Review Quiz…. On a clean sheet of loose leaf paper. 1. Write down the type of internal force that is happening. B. A. Write down the type of internal force that is happening. D. C. Write down the type of internal force that is happening. E.
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Review Quiz… On a clean sheet of loose leaf paper
2. Describe the difference between a dead load and live load. 3. The weight of an African Elephant is 55 000 N. What would the mass be? 4. An apple has a mass of 180 grams. What is the weight of the apple? 5. Describe the difference between mass and weight
Stresses: What materials and structures do when they fail…
What materials and structures do when they fail • Snap • Twist • Buckle • Bend • Shear how do we get theses failures?
Bend or Buckle • When you compress something, it can bend or buckle under the force (ex. Pop can) • Compression forces cause material to bend on the inside of the curve, and pull and snap on the outside of a curve
Torsion / Twist • Materials of structures often shear when twisted • Sections of a structure slide past each other and the structure cracks or breaks in two.
Shear • Solid materials are never perfectly uniform, there are microscopic cracks or weaknesses • When a solid material is compressed, the crack can enlarge or break apart.
Common Shear Example • Weight of a building may compress the soil causing the soil to shear or slip sideways.
Making use of stress • BUCKLE: car bumpers, grass, cardboard box flaps • Buckle in a crash so people are not injured • SHEAR: gear shafts, propellers in boats • Shear pin • Gears slip past each other • TWIST: twisting yarn as opposed to straight fibers • controlled twisting turns hair into braids, string into ropes, and wires in to cables
Videos • Tacoma Narrows • Bonny Brook Bridge: http://www.cbc.ca/news/canada/calgary/cp-responds-to-calgary-mayor-s-anger-over-rail-bridge-failure-1.1302523 • Leaning Tower of Pisa
Table Group Brainstorm • In your table groups you are going to put your heads together and come up with as many different ways you can think of to combat stress and failure • WAYS TO ENSURE STRUCTURES ARE SAFE, STABLE, RELIABLE…. And SAFE • AS MANY AS POSSIBLE
Center of Gravity or Center of Mass • A structures most stable point • Where the weight is evenly dispersed and all sides are in balance
Unbalanced Structures • To determine whether a structure is balanced or is likely to fall, locate its center of gravity and draw a line directly down towards the earth. • If the arrow points to a solid foundation, the structure is stable