Building a Bridge

[Rob Bodenschatz]

Since I'm using wood I figure it's ok to post this here...

One of my summer projects is building a bridge across the creek that goes through my property. I'm looking to span about 20-24 feet. Does anyone know of a good calculator that I can use to determine the size of the beams that I should build for this thing?

[Maurice Metzger]

Hi Rob,

There are various tables here:

http://www.globalsecurity.org/military/library/policy/army/fm/3-34-343/appc.htm#tabC-1

This is from the US Army field manual FM 3-34.343 Military Nonstandard Fixed Bridging.

The table of contents for the manual is here, there is probably more info elsewhere in the manual:

http://www.globalsecurity.org/military/library/policy/army/fm/3-34-343/index.html

Maurice

[Jamie Buxton]

Unfettered by any knowledge of your project, I think you should build a covered bridge -- y'know, timber-framed, with a roof to keep the elements off the timber. For a woodworker, they are way, way cool!

Pennsylvania is a hotbed of covered bridges. Google with "covered bridges pennsylvania" for scads of sites about them. You could have a real fun weekend driving around to look at a few.

[Tom Jones III]

My BIL bought some land very cheap b/c there was a small creek that had a very deep channel running in front of the property. Nobody was willing to pay for a bridge to be built just so that they could then build a house. BIL found a source for free/very cheap telephone poles. He used several set in concrete for the legs and then laid more across to form the base of the roadway. Cement trucks were driving over it to build the house with no problem. The county bridge inspector said that it was ridiculously overbuilt and that he would never have problems with it.

[Rob Bodenschatz]

Maurice, I started to look at that manual and it made my head spin. Maybe I'll try again later.

Jamie, yes, there are a lot of covered bridges in PA. In fact I drive across a few on my way to work. I like you're thinking and I may do that in the future. However, I wanna get something up quick so that'll have to remain in my dreams.

Tom, I have looked into that but have not been able to locate any. I'll keep trying. If it's overkill for cement trucks, it's gotta be overkill for my three and five year olds to walk across.

Let me throw this out there more clearly. I want to build a bridge that will only be used for walking, no cars or cement trucks. 24 feet long by 4-6 feet wide. It must span that length. No cover. Nothing fancy. I was going to bind together a bunch of 2XSomethings to make the spans and lay planks perpendicular across the top.
Would 4"x8" be sufficient? What type of wood would you use?

[Kyle Kraft]

If I were to make such a structure, I might go to a house which has floor trusses, the kind that will clear span a 24' basement and reverse engineer three of them out of green treated lumber. Slap on some decking and enjoy....should hold foot traffic as long as you break your step instead of marching.

Kyle in K'zoo

[Doyle Alley]

I would go with the telephone pole route. No worries about rotting, etc. I've seen several built this way. A tractor/loader or backhow would help moving and setting the poles.

[Greg Jurrens]

Here in the mountains of northern California where I live there are numerous creeks. When we need a bridge in a hurry we use a scrap railroad flatcar.
Greg Jurrens

[Jamie Buxton]

Rob, I think you aren't going to find 4xsomethings which are 24 feet long at your local lumber emporium. To get beams that long, you may need to order engineered beams, like glu-lams. When I last looked at them (quite a few years ago now), the manufacturers didn't want to guarantee glued-up beams for exterior use. Timbers shorter than the span is one of the motivating factors in covered bridges. They built timber trusses from short lumber in order to span wide rivers.

Telephone poles or railroad flatcars are expedient solutions, but they're sure inelegant compared to timber-framing!

[Ken Garlock]

A friend about 6 miles away had a gully to put a small bridge across so that he could access his auto workshop. He put a concrete retaining wall on each side, and then bought steel I-beams to span the gully using the walls as support piers. Across the beams he put 2x10s to form a road bed, and then came back with a wide wooden tread for car tires. I have been over the bridge in my pick-em-up, and it is steady as a rock.

BTW, he will not have to replace the steel beams in his lifetime.

[tod evans]

24 foot span, weather, cost effective= steel..02 tod

[Matt Meiser]

A house the next road over from us has a covered bridge on their driveway that crosses a small creek. It looks really cool.

Personally, I think the pole idea sounds like a great one. It will be way overbuilt for a walking bridge but you won't have to worry about it. You could put some kind of a skirt on the sides to hide the poles if didn't want to see them.

[Rob Russell]

I did a quick search for floor span tables. Douglas fir 2"x14"s will span 24'+, assuming you're using #1 or better lumber. The only thing is does pressure-treating lumber change its load-carrying specs.

Where you'd find 24'x2"x14" pressure treated lumber is a good question.

As far as using PT lumber and building floor trusses - I'd want assurances from the truss manufacturer that the plates wouldn't rust. If they do, your trusses fall apart.

I'd guess that the steel would have the advantage of producing a lower bridge deck.

[Jim Becker]

Three telephone poles...some side skirts as mentioned to "clean it up and...some 3/4" modified to make an even surface for walking and biking across. The skirts would also allow you to construct a rope railing for safety, too, if you want.

[Dick Bringhurst]

I notice you're in Penn. If you get snow and ice and your only talking about foot traffic be sure you"re taking into account a snow and ice loading. It may be the heavist load you'll have. A 48" deck (walkway) can hold a lot of anow and ice. Dick B.

[Andy Hoyt]

Lots of ways to skin this cat.

What's the budget? On the cheap - Nice but, but geez! - Sky's the limit.

Pair of waders might be easiest.

[Ben Grunow]

Rob- I would use pressure treated PSL's or steel depending on your level of comfort with steel. I built a bridge using steel and we bolted 3x12 PT to the top and nailed 3x12 boards across for the deck. 2x would work but the span should be engineered. THe span tables from the manufacturers should be all you need. Look on the timberstrand, georgia-pacific, or who ever supplies engineered lumber to your local lumber yard to see them. THe lumber yard salesman should be able to steer you through this as most of them have computer programs from the mfgrs of the engineered lumber that enables them to size joists and determine spacing for residential jobs.

BTW, make sure to pour some solid (thick and below the frost line if there is one where U live) footings and head walls with anchor bolts. Is there any chance that the stream could wash the bridge out during high flow? Just thinking. Good luck.

[Paul Downes]

Rob, here is another bridge design to consider.
http://pacifi.ca/home.html
this design looks really interesting and somewhat simple to build. I have been considering building one on a friends property so he can get into his woods across a creek. It's a perk for letting me hunt his property.

[Dick Strauss]

Rob,
A suspension bridge might be the least expensive route. In addition, the suspension cable can also be used as a hand rail. Many farmers (who allow snowmobiling across their property) have allowed the snowmobile assoc to build these across the rivers and streams. I know of many that span 100' or more across the Winooski River in Central VT between Burlington and Montpelier. Obviously if they can be built to span 100' and hold a snowmobile alolng with lots of snow/ice, they should be safe for a few people at a 24' length.

[Roger Bell]

If this is to be a footbridge for your children, stop and take the time to develop an adequately engineered design. This is especially the case if the creek is such that structural failure might result in drowning. If this is the case, then this is not a question that should be directed to woodworkers or building contractors, but to a licensed structural engineer.

You absolutely must factor in snow load for your area as well as the dead load of any decking or railing. You might also consider adding a handrail on at least one side. Resist the temptation to cobble something together. Before everyone starts rolling their eyes and muttering, I suspect that a single hour of consultation with an engineer will likely get you what you want as long as you keep it simple.

If there is a National Forest near you, contact their engineering people. They specialize in constructing pedestrian footbridges. Their structural engineers will likely know the snow loading and will likely have quite a few stock plans available incorporating a variety of materials, including logs, dimensional lumber, etc. If you ask them nice, they might be willing help you refine your ideas and come up with something that is consistent with simple and cheap.