Pro Walk Pro Bike Pro Bridge – Part 3: The Glenwood Marsupial Bridge

Part 3 of the series leading to next week’s Pro Walk Pro Bike Pro Place 2014 expounds on a bold idea: impregnating the Glenwood Bridge with a bike/ped catwalk – such an addition has been called a “marsupial”.

Glenwood Bridge (pghbridges.com)

Glenwood Bridge (pghbridges.com)

The Glenwood Bridge, built in 1966, carries Second Avenue (PA885) over the Monongahela River at mile 5.9 (from The Point).  The Great Allegheny Passage (GAP) flanks the river’s south bank.  At the north end, in Pittsburgh’s Hazelwood neighborhood, the Duck Hollow trail dead ends where the bridge crosses railroad tracks.  In the works is a plan to extend the Eliza Furnace Trail through Hazelwood to Duck Hollow.

Glenwood Bridge (Google  Maps)

Glenwood Bridge (Google Maps)

Currently the only proper trail crossing upstream of the Hot Metal Pedestrian Bridge is the wonderfully rehabbed Riverton (former railroad) bridge at McKeesport.  Consultants for MOVEPGH, the city’s transportation plan, recommended a bike/ped crossing at the Glenwood Bridge.  (I believe the trail network would also benefit from a trail-to-trail river crossing at Homestead connecting to Frick Park and Squirrel Hill.)

The existing Glenwood structure is essentially a highway bridge, with a narrow sidewalk on the downstream side, and virtually no connections to the river trails.  But the structural configuration appears to present an opportunity: build a pathway as a lower deck.  Build a marsupial bridge.

In a nutshell, the existing bridge is referred to as a “deck-truss” because the deck (roadway) is supported by trusses – two trusses – one at the upstream edge of the deck and one at the downstream edge.  The two trusses are connected by diagonal and horizontal cross framing.

When looking through the cross framing from one end of the bridge towards the other, there is a clear line of sight.  I contend that it is possible – even feasible – to construct a pedestrian bridge through the cross frames.  A Glenwood Marsupial Bridge very well may be the most cost-effective way to span the Mon anywhere between Southside and Homestead.

Looking through the cross frames (photo by author)

Looking through the cross frames (photo by author)

Additional to the marsupial structure itself, the project would require access ramps at the north and south approaches, and a new bike/ped bridge over the railroad at the north end.  The south ramp (to the GAP trail) would be a tall structure and would constitute a significant portion of the cost.

North Approach (Google Maps)

North Approach (Google Maps)


South Approach (Google Maps)

South Approach (Google Maps)

There are precedents.  In Richmond, VA the Belle Isle Bridge over the James River hangs from the underside of US1.

Belle Isle Bridge (photo by author)

Belle Isle Bridge (photo by author)

In Milwaukee, WI the Marsupial Bridge is suspended beneath the Holton Street Viaduct over the Milwaukee River.

Milwaukee Marsupial Bridge (ladallman.com)

Milwaukee Marsupial Bridge (ladallman.com)

Happily, a group of civil engineering students in the University of Pittsburgh’s Swanson School of Engineering are considering the Glenwood Marsupial Bridge as part of their capstone design project this fall.  Dr. John Oyler teaches an excellent senior design course that tackles real infrastructure problems in the Pittsburgh area.  In the past they designed structures related to the Hot Metal Bridge, Davis Avenue Bridge, Allegheny Commons Footbridge, and a Montour Trail crossing.

As the students’ project advisor, I would guide them as they investigate questions such as:

  • How will the Glenwood Marsupial Bridge work spatially?
  • Can the overall structure support the required additional loads?
  • Can local cross framing members support additional loads?
  • Can the added weight be minimized by using a fiberglass-reinforced polymer deck (instead of concrete)?
  • How would this crossing benefit the trail network and neighborhood connectivity?
  • How much will it cost to build the Glenwood Marsupial Bridge and accessory structures?
  • What questions or concerns might PennDOT have about the idea?

(This post was conceived, composed, edited, and published while riding Amtrak’s Pennsylvanian from Philadelphia to Pittsburgh.)

5 thoughts on “Pro Walk Pro Bike Pro Bridge – Part 3: The Glenwood Marsupial Bridge

  1. Pat,

    I am part of the senior design team at Pitt considering this option. While we are planning to do at least feasibility study on it, this project has not necessarily been selected for our final design. We are also considering doing a rehab of the Carrie Furnace Hot Metal Bridge as a way of crossing the Mon just East of Homestead. With plans having been proposed to bring the Duck Hollow Trail down to the historic Carrie Furnace site, this option seems equally or more feasible and practical (as the Glenwood Marsupial Bridge) to us. We would love to know your thoughts. Looking forward to working with you on this project!

    -Don Cunningham

    • Hi Don – thanks for the clarification – post updated.

      I am thrilled that you are considering these projects. Definitely spend your time working on whatever is most interesting to you and the best learning opportunity.

      In terms of trail network connectivity, I think a bike/ped crossing right under the Homestead Grays Bridge would be ideal. I agree that rehabbing the Carrie Furnace Hot Metal Bridge would be equally (if not more) cost-effective than Glenwood, and really beneficial to the trail network.

      So I guess my assertion still holds, that the Glenwood Marsupial would be the most cost-effective crossing between Southside and Homestead. But of course Carrie Furnace is not far upstream from Homestead.

      You may already have this link to some mapping studies by others:

      http://tinyurl.com/cch2ldz

      Looking forward to working with you too!

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  4. Hi Patrick. I just discovered this today. Good stuff! Did you or the Pitt students write up the conclusions of their study?
    About the ramp at the south (Hays) end of the Glenwood Bridge. When I looked at this a few years ago I was thinking that the best location for the ramp is not at A (near the GAP trail) but at B (where the truss ends, onto Sandcastle property). See picture: https://flic.kr/p/PZuftm

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