Saturday, April 27, 2013

Bridges with wings...?

Over the past few years I've designed and built a number of bridges for a local park. I'm often asked what purpose the funny little wings I design into them has on the bridge, (other than they just look kinda cool.)

Well it it really is a kinda major, but simple structural improvement.

The #1 issue I've notice when using 2X materials to build bridges out on trails in my 40+ years of hiking and biking is the joist roll out. It usually happens long before the materials have rotted and could be used for many more years. Joist roll out is caused but the weight traveling over the bridge twisting the joist out. A joist under weight doesn't just bow down but actually twists side to side and over time the screws and/or nails flex, bend or just break off allowing the joist to roll away and the bridge to sag and fall apart.

Many people add the cross brace board between the 2 joists, but the issue is the screws or nails are going into end grain of the brace, which doesn't hold well and will usually pull out rather quickly, but, As the image above shows, by making a slightly longer deck board and adding the "wing" piece underneath in conjunction with the inner cross brace the joist is trapped in place making it near impossible for it to roll out or in. Also the whole structure will be a good bit stiffer because as I mentioned earlier when a bridge bounces as you walk/ride across it the joists are actually rolling side to side much more than they are sagging or bowing under your weight. With the wings they can't twist or flex as easily, making the whole system stronger and stiffer.

By adding a few pieces of scrap material you can honestly add many, many years of life to the bridges that you build.

1 comment:

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