Bridges are designed to connect landscapes, but movable bridges must do more. They must serve both land and water traffic without compromising structural stability. Among the most versatile of these designs is the swing bridge, a structure that rotates horizontally around a central pivot pier to allow maritime passage while continuing to support rail or vehicular traffic. Though first introduced in the United States in the nineteenth century, swing bridges remain an important component of modern transportation systems. Their longevity depends on one critical principle: equilibrium.
A swing bridge consists of two spans, often referred to as swing spans, that rotate about a vertical axis located at a central pivot pier. In the closed position, the bridge functions as a continuous structural system supporting dead and live loads from vehicles or trains. In the open position, the spans rotate, effectively behaving as cantilevered structures. This transformation requires careful consideration of load distribution, balance, and mechanical design.
This student research presentation explores how swing bridges maintain static equilibrium in both operational states. Central to the analysis is an understanding of forces, moments, shear, and load bearing points. Whether open or closed, the bridge must satisfy the conditions of equilibrium, meaning that the sum of forces and the sum of moments must equal zero. Achieving this balance requires precise structural calculations and thoughtful mechanical integration.
Two primary mechanical systems are examined: rim bearing swing bridges and central bearing swing bridges. While both designs accomplish horizontal rotation, they differ in how loads are carried and distributed. In rim bearing systems, loads are transferred through rollers located around the perimeter of the pivot pier. In central bearing systems, the primary support occurs at the pivot point itself. These differences significantly influence structural behavior, maintenance considerations, and design complexity.
The research identifies five essential factors required for analyzing any swing bridge design. These factors guide engineers in evaluating total load, dead and live load interactions, pivot pier capacity, truss configuration, and the resulting shear and moment diagrams. Free body diagrams and structural modeling techniques support this evaluation, ensuring that the bridge remains stable throughout its operational cycle.
Industry insight further strengthens the analysis. An interview with a professional engineer at American Bridge Company provides real world perspective on the challenges and design decisions involved in swing bridge construction and rehabilitation. Economic considerations often favor rebuilding existing swing bridges rather than constructing entirely new movable systems. Even so, equilibrium principles remain the foundation of both retrofit and new design efforts.
For students and structural engineering professionals, this session underscores the importance of foundational mechanics in complex infrastructure systems. Swing bridges demonstrate how classical equilibrium theory is applied in dynamic, real world conditions. By carefully managing load distribution and mechanical rotation systems, engineers ensure that these structures continue to serve both transportation and maritime needs safely and efficiently.
Author and Affiliation
Leah Huxhold, New England Institute of Technology
This presentation will be delivered in person at the SAM International Business Conference as part of the Invitation Only Submissions track. Attendees will examine the structural mechanics and design principles that allow swing bridges to maintain equilibrium in both open and closed positions. For more information visit www.samnational.org/conference