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A VIEW FROM THE BRIDGE:                                                                                  

Bridging the Gap Between Practical Spirituality and Science

*The purpose of this blog is to help expand perspective in order to experience a deeper life connection.

Volume 1

How Bridges Work

by Robert Lamb and Michael Morrissey

We are a species of bridge builders. Since time out of mind, humans have engineered structures to surmount obstacles. We've tamed steel, stone, lumber and even living vegetation, all in an effort to reach the places, people and things we desire.

Although the concept itself is as simple as felling a tree across a creek, bridge design and construction entails serious ingenuity. Artists, architects and engineers pour vast resources into bridge construction and, in doing so, reshape the very environment in which we live.

As a result, we inhabit a planet of bridges, some as ancient as Greece's 3,000-year-old Arkadiko bridge or as unchanged as India's 500-year-old Meghalaya living bridges, which are coaxed into existence from growing tree roots (more on that later). Countless others have fallen into the ravines and rivers they span, as humans continue to tackle ever more ambitious bridges and construction.

The Basics of Bridge Design

If you're going to build a bridge, you'll need some help from BATS -- not the furry, winged mammals that so often live beneath bridges, but the key structural components of bridge construction: beams, arches, trusses and suspensions.

Various combinations of these four technologies allow for numerous bridge designs, ranging from simple, beam bridges, arch bridges, truss bridges and suspension bridges to more complex variations.The key differences between these four bridge types comes down to the lengths they can cross in a single span, which is the distance between two bridge supports, the physical braces that connect the bridge to the surface below. Bridge supports may take the form of columns, towers or even the walls of a canyon.

Modern beam bridges, for instance, are likely to span up to 200 feet (60 meters), while modern arch bridges can safely cross 800-1,000 feet (240-300 meters). Suspension bridges are capable of extending from 2,000-7,000 feet (610-2,134 meters).

Regardless of the structure, every bridge must stand strong under the two important forces we'll talk about next. 

Tension and Compression: Two Forces Every Bridge Knows Well

What allows an arch bridge to span greater distances than a beam bridge, or a suspension bridge to stretch over a distance seven times that of an arch bridge? The answer lies in how each bridge type deals with the important forces of compression and tension.

Tension: What happens to a rope during a game of tug-of-war? Correct, it undergoes tension from the two sweaty opposing teams pulling on it. This force also acts on bridge structures, resulting in tensional stress.

Compression: What happens when you push down on a spring and collapse it? That's right, you compress it, and by squishing it, you shorten its length. Compressional stress, therefore, is the opposite of tensional stress.

Compression and tension are present in all bridges, and as illustrated, they are both capable of damaging part of the bridge as varying load weights and other forces act on the structure. It's the job of the bridge design to handle these forces without buckling or snapping.

Buckling occurs when compression overcomes an object's ability to endure that force. Snapping is what happens when tension surpasses an object's ability to handle the lengthening force.

The best way to deal with these powerful forces is to either dissipate them or transfer them. With dissipation, the design allows the force to be spread out evenly over a greater area, so that no one spot bears the concentrated brunt of it. It's the difference in, say, eating one chocolate cupcake every day for a week and eating seven cupcakes in a single afternoon.

In transferring force, a design moves stress from an area of weakness to an area of strength. Different bridges prefer to handle these stressors in different ways.

 

Next month we will continue our exploration.....

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