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I'm building a small bridge over a creek on my property and using some forest service plans which recommend 8x8 ground contact lumber kept in place by earth anchors for the sills that the bridge stringers rest on. It's not out of the budget to use the 8x8s, but to move them in place might be a little difficult with the machinery I have access to (basically none). If the main force on these is a downward pressure, could I substitute four 2x8s stacked and attached together at bridge site? The only concern I could think of would be water penetrating between the boards and causing them to decay faster.

Edit: the 8x8 (or built up beam) would be fully laying (lying? 😆) on a solid bed of compacted gravel (or concrete) and anchored using a duckbill anchor. Since none of this is suspended in air, I don't think deflection of the beam is a concern. However, it's possible that it'll be completely submerged in water at flood stage, which happens 1-2 times per year and lasts for less than a day.


Edit #2

Here is a link to the page with the plans I'm using with some modification (just adding railings and using more stringers). https://www.fs.fed.us/t-d/pubs/htmlpubs/htm12232316/.

Below is an image that illustrates the sill. In this image, they have a heavy duty parking curb, (which is a possibility to use but those things are heavy and I don't have easy access to a machine to easily move them), but they specify that an 8x8 pressure treated southern pine could be used.

Bridge Sill

I do understand that a laminated beam spanning an opening should be vertical boards, but in this case, I thought the horizontal stacked approach might be ok and just make it a little easier to handle it as I do it myself.

Thanks!

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  • Can you post a drawing or picture of the plans? Particularly the sills and stringers, etc. – Greg Nickoloff Aug 15 '19 at 3:04
  • @GregNickoloff - Yes, sorry I only had a few minutes to post that update. I'll get them in a moment. – Drewsonian Aug 15 '19 at 3:18
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I’ve posted several issues with other answers...I won’t repeat here.

Recommendation:

I prefer the solid 8x8 member idea, but if too difficult for the OP, due to physically trying to move the members around, then laminated members laid FLAT and anchored with straps (as suggested by @Solar Mike) and anchor with straps or hooks over the top , rather than any bolted or nailed connectors.

Having the members flat will help with the shifting of the bridge, due to movement (dynamic load) or water movement.

I’d definitely have the straps or hooks installed over the top of the “beam” so it doesn’t float away and doesn’t move off the solid base.....but I’d avoid using bolts to hold everything down.

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  • Honestly, it sounds like less work to just put down the parking curb or 8x8, so I'm going to scrap the laminated member idea. Thank you! – Drewsonian Aug 15 '19 at 3:40
  • Although if the picture I recently updated sparks any other thoughts, I'm open to hearing them :-) – Drewsonian Aug 15 '19 at 3:41
  • even with a solid 8x8, it would be wise to use straps because bolting or nailing through the member could compromise the pressure treatment? – Drewsonian Aug 15 '19 at 17:22
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    Yes! And be extra careful with the ends, because “end grain” soaks up moisture much more than “side grain”. – Lee Sam Aug 15 '19 at 18:09
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    If straps are not available or would decay too quickly (depending on material, etc.) I’d use steel straps that fit tight over the top of beam, but not bolted to beams. Bolt the steel straps to the concrete base. – Lee Sam Aug 15 '19 at 18:12
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I would put 2*8 vertically as shown, then you will only need a few fixings as the "layers" won't be sliding over each other and you can even put bands around (shown in red). This is a common technique.

enter image description here

You only have to put a plank across two bricks and see the difference between the orientations...

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  • But they will. They will want to bow sideways. You should exclude horizontal orientation from this discussion, as no one would do that, it sacrifices strength for no reason. – Harper - Reinstate Monica Aug 14 '19 at 21:52
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Use Trex for that

Given the ground contact, just use plastic lumber such as Trex. The ideal thing would be a plastic curb, but if you think that is too heavy, just layer Trex horizontally. You don't need 8" of depth, you don't even need 8" of width.

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  • I would stack them vertically : I I I I , not as horizontal layers, which means that the bowing, laminating and fixing issues will be reduced... – Solar Mike Aug 14 '19 at 21:35
  • @SolarMike the bowing I am referring to is in the context of already having done that. Look at a highway bridge sometimes, they have several steel girders stood vertical, and also cross braces to keep the girders from flopping sideways. – Harper - Reinstate Monica Aug 14 '19 at 21:45
  • So put 4 planks in both orientations and test... To prevent the layers sliding is more challenging in the horizontal orientation for wood - the steel has the bracing or think of an I beam the web stops the relative movement... – Solar Mike Aug 14 '19 at 21:48
  • Given the goal of a square-ish member, there's no reason whatsoever for anyone to choose a horizontal orientation, ever. Horizontal is simply wrong. So cross it off the list and what's left? – Harper - Reinstate Monica Aug 14 '19 at 21:51
  • You put "Just like sheets of paper in a stack" and stacks of paper tend to be horizontal... Having edited your question to cover... – Solar Mike Aug 14 '19 at 21:55
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Would pouring a concrete sill work in this context? Essentially make a "parking curb" out of reinforced concrete in place and build on top of that.

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  • Yes, that's definitely an option. I just would have to get a buggy to cart the concrete back to that part of the property and I'm not experienced with concrete. – Drewsonian Aug 15 '19 at 14:20

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