Six years ago the Civil Engineering Department at Pitt was contacted by the Hollow Oak Land Trust with a request for a preliminary design of a bridge over Montour Run, connecting Montour Woods with the Montour Trail. This quickly became a Senior Design Project, with our team charged with developing alternate designs in steel, timber, and fiberglass reinforced plastic (FRP).
We soon learned that the project had several unique requirements – the goal of maximizing the use of volunteer help, and a similar goal of minimizing the use of major powered equipment and its impact on the natural environment. Ideally the bridge would be shipped in pieces light enough to be handled manually, bolted together, and then manhandled into place.
The first step in the project was a major hydrological study of the Montour Run watershed to confirm there would be adequate clearance under the bridge to accept the runoff from a “one-hundred-year storm”. Although flow in the stream is modest during normal conditions, it is obvious from observation of its floodplain that the possibility of a major flash flooding requires the bridge to be at least twelve feet above the normal stream level. Another requirement was a geotechnical study to determine the suitability of the soil on both banks of the creek to support the loads from piers and the bridge. The students dug test pits to sample the soil, then tested the samples in our geotechnical lab.
Based on these studies it was possible for the team to layout the bridge and its approaches, permitting the structural engineers to proceed with their designs. The result of their efforts was a comprehensive design in each of the three materials, with each design satisfying the client’s desire to maximize assembly at the jobsite.
Our estimated initial cost for either the steel or the timber alternative was about $480,000; for FRP, $630,000. We also determined that the life cycle maintenance costs for the FRP alternative would be dramatically less than for steel or timber.
After submitting our recommendations to Hollow Oak, we heard nothing more about the project for five years. We then learned that they had acquired the necessary funding and that the bridge would be installed early this year. Sure enough, the Hollow Oak May 2022 newsletter announced a ribbon-cutting ceremony and the opening of the bridge on May 26. I was unable to attend that event, but did manage to visit and inspect the finished product recently.
I was immensely impressed with what I saw. The FRP portion consists of a 75’ main span over the stream, with a 20’ long approach on the west side and a 30’ long one on the east side. The main span is supported by tall concrete piers on each bank; both approaches are supported by cast in place abutments at the other ends. A 30’ long timber approach span, supported every five feet by a pair of eighteen inch diameter “Sono tubes” (vertical concrete cylinders) ties into the bridge on the west side; a similar one, 45’ long, ties into the east end. All told, the system is 200’ long, elevated sufficiently to survive flash flooding.
The inside width of the bridge is 5’-6”, certainly sufficient for foot traffic. While I was there, several bicyclists crossed in single file. The width of a pedestrian bridge is a major parameter in its design requirements. Regardless of the anticipated usage of a pedestrian bridge, the designer must assume that, at some point in its life, it can be subjected to “wall-to-wall” traffic. Consequently, for a truss bridge, the design load is directly proportional to the deck width. I always felt this requirement was unreasonable; then I saw the bridge over Forbes Avenue at Pitt jam-packed with students trying to get a glimpse of some celebrity passing below in a convertible.
The main span of the Hollow Oak bridge is supported by a pair of trusses, each five feet deep with posts spaced at five feet and “X-bracing” between them. The top and bottom chord members are pairs of ten inch channels (“C-shaped”) separated by 2” hollow square tubes as posts. The X-bracing members are also 2” tubes. Under the deck, at each post, are a pair of transverse channels with outriggers extending two feet beyond the trusses; diagonals from their ends to the top of the posts provide lateral stability for the trusses.
The deck consists of transverse 3 by 12 timber planks. A series of three inch channels cover the face of the trusses and serve as horizontal safety midrails, limiting the opening to nine inches. Connections throughout are stainless steel bolts and self-tapping screws. The resin in the FRP is an olive-green shade which blends nicely with the natural setting. As far as we know, this material has superior durability and will require very little maintenance for many years to come.
We have determined that individual bridge components were shipped to the site and that all assembly was accomplished there. It does appear that a short temporary access road was built at the east end, probably to permit access by concrete mixer trucks. Nonetheless there has been no significant permanent damage to the natural environment in the immediate vicinity of the trail.
This is the fourth case where a bridge for which our students prepared a preliminary design eventually has been constructed. It comes as close to being “near-real-world” as is possible. In every case, our work has been accompanied by a disclaimer – “for reference only” – acknowledging the necessity for a licensed engineer to perform final design.
It is interesting to compare and contrast our preliminary design with the finished product. Our final estimate for the FRP alternate is remarkably close to the actual cost reported in the Hollow Oak newsletter. I hope each member of our team has the opportunity to visit the site and witness the fruition of their efforts.