Wooden bridge span question

[RobA]

I want to build a wooden bridge across a creek for people, ATV's and horses. I was going to use pressure treated 2x10's for the joists of the bridge which would be supported by concrete piers using Sonotubes. If I double up the 2x10's making the joists 3" thick how far do you think I could safely make the span and still support up to 2,500 pounds?

Is 16' too much? I want to avoid adding supports mid-way through the span since that would be in the creek.

(I've considered other alternatives but like the idea of wood both for looks and ease of construction)

 

[mboulais]

I once built one with Cedars for beams. Three logs each about 15" on the butt end spanning a total of 20' with a center suport. I decked it with 2x's and as far as I know 4 wheelers are still crossing it today (15 yrs later). How far to the water? How deep? I am trying to get a feel for the severity of a complete collapse with a machine/ horse/ people on it. Regardless, I will do some math on a design for 3 beams each consisting of double 2x10's. I will post again soon.

 

[RonMar]

Span engineering can be tricky and can make or break the engineer that designs them. How many joices, spaced how far apart? How long do you intend on using it? Who is going to be qualified to inspect it for safety? How long are you willing to accept liability for it's inspection, safety and use? A horse is a difficult engineering problem in itself. Since it's weight is concentrated in a small area at the hoof, you will need heavier decking, adding to the overall stress borne by the span. 16' sounds like an awfull lot for your 3X10 beams. How deep is this stream that you want to cross. How deep is the canyon that it has carved(how high will bridge deck be above water)? What is the peak and average water volume(depth and width of the water throughout the year). how do your local codes apply to bridging streams?

Would it be possible instead of a traditional bridge span, to lay large gauge culvert pipe across the stream bed and then fill in over the culvert with gravel and stone on the flanks? This is a modern variation of some of the oldest stone arch bridges on the planet. Constructed by the Romans, there are still some in existence today. IF the stream flow and local codes will allow it, it would probably be the safest long term low maintenance solution.

large stepping stones and a foot plank for the pedestrians, and the horses and ATV"S get to get wet fording the stream is also another option.

I personally would probably pass on the liability of a traditional span for the situation/use you described.

Good Luck

 

[porky69]

Instead of making a full wooden structure how about using the deck off of a flatdeck semi-trailer?

If you bought a 20foot high boy flatdeck and cut the running gear and other garbage off the trailer and just use the deck and frame rails set on the Sonotubes you would have a super strong bridge deck. Old trailers can usually be bought for dirt cheap.

 

[mboulais]

Span engineering can be tricky and can make or break the engineer that designs them. How many joices, spaced how far apart? How long do you intend on using it? Who is going to be qualified to inspect it for safety? How long are you willing to accept liability for it's inspection, safety and use? A horse is a difficult engineering problem in itself. Since it's weight is concentrated in a small area at the hoof, you will need heavier decking, adding to the overall stress borne by the span. 16' sounds like an awfull lot for your 3X10 beams. How deep is this stream that you want to cross. How deep is the canyon that it has carved(how high will bridge deck be above water)? What is the peak and average water volume(depth and width of the water throughout the year). how do your local codes apply to bridging streams?

Would it be possible instead of a traditional bridge span, to lay large gauge culvert pipe across the stream bed and then fill in over the culvert with gravel and stone on the flanks? This is a modern variation of some of the oldest stone arch bridges on the planet. Constructed by the Romans, there are still some in existence today. IF the stream flow and local codes will allow it, it would probably be the safest long term low maintenance solution.

large stepping stones and a foot plank for the pedestrians, and the horses and ATV"S get to get wet fording the stream is also another option.

I personally would probably pass on the liability of a traditional span for the situation/use you described.

Good Luck

Unless he is opening this up to the public, his liability is pretty limitted. I did run some math and I am not confident the even 3 beams of double 2x10's would be enough to satisfy me. At least no one suggested a house trailer frame yet!

This is what I looked at:

3 beams each constructed of 2 2x10's , each wieghing around 120 lbs
A wood deck 5' wide x 3" thick wieghing 700 lbs
2500 lbs additional load
Total load per beam = 1200 lbs

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

I think you need to go a little heavier. As far as the small foot print of the horse goes, the concern there would be shear forces

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.

I would not hesitate to build a wood deck, I would inspect it for rot every year, but it should be fine. I did not try any math with larger beams, but 2x12's seem a better choice. bolt them together with two bolts every 20-24" and it should act as a single beam. I am not a PE, but I am a mechanical engineer and I disagree with the attitude of most engineers that it is too risky to give advice.

 

[grnspot110]

Just an option, I have a bridge for the same purpose, I used a section of an old trailer house frame, bolted 2" x 4"'s to it to nail the flooring to. Have used it for several years with no problem. I also nailed a row of landscape timbers along each edge to aviod slipping off with UTV's. ~~ grnspot110

 

[RonMar]

Unless he is opening this up to the public, his liability is pretty limitted.

I was not even thinking of public use, there would be a whole lot more hoops to hop thru for that type use. I was thinking purely from a family safety standpoint... I am liable for my own familys safety, just as I am for any guests I invite into my home or property. Would I want to be responsible for a design error on my part injuring or killing one of my family, friends or animals? absolutely not! Would I want a missed or invisible detail when I do an inspection that leads to a falure, to injure or kill one of my family or animals a few years down the road when the structure gets old? NO! Then heaven forbid it should collapse when a tresspasser rides their horse or ATV across it and gets hurt when it fails... IF It is on my property and I build it, I am responsible for it for the rest of our lives! There is a lot on the line when you engineer a structure that defies gravity, holds people and also must be exposed to the elements. The greater number of variables you can remove, the better off you are. Gravel and stone over culvert pipes dosn't really defy gravity, and is fairly easy to inspect. Either the material is there or it isn't. It dosn't rot or decay appreciably(at least in a typical human lifespan) regardless of temperature or moisture content. It can be erroded by water flow, but again, those type damages are readilly apparent in the missing or displaced material.

 

[mboulais]

I was not even thinking of public use, there would be a whole lot more hoops to hop thru for that type use. I was thinking purely from a family safety standpoint... I am liable for my own familys safety, just as I am for any guests I invite into my home or property. Would I want to be responsible for a design error on my part injuring or killing one of my family, friends or animals? absolutely not!

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. I think that most of the time these requests about bridges are met with a lot more resistance than they deserve. He is not trying to build a bridge strong enough for a tractor and log wagon or for a fully loaded fuel truck (yes, I ran into a guy trying to build a bridge for that on about a $500 budget). I think his requirements are reasonable and achievable by a homeowner with reasonable mechanical ability. I did ask about hieght / depth for the reason oof assesing risk. If the bridge is two feet above two feet of water the risk is a lot lower than 4' above 6' of water. If I didn't do anything that involved risk, I wouldn't do anything.

 

[RobA]

Wow. Good responses/advice/thoughts. Thank you very much. Here are some more specifics:

- part of the reason for the wooden bridge in this location is aesthetics so I'm not considering culverts, flatbeds, etc for this crossing
- the bridge will not be the only crossing, I'm having a low water crossing installed in a few weeks (just got approval)
- bridge will not be open to the public and is not in the view of the public
- I was thinking 5' wide so it would be 5 joists (more weight) 16" on center and railings (even more weight)
- creek flow is gentle under normal conditions
- the bridge would be about 3 feet above the normal water level
- under normal conditions the water is only 4 feet wide at this location and only 12-18" deep
- high water in this location does not spill the creek banks, if the creek floods it spills its banks upstream and further downstream
- the reason for the longer span is due to the contour of the stream bank and to avoid higher water beating on the bridge supports
- I would use pressure treated for all wood components

By the way, don't worry, I'm not a lawyer. I respect everyone's opinions and advice here but would never hold anyone to it.

 

[mboulais]

Wow. Good responses/advice/thoughts. Thank you very much. Here are some more specifics:

- part of the reason for the wooden bridge in this location is aesthetics so I'm not considering culverts, flatbeds, etc for this crossing
- the bridge will not be the only crossing, I'm having a low water crossing installed in a few weeks (just got approval)
- bridge will not be open to the public and is not in the view of the public
- I was thinking 5' wide so it would be 5 joists (more weight) 16" on center and railings (even more weight)
- creek flow is gentle under normal conditions
- the bridge would be about 3 feet above the normal water level
- under normal conditions the water is only 4 feet wide at this location and only 12-18" deep
- high water in this location does not spill the creek banks, if the creek floods it spills its banks upstream and further downstream
- the reason for the longer span is due to the contour of the stream bank and to avoid higher water beating on the bridge supports
- I would use pressure treated for all wood components

By the way, don't worry, I'm not a lawyer. I respect everyone's opinions and advice here but would never hold anyone to it.

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.

 

[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.