We've been asked to design a small covered bridge, across a ravine, as part of a landscape plan. The owners would like it to be of timber and look pretty traditional. Span from bank to bank is roughly 20 feet, so not big. We could make it as a simple span I'm sure, but it seems like it would be more fun to do a truss of some sort. I'm imagining a parallel cord truss, with the lower cord supporting the bridge deck, and the upper cord being 42" above the bridge deck, so as to double as a handrail. We'll need to run posts up to support a gable roof of ~ a 4 pitch. Should have 4-5' of walkable deck between the trusses.
I'm looking for ideas on how to configure the structure, and examples of similar bridges you may know about. I will likely cut and install the frame, should the owners bite. Thanks!

Hey,
Most authentic covered bridges use trusses for the entire heigth of the side wall. The short span could easily be done with a king-post truss.

Here are some links to king post designs, and one of a TF, 51ft, multiple king-post truss bridge that I built out of reclaimed barn beams.

http://www.nycoveredbridges.org/page41.html
http://www.tfhrc.gov/structur/pubs/04098/04.htm
http://www.coveredbridgesite.com/ny/forge.html
http://www.coveredbridgesite.com/ny/tappan.html

http://i453.photobucket.com/albums/qq254/crwtimberframe/Coveredbridgewinter.jpg

Rooster

Hey Craig- Thanks for the info!
One of the parameters is for users to be able to take in views from the bridge while crossing. One of the designs we've considered is a lattice truss, but one that's only 42" high. Designer (wife...) thinks the lattice truss a bit busy and prefers a more traditional handrail and spindles (which does go more neatly with existing buildings).
I'd like to play around with a king post design along with a shear keyed beam:

I'd use ~4X4 hardwood shear keys at ~16" o.c., let 1" in to each beam, creating a 2" space between the beams, which would also allow for deck bearing.
Anyone care to comment? Cheers!
Chris Koehn

What's the build date?

Hi Chris - Happy to help in any way I can.

Your skp doesn't suggest, but I take it your trusses are longer than the span , and the braces (bridgespeak – struts in TF-speak) fall over the the bed timbers that sit on the abutments? This is traditional and good design, keeping the portals and their gables and thereby the weather well away from the bridges underpinnings. Also gives you a direct load path for the moment imparted by the braces.

Don't see any relish, Bottom Chord beyond the posts, King beyond the Bottom Chord. Good bridge design is all about relish and airspace / airflow.

How will the Floorbeams interact with the Chords? I'd think four can be made to suffice, one either side of the King and one at either end, these carrying four longitudinal sleepers and those some maybe 2 ½” flooring. (More bridgespeak – It's a floor not a deck) Best to avoid running the flooring over the Chords, it inhibits airflow and the ability to clean dirt and cobwebs away and makes it harder to visually inspect them with regularity.

How will your Ties and Upper Lateral bracing be configured?

I'd think the Shear Blocks might be overkill for such a short span, (you'll have the posts and the braces double-dapping & keying the Chord Lams) but it's a traditional solution and a good one – Bottom Chord scarfs are the first thing I look for when I walk into a new to me bridge. I'm a student of Chord splices, but it's also the first outward manifestation of the bridge geek in me.

Think airspace betwixt the Chord Lams !!!

We've long sourced traditional hardware (squarehead bolts, cast ogee washers etc.) from KL Jack in Manchester and Portland, But here's a source for some out there on the WC - http://www.portlandbolt.com/projectresume/coveredbridgeprojects.html

The approach to cutting is a little different in bridgework. The forces and loads are constant, and crush is an issue not typical of most framing.

I'm gonna send you a PDF of a 60' ped bridge I designed, it's more truss than you need, but it's a good visual of how the various systems interact and that I think, will help as you design this one.

Hire Will T!