Upload
jocelyn-hopkins
View
235
Download
0
Embed Size (px)
DESCRIPTION
Types of Forces Internal External 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)
Citation preview
Unit D: Structures and
ForcesTopics 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 loadChanging or non-permanent force acting
on a structure (wind, weight of things in a structure)
Forces & Loads
External Forces• 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 original shape
See pg 308, forces on a bike
Use big marshmallows to show forces
See pg 309, samples of forces
Pg 309 Internal ForcesExample 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 ForcesExample Compression, tension,
torsion, shear, bendingTent 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 ForcesExample 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 ForcesExample Compression, tension,
torsion, shear, bendingWires in braces
Wringing out a wash cloth
Spider web
Elastic band
Chain connecting a ship to the anchorSitting on an exercise ball
Bolt you are tightening with a wrenchLegs of the chair you are sitting on
Unit D: Structures and Forces
Topic 5: How Structures Fail
Types of Failure Shear
Crack, break Due to compression, twisting (torsion),
bending Bend and buckle
Compression
Types of Failure Torsion
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)
Making Use of Stress
How can forces such as buckling, twisting, bending and shearing be
used in a positive way?
Buckle 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
Twist 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 321-324,326