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About this Article
Written by: Ryan Kelly
Written on: October 1st, 2000
Tags: building & architecture, civil engineering, transportation
Thumbnail by: German Wikipedia/Wikimedia Commons
About the Author
In Fall 2000, the author was a student at USC.
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Volume II Issue IV > Shaky Ground: The Design of Suspension Bridges
The Tacoma Narrows Bridge, which collapsed on November 7, 1940, in winds of only 40mph, gained the attention of the entire scientific community. The stability of a suspension bridge is paramount; consequently engineers must scrutinize every aspect of a bridge design. One of the most important considerations in bridge design is aerodynamic force, which, even when very small, can have great effects on a suspended structure. This article presents some of the design characteristics of bridges, a short history of the Tacoma Narrows, and explores what can be done in bridge design to prevent unnecessary undulation, damage, and total collapse.

Introduction

There are 144 suspension bridges in the United States. When designing suspension bridges, engineers face many natural problems. The wind does not initially seem able to move structures as massive as bridges; however, the summation of small forces that the wind exerts can result in displacements by inducing resonant effects in the span. Suspension bridge engineers need to take these effects into account because the added effects of wind shear can result in disaster.

Bridge Design

At a basic level, all suspension bridges are similar. They are usually composed of two or three towers built on solid ground at each end of the span and two main cables attached to these towers. From these large cables hang many smaller cables, where the roadway or deck rests. However, bridges are not limited to these components. Suspension bridges need stiffening to assist with efficient weight distribution. If the roadway were simply hanging from the main cables a large, heavy-loaded truck passing over the bridge would exert all its weight on the cables immediately around it. This would compromise the integrity of the structure, placing too much weight on a single hanging cable, and stressing the roadway. This CAN result in cracking and bending. However, regardless of the location of the load on the bridge, stiffening prevents the bridge from bending and allows for better distribution of weight over more cables and a larger section of the roadway. At the end of each main cable is a huge concrete structure, called a "cable stay." The cable stay is firmly implanted into the ground, where each cable is securely fastened into the concrete, and can then transfer the huge load of the bridge into the ground. Thus, the part of the bridge that now bears the weight is shifted from the roadway and hanging cables to the main towers and cable stays.
German Wikipedia/Wikimedia Commons
Figure 1: Golden Gate Bridge in San Francisco, California.