SDOT considering a carbon fiber walk/bike bridge at Northgate

A carbon fiber bridge over I-5. Image from SDOT.

A carbon fiber bridge over I-5. Image from SDOT.

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.

From SDOT:

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.

About Tom Fucoloro

Founder and Editor of Seattle Bike Blog.
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18 Responses to SDOT considering a carbon fiber walk/bike bridge at Northgate

  1. Cheif says:

    Crabon bridges are hecka fast but they never call out when passing at speed on the burke.

  2. Al Dimond says:

    Truly a shame we can’t go under…

    • virchow says:

      I think an underpass would become a pike-place style shooting gallery in about 0.1 day. Keeping it clean and family friendly would cost beaucoups money. I could be wrong (the I-90 bike trail doesn’t seem to fair too poorly) but that’s my suspicion.

      The carbon fiber bridge does look pretty cool! I hope something comes out of it!

      • Gary says:

        Hmm, depends on who’s packing… but I for one would not like to use an underpass here.

      • Al Dimond says:

        The engineering challenges of the bridge all have to do with going over the top of a freeway that’s already much higher than street level. An “underpass” would actually be close to level with the land on either side of the freeway — turning the freeway into the bridge it always should have been! Streets that run under bridges exist everywhere there are bridges and aren’t generally bad places to be.

        Converting part of a freeway as big as I-5 from an embankment to a bridge would be a really huge project. But then climbing up to the level of the freeway embankment cuts into the usefulness of the route a lot — the distance of a 1500′-long approach ramp for a 40′ climb doesn’t get you all the way to 92nd Street but it’s surprisingly close (a 2100′ run with about a 60′ rise).

        There’s really not much that can be done at this point, so our situation should serve as a warning to future urban planners and engineers. If you must build freeways, make sure they’re permeable! If you don’t, you’ll end up with access gaps that you can scarcely improve, even with a lot of resources.

      • Tom Fucoloro says:

        I think it would be cool to get an estimate on a tunnel, just so we can know how much money we’re talking about. But I have a feeling it will be a lot. Then again, the bridge ain’t cheap…

        Ugh, WSDOT really is the most logical source of funding for this project (though perhaps not the most politically-feasible). After all, it’s their big stupid highway that caused the problem.

      • jay says:

        Just need to put a bunch of art in the tunnel, then SAM can pay for 24/7 security. Of course they’d probably want to close it at night.

        I’m not an engineer, (though I play one at work) I wonder if longer spans and smaller foundations could really make up the cost of carbon fiber?
        “Traditionally these bridge types are steel, and that is what the design codes reference”
        Well, I guess compared to the engineering, the cost of the carbon fiber itself may be no big deal. At least one thing going for carbon fiber is that the 787 came in on time and ahead of budget! or am I thinking of something else? Maybe the steel and concrete second Tacoma narrows bridge? (oh, relax, I’m joking, the carbon fiber fabrication was only a small? part of the problem)

        In addition to the many millions that still need to be found, one other issue I see is that in one of these stories about a bridge it was written that Sound transit’s 5 million goes to either a bridge or other pedestrian/bike infrastructure. Either…not both, As a bicyclist, I think even a modest improvement to the 92nd overpass and access thereto would beat crossing a high bridge. Granted, that wouldn’t help people walking to the Community College much, on the other hand a bridge would cost something like five times as much as the entire existing Pronto bike share.

        Even with a bridge, once one is across, then where does one go? if ones final destination is very much north or south of the bridge one may not have gained much over (an improved) 92nd or Northgate Way, if one is going much farther west, one might like to ride a bike anyway , then on a bike, even if going due west, the jog to (an improved) 92nd or Northgate way doesn’t seem so very far out of the way. It’s not like crossing the Fremont bridge to get to Trader Joe’s from interbay. (for the record I doubt spending well upwards of 20 million on the Ballard bridge would be best use of finite/non-existent funds either)

      • Ben P says:

        What casual cyclist or ped is going to want to climb that high? The freeway is a problem, but I feel this bridge isn’t the solution. From an engineering perspective, factoring in the creek, would a 12′ wide underpass be more expensive than 20′ wide bridge?

  3. Ints says:

    Boeing is cranking out large crabon fiber tubes daily and flying them through the sky as 787s. WSDOT should look for a partnering opportunity with them to get some 787 fuselage blanks before windows and hatches are cut out and repurpose them for this bridge! It would be an awesome “Seattle” bridge.

  4. Gary says:

    Nice drawing, but it’s missing the usual cage that is added to keep people from dropping stuff on the cars below.

    And somehow I don’t see how using carbon fiber would lessen the cost…. (less weight, less supports?)

  5. Doug Bostrom says:

    I wonder if SDOT has considered that somebody with a framing hammer could bring down the bridge as pictured within a few minutes, given the selectively directional strength of carbon fiber? That is, if the truss shown midspan is actually doing anything, as opposed to being an annoying reminder of the practical rain roof that might be there if the bridge was not made of black gold.

  6. Kate Martin says:

    I made a site visit and took a look at the underpass idea – it would be separated, but not at all a tunnel because you don’t go below grade to access it, you’re on grade with the freeway above you – just like any other on-grade underpass. My only question was whether the light rail route actually precludes this solution? Can someone verify that? In addition to the North Seattle College – , Neighborcare is planning a very comprehensive healthcare facility to be built on the Seattle King County Public Health site just north of there. It seems like a multi-modal repair of the grid would be a more comprehensive solution rather than only peds and bikes. It’s such a huge gap in the street grid from 92nd to Northgate Way.

    • Al Dimond says:

      Did you make a site visit with an engineer? I’m not a structural engineer (only a decorative engineer), but… the freeway is built on an embankment, which is what holds it up; therefore it doesn’t have support built into it like a bridge. There may be a good way to excavate what would amount to a tunnel through the embankment without modifying the structure of the roadway, but I wouldn’t assume that’s the case.

  7. RossB says:

    I appreciate the ingenuity in what SDOT is considering. But one statement stands out:

    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.

    OK, but let’s take a step back now, shall we. As I see it, a bridge should do three things:

    1) Provide pedestrian access from the new train station (which will be substantially above the the ground) to the other side.
    2) Provide wheel chair accessible access to the other side as well.
    3) Provide bike access to the bridge.

    Ideally, you would have a big ramp, so that bikers could cruise over to the other side. But that isn’t really essential. Bikes and wheel chairs could ride an elevator, then go over to the other side. Less than ideal, but a lot cheaper. Is it worth it to spend millions of dollars so that bikes can have a nice smooth ramp? Or am I missing something here?

  8. Gary Anderson says:

    The bridge could use the approach ramp design of the Golden Ears bridge crossing the Fraser river at Langley. Rather than a long linear ramp the approach is configured in a spiral fashion that’s very interesting to ride or walk. Since the ramp is stacked, it takes up much less land area.

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