Wooden bridge span question

[Zinno87]

Your loading won't work, and I don't think that considering a point load is necessary because the horse will be moving across the bridge. You should at least estimate the area the horse will take up, divide the weight of the horse by that, and then add a safety factor to get your pounds-per-square-foot (psf).

 

[RobA]

Your loading won't work, and I don't think that considering a point load is necessary because the horse will be moving across the bridge. You should at least estimate the area the horse will take up, divide the weight of the horse by that, and then add a safety factor to get your pounds-per-square-foot (psf).

Unless I'm missing something there has to be more to it than that. My wife's horse weighs about 1,000 pounds. His hoofs are roughly 25 square inches each (even less since just his shoes are contacting the ground). That's a total of 100 square inches or 0.7 square feet. 1,000 pounds on 0.7 square feet is 1,428.5 pounds per square foot. That may be a useful for the force exerted on the decking boards but not the beams. I would imagine you would have to take into account how the bridge is tied together (decking, bracing) since that would transfer the weight to the all of the beams. I might agree that the 1,428.5 pounds per square inch measure is appropriate for determining if the decking will hold without the horse's hoof breaking through. Even then 2x lumber spanning less that 16" should a horses hoof.

 

[MikeD74T]

RobA, I don't know if you looked at the (giant) link to Rainbow Bridges I posted earlier. If not, take a look at "pick a bridge" then "loads". It gives a good explaination of what you're looking for. Keep in mind these are 2 truss bridges, load capacity can be doubled by using 4 trusses.
You're right about footprint. A 100 pound woman in high heels creates more ground pressure than elephant but can cross a much lighter bridge. MikeD74T

 

[RobA]

RobA, I don't know if you looked at the (giant) link to Rainbow Bridges I posted earlier. If not, take a look at "pick a bridge" then "loads". It gives a good explaination of what you're looking for. Keep in mind these are 2 truss bridges, load capacity can be doubled by using 4 trusses.
You're right about footprint. A 100 pound woman in high heels creates more ground pressure than elephant but can cross a much lighter bridge. MikeD74T

I did look at that site and have seen pictures of those bridges before. I'm sure they can hold a lot of weight relative to their size but something makes me nervous about them. Call me a worrier but if it were a pedestrian only bridge I may consider it more. Maybe I should reconsider.

 

[MikeD74T]

RobA, I suggested Rainbow Bridges because you seemed committed to all wood. I did this bridge a few years ago for less than $1000 materials cost. Was spec'd to a 8000 lb rolling load. Span is 24', 4 beams 14"x 26'. 2x10 rough hemlock plank deck. The lengthwise strips are to keep snowmobile carbide runners from sawing thru the deck. MikeD74T

 

[RobA]

RobA, I suggested Rainbow Bridges because you seemed committed to all wood. I did this bridge a few years ago for less than $1000 materials cost. Was spec'd to a 8000 lb rolling load. Span is 24', 4 beams 14"x 26'. 2x10 rough hemlock plank deck. The lengthwise strips are to keep snowmobile carbide runners from sawing thru the deck. MikeD74T

VERY NICE!! Beautiful bridge. $1000? I'll take 2!!

I do like the look of wooden bridges. Funny thing is I asked my wife a few days ago about a covered bridge and she said she liked them. I'll have to show her your pictures.

 

[chucko]

"and I don't think that considering a point load is necessary because the horse will be moving across the bridge. "

I don't know bridges but do know Floors and Roofs and for a floor or roof we would do a moviable point load. basicaly check every panel point and mid panel to make sure it could support that load.... just my 2 cents

 

[AlanB]

Beautiful bridge Mike. Is it covered because of snow and stuff to make it safer, or just for the beauty of it?

Very nice.

On the whole lawsuit thing, the original poster just has to find his comfort level. I have a freind that has a farm, that had a party. He told everyone NO fourwheelers. One showed up. He told them to leave. They did not, he said leave, dont get it out, they got it out, they then hurt themselves. They then sued.

I don't think they per se "won" in the end, any great windfall or what not, but they still cost him a pretty penny.

That said, does that mean I should not have a farm or parties? There are just some people that are idiots out there.

On the other side, we built a wood bridge about a 20' span for an area I have a motorcycle event on every year. No handrails, sloping, slippery, 10' over the water. One year, the bridge was about 3' under water, you could not see it, and I kid you not a bit, I still had folks riding across it (after I told them absolutely not too) Never got sued over that

I guess my short thing is if you could pull of a bridge like Mike built, Man, how cool.

As too the legal ends, no amount of building will remove any liability, and there is a liability there that someone could sue if they fell in now because you should have built a bridge. Do what you think is right, and reasonable, and hope like heck you do not run across those lawsuit happy idiots that we all seem to read about.

 

[RobA]

....I have a freind that has a farm, that had a party. He told everyone NO fourwheelers. One showed up. He told them to leave. They did not, he said leave, dont get it out, they got it out, they then hurt themselves. They then sued.....

If I build this bridge I have thought about adding a gate on either end with 'no trespassing' signs on it for occassions such as that. I'd leave it open most of the time. However, "Murphy's Law" would have a guy like that showing up unannounced.

 

[zeuspaul]

I haven't seen mention of strength of materials. Doug Fir is a lot stronger than pine. Pressure treated lumber is weaker than the same species which is not treated.

You may find span tables for home building are based on untreated Doug Fir. Use extra wood if using pressure treated.

Zeuspaul

 

[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'.