Wooden bridge span question

[2manyrocks]

The covered wooden bridge is beautiful.

This discussion brings back 40 year old memories of when my Dad replaced our wooden bridge with concrete and steel. About 20 years later, we had abnormally high water that washed our neighbor's wooden bridge down the creek into our bridge. It partially blocked the water flow so that the water ran over the road and washed the approach out on one side, but the bridge held and is still in use today. I don't know the right term for them, but the bridge was built on rock and mortar walls on each side of the creek.

I never expected to see flood water like that or to ever see it again, but that explains why our creek bottom is flat. Nature washed it flat, and I guess will wash it again someday.

 

[MikeD74T]

AlanB, Originally the bridge was an open deck with no railings. Wife worried, appropriately, about the grandkids so I put log railings on. After 4 years in place the deck planks started to rot thru. Since we have two 1800's covered bridges in town that the grandkids love, covering our bridge was the only option. The roof essentially puts the deck inside out of the weather. I planned to put solid waist high sides between the posts but we got a flood that was 18" over the deck. Realized that with solid sides the current would probably carry everything downstream. Have had another similar flood since. Covering the bridge was pretty easy in spite of that being an afterthought. MikeD74T

 

[AlanB]

Mike, for an afterthought, it sure is beautiful and something to be proud of.

 

[Aquaman]

Mike,
Excellent job on the bridge!!

 

[tree grower]

Some very good thoughts and recommendations, and photos of some great bridges. My best project was a 47 foot span using two 30" deep I beams, and a structural deck of 4x10 PT on edge. I told the engineer that it should support 100,000 lbs at the center. We've hauled over 1MM bd ft of logs over it so far, and it holds up just fine. We included no safety features on purpose--no curbs, no railings, no signs. That way no lawyer could claim our safety features are inadequate.

Rob--being from southern PA you should have no difficulty finding black locust logs for stringers. These are mostly straight, extraordinarily strong, and rot-wise will last 10 years longer than granite.

If you want a reasonable alternative to PT for the decking, try 3" hemlock. It should be available from a local sawmill. Hemlock won't last as long as PT, but it certainly is strong, and will last a long time. I wouldn't use white pine for any of it!

Give special heed to the Mainer who cited snowmobile bridges. Snowmobilers have too much invested in their machines to risk them on half-fast bridges. Also, the US Forest Service links to wooden bridges are good.

Building bridges is fun!!

 

[2manyrocks]

One thing I've read is that most of the strength of a log is in the outer layers, and a log is supposed to be stronger than its equivalent size in dimension lumber because it still has the outer layers. Maybe dimension lumber is easier to buy at the lumber yard and get to the site, but logs should be stronger and locust has a reputation for lasting. About all the wooden bridges privately built around here were made with logs and sawmill lumber.

 

[MAYOR8020]

View attachment 92795

Any pictures?

yes here they are

 

[cmhyland]

Try looking at oak crane mats

North Pacific—Pole & Piling Divsion—Crane/Laminated/Dragline Mats

Regards,
Chris

 

[Coyote]

I built a similar bridge spanning 12 ft, with an unsprung distance of 10 ft. Building for ATV's is easy, horses are a diferent story because of concentrated heavy loads as they trod across. I used 3 2X10's bolted together to make one joist, used 3 joists, and decked it with 2X8's. Have driven a diesel F350 over it and a 1000lb horse with a fat guy in the saddle and worked fine!

 

[biggerten]

If you sister the joists as described, consider ripping some sheets of marine plywood and sandwiching the ripped pieces between the joists. Dont forget to stagger all the joints and use a marine adhesive as well. The plywood should make the result much 'stiffer'.