Mechanics and Structures

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Mechanics and Structures
IENG 331, Safety Engineering Fall 2006
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Assignment

• Read Chapter 10 from Brauer Text
• Do the Review Questions (HW10)
• 1-9

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Introduction Historical Events

• 4/27/78 West Virginia scaffold collapses
• Cooling tower construction project
• 170 fall, 51 deaths
• Lack of bolts, inadequately cured, insufficient
  strength in concrete
• 5/30/79 DC-10 crash, Chicago
• 271 deaths
• 3/8 engine support bolt failed, engine broke
  away ripping 3 redundant hydraulic flight control
  lines

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Historical Events

• August 1989
• It was discovered that substandard bolts
  (strength, etc.) were marketed for aircraft,
  trucks, etc. Some companies were criminally
  charged. The problem is virtually unsolvable.
• 1995
• A worker steps on a plastic skylight, which
  fails, falling 20 feet to death on concrete.
  Many years of ultraviolet light exposure caused a
  weakening of the plastic.

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Historical Events

• 1998 and 1999
• After six deaths and many injuries to auto racing
  spectators, designers re-evaluated standards for
  separating vehicles and crash debris from fans.
• December 26, 2004
• 9.0 quake northwest of Sumatra creates a tsunami
  throughout the Indian Ocean as far as 1000 miles.
  Death toll 170,000. 2 million people lost
  their homes. 5 million people needed relief
  assistance.

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Contact Stress

• S F / A
• S, stress, force per unit area (e.g., psi)
• F, force
• A, contact surface area

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(No Transcript)
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Safety Factors

• A safety factor is often applied
• Makes an allowance for unknowns related to
  materials, assembly, use, age, corrosion, wear,
  etc.
• SF failure producing load / allowable stress

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Understand tension, compression, shear, torsion,
bending, bearing, punching shear, column buckling
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Other Structural Failure Considerations

• Static loading vs dynamic loading
• Instability of base or foundation
• Weather, e.g., Wind loading, snow on roofs

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Causes of Structural Failure

• Design Errors
• Faulty Materials
• Physical Damage
• Overloading and Inadequate Support
• Poor or Faulty Workmanship
• Poor Maintenance, Use and Inspection

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Poor or Faulty Workmanship

• Hyatt Regency Hotel Kansas City
• 1981 Walkway collapsed

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Applications Static Mechanics

• Example 10-1
• Bolt is in tension holding two elements together
  weighing 10 lb
• 20 lb force caused by tightened nut
• Total effective load on bolt
• 1001020 130 lb

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Applications - Friction

• Suppose someone is about to push a large box,
  assuming it will slide. It might also tip over.
  Which will occur, the sliding or the tipping?
  Assume the coefficient of friction between the
  box and floor is 0.6.

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Multiple Modes of Failure