The Mad River Path Association (MRP) manages a series of trails in the towns of Warren, Waitsfield, and Fayston. The trails, nearly 10 miles in all, range from the flat, pastoral Mad River Greenway along the bank of the eponymous river to the steep Vista Trail that gains an overlook behind Warren School.
Last year MRP removed a failing bridge on the Millbrook Trail, which runs 2.1 miles from the Mad River Barn to Tucker Hill Road. The missing bridge breaks the trail in two for everybody except those hardy hikers and mountain bikers willing to ford a stream. Before proceeding with construction of a replacement bridge, MRP needed a Professional Engineer to approve a new design.
In this case PERCH did not design the bridge – an architect did. But PERCH did what an architect is not qualified to do, which is to verify that the design is structurally sound. The starting point was a set of sketches by the architect, Charlie Hosford, which show the bridge from several perspectives and label the materials and dimensions. Conversations with Charlie helped PERCH determine what was needed: an analysis of the structural components of the bridge, and a design for key connections.
The primary loads on the bridge are its own weight (dead load), pedestrians (live load), snow, and wind. According to NCHRP Guide Specifications for the Design of Pedestrian Bridges, the standard live load on a footbridge is 90 pounds per square foot, which equates to adults standing shoulder-to-shoulder across the entire span. This load case might occasionally happen in a city park, where pedestrians might crowd onto a bridge to watch fireworks, but it’s ludicrous for a rural trail. MRP proposed that they could post the bridge for a load limit and bypass the NCHRP provision, which in any case is a guide rather than a legal requirement. This allowance makes a slimmer (and cheaper) design possible.
PERCH confirmed that the decking and girders are adequate as designed for a live load of 30 pounds per square foot, or up to 25 persons spaced equally across the bridge. PERCH also issued a connection design for the girder splices, using Simpson metal plates and a specific nailing pattern. An Engineering Report was submitted to the Town of Fayston, and I’m proud to report that they issued a building permit right away. Construction will proceed this summer.