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SECTION 2.0 Internal and External
Forces Act on Structures
Reminder! What is force?
A force is a push or pull that causes an object to change its movement or shape
The effect a force has on a structure depends on 3 things:
1) Magnitude The stronger the force the more effect it will
have on a structure
2) Direction The direction of the force acting on a
structure will determine its effect
3) Location The location on a structure where the force
acts will effect the outcome
Measuring A Force
The unit used for measuring force is called a Newton (N)
1 Newton is the amount of force needed to hold a mass of 100 grams
How many Newtons would be needed to hold a 1 kg book?
External Forces
An external force is a force that is applied on a structure by something outside of the structure
Gusts of wind, waves in the ocean, and the Earth’s gravity are examples
Mass The amount of
matter in an object
Weight The amount of
gravity acting on an object
Centre of Gravity
Gravity acts on all parts of a structure
The Centre or gravity is the specific point where the mass of the structure is evenly distributed
To make a structure more stable, it must have a wider base
A wider base puts most of the mass near the ground, and lowers the Centre of gravity
A load = An external force
Static Load The weight of a
structure These forces stay
the same for long periods of time
Examples are the wood, nails, screws used to hold a bookcase together
Dynamic Load An external force
that moves or changes over time
Examples include the powerful gusts of wind that blow in Grande Cache or cars driving over a bridge
Types of Bridges
Beam Bridge
Arch Bridge
Suspension Bridge
Truss Bridge
Internal Forces
An internal force is when one part of a structure puts force on another part of the same structure
Push the palms of your hands together and you can feel the internal forces within your muscles
There are 3 types of internal forces: Compression Tension Shear
When more than one of these forces act on a structure they are called complementary forces
Structures that “Stand” the test of time
The Parthenon was built in 432 BC in Athens, Greece for
the goddess Athena and stood intact until 1687 when
gunpowder stored inside was detonated
This Roman aqueduct was built between 60-80 AD in Spain and
stretches over 16 km. It is called El Puente which means
“The Bridge”
What’s the Strongest Shape?
TRIANGLE
Structural Components - Arches The arch can
support a large load b/c the force of the load is carried down through the arch to the foundation
The keystone is the center piece of the arch and is the most important part of the arch
Structural Components - Beams I-beams are stronger
than simple beams and have less mass
Girders are long, hollow rectangular prisms
A truss is a framework of beams usually arranged into interlocking triangles
A cantilever is a beam supported only at one end (a diving board)
Structural Components - Columns
A solid structure that stands by itself or can support beams
Usually circular
Very common in ancient architecture
Stress, Fatigue, and Failure
Structural Stress The combination of internal and external
forces that act on a structure and weaken it
Structural Fatigue A permanent change in the a structure caused
by internal forces (compression, tension, or shear)
Cracks in a structure are a sign of this
Structural Failure Occurs when a structure can no longer stand
up to the forces acting on it Think of the video of the collapsing bridge
