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”.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. 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.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.
There are precedents. In Richmond, VA the Belle Isle Bridge over the James River hangs from the underside of US1. In Milwaukee, WI the Marsupial Bridge is suspended beneath the Holton Street Viaduct over the Milwaukee River. 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.)