Nearly all engineering designs serve a purpose. When they fail to serve the purpose for which they were designed, they are considered to have failed. Failure is an expectation, as nothing has an infinite lifetime. As such, all engineering designs have an expected or intended lifetime after they are built and delivered, which may be thought of as a best-before date.
Things with moving parts, like cars, may have a lifetime of fifteen years and 200,000 km. That is because a very popular mode of failure is fatigue. The strength of a material actually decreases when it is stressed, even at low values. On the other hand, static objects, like chairs, tend to survive longer than is required – typically, the paint will fade long before a leg breaks. A chair tends to go out of style before it falls apart. This is not surprising: whenever it is possible, engineers apply a high safety factor to their designs.
A safety factor is a number that represents the extent to which a product is over-designed. If a chair is designed to hold a 250 lb individual before yielding or buckling, a safety factor of five (typical when human life is at stake) may be applied when determining the thickness of the wooden legs. As a result, a failure would occur if a 1,250 lb individual were to sit in the chair. The weight and material cost associated with thickening the legs of the chair is very small. In fact, a chair is first designed for aesthetics and ergonomics alone. The last step of the design process is to check the extent to which the design is structurally sound. It is so easy to design a chair that its factor of safety becomes an after-thought.
It is not as trivial to design an airplane, for example. Here, aesthetics do matter, but are of lesser importance than proper function. Safety is of the highest importance for many products, and the first order of business for these is to assign minimum allowable safety factors for the various failure modes of the various parts. If an airplane is intended to take 2,000 flights in its designed lifetime, the wings may be sized so that they ought to survive 10,000 flights without a failure. Still, the plane will be grounded after 2,000 flights so that its safety factor remains at five.
When human life is not at stake, the safety factor may be reduced to 1.4, or when necessary, 1.25. These figures may seem arbitrary, but they are chosen based on statistics, and insurance companies that ensure the parts require minimum values. Creating aggressive designs of this type is the closest most engineers get to living on the edge.
An example of an engineering design with a very low safety factor is a satellite. It is merely a hunk of metal with some wires, and carries no life as it orbits the Earth. Insurance companies still charge huge sums to ensure satellites, but far less than space missions, which transport living people. The trade-off for the engineering firm goes as follows: which is cheaper, to pay launch fees for extra mass (more mass will allow for a higher safety factor) or extra insurance costs for a design that is tight on safety? In all cases, a tight design with careful engineering is chosen, simply because the cost to place a satellite in Geo orbit is around $20,000 US per kilogram payload.
Although safety factors are associated with engineering, we all make use of them when we make decisions in our day to day lives. The best example of this is when we buy a home.
In choosing a reasonable mortgage to undertake, those looking to purchase a home are often advised (and even encouraged) by bank employees to undertake the maximum mortgage their current incomes can support. This is ridiculous, as it corresponds to a safety factor of 1.0, and an engineer would never place themselves in such a situation with regards to any design, even one that does not place one personally at risk. A mortgage does place the home owner in financial risk, and as such, should be taken very seriously.
The recent economic crisis has really hurt first-time home buyers. Many of them have lost their jobs, and their unsubstantial safety factor associated with their mortgages has left them in big trouble. So many Americans have declared bankruptcy, and a sad number of suburban streets have a very low occupancy rate. We need to learn from this.
It is prudent, and very smart, when undertaking a mortgage, to apply a safety factor of two. How can one do this? Pretend that one of the two people involved in paying for the mortgage were to lose his or her job. Choose the maximum mortgage that would still be affordable in this case.
This sort of conservative approach would of course delay many people from buying their first home, but it would allow them to sleep easier at night once they do invest. Risk is among the top concerns for a program manager at an engineering firm, and ought to be considered when it applies to major life decisions.
The decisions we make have a corresponding safety factor. In the name of pragmatism, we must all apply reasonable safety factors to our lives. How would you feel if you travelled in a car with a safety factor of 1.25? It could fail if the temperature were to be lower than had been designed for by just a few degrees, or if the engine were revved at just a slightly higher rate than the automaker had predicted.
On the other hand, there is a balance that we must strike. It would be prohibitively expensive to apply a safety factor of ten (if not impossible) to all facets of an airplane. We must realize that there is some inherent risk associated with everything that we do, and make decisions that we are comfortable with. As in life, there is an important balance to strive for as an engineer, whose holy grail is an optimal design.
It’s like the old joke. The pessimist says the cup is half empty. The optimist says it is half full. The engineer points out that the cup is twice as large as it needs to be.
A safety factor of two, as it turns out.
As an hopeful engineer, it was interesting to learn about safety factors but more importantly, I never taught of using such an application for buying a house. I have always tried to be financially prepared but this had never corssed my mind. Good article and very helpful.
I suppose we can, if we so choose, apply a safety factor to many aspects of life. Like when we give ourselves 45 minutes to travel by car to a job interview that is just 15 minutes away. SF of 3 in case of traffic.
Hey I am designing the mechanical chair what factor of safety should I take?
Sanket, what is a mechanical chair? Is it just simply a chair, or does it have moving parts? Usually, a chair has a high safety factor, like 2 or even 4 (so a chair designed for 300 lbs could actually support 600 lbs or more). The focus for a chair is often aesthetics, and the dimensions are over-designed... this makes them last longer.
hola buenas, somos unos estudiantes de la universidad de barcelona y queremos saber el factor de seguridad que tiene que tener un somier de una cama, alguien nos podrí ayudar? muchas gracias
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