# How to Build a Suspension Bridge

You can build a simple suspension bridge of of everyday classroom materials in order to demonstrate the basic underlying physics. A suspension bridge includes two towers, two cables hanging in a parabola, a deck, hanging cables and sometimes a truss under the deck to provide wind resistance.

Few human-made constructs can instantly inspire awe from a distance (or up close, for that matter) in the way a suspension bridge can. This category of very long, elegantly symmetrical structures includes some of the most famous bridges in the world, including the Brooklyn Bridge in New York City, the Golden Gate Bridge in San Francisco and the Akashi Kaikyo Bridge in Japan.

The reason suspension bridges can be built to spectacular lengths (close to 4 km, or 2.5 miles, as of 2019) is that their unique tower-and-cable support system allows some of the forces the bridge must withstand to be displaced laterally via the cables' anchor points on the land on either side of the bridge. You can explore this principle yourself in a do-it-yourself experiment.

## Suspension Bridge Design

Suspension bridges have a characteristic appearance, the most notable feature being the pair of towers anchored into the ground (usually under river, lake or sea water). These support the two main parallel cables running from end to end of the bridge over the tops of the towers, forming a U-shaped curve in the middle mathematically known as a parabola.

An array of vertical cables are suspended from the main cables and support the deck that serves the bridge's primary function of allowing passage over a gap. Typically, there is at least one roadway here. The length of the deck between the towers is called the main span. The lengths of the bridge on either side of this span often add up to about the length of the main span.

## General Concept of a Suspension Bridge

Picture yourself and an identically sized companion standing facing each other at a distance of about 15 feet (about 3 meters). Imagine that each of you is holding both hands high overhead, and that each hand – four in all between you – supports a long rope that hangs almost to the floor between you.

If people standing behind each one of you were to slowly apply an equal horizontal tension to the ends of both ropes, pulling away from the center of this impromptu physics party, the ropes would slide backward through your and your friend's hands, with the "U" of the ropes gradually approaching a horizontal line between your and your companion's hands.

Now imagine a a series of smaller ropes draped over the two main ropes on each side and attached to a 20-foot-long, lightweight board sitting the floor between your and your friend's legs. If the tension were sufficient, the board would be raised off the floor as the main ropes became more taut. If you and your friend were to "clamp" the board into place between your knees, this "deck" would be supported by both the towers and the cables.

Suspension bridges are light and flexible, since so much of them consists of cables and they are built to sway with wind and other stresses. Excessive stiffness (and this may be counter-intuitive) is a disadvantage in bridges and other structures that experience shear stress and other unusual forces.

The deck is built from materials that are both compressible and able to withstand tension (extension), allowing it to bend upward and downward slightly without breaking. Of course, excessive sway would produce hazards in its own right.