Grade 7 Notes for section 4.2

Forces That Can Act on Structures

• Structures constantly experience forces; they must be designed to withstand the forces they will face (Force: any push or pull)
• If the structure isn’t strong enough it may experience structural failure.
• If the structure is too strong, time and resources may have been wasted.

Internal and External Forces

• External force: acts on an object from outside the object; e.g., wind, gravity, earthquakes
• Gravity
• is a natural force of attraction
• acts on all structures all the time
• pulls structures towards Earth’s centre

• other external forces
• a person on a ladder
• pulling a drawer open
• sitting on a chair
• cars on a bridge

• Internal force: one part of a structure acting on another part of a structure; e.g.,
• tension in stretched cables on a bridge
• compression by the weight of a roof on the walls

Describing Forces

To describe forces, engineers use three main things:

• The force’s:
• Magnitude (strength)
• The size of force compared to the size and weight of object
• Direction
• Where the force is coming from
• The point and plane of its application
• Point of application: the exact location where the force meets the structure
• Plane of application: the side of the structure affected by the force

External Forces and Loads

Every structure supports a load

• Total load: the sum of all the static and dynamic loads
• Static load: the effect of gravity on a structure (the weight of the structure itself) also called dead load
• Dynamic load: the force that moves or changes while active on the structure
• Called “dynamic” because they change their magnitude, direction, and point and plane of application.
• Wind is considered a dynamic load because its magnitude can change

E.g., a bridge: The static load is the weight of the bridge itself (dead load).

The dynamic load is the weight of the moving cars across the bridge (also called live load).

DRAW PICTURE OF TRUCK ON BRIDGE WITH LABELS HERE

Internal Forces

• Can be classified as:
• Compression – a force that squeezes or presses something together
• Tension – a force that stretches apart to expand or lengthen
• Shear – a force that pushes in opposite directions
• Torsion – a force that twists
• Bending – A force that acts to bend a component putting one side of the part in tension and the opposite side in compression

The forces occur within the human body as well since the human body is a structure; e.g., when a skater twists, torsion occurs within her body.

Designing for Forces

Engineers have to consider all these forces when designing structures, e.g., they have to consider the load of the snow on a bridge in addition to the truck.

Engineers design structures to withstand a hundred year storm, a large storm that occurs once in a hundred years.