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Why Structures Fail

Why Structures Fail. An Introduction to engineering analysis. Building a Tower. Each team will design a tower made of balsa sticks Each team will do a structural analysis of their tower to predict where it will fail Each tower will under go ‘destructive testing’.

paul-compton
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Why Structures Fail

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  1. Why Structures Fail An Introduction to engineering analysis

  2. Building a Tower • Each team will design a tower made of balsa sticks • Each team will do a structural analysis of their tower to predict where it will fail • Each tower will under go ‘destructive testing’

  3. The Language of Engineers Mass Position Velocity Acceleration Force Pressure Stress Deformation System Failure

  4. Mass • A measure of how much matter an object is made of • Does not change with environment Metric Unit: ? English Unit: Pounds (lb or lbm) Grams (g) Weight: (in pounds force) 200 lbf 90 kg 200 lbm 90 kg 200 lbm Weight: 33 lbf

  5. Position Metric Unit: Meters (m) English Unit: Feet (ft) • Position fully defines an object’s location in space (X, Y, Z) = (.2 m, .5 m, .6 m) (X, Y, Z) = (1 m, .3 m, .4 m) Y Z (X, Y, Z) = (.5 m, .1 m, .2 m) X

  6. Velocity • Velocity describes the rate of change of an object’s position 30 m/s Metric Unit: Meters per second (m/s) English Unit: Feet per second (ft/s) • Includes both a magnitude and direction 30 m/s 10 m/s • Can also be expressed as a ‘vector’ • (x,y) = (3 m/s, 4 m/s) 4 m/s 3 m/s

  7. Acceleration • Acceleration describes the rate of change of an object’s velocity Metric Unit: m/s/s (m/s2) English Unit: ft/s2 V = 10 m/s a = 0 m/s2 V = 10 m/s 1 sec 0 sec 2 sec 3 sec 4 sec 5 sec 0m 10m 50m 20m 30m 40m 0 sec 3 sec 1 sec 2 sec V = 10 m/s V = ? 16 m/s a = 2 m/s2

  8. Force • Any influence that causes an object to change speed, direction, or shape • Force = mass * acceleration F = m*a m = 90 kg a = 9.81 m/s2 m*a = 883 N Metric Unit: Newton (N) English Unit: Pounds Force (lbf) a = 5 m/s2 F = 500 N 100 kg

  9. Pressure • When a force is exerted over a defined area • Pressure = Force / Area P = F / A Metric Unit: Pascals (Pa) English Unit: Pounds per square inch (PSI) Force = 500 lbf 24 inches 24 inches Area = 24 in x 24 in = 576 in2 P = 500 lbs / 576 in2 = .87 PSI

  10. Stress • A measure of the internal forces acting on a deformable structure Metric Unit: Pascals (Pa) English Unit: Pounds per square inch (PSI) F = 5000 kg * 9.81 m/s2 = 49050 N F = 883 N 20 m 20 m 20 m * 20 m = 400 m2 Stress = P = 49050 N / 400 m2 = 122 Pascals Stress = P = 883 N / 400 m2 = 2.2 Pascals

  11. Stress • Types of stresses Tension Compression Shear

  12. Deformation • Occurs when the stress in an object exceeds the yield strength of the material 500 kPa Material: Purple Block Yield Strength: 1000 kPa Stressed 1200 kPa kPa = kilo-pascal Or 1000 pascals Deformation!

  13. System Failure • Occurs when the stress in an object exceeds its ultimate strength (Compressive or Tensile) 500 PSI Material: Purple Block Yield Strength: 1000 PSI Ultimate Strength: 2000 PSI Stressed 1200 PSI Deformation 2500 PSI System Failure!

  14. Tower Failure • Next Week: Where will your tower fail? Boom!

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