As bicycle makers keep pushing the envelope on how many bike parts can be made out of carbon fiber to shave a couple grams off mega-high-budget bicycles, Seattle is looking to use similar technology for its bridges. Specifically, the city is looking into a carbon fiber design for the Northgate walk/bike bridge over I-5, which poses some engineering challenges for bridges made of standard building materials.
The bridge remains underfunded after a failing to win a Federal TIGER grant. SDOT and Sound Transit have each pledged $5 million to the project, but early design cost estimates suggest that $15 million more is needed to make it happen.
Without a walk/bike bridge, the number of homes and destinations within an easy walk or bike ride to the planned Northgate Link Station will be dramatically reduced. This includes North Seattle College.
Because I-5 is slightly elevated at this point and Interstate rules require ample clearance, the bridge will need some fairly dramatic approaches to provide an accessible and easily-bikeable ramp. The crossing of I-5 is also fairly long because the highway is so wide in North Seattle.
But these are engineering challenges that the city needs to overcome if Northgate is ever going to be a walkable and bikeable neighborhood. So whether or not a carbon fiber bridge proves to be the best option, it’s great to see the city exploring creative options to reconnect the community and provide access to transit.
Now the city and Sound Transit just need to come up with a funding plan to make sure it actually happens, including a backup plan if they can’t win a TIGER grant next year either.
The proposed pedestrian and bicycle bridge over I-5 at Northgate – linking the North Seattle College on the west with the bus and (future) light rail transit center on the east – has to be pretty high for vehicles on the freeway to pass underneath. That height (about 40 feet above 1st Avenue NE) makes for a looong approach ramp, over 1500 feet, most of it up in the air.
Traditionally these bridge types are steel, and that is what the design codes reference, but SDOT’s team is considering the possibility of using carbon fiber – the stuff that Boeing uses in the 787. Carbon fiber is ten times as strong as steel at less than a quarter of the weight which enables longer spans, smaller foundations, faster construction and less traffic disruption. Andy Bridge, Director of Research and Development for Janicki Industries, says other advantages include reduced visual impacts due to a thinner support structure, easily formed organic shapes, and lower maintenance costs.
The SDOT Northgate Pedestrian and Bicycle Bridge team will be considering many factors – principally safety – in making design decisions, but is excited about the potential of new materials and methods. This is just one way in which SDOT is seeking to take advantage of innovations in design to reduce costs and provide great service.