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Build Your Own Bridge

Construction How-To, Stone and Concrete June 28, 2011 Sonia


bridge lead

 

 

By Larry Walton

 

 

 

A bridge is one of those things that are often taken for granted until you don’t have one, especially if you live on a rural property and there’s a creek between your house and the county road. Jon Ford had plans to build a new bridge along with building a new house on his property, but plans for the bridge were made top priority when a wayward truck carried too much weight over the old bridge and it collapsed.

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Red Tape

Most bridge builds require considerable planning, especially when it is located over a waterway that is under the jurisdiction of several different government agencies. It doesn’t have to be a significant river for several governing bodies to claim control over the rights to build any kind of structure near the waterway. There are agencies that regulate flood control, water quality, fish habitat, etc., and each one of these agency’s regulations must be satisfied before the project can proceed.

Jon had to file a riparian landscape plan, file an application with the Oregon Department of Fish and Wildlife, deal with an agency called the National Marine Fisheries Service, submit a fish passage plan, get the approval of the Department of Forestry, and deal with the County Planning and Development Department. He had to certify a base flood elevation and hire an engineer to draw up the bridge plans as required by the County.

The local Fire District also got involved by requiring that the bridge hold a loaded fire truck (weighing 50,000 lbs.), that the approach angles to the bridge be within specifications, and that a turnaround was provided on the other side of the bridge. Ford already had plans to engineer the bridge to handle a 75,000-lb. loaded concrete truck for the construction of the house, so he made sure the other fire-related requirements were factored into the driveway plans as well.

Also added to the government agencies was FEMA, which presented an inaccurate map showing that the entire property was in the flood plain. Part of Ford’s permitting process was getting the map amended to reflect that his property was indeed not under water.

A variety of agencies meant a variety of concerns ranging from impact on water quality, on fish migration, on flood mitigation and on vegetation frustration. Just when Ford thought all the bases were covered, one of the agents decided that they would need a 20-foot bridge, which was a big change to what everyone had said all along would be a 15-foot bridge. That additional length of the bridge had potential to add thousands to the cost of the bridge in engineering costs, added materials and stronger beams.

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The main components of Ford’s bridge includes [1] concrete abutment, [2] steel I-beams, [3] pressure-treated 4×12 stringers, [4]pressure-treated 4×12 decking, [5] pressure-treated 2×12 runners and [6] pressure-treated 4×6 curbs.

 

 

Admittedly, there are good reasons for many of these regulations. In fact, I live downstream from the property, so my interest in protecting our stream was more than just a passing curiosity. I wouldn’t want it dammed up or contaminated and I certainly wouldn’t want to see his new bridge coming my way in the event of a record-breaking flood.

In the unlikely event that a government agency is not giving you specifications for elevation and span on your bridge project, make sure you are above the 100-year high water mark. A bridge that is built too low, or with a span too narrow, can cause a significant choke point during those winter storms or spring thaws that bring streams out of their banks. Debris can hang-up on a bridge, which can catch more debris and eventually dam up the stream, causing a great deal more damage than many people can imagine.

Eventually you’ll even need to address the logistics involved with making something so large that it is structurally designed to handle the stresses of vehicle traffic year after year. All of these considerations must be satisfied before the first shovel can touch the banks of the stream.

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A small stream such as this one may not seem like much of an obstacle to the casual observer, but a little local history can tell you that most streams can get very big when the weather gets really nasty.

 

Strong Foundation

Once all the concerned government agencies, community input, surveying and engineering aspects are completed, the actual construction of the bridge can begin. Just like with house construction, the sub-base for the foundation must be shaped and compacted first. For this bridge, which was designed with I-beams, the two foundation pieces that support the ends of the beams are called abutments.

The father and son team of Mike and Jon Ford poured each of the abutments in two parts on top of thoroughly compacted sub-bases topped with3/4 minus crushed rock. Using the surveyor’s base flood-elevation mark to set the top of the footing portion of the abutment, Mike and Jon built their forms to make the footing 3 feet thick and 2 feet high, which made it top out at 1 foot above the base flood elevation. They also formed 2-foot wing walls at 45degrees on each end of the abutment.

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A laser level is the primary tool to transfer the elevations determined by the surveyor to the actual form elevations on the bridge site.

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The project begins by establishing the elevations for both the top of the bridge and the bottom of the bridge supports that span the stream. Once this has been determined, an adequate footing structure for the bridge supports must be designed and laid out. The footings themselves must be engineered to have the proper mass and structural support to bear the bridge along with its expected loads.

They constructed the forms of 2x4s with 3/4-inch CDX plywood skins on the insides. Mike’s form design included a 2×8 base, which allowed for fine-tuning the locations and for staking the forms into the sub-base to keep them in position. Splices and joints in the base plates were offset from connections in the panels, which made the entire assembly stronger.