Mechanics and Structures
IENG 331, Safety Engineering
Fall 2009
Assignment
Read Chapter 10 from Brauer Text
Do the Review Questions (HW10)
P. 136
Odds
4/27/78: West Virginia Scaffold Collapse
Cooling
tower
construction
project
170 fall, 51
deaths
Lack of
bolts,
inadequately
cured,
insufficient
strength in
concrete
DC-10 Crash, Chicago, 5/30/79
271 deaths
3/8” engine
support bolt failed,
engine broke away
ripping 3
redundant
hydraulic flight
control lines
Substandard Bolts
Bad
Good
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.
Plastic Skylight
Death, 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.
Autoracing – safety for fans and
drivers
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.
Indian Ocean Tsunami, 12/26/04
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.
US Bridges
Decaying
5/12/82: A
Congressional
report stated than
>212,000 of the
nations 526,000
bridges (~40%)
were structurally
deficient or
functionally
obsolete.
Minneapolis I-35W Bridge Collapse
8/1/07: Mississippi River during rush hour
13 fatalities.
Contact Stress
S = F / A (Equation 10-1)
S, stress, force per unit area (e.g., psi)
F, force
A, contact surface area (e.g., sq in)
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 (Equation 10-2)
Understand tension,
compression, shear,
torsion, bending,
bearing, punching
shear, column buckling
Other Structural Failure
Considerations
Static loading vs. dynamic loading
Instability of base or foundation
Weather, e.g., Wind loading, snow on
roofs
Causes of Structural Failure
Design Errors
Faulty Materials
Physical Damage
Overloading and Inadequate Support
Poor or Faulty Workmanship
Poor Maintenance, Use and Inspection
Poor or Faulty Workmanship
Hyatt
Regency
Hotel –
Kansas
City
1981 –
Walkway
collapsed
114 killed
>200
injured
Applications – Static Mechanics
Example 10-1
Bolt is in tension holding
two elements together
weighing 100 lb and 10
lb suspended elements
20 lb force caused by
tightened nut
Total effective load on
bolt
100+10+20 = 130 lb
Example 10-1 Continued
Mild steel bolt, ultimate tensile strength (from tables)
is 60,000 lb/in2
¼” diameter bolt, cross sectional area is 0.049 in2
(Note: book has 0.196 in2)
Stress in bolt using Eq 10-1 = 130 lb / 0.049 in2 =
2653 lb/in2
Applying Eq 10-2, actual safety factor = (60,000
lb/in2) / 2653 lb/in2 = 22.6 (Note: Text has 90.5)
Assume a reasonable safety factor for this application
is “3”, 22.6 >> 3, therefore the bolt should safety
carry this load
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.
Multiple Modes of Failure
Assignment
Read Chapter 10 from Brauer Text
Do the Review Questions (HW10)
P. 136
Odds
Due Date: ?