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Fundamental Concepts & Terms. IENG 331 Safety Engineering. Why Safety?. Read Chapter 3! Humanitarian Reasons Regulatory Reasons Economic Reasons. What is an Accident?. An event that is not expected or intended Could cause injury loss Implies “chance”. Fundamental Accident Causes.

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fundamental concepts terms

Fundamental Concepts & Terms

IENG 331

Safety Engineering

why safety
Why Safety?
  • Read Chapter 3!
  • Humanitarian Reasons
  • Regulatory Reasons
  • Economic Reasons
what is an accident
What is an Accident?
  • An event that is not expected or intended
  • Could cause
    • injury
    • loss
  • Implies “chance”
fundamental accident causes
Fundamental Accident Causes
  • Unsafe Acts
  • Unsafe Conditions
  • Both
  • “Not necessarily a ‘chance’ event”
types of losses




damage to property, equipment, materials

cost of replacement

legal & medical services

Loss of time, production, sales

time to complete forms




hospitalization, rehab

public image damage

Types of Losses
losses direct vs indirect costs
Losses: Direct vs. Indirect Costs
  • Direct (Obvious)
    • medical expenses, WC, repair or replace damages
  • Indirect (Not Obvious)
    • 4:1 Ratio (Iceberg Theory)
    • Injured worker’s wages, lost supervisory time, co-workers’ lost time during emergency, damaged equipment, ruined product, overtime for production to catch back up, learning curve for replacement worker, clerical costs, payments made to injured under benefits program
unsafe acts vs unsafe conditions
Unsafe Acts vs Unsafe Conditions
  • Heinrich analyzed 75,000 accidents
  • 88 : 10 : 2 ratio
  • 88% unsafe acts
  • 10% unsafe conditions
  • 2% unpreventable causes
  • Engineers can attack unsafe conditions
  • Must understand human behavior and management principles to attack unsafe acts
accident injury relationship
Accident - Injury Relationship
  • Heinrich’s 300 : 29 : 1 ratio
  • For 330 accidents
    • 300 result in no injury
    • 29 produce minor injuries
    • 1 produces major, lost-time injury
  • Opportunities to improve are great
  • Many accidents are “rehearsed” many times
accident costs relationship
Accident - Costs Relationship
  • Pareto Relationship
  • 80% - 20% rule
    • 80% of the costs are related to 20% of the injuries
    • for example, low back lifting injuries represent 20% of all accidents, but represent 80% of the costs
    • if you can manage and control that 20% of accidents, you can control 80% of the costs
  • Figure 3-1 shows 50% of the costs are related to 2% of the injuries
    • “the powerful few”
  • Safety: being relatively free from harm, danger, damage, injury
  • Risk: measure of both frequency and severity of hazards
  • Hazard: unsafe condition, the potential for an activity or condition to produce harm
  • Safety Engineering: application of engineering principles to the recognition and control of hazards
accident theories
Accident Theories
  • Domino Theory
  • Energy Theory
  • Single Factor Theories
  • Multiple Factor Theories
domino theory heinrich
Domino Theory (Heinrich)
  • Injury is caused by
  • Accidents which are caused by
  • Unsafe acts or conditions which are caused by
  • Undesirable traits (e.g., recklessness, nervousness, temper, lack of knowledge, unsafe practices) which are caused by
  • Social environment
domino theory cont
Domino Theory Cont.
  • Stop the sequence by removing or controlling contributing factors
  • Strong emphasis is placed on the middle domino: unsafe acts or conditions
energy theory haddon
Energy Theory (Haddon)
  • Accidents & Injuries involve the transfer of energy, e.g., fires, vehicle accidents, projectiles, etc.
  • Transfer of energy from a “potential” to “kinetic”
  • Attack problems in parallel rather than serial (as is presumed in Domino Theory)
energy theory cont 10 strategies to prevent or reduce
Energy Theory Cont.10 Strategies to Prevent or Reduce

1. Prevent the marshalling of energy.

- don’t produce the energy

- don’t let kids climb above floor level

- don’t produce gun powder

2. Reduce the amount of energy marshalled.

- keep vehicle speeds down

- reduce chemical concentrations

- don’t let kids climb above 3’


3. Prevent the release of energy

- elevator brakes

4. Modify the rate at which energy is released from its source or modify the spatial distribution of the released energy.

- reduce the slope on roadways

5. Separate in space or time the energy being released from the structure that can be damaged or the human who can be injured.

- separate pedestrians from vehicles


6. Separate the energy being released from a structure or person that can suffer loss by interposing a barrier.

- safety glasses, highway median barriers

7. Modify the surfaces of structures that come into contact with people or other structure.

- rounded corners, larger surface areas for tool handles

8. Strengthen the structure or person susceptible to damage.

-fire or earthquake resistant structures, training, vaccinations


9. Detect damage quickly and counter its continuation or extension.

- sprinklers that detect heat

- tire tread wear bands

10. During the period following damage and return to normal conditions, take measures to restore a stable condition.

- rehab an injured worker

- repair a damaged vehicle

single factor theories
Single Factor Theories
  • Assumes that when one finds a cause, there is nothing more to find out.
  • Weak theory, there can be so much more to learn!
multiple factor theories
Multiple Factor Theories
  • Accidents are caused by many factors working together
  • The theory and the analysis is more complex, but more realistic than Single Factor Theory
  • Consider the Four M’s:
    • management, man, media, machine
    • And their interactions
preventative strategies
Preventative Strategies
  • Proactive vs. Reactive
  • Frequency strategies
  • Severity strategies
  • Cost strategies
  • Combinations
  • Three E’s of Safety: engineering, education, enforcement
three lines of defense
Three Lines of Defense
  • Engineering Controls
  • Administrative Controls
  • Personal Protective Equipment
safety factors
Safety Factors
  • Since there is a chance element in safety, we can improve our chances by implementing a “safety factor”
    • Scaffolding – 4:1
      • Designed to withstand 4 times the intended load
    • Overhead crane hoists – 5:1
    • Scaffold ropes – 6:1
  • Why not use 10:1 as a standard?? $$$$$
  • Beware when using field tables or computer programs. Are the safety factors applied or not??
fail safe principles
Fail-Safe Principles
  • General fail-safe principle
    • The resulting status of a system, in event of failure of one of its components, shall be in a safe mode.
    • Read Case Studies 3.5 and 3.6
  • Fail-safe principle of redundancy
    • A critically important function of a system, subsystem, or components can be preserved by alternative parallel or standby units.
  • Principle of worst case
    • The design of a system should consider the worst situation to which it may be subjected in use.
    • Murphy’s Law: If anything can go wrong, it will.
engineering pitfalls
Engineering Pitfalls
  • False sense of security in technology
  • Read Case Study 3.7
    • Human error is involved heavily
  • Recommendation for Reading:
    • Set Phasers on Stun: And Other True Tales of Design, Technology, and Human Error. Steven Casey. Aegean Publishing Company, Santa Barbara, CA. 1993. ISBN: 0-9636178-7-7
how safe is safe enough
How Safe is Safe Enough?
  • Can absolute safety be achieved?
  • Remember the concept of “risk”.
  • What is “acceptably safe”?
  • Remember the Risk Assessment Matrix: Severity vs Frequency
hw3a chapter three
HW3A – Chapter Three
  • Review Questions (p. 24-25)
  • 3, 6, 9, 12, 15, 18, 21
  • 15 points
  • Due 9/19 (beginning of class)
hw3b hazard awareness advisor
HW3B – Hazard Awareness Advisor
  • Go to
  • Select “e-tools”
  • Go to “Expert Advisors”
  • Select “Hazard Awareness” and download it to your computer
  • Run this “expert system” to evaluate your workplace or someplace you have worked (or someplace you want to work).
  • Write a one page memo that describes what you have learned from this exercise. I especially want to know if you’ve learned of some new workplace hazards. Do you think this is a useful tool for industry?
  • 30 points, due asap