More design information

The design criteria of the bridge was that it be “cute”, constructible by the two of us, look like a bridge (instead of like some long, skinny deck), be one complete open span (no supports in the middle) and be made of wood. Towards that end we began researching different kinds of small bridges. A search of the internet determined that we could buy small bridges for gardens but they were no longer than about 12 feet. As the spot we had chosen for the bridge would require a span some 20 feet long we determined they would be unsuitable. Additional searching on the internet found a man in South Africa who had plans for bridges up to 50 feet using just 2 x 4’s. A more detailed look at his plans determined that the slope of the arch bridge was too steep (4 foot rise in an 18 foot span), so we continued looking. [http://www.pacifi.ca/2040.html]

We decided that an arch bridge looked good and so for awhile we concentrated on just looking at arch bridges. We found a gentleman from Wisconsin who had blueprints of arch bridges for sale and we bought one. This was a mistake for two reasons. First, the dimensions of the two main support beams were undersized (this was determined first by comparing the thickness to the required span and second by a nice lady (Jeanine) in Wyoming who an Auto-CAD program) and secondly by the complete lack of any plans for actually making a beam of the required dimensions in an arch. Fortunately this mistake only cost us $35.90.

We looked at arch bridges on the web (again) and found several companies that made bridges for more industrial purposes like golf courses and parks. An example of this would be York Bridge Company at http://www.ybc.com and the Timberweld Company at http://www.timberweld.com. I called Timberweld and asked them to quote me price for a glue-laminated arch bridge with a 20 foot span. Jeanine recommended some values for live load (people - 45 pounds per square foot) and dead load (wood, snow, etc - 15 pounds per square foot) along with recommending the dimensions for the main support beams she quoted me a delivered price of $2,900.00. Following my long disappointed silence, Jeanine then quoted two lower prices for a slightly different design ($2,600.00) and a slightly different delivery method ($2,300.00). She pointed out that rather than using the more expensive Douglas Fir or Alaskan Cedar they would use treated Southern Yellow Pine. Thanking her profusely for all of her help I moved on.

Giving up on the arch bridge we moved on to consider Kingpost bridges. Kingpost bridges were most commonly used in covered bridges and we could find many pictures of covered bridges. What I could not find were plans for Kingpost bridges nor could we find any detailed construction guidance. Ditto for modified Kingpost bridges and even Queenpost bridges. We grew frustrated again and once again moved on.

We decided on a simple steel beam supported bridge. We were woried that the weight of the beams would be so great the two of us could not place the beams by ourselves. We calculated that a 4" W-flange beam of 13 pounds per foot would have a maximum deflection of 3/4" [code allowable 1"] and a natural frequency of 19 hertz. We then moved forward on the installation.