# Structures

## Structures

A structure is rigid meaning that it can support weight.

Triangular structures are more rigid than others. So a rectangular structure can be made rigid by adding a diagonal piece so that the rectangular change into two a triangular structure, which are more rigid. This is why doors, water tanks and roofing tuffs are made with triangular shapes.

Struts and Ties
In any structure, there are parts, which are under action of tensional forces and others under action of compressional forces.

Ties are girders, which are under tension.
This occurs when a girder results in the points it joins to moves further apart on the removal of such girder in a tie.

Properties of ties:
It is under tension
When removed, the point it joins move further apart.
It can be replaced by a rope or strong string.

Struts are girders, which are under compression.
This occurs when a girder result in the point to move closer together on removal girder in struts.

Properties of struts:
It is under compression
When removed, the point it joins move closer to each other.
It cannot be replaced by a rope or strong string.

In order to determine each of the girders whether its a tie or a strut, each of the girders is removed and the effect is noted.

If the points move further apart then the girder is tie and if the points move closer together then the girder is strut.
When BC is removed, point B moves close to point C showing that girder BC is strut.
When AB is removed, point A moves close to B. so girder AB is strut.
When AE is removed, point A moves further apart from E meaning that girder AE is tie. Similarly, girder ED is tie

For structures
When BE is removes point B move further apart from E meaning the girder BE tie.

When BF is removed, the structure turns about point G. B will move further away from F hence BF is a tie.
When BC is removed, the structure will bend at E. Thus, C will move in the direction of the load, far away from B. This means BC is under tension and hence it is a tie.

When CD is removed, point D moves down wards with the load. Point D moves away from C, so CD is a tie.

When DE is removed, CD will be vertical due to the load. Thus, point D moves nearer to E meaning that girder DE is a strut. Similarly, girder EG is a strut.
When CE is removed, the load moves down wards and part BCD will be straight due to the load. Thus, point C moves nearer to E meaning that girder CE is a strut. Similarly, girders BE and AB are struts.