2

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!

Drewsonian
  • 85
  • 1
  • 6

4 Answers4

1

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.

Lee Sam
  • 23,238
  • 3
  • 20
  • 44
  • 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 03:40
  • Although if the picture I recently updated sparks any other thoughts, I'm open to hearing them :-) – Drewsonian Aug 15 '19 at 03:41
  • 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
  • 1
    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
  • 1
    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
0

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.

Harper - Reinstate Monica
  • 300,628
  • 26
  • 286
  • 734
  • 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
  • I had to, because I was unclear. I hadn't disclaimed horizontal because laying rectangular members horizontal is inherently so useless that I didn't think I would have to explicitly say so. – Harper - Reinstate Monica Aug 14 '19 at 21:56
  • @SolarMike No, No, No. Harper is incorrect on several issues: 1) vertical orientation of 2x8’s is incorrect, 2) plane the 2x8’s so they fit together better, 3) the 8x8 is stronger than laminated 2x8’s, 4) bolt the 2x8’s together is ok, (This will be a long comment, so let me finish before any rebuttals.) – Lee Sam Aug 15 '19 at 02:08
  • @Harper 1) Installing joists in a vertical orientation is correct in a “static structural design” (a typical building WITH span requirements), but NOT in a “dynamic structural design” (a typical bridge design WITHOUT span requirements). The OP clearly states that the plans call for 8x8’s are to be laying on a full bed of compacted gravel (or concrete). Therefore, there is no span for the beam and no advantage for the 2x8 to be laid vertical. In fact, the “dynamic structural considerations is clearly sideways motion (due to transport movement) and the 2x8’s should be installed horizontally. – Lee Sam Aug 15 '19 at 02:26
  • @Harper 2) There are several types of pressure treatment. The OP has indicated “ground contact” (which I suspect he took from the specs), which is significantly better than standard pressure treatment used in buildings where the wood is in contact with masonry or concrete, but not buried or submerged. When you plane the edges, (or use bolts) you destroy that higher grade of treatment. The treatment does not go completely through the member. Just cut the end of a PT member off and look at it. You’ll see the perimeter is very embedded and the interior not so much. And you want to plane that off? – Lee Sam Aug 15 '19 at 02:38
  • @Harper 3) Laminated material is far stronger than cut-timbers. All of the structural values (and charts) show higher values because more of the laminated members can be seen and graded accordingly. Graders cannot see the interior of the 8x8’s, so they account for that by down grading the 8x8’s. The glue is actually stronger than the wood. With the boards in the horizontal position, even the OP could successfully apply epoxy...and because it’s not spanning anything, it doesn’t even have to be that good. – Lee Sam Aug 15 '19 at 02:50
  • @Harper 4) Bolting the pressure treated members together ruins the exterior treatment, similar to the idea of planning the material. However, this is far worse because water can penetrate all the laminated layers and develop rot much quicker. – Lee Sam Aug 15 '19 at 02:53
  • @LeeSam I am pretty sure OP is building a bridge. He is saying the ends will rest on gravel etc. But the middle will be over air. – Harper - Reinstate Monica Aug 15 '19 at 02:59
  • @Harper No, he specifically says “none will be suspended in air”. – Lee Sam Aug 15 '19 at 03:22
  • Wow, great comments here and several things I had not considered, 1) keeping the 2x8s from sliding when horizontal, 2) pressure treatment only penetrates the wood to a certain point, 3) planing the wood to fit is a good idea, but you lose treatment. I think I'm just going to put on my big boy pants and use a concrete parking block or the actual 8x8. :-D – Drewsonian Aug 15 '19 at 03:37
  • @Harper BTW, we often install beams horizontally in retrofit projects to horizontally reinforce roofs, floors, etc. in high wind areas or seismic upgrades. We also use a horizontal installation to laterally brace extra tall independent columns. – Lee Sam Aug 15 '19 at 04:32
  • Al the linked content look like a bridge bridge to me, but whatevs. A bridge laid on dirt that has no bridge related duties. I don't know who's responsible for that idea, but check please. @LeeSam you really oughta pull your very smart comments into an answer of your own, coz Imma delete this answer. – Harper - Reinstate Monica Aug 15 '19 at 05:27
  • Thanks for the update to Trex, that's a good idea. The plastic curb cost is a little high and I've had a hard time finding a supplier locally so far, but I'll look around again. – Drewsonian Aug 15 '19 at 07:33
0

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...

Solar Mike
  • 27,371
  • 2
  • 32
  • 61
  • 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
0

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.

gnicko
  • 5,605
  • 1
  • 21
  • 38
  • 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