Science 7 – Unit D: Structures and Forces

Topic 6: Designing with Forces

How to Design Stable Structures   Structures must deal with loads (structural strength) and external forces (structural stability). Designers/engineers must recognize, analyze, and deal with the types of loads and external forces that their structures will deal with. Remember external forces cause internal stresses. They can deal with these external forces in one of three ways:

I. Spread the Load   Distribute the load throughout the structure so that no single part is carrying most of the load.

Domes are good examples of this strategy.

II. Use Angled Components   Engineers may use angled components so that forces are directed along them and are used to hold pieces together rather then pulling them apart. Triangular structural components are a good example of this strategy.

III. Shape the Parts to Deal With the External Force.

  Engineers shape the parts to stand up to the specific type of force they are likely to experience.

Eg. The Vasco Da Gama Bridge, the longest bridge in Europe is curved to better deal with tsunamis and earthquakes.

How To Support Heavy Loads    Different Structural Components can be used to help support heavy loads.

Arches, braces, cantilevers, large columns, flying buttresses and triangular corrugations give added structural strength to a structure.

Eg. A double cantilever bridge uses the cantilever component to support the loads of the vehicles on it.

You need to use the right materials to deal with specific internal forces.

   Certain materials such as concrete are very good at resisting compression, but not at resisting tension.

Others such as most metals can handle tension a lot better than compression.

Finally, certain materials can handle a number of different forces such as steel.

Friction can be a good thing too!

  Friction is a force that resists movement. Friction is very important in structures. Unless you’re talking about mobile joints, you want a lot of friction in your structures.

Nails, screws, bolts, glue, staples, mortar etc. are all types of rigid joints that use friction to help hold structural components together.