Beam Bridge: Description, Mechanics, Examples, & Facts

Overview

• A beam bridge is a simple type of bridge with a horizontal beam supported at each end.
• It is the most common and oldest bridge form.
• Initially made of wooden beams, later supported by wooden or stone piers.
• Modern beam bridges are usually constructed from steel or reinforced concrete.
• Useful for highway overpasses and railways.

Mechanics

• Beam bridges carry vertical loads by bending.
• The top of the beam experiences horizontal compression.
• The bottom undergoes horizontal tension.
• Supports transfer loads from the beam by vertical compression to the foundations.
• Prestressed concrete is effective due to its ability to handle compression and embedded steel for tension.
• Typically limited to spans less than 76 meters (250 feet) due to bending.

Construction

• Typically involves casting concrete footings for piers and abutments.
• Weak soil requires wooden or steel piles to support footings.
• Steel beams are factory-made, shipped, and placed using cranes.
• For short spans, beams are a single unit; longer spans use plate girders.
• Plate girder: An I beam with separate top and bottom flanges welded or bolted to a web.
• Longer spans can be haunched, being deeper at supports and shallower mid-span.

Examples of Beam Bridges

• Lake Pontchartrain Causeway, Louisiana: Nearly 39 km (24 miles), one of the longest overwater bridges.
• Manchac Swamp Bridge, Louisiana: Almost 23 miles long, supported by concrete trestles.
• Donghai Bridge, China: Over 32,500 meters (20 miles), connects Shanghai with a port in Zhejiang.
• Tianjin Grand Bridge, China: 113.7 km (70.6 miles), serves as a railway viaduct.
• Labajin Bridge, Sichuan, China: Tallest pier at 183 meters (600 feet), designed for seismic activity.

Notable Facts

• Beam bridges are also known as girder bridges.
• Stability usually limits the length between supports.
• Used in both military and civilian infrastructure.

By understanding the mechanics and construction of beam bridges, as well as recognizing notable examples, one can appreciate their contribution to infrastructure and engineering. These bridges remain integral in connecting places while showcasing engineering advancements.