I'm building a bridge and I need some Advice.... 1

[Cog]

A freind of mine recently asked me to build him a foot bridge at his acreage. I'm a Gear, so I don't really know the first thing about it. I guess people just assume that as engineers we can do anything and everything. Anyways, it sounded like fun, so I took him up on the offer.

Here's the design parameters and environmental constraints:

- Needs to span a creek approx. 25 feet wide with up to 2 feet of fairly fast water flowing under it
- Creek bed is composed mainly of rock, with clay/sand banks
- Needs to be a low impact structure, easily removed if needs be
- Lastly, out of economic concerns, the brigde will be built with locally harvested logs (timber bridge).

Perhaps due to my lack of expertise, my first instinct was to just flop a big tree across the creek and call it a day. Simple, at least. But my freind wishes something more elegant.

So this is where I need some help from you guys. What kind of bridge should we build, and how do we do it? And is there anywhere I can get info on this sort of thing? Big questions, I know, but any help is appreciated.

Thanks for all your help,

Greg.

 

[Qshake]

You were right on with felling a tree over the creek and moving on! This is a task and by using local timber, its even more daunting.

A simple foot bridge huh? Well, make sure that some time in the future your friend won't see the need to drive across it whether in a golf cart or semi. So be mindful of the loading.

Lastly, there are many guides out there on timber bridges and projects such as these. In fact, I'm almost sure there is a website with information on backyard projects like this that will be simpler and easier to understand. Some of that information would be particularly appealing since you'd like to work with insitu timber and process it yourself.

Having said that, there are some good references on timber structures and even timber bridges. Though those that come to mind are for driving highway vehicles across. Design of Wood Structures by Breyer and Structural Design in Wood by Stalhnaker.

 

[georam]

For timber bridges, etc, check the following sites :
USDA Forest Services - Wood Transportation Structure Research

http://www.fpl.fs.fed.us/wit/

Canadian Wood Council

http://www.cwc.ca/english/index.htm

Some basic info about bridges :

http://www.matsuo-bridge.co.jp/english/bridges/basics.shtm

or check with bridge website for various links :

http://www.bridgesite.com

I think there was also info about wooden bridge in ASCE Civil Engineering recently. ... or about US National Wooden Bridge Competition.

Hope this helps

 

[Cog]

Thanks for the replies and the links! The Canadian Wood Council link was a particularly good springboard. I'm having trouble finding info on small scale DIY stuff; everything's commercial and big.

Loading was, and is, my biggest concern. I'm going to make it as narrow as practical, to prevent big things (quads, horses, trucks...) from crossing.

I've come up with a design, and I just want to run it by you guys to make sure I've not made any gross oversights.

The general idea:

A basic beam bridge. Essentially, two trees of suitable size are felled, limbed, and cut to proper length. They are dropped across the creek with about 3 feet between them. Slats are nailed on to fill the gap between the logs and provide a walking surface. Done and done. Nothing earth shattering, but my profs always tell me to keep it simple.

Here is the basic assembly process:

After the trees are felled, limbed, and cut to size, a strip is cut off the top of the logs. The strip (kinda like a chord on a cirle) has a thickness of 1/3 the diameter of the log.

This is done to both logs, so there are two logs with a flat side, and two long strips of wood. The long strips of wood are then cut into 3' lengths to provide the slats to walk upon.

Next, the logs are set flat side up on the ground. Two slats are tacked 1/3 of the way from either end. So if the the logs were 24 feet long, you would have a slat tacked 8 feet in from either end, holding the logs parallel. Then two more slats are added directly underneath the first two. Holes are drilled and bolts are run, effectively turning the four slats into two big compression clamps. I think this will provide adequate torsional rigidity, but I could be wrong (?). The brigde frame is then towed to the bridge sight, dragged into place, and settled in. The remaing slats are screwed or nailed in place, and the bridge is done.

This is the best design I could come up with. It is economical in terms of materials, with almost zero waste. It should take only a day or so to complete. And it will hopefully have a decent service life.

However, I'm a just a lowly engineering student (3rd year), so if my youthful enthusiasm or lack of experience has caused me to miss something, Please let me know. I also welcome any othe ideas or feed back on the design in general.

Thankyou once again for your help,

Greg.