Monday, May 2, 2016

Bridge Building Series - Beam Bridge 1

Before we go on to the topic of beam bridges, I'd like to discuss the type of forces acting on a bridge. 

At any given time, two types of forces act on a bridge: compressive force (compression) and tensile force (tension).

The compressive force is a force that compresses or shortens the thing it is acting on. The tensile force is a force that expands or lengthens the section it is acting on.

Beam Bridges

These bridges are everywhere.

They are the simplest and least expensive bridges to build. 

When a load is placed on the beam bridge, the top of the beam bridge is pushed down by a compressive force as a tensile force stretches the lower portion of the beam bridge. The farther apart the supports structures are, the weaker a beam bridge becomes. 

A simple beam bridge experiment

Supplies:
·         A sponge 
·         A permanent marker
·         A ruler
·         2 books (or anything that can create a gap to lay a sponge across)

 Build instructions:
1.   Draw lines 1/2” apart on the sponge.
2.   Lay it across a gap created by books, desks, or other flat objects.
3.   Tape/secure the ends onto the books, desks, or other objects.
4.   Now, press down on any part of the sponge. What happened? Do the lines curve inward and closer or curve outward and farther on top of the bridge? How about the bottom of the bridge?

Top View


The 1/2" gap between the two lines is beginning to close in and shorten.

The 1/2" gap between the two lines is closing and is shortened noticeably compared to the gap between the other lines.

The 1/2" gap between the two lines is invisible. It looks as if the two lines are one.



Bottom View

The 1/2" gap between the two center lines is just beginning to lengthen.

The 1/2" gap between the two center lines is lengthening more.

The 1/2" gap between the two center lines has expanded and lengthened.

Our bridge has collapsed due to too much load on the bridge.

I never thought to do this simple experiment because it seemed so...simple. But it easily and clearly demonstrates tensile and compressive forces acting on a bridge. 

This simple understanding will lay the foundation for the bridge experiments to come.

Have a great day!