Arch Bridge

Overview

	Arch Bridge Bixby Creek Bridge, Monterey, CA

Arch bridges are one of the oldest types of bridges and have great natural strength. Instead of pushing straight down, the weight of an arch bridge is carried outward along the curve of the arch to the supports at each end. These supports, called the abutments, carry the load and keep the ends of the bridge from spreading out.

The Romans developed considerable skill in constructing arches and many of their structures remain intact today. However, they were not able to calculate the strength of their-arches although their knowledge of geometry inherited from the Greeks led them to adopt semicircular spans. Their arches were constructed of stone made into the form of wedge shaped segments, or voussoirs, built up side by side on temporary timber supports until the top piece at the crown of the arch is pushed hom(keystone). One of the most famous examples of their handiwork is the Pont du Gard aqueduct near Nîmes, France. Built before the birth of Christ, the bridge is held together by mortar only in its top tier; the stones in the rest of the structure stay together by the sheer force of their own weight.

The Pont du Gard bridge The Pont du Gard aqueduct

Modem arch bridges are built of concrete. It is an effective way to transfer loads to the ground where foundation conditions are adequate to resist horizontal loads. A large nurnber of concrete arch bridges have been constructed in the 20th century in both in-situ reinforced concrete and precast concrete.

The New River bridge

Today materials like steel and pre-stressed concrete have made it possible to build longer and more elegant arches, including a spectacular 1700 foot span in New River Gorge, West Virginia. (More typically, modern arch bridges span between 200-800 feet.)

A prime consideration in the erection of an arch bridge is the method of supporting the voussoirs during construction. The cost of staging required to support the arch centering during the placing of the voussoirs represents a substantial proportion of the cost of construction. Their design and construction therefore requires detail investigation.

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