how to brace parallel beams against concentric curvature?

Question

I am trying to rehab an old wooden footbridge that got trashed by a falling tree. It's based on a beam on each side, made up of two 2x12s. One got broken, and we've jacked it up and will patch it with a "sistered" board.

The other issue is that both twin-2x12 beams have taken on a significant curve. They're both curved in the same direction, so they describe more or less concentric arcs.

I am able to pull on the middle of the thing with a come-a-long or a tractor and straighten it out. The question is: how best to brace it to keep it straight ?

First thought was a simple diagonal. But this isn't the usual carpentry issue of trying to keep a rectangle from turning into a parallelogram. Perhaps two diagonals forming a 'V' ? And almost certainly it's not sufficient to attach whatever braces to the bottom edge of the 2x12 beams; they need to be attached to the sides of the 2x12 beams, so they cannot twice back into curvature.

Answer 1

Stressed bracing between the curved parts will push the beams apart. The inside curve may be pushed straighter, but the outside curve will be pushed out farther:

An extra beam in the middle with cables or such tied to the inside curve's foundation like this will pull the center beam to the center of the curve:

As the pseudo-dimensions show, the shortened cables pull the center beam, and both curves pull to the center of the curve, too:

One problem with this is the strength of the cable tie points at the foundation of the inside curved beam. Depending on how much curve you're trying to take out, and what you have on site to tie to, you might find that there is too much pull on the tie points.

One way around the tie point strength problem is to use multiple cables. Each cable cuts the strength needed at the tie point for that cable, and contributes to safer cabling, allowing multiple points of failure.

Answer 2

Remove a beam and spin it around so that both beams curve outward. Then pull them together with a ratchet strap or a come along. Brace with perpendicular and diagonal braces. Keep the beams pulled together with threaded rod.

Answer 3

Turn each beam 90 degrees.

Now the curve is upward. It is downward? Turn it the other 90 degrees so it is upward. The weight of the bridge itself will pull the curved beams down to straight. If it is not quite straight and there is a raised hump in the middle of the bridge that is ok.

Answer 4

You have two problems.

1. Fixing the foot bridge that is broken now.

2. Figuring out source of tension/compression that caused the bowing once it was unbalanced.

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Fixing the bridge

Depends how far you want to go with patching this structure for the short term.

The long term fix is to rebuild it with two new beams, and to reuse deck timbers and handrails on the existing ground abutments.

As a short term fix over winter you might attach one/two guy wires on the inside of the curve, and pull them taut to a couple of stout pegs on the bank - Steel warratahs or similar would be adequate as posts. Leave some mechanism so you can add tension to the ropes/straps over time.

Plan view from above

     |        |
    o|        |o  <-- posts
     \        /
     |\      /|
     | \    / |
     |  \  /  |
     |--------|
bank |        | bank
     |--------|
     | stream |

Another fix would be to reconstruct the bridge using regular X bracing for the entire length, along with sistering/replacing the broken beam.

     |--------|
bank |XXXXXXXX| bank
     |--------|
     | stream |

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Dealing with the Compression Stress

I'm no engineer, but I've lived in an earthquake-damaged city for ~10 years. Almost every watercourse has suffered accelerated erosion and landform changes. Most bridges have had damage, from their abutments dropping (the bridge is a step up from the roadway) River banks have all slumped.

I suspect from the description that your watercourse has narrowed over the time since the bridge was built. This narrowing is slow, you wouldn't notice it.

Thus, the two banks are pushing on the ends of the bridge, compressing it. When the tree fell and broke one side, the bridge was able to release the pressure sideways.

Road bridges deal with contraction/expansion, but your footbridge is probably secured to each abutment on the bank. The fix is to make one or both ends somewhat "free floating"

Side view from upstream

 \      Securing pins                     /
  \     /                               /
   +---|-------------------------------+
   |   |                               |
   +---|-------------------------------+
    ---|--|                  |------    <  Not secured, free to move
 Concrete |                / | Concrete
  Footing  \             /   |  Footing
             00 water 00

The "free-floating" side would be constrained to stop it moving sideways. Or you might prefer to secure both sides using slots to allow for future movement.

Another option is to replace the entire bridge with some pipes to carry water flow, and cover them with soil. This would be a culvert-bridge and could resist the compression from the banks better. Also, concrete/metal pipes are probably a lot cheaper than wood these days, so it might be cheaper. Culvert bridges would potentially carry heavier loads too, but the exact numbers need to be figured out.

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Related - plan on doing tree maintenance around your property, at the bare minimum a tree inspection seems wise, which is easiest before leaves come in at the start of Spring. If one tree has broken, others might be simi