This bridge design is for driveways that must go over a creek. Its simple design features concrete foundations, steel girders, and a wooden floor.
Planning for a driveway with a bridge over a creek
Sometimes there is no option except for a driveway to have a bridge that spans over a creek. The first step is to contact the Army Corps of Engineers and find out what regulations govern the site you wish to build on. More often than not, this is little more than approval permit which grants permission to build.
It is wise to consider the spans for the girders when deciding the exact location for the bridge. The narrower the creek, the shorter the spans for the girders. This also reduces the amount of wooden flooring it will take. The only downside to this is that during flood conditions, the water travels faster where the creek is narrower. To combat this, the bridge should be built as far above the flood level as possible.
Designing the bridge can be as easy as adjusting the drawing below to fit the dimensions for your site. Beware with this however, without an engineer much of the design will be guesswork. Know that a loaded concrete truck is going to weigh in at about 80,000 lbs. This is a considerable load that should not be taken lightly. It is far easier if there is an alternate route for heavy trucks, such as driving through the creek. This way the bridge will not have to be built to support such great loads.
While there are a number of options to consider when designing the foundations, using poured concrete walls is the best. The walls should be at least 12″ thick, as wide as the bridge, and a height to allow for the girders to rest on top of them. The walls must be poured directly on top of bedrock. If the bedrock is very deep, then that opens up a whole new set of problems. An engineer must be consulted if this is the case.
The foundations must also be anchored to the bedrock to reduce the chance of the bridge washing downstream during a flood. 3/4″ holes should be drilled into the bedrock at least 8″ deep. The reinforcing steel bars should be driven in tightly into the holes. The more of these pins, the better.
If the steel girders are more than 12″ wide, then it will be wise to adjust the foundation forms so the girders will rest inside of notches. This along with wedge anchors will hold the girders well.
It is obvious that the longer the span of the steel girders, then the wider they will have to be. “I” beams are the best choice. There shape makes them naturally resistant to twisting during installation, and under heavy loads. “I” beams such as these can sometimes be found at recycling centers at a fraction of the cost of new ones. It may be worthwhile to check around before spending a bunch of unnecessary money.
Consider the width of the wheels for the vehicles which will be crossing when spacing the two girders. It is best place them in a manner so that for the most part they are directly beneath the wheels. Know that very large trucks have a wheel width of around 8′ 6″, pickup trucks around 6′, and compact trucks and most cars about 5′. This becomes far more important when using a single layer of wood flooring; especially when it begins to age.
Now comes the wooden floor for the bridge. It is wise to use a minimum of 3″ x 6″ oak timbers. Bolt each board to the girders with a minimum of 3/8″ carriage bolts. For an even better result, install more planks for the wheels to roll on. This will make for a much smoother ride. Variable thicknesses in the oak boards will otherwise make for a rather bumpy crossing over the bridge. Some folks believe in treating the oak boards with a mixture of used motor oil and diesel fuel. This will enhance the lumber’s ability to repel water and insects. It also gives it a rather attractive stain.
In the end, don’t forget what this project is all about; providing a safe, reliable, and long lasting means for the driveway to cross a creek. Remember that no matter how the bridge is built, it is still subject to mother nature. The wooden floor will decay over time, the steel girders will corrode, and the concrete will eventually crumble, but if maintained properly, none of this will happen any time soon.
Bridges are in effect a symbol of our modern technology. They save millions of gallons of gasoline each day along with countless hours on the road. Whatever your advantages will be for building a bridge, don’t forget to make it look good!