mechanical · 30 minutes · Official OETT Aligned
Frames, Arches & Supports
Follow the weight down to the ground
What to know
- compression: squeezing force
- braces: parts that stop a frame leaning
- abutments: supports at the ends of an arch
- how a load reaches the ground
A structure needs a supported path for its load, all the way to the ground.
The core rule
Triangles help a frame keep its shape. Arches push down and outward on their end supports.
An unbraced rectangular frame can lean sideways. Adding a diagonal brace divides it into triangles that resist this change. An arch carries weight through squeezing forces, called compression. Its end supports, called abutments, must resist the outward push as well as the downward load.
Worked examples
Example 1: easy
A diagonal brace stops a rectangular frame from leaning sideways.
- An unbraced rectangle can lean sideways without changing the length of its sides.
- A diagonal brace makes two triangles and stops that shape change.
The triangular shape resists a sideways change better than an unbraced rectangle.
Example 2: medium
An arch carries squeezing force toward its two end supports.
- Follow the squeezing force from the top of the arch toward both ends.
- The arch pushes down and outward against its end supports.
The supports must resist both the downward load and outward push.
Example 3: hard
In a frame of connected triangles, follow the load to the supports to see which bars carry force.
- Start where the load enters the frame. Follow the connected bars.
- Trace the force through the triangles to the supports and ground.
A load needs a connected path to the ground.
Common mistakes
An arch only pushes straight down on its supports.
It also pushes outward. The end supports must hold against both directions.
Put it into practice
Use guided explanations, check your answers, and try a new variation.