Imagine having to install a new bridge in a rural area where there isn’t a lot of room for heavy equipment, like cranes. Imagine the location being so out-of-the-way that there isn’t an easy way to detour traffic around the construction site. Between the two concerns, you suddenly have a big problem replacing an old bridge in a manner that is efficient and as convenient as possible. Enter ‘bridge-in-a-backpack’.
The Washington State Department Of Transportation (WSDOT) found themselves in just such a position in early 2020. They were looking to replace an aging bridge that sat on top of a narrow concrete culvert. Not only was the bridge in need of some TLC, but engineers also wanted to get away from the culvert design in order to make it easier for fish to traverse Loutsis Creek.
Apparently, the culvert was narrow enough that it limited the movement of steelhead, coho, and other species in the creek. The new bridge was to dispense with the culvert design in favor of a more open span that would in no way inhibit the flow of water. Engineers made it happen with a bridge design relying heavily on carbon fiber tubing.
Concrete-Filled Tubes
One of the big problems that engineers faced was building a temporary roadway so that the old bridge could be completely removed. Building that roadway and bringing in the heavy equipment necessary to install a conventional bridge just wasn’t doable. So instead, they turned to the bridge-in-a-backpack design.
What is bridge-in-a-backpack? It is a concept based on the idea of building parts off-site, shipping them in, and then assembling them in place. The major component in this particular bridge design was a group of carbon fiber tubes designed and fabricated off-site. They were to provide most of the the support for the deck above.
On-site, they were hoisted and put into place using a forklift and straps. The installed tubes were then filled with concrete for extra strength and rigidity. Workers were able to construct the entire support structure in just four hours, as opposed to days had they poured traditional concrete supports.
Engineers did not have to bring in heavy cranes either as the empty tubes were light enough for forklifts to handle. This made them easier for workers to install as well. And because carbon fiber is stronger and stiffer than steel, there were no worries about their ability to support the bridge deck once filled with concrete.
Easy to Make
The bridge-in-a-backpack concept is brilliant on many levels. It allows for rather quick bridge building at a comparable cost. It relies on composite materials which, on average, offer a longer lifespan and fewer maintenance requirements compared to concrete and steel. The icing on the cake is how easy it is to build bridges using modern composites fabricated in a factory setting.
The carbon fiber tubes can be made in just about any modest commercial fabrication shop, according to the engineers at Salt Lake City’s Rock West Composites. And thanks to technologies like filament winding and pultrusion, fabricating can be automated. This brings the cost of producing carbon fiber tubes down considerably.
Next, bridge components can be easily shipped to the construction site on trucks. The materials are light enough to be handled with forklifts, eliminating the need for the heavy equipment typically associated with steel beam construction.
Steel and concrete have enjoyed a long and successful run in infrastructure. But their day has passed. Today, it is about composites like carbon fiber. These are the future of infrastructure. Just ask the engineers at WSDOT.
