Bridges – Joist Sizing

Bridge Joist Sizing

Have you ever walked across the floor of a new home and bounced up and down? The reason this happens is that the floor joists are flexing. It is natural for them to do this and is even permitted by the building codes. Just because the floor joists bounce doesn't mean they are going to crack and collapse.

If you want a stiff floor, you need to make sure the floor joists are sized correctly. Engineers have developed tables that tell you what types of wood floor joists are the strongest and produce the least amount of flex or bounce. You can get these span tables from any high quality lumber yard. They are not easy to understand so make sure you have someone show you how to use them.

Bigger is Better

Wood joists are simply beams. If you want a beam to have minimal deflection over a given span, then simply increase the height of the beam. For example, if you install upright a ten foot long 2 x 6 over two sawhorses that are about 9 feet six inches apart and then suspend 2 sacks of concrete from the middle of this beam, it will surely deflect or sag and might even crack. Substitute a ten foot long 2x12 and perform the same test and the joist's deflection may not be visible to the naked eye. Long bridge spans simply need large joists.

Suggested Spans

If you are going to build a small bridge that spans perhaps 8 feet or less, I would use 2x8's that are placed 16 inches on center. A span of 12 feet would require 2x10's and spans between 14 and 18 feet would require 2x12's. Anything over 18 feet would require 4x4 or 6x6 posts mid span to cut the actual span in half.

If you are thinking of building any bridge with a span greater than 20 feet, you better get a structural engineer involved, plain and simple.

Steel is a Great Substitute

A good friend of mine owns a Christmas tree farm. A creek runs through the property. He wanted to build a pedestrian bridge that would also support a small tractor. The span between the support piers on either side of the creek was about 24 feet. I suggested that we build it using two steel I-beams that were 14 inches high and weighed about 12 pounds per foot. An engineer specified the beams and the completed bridge is quite handsome. We had holes punched in the top flanges that allowed us to bolt treated 2x6 plates. We simply nailed treated 2x6's to these plates to make the bridge deck. The entire bridge took a little over a day to build. Keep steel in mind for your long bridge!

Companion Articles:  Bridges in Your Backyard, Bridge Building Tips, Bridge Planning