I'm building a bridge across a ditch in my nephews front yard. Shown here is an elevation of one of the four joists that will support 2 x 6 redwood treads, width of 5.5'. To achieve the curve, I will need to assemble the joists from 2 pieces that will need to be scabbed together in the center. The unsupported part of the span measures 11.5 feet, the vertical thickness of the majority of the span is 10", it measures about 8.5" where it leaves the supporting pads at the ends. The wood to be used is pressure treated Doug Fir, structural grade. My intent is to scab these two pieces with a 4' long piece of 2 x 8 PT Fir, same as above. enter image description here I'm a long time builder but I'm a bit nervous on this one. I've not, in memory, seen similar. Think this is sound? If so, how best to secure the scabs? Many screws? Carriage bolts? Use iron versus the wood scabs? (I'd rather avoid that.) Wood-scab both sides of the interior joists? I guess my biggest concern is stress, or wear from use, on holes going through the joist halves.

I could run the design flat with a single piece but that would require steps on the ends and would not look nearly as nice in its intended location.

EDIT: As per suggestion, I've lengthened and fattened the scabs.The left side shows bolts at 6" centers, 4" centers on the right. Comments? enter image description here

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    I like the modifications, but think there may be too many bolts. I would suggest eliminating the center line of bolts since the stresses are well absorbed at the top and bottom rows.
    – Ashlar
    May 1, 2019 at 21:46
  • @Ashlar Whoa! I added all those bolts at [what I thought was] your suggestion. On re-reading your post, I see it's 1 foot OC versus 1 inch OC! You have a point on the middle row, the top row will resist compression, the bottom row expansion. I think I'll stick with the 6" spacing versus 1' though. Thanks.
    – bpedit
    May 1, 2019 at 22:49
  • Giant half lap joinery and steel plate scab with through bolts.
    – Alaska Man
    May 5, 2019 at 20:22
  • Having a solid center and two joins at each side may provide a stronger structure when the joints are not as resistant to bending forces as the original beam.
    – Netduke
    May 17, 2019 at 19:34

2 Answers 2


Like you, I have built a great deal, but I am not a structural engineer so my recommendations are more intuitive than definitive. I would recommend extending the length of the scabs as much as possible. If you are creating the arches from 10' framing I would use the same for the scab and cut it to the curve. I would apply fasteners 1' OC. staggered top and bottom. Since it will be exposed to the elements for a long time I would use 3/8" galvanized bolts rather than nails which would be more likely to rust through over time. Drill the holes to 3/8" diameter for a tight fit.

  • Thanks. The 3/8" galvanized bolts were my first choice. Good idea to cut the scabs from the same 12" lumber (reduced to 10" after cutting radii). The 1" spacing seems a bit too close to me, more bolts but less wood. But I could, if cutting the scabs from 12" stock make then a bit longer to accomodate more bolts. (Nails were never on the menu, the other alternative was coated deck screws and plenty of them.)
    – bpedit
    May 1, 2019 at 1:59
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    Agree that the scabs should be as long as possible (2x12 good!) and that through bolts much much better than screws. If it was me, I'd do one scab on the interior of the two outside joists, and scab both sides of the two interior joists. Maybe that's overkill, but wood is relatively cheap. You might even think about cable or wood that acts like a collar tie at the bottom of the joist. May 1, 2019 at 14:38
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    @AloysiusDefenestrate Thanks. The double scab on the inside joists was a likely part of the plan. The collar tie iwould definately detract appearance wise. This ditch is not your typical country-fare, it was recently built, attractive for a ditch, lined with large white cobble. My niece and nephew have built a nice, flower lined pathway leading to this bridge which will land on a asphalt road. I might also mention the joists are 12.5" on center.
    – bpedit
    May 1, 2019 at 15:29
  • Thanks for the help. It turns out I planned with the wrong dimensions, the span turns out to be about three foot less than advertised. I can now fabricate the joists from one piece of lumber!
    – bpedit
    May 17, 2019 at 15:33

Hey i usually over engineer stuff cuz im paranoid, doesnt necessarily mean I have the best answer. I am a carpenter but have never done a bridge.

I would span the scabs to 8’ using plywood so i could easily cut the arch, and put 1-2 layers of 3/4” plywood on either side of the arches. If the scabs are visible and you want it to look good after just extend the outside pieces all the way to the ends, just keep a full 8’ piece centred on the joint.

  • Thanks. My current thinking is that the PT fir would be more durable than exterior ply and offer a bit more bearing surface for the bolts. I'm hoping, in fact counting on, cutting the radii with a 7.25" circular saw. I've already made saw guides to theis effect. It seems a 25' radius is less of a curve than my occasional error when freehanding with said saw!
    – bpedit
    May 9, 2019 at 1:47
  • Sorry I use plywood as an interchangeable term, your right PT would be the best. May 9, 2019 at 3:13

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