Timber bridge

[allank]

We have a bridge over a creek that has old timbers from the Sierra snow sheds on the deck.
Has concrete abutments and steel beams across.
The timbers are all 8" X 14 - 16" X 16ft long. We've always had the beams fairly close in spacing, maybe less than 1/2 in apart but the gaps always fill with gravel and dirt, so poor drainage. In the winter water collects and beams start to rot out.

Contrary to popular opinion, I believe we should increase the gap to as much as 1 1/2" between timbers to make it easier for water to pass and make it less likely to fill with dirt and gravel. Cars and trucks all cross now and even large concrete or gravel trucks.

Anyone have any experience on this?? See any reason why we can't increase the gap? Surely 1 1/2" isn't too much.
Thanks

[Dave S.]

Any trapped moisture will advance the rot. What species of timber are they? Here in the Northeast white oak and black locust are the choice for this type of work. You will always have rot where the wood touches the metal, as it won't dry as fast. Increasing the gap should help with the trapped gravel.

[allank]

Don't know what kind of timbers.
They are old Railroad Snowshed timbers from the Sierra Nevada. They have long "sheds" (artifical tunnels) for the trains going from Sacramento up towards Reno in the mountains. I'm sure they've been treated with creasote or some other long abandoned chemical.

Just not sure how much of a gap is reasonable. Car tires rolling over a gap should be bothered with 1 1/2".

[agmantoo]

Vehicle cross cattle crossings with wide gaps with no problem. I have a bridge with metal I beams that have a gap, again no problem. As you can see the gaps are much wider than 1 1/2 inch.

[allank]

Thanks very much. I think I can convince everyone now.

[fantasymaker]

Yep go with the gap of a size a little larger than your gravel.
BUT WHILE YOU HAVE THE PLANKS UP.
Why not change the spacing a bit and then GLUE the planks to the beams below. Not so much for the strength but cause the glue (or calking) will help seal moisture out of that joint.

[Bentley]

Originally Posted by fantasymaker

Yep go with the gap of a size a little larger than your gravel.
BUT WHILE YOU HAVE THE PLANKS UP.
Why not change the spacing a bit and then GLUE the planks to the beams below. Not so much for the strength but cause the glue (or calking) will help seal moisture out of that joint.

Afraid I strongly disagree with the glue/caulking idea. It sounds great on its face, but in practice, it sets you up for advanced failure. The reason is that, in practical application, there's no way to really keep the water out. The glue/caulk just inhibits drainage, keeping water in contact with the timbers AND the steel beams. This jalmost guarantees quicker rotting of timbers and corrosion of the steel beams.
If anything, cut or grind notches across the timber bearing surface where the timbers contact the steel beams. This will assist with water drainage.

If you are going to remove the timbers, you might want to treat the steel beams with a product called Trenton Wax. There are better products such as epoxy coatings, but most of them require sandblasting to obtain a decent anchor pattern. The wax does not require sandblasting.
It comes in square blocks, has a corrosion inhibitor in the wax, and you can melt it in a pot over a fish fryer, and apply with a paint roller. Use two coats.

The wax will flow into any voids or pores in the steel, and extend the life of the beams many times.

http://www.trentoncorp.com/

Good Luck

[Yvonne's hubby]

Originally Posted by allank

We have a bridge over a creek that has old timbers from the Sierra snow sheds on the deck.
Has concrete abutments and steel beams across.
The timbers are all 8" X 14 - 16" X 16ft long. We've always had the beams fairly close in spacing, maybe less than 1/2 in apart but the gaps always fill with gravel and dirt, so poor drainage. In the winter water collects and beams start to rot out.

Contrary to popular opinion, I believe we should increase the gap to as much as 1 1/2" between timbers to make it easier for water to pass and make it less likely to fill with dirt and gravel. Cars and trucks all cross now and even large concrete or gravel trucks.

Anyone have any experience on this?? See any reason why we can't increase the gap? Surely 1 1/2" isn't too much.
Thanks

We used a one inch gap on between the decking on the bridge we built, that seems to be sufficient to let the water and mud run on through for ours. It was built with 4x8 white oak decking so isnt quite as thick as yours. I see no problem with widening the gap to two or three inches though unless you have lil kids playing, they could get a foot stuck if its too wide.

[fantasymaker]

Originally Posted by Bentley

Afraid I strongly disagree with the glue/caulking idea. It sounds great on its face, but in practice, it sets you up for advanced failure. The reason is that, in practical application, there's no way to really keep the water out. The glue/caulk just inhibits drainage, keeping water in contact with the timbers AND the steel beams. This jalmost guarantees quicker rotting of timbers and corrosion of the steel beams.


Good Luck

Humm maybe the caulking was a better Idea than the glue but there must be a reason that caulking is used in log cabins and boats without causing excessive moisture retention and rot. Any idea why?