Wooden bridge span question

[quicksandfarmer]

The beam deflection exceeded .5" with that load, I was looking for less than that based on an L/360 deflection.

I'm not sure that L/360 is the right standard for this application. It's used in residential construction as the point where deflection is apparent, and brittle finishing materials are subject to cracking, but it is nowhere close to the point of failure. As long as the deflection isn't so great as to be unnerving, what you really care about is having a good margin of error against catastrophic failure.

Also, when calculating deflection it is customary to use only the live load. You don't care so much that the span sags slightly over its length when unloaded, what you worry about is the perception of bounce as the span is loaded.

I am around horses some so this isn't a blind guess.
A horse hoof is say around 12 sq in.
The horse and rider is probably in the 1500lb range together
The load per hoof is 375 lbs
The ground force is around 30 lbf/sq in.

But depending on the horse's gait, the weight is only on two or three legs at a time, so it could be twice that. By the standards of residential construction these loads are huge.

 

[RobA]

Is that 5 beams of double 2x10's? The added wieght of the beams and railngs should not be a problem and I suspect without doing the math that the deflection would more than meet L/360 and may be close to L/480. You should have a safe bridge to be proud of.

Yep, 5 double 2x10's, 16" on center. I could easily do 2x12's if it would help. My original thinking was 2x10's would do it - but that was just an initial guesstimate. Even tripling the 2x lumber to get 4-1/2" wide beams is not a big deal to do. In the scheme of things it's not that much more money.

 

[Zinno87]

I'm not sure about the span and the deflection, but I would make sure to add diagonal bracing between the joists.

 

[2manyrocks]

When people used to build wood bridges across creeks here, they used tree trunks as the beams. I'm guessing four or five 12" trunks over which they nailed bridge timbers as planks. They've gradually been replaced by steel and concrete over the years.

A friend of mine showed me a drawing last week of a culvert crossing that had been dressed up with side walls and a railing. Although the top of the culvert crossing was actually flat, they installed a radiused hand rail to give the illusion of a bridge and then put post lanterns at each end to dress it up.

 

[RonMar]

What about all the people that slap a half-*** deck off a second floor and host parties out there. Most peole don't lag decks into the house stucture properly but never think twice about risking the lives of loved ones on it.

Yep, they sure do, and occasionally the party comes to a(pardon the pun) crashing halt... Permits and codes are supposed to help prevent this. The main point I was trying to get at is that There are a lot of variables in a home designed and constructed stick built bridge.

They are replacing two wooden bridges in our town with concrete and steel structures.

I have seen some very attractive gravel and culvert affairs also. The sides are larger basket ball sized(half-man) rock walls with gravel fill in between for the roadbed. The sides are extended up above the roadbed to form side rails like a stacked rock wall alongside a path. The rock sidewalls are stacked over the end of the culverts, so you can't hardly see them.

 

[mboulais]

I'm not sure that L/360 is the right standard for this application. It's used in residential construction as the point where deflection is apparent, and brittle finishing materials are subject to cracking, but it is nowhere close to the point of failure. As long as the deflection isn't so great as to be unnerving, what you really care about is having a good margin of error against catastrophic failure.

Yeah, I was just using the L/360 as a guidline since that is usually noticable, but not unnerving deflection. In most cases I think beam wt is supposed to be included and in floor and roof systems, that's your dead load. Anyway, I've always added it in.

Yep, 5 double 2x10's, 16" on center. I could easily do 2x12's if it would help. My original thinking was 2x10's would do it - but that was just an initial guesstimate. Even tripling the 2x lumber to get 4-1/2" wide beams is not a big deal to do. In the scheme of things it's not that much more money.

I had orginally only looked at 3 beams, with 5 it should be more than adequate.

There are a lot of variables in a home designed and constructed stick built bridge. ........ I have seen some very attractive gravel and culvert affairs also. The sides are larger basket ball sized(half-man) rock walls with gravel fill in between for the roadbed. The sides are extended up above the roadbed to form side rails like a stacked rock wall alongside a path. The rock sidewalls are stacked over the end of the culverts, so you can't hardly see them.

I agree there are other options for building an attractive bridge, I can think of one bridge / culvert project on here that had custom ironwork in the railings too.

I just don't like to rule out wood so fast. It can be done safely with good fasteners, good finishes, and some planning. Either a suitable crossing can be made, but I suspect the cost of gravel and culvert tiles will add up fast and ther is the question of can you get the culvert to the building site (ground conditions, equipment, etc..) and can you get the gravel there (without 100 trips with a 1/2 yd bucket). Sometimes the easy answer is still the right answer.

 

[ktm250rider]

I used 2 large pines about 30 feet long for my bridge. Span is probably about 20 feet with the other 5 on each end for support. Ran 2x10 for the first decking and then 4 2x12 in pairs set at tractor wheel width. Doesnt move one bit with a load of fire wood in the bucket and a trailer on the back.

 

[ilarcher]

I think I would go 12" OC. 16" OC would be fine if it was going to be a walking bridge. If you plan on useing it with any kind of weight and a vehicle(atv, tractor, etc.) 12" OC would be better.

I am a carpenter by trade and I use 12"OC for alot of spans on floor in houses. It makes a huge difference in deflection. Where I build houses at if I use 2x10's I can only span 15'9" at the max, anything over that has to be 12" OC.

I do recomend the cross bracing between the joist, it will only help.

You can also put diagonal braces down to your pillars(or ground). Just divide your span into 3 parts and it will stiffen the structor greatly.

 

[meadowlarkponds]

RobA,

This thread may or may not be of use to you, but I thought I'd offer it and a couple of thoughts.

http://www.tractorbynet.com/forums/projects/105232-another-bridge-project.html?highlight=bridge

Whatever you decide, I would "overbuild it". The calculations are somewhat less than perfect because of a lot of variables. In my case, the bridge mentioned above has supported an 85 hp tractor w/bat wing, my Case dozer, and many other lessor things. It will be in place and safe for many years after I'm long gone from this Earth. I don't have to ever worry about what I'm taking across or some other person may be taking across. It's just not worth it to build at the design level...overbuild it and sleep good at night.

 

[RobA]

Whatever you decide, I would "overbuild it". The calculations are somewhat less than perfect because of a lot of variables.

Good advice. Thanks. And yes, your link was helpful. As they say a pictures are worth a miilion words.