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Topic – Forces and Structures

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Forces and Structures

What Is a Structure?

A structure is something that holds or supports a load. For example, a garden shed has four walls that support a roof.

Structures are built for one or more purposes. A garden shed provides a place to keep garden tools dry. A locked shed will also keep the tools safe so they will not be stolen.

What Types of Forces Act on Structures?

Two types of forces act on structures—internal forces and external forces.

Internal forces are forces that are created from parts of the structure. For example, the roof of a house is heavy and pushes down on the outside walls. The roof is part of the structure of the house. If the walls are not strong enough to hold up the roof, the house will collapse. So the pushing force created by the weight of the roof is an internal force.

External forces come from things that are not part of the structure. For example, snow will push its weight down on the roof. The walls also have to be strong enough to support the weight of snow on the roof. Because the snow is not part of the structure, snow is an external force.

Two Types of Loads

Loads are forces that act on a structure. All structures need to be strong enough to hold up under the loads placed on them. Loads are divided into two categories—dead loads and live loads.

A dead load is a permanent force acting on a structure. The weight of the materials used to build a structure are part of the dead load. Anything permanently attached to a structure is part of the dead load. Carpet and window planters are dead loads.

A live load is a temporary force acting on a structure. Objects that carry temporary loads are also part of the live load. Here are some examples:

  Furniture is part of the live load in a house. Furniture can be removed from the house if you move to a new house.

  Snow that builds up on a house roof is a live load. Snow is temporary because it can melt away or be removed.


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