Building a Bridge...help

Greetings jcobbler and welcome to the community!

Building a bridge huh?

Wow what a project:smileyhappy:

Well this is one of those projects I can say that I haven’t worked on yet although I do have plenty ideas that I can appoint  i haven’t build a bridge yet.

I did build decks, docks, pergolas, roofs, balconies and more and I do have an idea what it would take to build a bridge on a SMALL scale.

First what is the purpose of the bridge you are planning on building?

If this is something outside of the normal residential use dead load (40-60lb a SF) and outside the safe heights I would recommend going over this with your local building department.

If this is something you are adding as a decorative landscape feature over the swale, pond or stream I have some ideas I would  consider  implementing.  

These are two most sensible methods i can think of;

1.  Pour continues footing 3-4 feet long and have you joist rest on the footings(first  picture on a smaller scale) or  

2.  Pour four separate posts, two on each side and have a joist on top of the beam connection.

Footings should be built according to your local requirements and soil conditions. Here in Chicago we are usually asked to have our piers  42" deep bellow the frost line and  back filled  with 4 " of gravel. Contact your local building department for the code requirements in your area. Assuming that this is just a small bridge that would be essentially only an extend walkway I would say that concrete pier or joist on top of the beam method would be adequate.

Tricky part with this method is to get all post at the same LEVEL. To accomplish this I would recommend using sonotubes and a water or laser level.

Once you get all four post at the same level I would attach a ABA type of post base on top of each post to keep wood away from concrete although pressure treated wood is chemically treated to resist rot and decay they still need to be separated from direct ground (concrete) contact.

Use double beams in between posts and attach them to the base using hot dipped galvanized fasteners.

With beams installed you can now install your joists and BRIDGE perpendicularly from double beam to double beam. Note that blocking should be used in between joist especially at the railing post connection detail.

Floor joists in residential construction are usually spaced 12", 16", 19.2” or 24" inches on center, using a

 SPAN CALCULATOR you can determine spacing, deflection, and size of the joist needed for this project.

For the railing supports I would definitely use 4x4 posts.

Depending on your local code these are usually 42" high and bolted to the joist with 1/2 Ø +galvanized carriage bolts. I would recommend for this project to have three equally spaced post on the each side. For the top rail I usually use 2x6 materials 2x4 nailer and 2x4 bottom plates.

Hope this helps and please post some before and after picture.

George