total weight of CK20 is ~2000#, so 2500# with the tiller.
The tractor would not be a true point load,, but what size beams (2 beams) would handle 2500# point load for 12' span.

 

[bcp]

Whether using solid timber beams or laminated, I would add a weatherproof cover over the beams before adding deck boards. I've seen tar, galvanized steel, aluminum, and synthetic material. Extend it an inch or two past the edge.

Wood will last far longer if the top never gets wet.

Bruce