Fanny Appleton Pedestrian Bridge: From Base Technical Concept to Final Design

The Longfellow Bridge Rehabilitation Project is a design-build project, undertaking the rehabilitation of a century old multispan steel arch structure and its iconic “salt and pepper shaker” towers. Within the same project, sitting in the shadows of the Longfellow, sits a nondescript and inadequate pedestrian bridge, soon to be replaced with the Fanny Appleton Pedestrian Bridge, named after Longfellow’s wife. The Appleton, a 750 foot long structure, with 550 feet of elevated bridge including a 222 foot main span which mimics the arch spans of the Longfellow but with a very different and contemporary approach curving “ribbonlike” through existing trees. The main span, a spandrel arch supported deck made up of diverging 18 inch diameter pipe arches and HSS spandrel columns, was analyzed as a Vierendeel truss and the entire superstructure modeled in 3-D to capture stresses of steel erection through the temperature induced stresses of the jointless, curved superstructure. The architectural design also included steel Wye approach piers with an inset detail. This geometrically challenging form led the contractor, fabricator, and designer to choose a steel casting used within the steel pier assembly to create the unique detailing. On a daily basis, the route is a heavily traveled multi-use path for walkers, runners and bikers but on the 4th of July, it carries shoulder to shoulder crowds from the city to the Esplanade for the televised Boston Pops Concert and Fireworks. The project’s goal was to mitigate pedestrian induced vibrations without the use of tuned mass dampers while still meeting the architect’s aesthetic intent. The bridge’s function and aesthetics led beyond vibration criteria of the AASHTO LRFD Design Guide Specifications for Pedestrian Bridge to SETRA (Service d’Etudes Techniques des Routes et Autoroutes). A 3-D multi-mode vibration analysis was performed applying dynamic loads to target multimodal frequencies. Each element, including the foundations, was evaluated for its strength and stiffness in contributing to vibration performance. Piles were assessed using a bounded range of soil stiffness for stress and vibration performance. STV Inc.’s design balances strength, service, and user experience within the confines of aesthetic requirements, contract requirements, and constructability. The bridge is currently under construction.