Pond Boss Forums GENERAL QUESTIONS Pond Boss Bridge span question

I want to build a wooden bridge across a creek - similar in design to Schroeder's . It will be used 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)

Hey RobA...I just bumped into this dormant request and thought I'd breathe a little life into it. I remember back a couple years ago and another PB'er forwarded me this link for a very interesting engineering design for bridge spans. He said he bought plans for $25.

Rob,

Generally speaking, you get more bang for the buck with wider lumber (like single 2x12's) than thicker lumber (like doubled 2x10's).

16' is a fairly short span. It lends itself to building a very strong span that will not bounce or sag, and is plenty strong to support 2 horses. Just moving up 4' to a 20' span starts to get dicey. It probably takes 2x more lumber (or even metal beams) to build a 20' span than a 16' span.

Rob, ideally, how far would you like to span?

The spot I want to cross would need a span of about 15 feet (+/-). I was going to use Sonotubes to create concrete support piers. From that I was going to anchor 6"x6" posts to get to the necessary height to add the joists and decking. I figured that if I set the concrete piers close to the water's edge and surround them with rip rap (to protect from the water eroding around it) I would have a stable base from which to build. If I could get the concrete pier closer then the span would be less. I have a post hole digger for my tractor but I'm not sure how close to the edge I could drill. I wanted to avoid drilling in the creek if possible (don't want to raise any 'gubmint' eyebrows).

In order to spread out any load on the bridge I was going to use 5 doubled-up 2"x10"x16' joists spaced 16" on center making the deck a little more than 5 feet wide.

bobad: 1 2"x12" would be better than 2 2"x10"'s? Or just a less expensive option to accomplish the same thing?

brettski: I have seen 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.

Rob I suggest you lesson to Ryan's advice I think he does this for a living. With that said I'll take a stab at it.

Based on a platform of 5'x16'(80 sq ft) and a live load of 2,500 lbs I come up with a 31.3 psf live load. Assuming equal load distribution. The span tables I have based on a 30 psf live load has the following.

2x10 @ 16 oc max span is 15'-11''.
2x12 @ 16 oc max span is 19'-2".

Wood species have different strengths. Yellow pine is the strongest but hard to get these days.

Based on you doubling the 2x10 I believe you'll be fine. But personally I'm with Bobad I'd go with 2x12's and double them and never worry with it again but I'm the ex KOOK in these parts so don't lesson to me. At only 5 ft wide that must be one heavy horse or I don't think that bridge will ever see 2500 lbs.
Good luck and keep us posted on your progress.

You never saw how much my wife feeds animals!

That raises another question. I was thinking 5+ feet wide and adding railings. Maybe I should make it wider so that it's safer to lead a horse across without getting squeezed against the railing. 6'? 7'? 8'?

Take a look at how my dock was constructed. It uses a combo of steel, treated lumber and composite decking. It's 4'x16'. I haven't done the calculations I've had 6 people on it at once and felt very safe. It is simply 4"x4"x1/4" angle welded into a rectangle with 4x4 joists layed 16" cc. The composite decking was screwed to the 4x4s. I wanted a low profile dock. It's constructed similarly to how railroad bridges are built. If welding isn't practical for you, bolting the frame together would be an option and facing the frame with rough cut 1x4 cedar and wood decking could give it a more natural look.

No,,, doubled 2x10's are definitely stronger than a single 2x12. I'm just saying that strength increases a lot more with width than with thickness.

If I remember right, a 16' pine 2x6 will support ~200# hanging from it. That means that a doubled 2x6 will support ~400#. A single 16' 2x12 will easily support 600#.

Rob, could you post a picture of the area you want to cross?

Sure. I'll take a few pictures this weekend. Thanks for the interest.

This is looking down from the pond...

The creek bank on the opposite side drops about 4' straight down to the creek.

This is taken from the other side at the edge of the bank that drops off...

The opposite creek bank is nearly flat for a few feet, gently slopes for a few more feet then slopes higher to the pond. The distance from the near bank (which drops straight down) to the opposite water's edge is about 8 feet. I figure I would have to come back a few feet to sink the concrete piers and install the piers on the opposite side right at the water's edge or a foot or so back.

By the way, I asked the same question on a TractorByNet post. Someone there used 4 doubled-up 2x12's that were 24' long. He made them by splicing together shorter boards (staggered seams). The doubled-up beams were bolted every 12". Here is his description:

We just finished our fifth wooden bridge this fall. It spans 24 feet. I used two 2x12 glued and bolted together for the trusses, and used a total of four trusses. I used three concrete piers on each side 30 inches deep and about 12 inches diameter. I did not use tubes. I did frame out a sqaure with 2x4's on the top to get it level to I could put steel across the three concrete posts. For the steel I welded two five foot "lentils" that were 3/8" thick. The decking is 2x6. The mule weighs about 1000 pounds, and we have had it loaded with about another 1000, and the bridge does not move.

How about a culvert pipe? They're pretty hard to beat for low maintenace and simplicy and pretty inexpensive. They can also be dressed up with retaining wall blocks or other masonry.

If you're set on a bridge. To get a 6' wide 16' span: Steel beams standard length are 40' you could use 1EA 8w15 cut into 2EA 16' and 2EA 4'. the 16 footer for the support beams and the 4 footers as diaphrams (cross bracing) to keep the support beams plumb, placing the support beams 4'cc. 13 EA 2"x6"-6' treated pine joists 16"o/c overhanging each steel beam by 10" and 2EA 2"x6"-16' end joists and 35 EA 2x6 decking boards, plus fasteners would give you a very strong long lasting bridge. Keep in mind the sonotubes will need a footer under them, say 2'x2'x6", to keep them from sinking in over time or under heavy loads. Use pleny of rebar too. The beams would be manageable at around 240lbs each and will hold a bunch of weight. Use a good paint system on the beams. In 20 years you can replace the decking if it deteriorates, the beams and foundation should last a couple of lifetimes. It's pretty high, I'd add a railing too.

How about a culvert pipe? They're pretty hard to beat for low maintenace and simplicy and pretty inexpensive. They can also be dressed up with retaining wall blocks or other masonry.

If you're set on a bridge. To get a 6' wide 16' span: Steel beams standard length are 40' you could use 1EA 8w15 cut into 2EA 16' and 2EA 4'. the 16 footer for the support beams and the 4 footers as diaphrams (cross bracing) to keep the support beams plumb, placing the support beams 4'cc. 13 EA 2"x6"-6' treated pine joists 16"o/c overhanging each steel beam by 10" and 2EA 2"x6"-16' end joists and 35 EA 2x6 decking boards, plus fasteners would give you a very strong long lasting bridge. Keep in mind the sonotubes will need a footer under them, say 2'x2'x6", to keep them from sinking in over time or under heavy loads. Use pleny of rebar too. The beams would be manageable at around 240lbs each and will hold a bunch of weight. Use a good paint system on the beams. In 20 years you can replace the decking if it deteriorates, the beams and foundation should last a couple of lifetimes. It's pretty high, I'd add a railing too.

Here is a link from the American Wood Council that allows you to insert the details of your construction, and it will calculate the maximum span distance. This should allow you to get a good estimate of what you need to suport the live weight of the bridge.

http://www.awc.org/calculators/span/calc...Horizontal+Span

I've seen that site before which gives the maximum span of 2x lumber. I can't find anything that gives me maximum spans of 2 pieces of 2x lumber joined together which is probably because there are probably too many variables for a decent calculation: glued? glued with what and how? bolted together? how many bolts? etc....

I think you're correct in saying it will allow me to get a good estimate. If I go the all wood route I'll have to make some estimates and assumptions - and over engineer it.

I guess you could use good carpentry rules and assume that a glue joint is stronger than natural wood fibers joined with lignin. considering that headers are made by gluing two 2X lumber together with a 1/2" plywood core, i would guess this is more than double the strength of a single 2X. You could always ad cantilever bracing to be sure.

Hmmmm, adding a plywood core. I like that. Double 2x10's or 2x12's with a piece of exterior plywood sandwiched in between. Glue together with a constuction adhesive (Liquid Nails?) and bolted together. That would have to create a strong joist capable of the span I'm thinking of - wouldn't it?