Unit D: Structures
and Forces
Topics 4-7
Topic 4: Forces, Loads,
and Stresses
Types of Forces
 Internal
 Tension, compression, shear, torsion,
bending
 External
 Dead Load
 Permanent force acting on a structure
 Includes the weight of the structure itself
 Live load
 Changing or non-permanent force acting on a
structure (wind, weight of things in a
structure)
• Dead Load:
• Permanent Force Acting on a
Structure
• Includes the Weight of the
Structure Itself
• Can cause Structures to break
• Live Load:
• Changing or Non-Permanent
Force Acting on a Structure
Internal Forces
 Tension Forces:
 Stretch a material by pulling
its ends apart
 Tensile Strength:
 Measures the largest tension
force the material can stand
before breaking
 Compression Forces:
 Crush a material by
squeezing it together
 Compressive Strength:
 Measures the largest
compression force the
material can stand before
losing its shape or breaking
 Shear Forces:
 Bend or Tear a Material by
pressing different parts in
opposite directions at the
same time
 Shear Strength:
 Measures the largest shear
force the material can stand
before ripping apart
 Torsion Forces:
 Twist a material by turning
the ends in opposite
directions
 Torsion Strength:
 Measures the largest torsion
force the material can stand
and still spring back to its
 See pg 308, forces on a bike
 Use big marshmallows to
show forces
 See pg 309, samples of
forces
Pg 309 Internal Forces
Example
Dead load? Live Load?
River (water)
Live load (shape the waterway
through erosion)
Beaver/bear
Live load (he moves around, not a
permanent force acting on the
ground)
Water wheel
Live load
Bridge
Dead load (people or cars on bridge
would be live loads)
People sitting on a stool
Live load
Precipitation from clouds
Live load
Wind sock
Live load (wind inside)
Pg 309 External Forces
Example
Compression, tension, torsion,
shear, bending
Tent ropes
tension
Fishing line
tension
Saw
shear
People sitting on a stool
Compression (tension on the bottom
surface of the stool seat)
Dog chewing on stick
Compression (shear if it breaks)
Bike (see pg 308) handle bars
torsion
Handle on the well
torsion
Internal Forces
Example
Dead load? Live Load?
Wind blowing against a tree
Live load
Tree rooted in the earth
Dead load
Compression of a spring (pen)
Live load
External Forces
Example
Wires in braces
Wringing out a wash cloth
Spider web
Elastic band
Chain connecting a ship to the anchor
Sitting on an exercise ball
Bolt you are tightening with a wrench
Legs of the chair you are sitting on
Compression, tension, torsion,
shear, bending
Unit D: Structures
and Forces
Topic 5: How Structures
Fail
 Shear
Types of Failure
 Crack, break
 Due to compression, twisting (torsion),
bending
 Bend and buckle
 Compression
 Torsion
Types of Failure
 Twisting brittle items (spaghetti, plastic
cutlery) causes it to shear
 Flexible structures such as cords, hoses,
rubber bands, shear less easily
 Metal Fatigue
 Bending a material back and forth, weakening
the metal, causing it to break (staples, paper
clips)
How can forces such
as buckling, twisting,
bending and
shearing be used in
a positive way?
 Car bumpers and sheet
metal buckle in a collision.
They absorb the crash
impact
 Grass on a soccer field will
buckle, so it absorbs the
impact when someone falls
Shear
 Used in boat motors to help
keep them from getting
tangled in weeds
 Used in the clutch and
automatic transmissions of
cars
 Yarn is twisted together to
make it stronger
 Wires are twisted into cables
 Hair is twisted into braids
Unit D: Structures
and Forces
Topic 7: Stable Structures
How can we make structures
more stable?
 Account for the center of gravity (need
to be balanced)
 Have a solid foundation
 Use pilings
 Make a solid layer (underneath roads there
is gravel, rough ashphalt)
 Cement “foundation” for house
 footings
 Make them symmetrical, or use a
counter-weight (cranes)
• Stable - Less Likely to Tip
• Centre of Gravity - The point
where all of the gravitational
• Pilings – Large metal,
concrete or wood cylinders
• Footing – Concrete
foundation under basement
walls
Pilings
Spin Stabilization
 Gyroscopes will balance if
they are spinning
 Throwing footballs in a spiral
helps them travel in a straight
line
 Space satellites spin to keep
the antennas pointed at earth
 Frisbee/yo-you, top
Assignments
 Review Q’s T 4+5
 Pg 320 # 1-5
 Topic 6: Designing with
Forces
 Read Topic 6 pg 321324,326