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Re: journalist query - factors of safety

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John,
 
Factors of safety are not cut and dried single digit numbers.  Developing an appropriate factor of safety depends on three aspects; material uncertainty, loading uncertainty, and construction uncertainty.  Construction uncertainty is usually lumped into material uncertainty when considering building systems.
 
The factors of safety used are contained within the building codes, which are developed by consensus based on probability studies and are typically adopted into code directly from professional recommendations.  The most notable of these would be the American Society of Civil Engineers, American Institute of Steel Construction, and the American Concrete Institute.  We use reduced allowable capacities for materials in combination with amplified loading depending on the type of materials and structure and our level of certainty.  Frequently an engineer may apply some judgment and build-in an additional factor of safety where he / she believes this is warranted.
 
Wood materials have a greater degree of uncertainty than steel or concrete, and therefore a higher factor of safety built in to the allowable capacities.  Also, the factor of safety is different depending on the type of loading (bending, axial tension or compression, torsion, or shear) and our experience record regarding predictability of behavior of the different materials under these different stresses. Elements that are more critical to the performance of the structure, like columns, will have a higher factor of safety than other parts of the structure.  Again, elements that have a higher degree of uncertainty such as soils typically have the highest factor of safety.
 
In addition to amplifying loads based on probability of occurrence, we also look at the different conceivable combinations of loads that may occur and how patterns of application of these loads will affect the structure.  Loads we are confident of, like the self weight of the structure, have a lower factor of safety than earthquake loading. 
 
The other aspect of loading is probability of occurrence and in many ways are a function of economics.  Earthquake and hurricane loading are highly uncertain and in a counter-intuitive way may actually have a lower factor of safety.  If designing for extreme forces which may or may not occur over the life of a structure, we will allow materials to go closer to their absolute limit.  The goal shifts from uninterrupted continued use too limiting loss of life and catastrophic failure.  The structure may be completely worthless and un-useable after the event, but if it did not collapse and the people were able to exit safely it was a successful design.  We can design structures to amazing levels of strength able to withstand virtually any calamity, the catch is no-one can afford to build them.  Tornadoes are an excellent example of this; it is pretty much understood in our society that buildings do not stand up to tornadoes.  Only the most critical structures consider tornado type loading in their design.
 
So to directly answer your question, what does the FOS of 1.3 stated in your post actually measure?  Is this the typical FOS to be applied to loading or an absolute factor of safety of the system?  And if a measure of the system under what level of conditions? Soil properties typically have a FOS of 3 or 4.  If the levee is a soil structure that has a FOS of 1.3 on loading, the system FOS is much higher.  Is this the FOS for hurricane level with a probability of occurrence of once every 100 years?
 
I stated economic aspects earlier.  The Army Corp can tell a jurisdiction that they can construct levees designed for the 50 year flood for x dollars, the 75 year flood for y dollars, and the 100 year flood for z dollars, and they strongly recommend the 100 year flood level.  The jurisdiction than has to decide how much to spend, and all too frequently will choose the lower cost higher risk option.  When the 100 year flood occurs there are all kind of recriminations, though rarely are they directed in the appropriate direction.  Extreme risk, loss of life, and property damage are taken into account in the engineered approach, with the limit being to prevent the loss of life.  Property damage is a given.  The recurring floods on the Mississippi around ten years ago are an example of this.
 
As a society we decide which structures are critical and where to spend our collective wealth.  In California, Hospitals and Schools are built to a higher standard (greater FOS) than commercial or residential structures.  Dams obviously should have a high factor of safety due to the potential for catastrophic loss of life in the event of failure.  If I was in New Orleans, I would advocate the critical nature of the levees surrounding the city.  If the bill was put forth to the voters tomorrow, you may actually get appropriated funds to create proper levees.  10 years from now when memory has faded, the same voters may not opt to spend their money on the levees and be willing to assume greater risk. 
 
Could be I'm getting cynical in my old age.
 
Sorry for the long post.
 
 
 
 
 
----- Original Message -----
Sent: Friday, November 04, 2005 7:33 AM
Subject: journalist query - factors of safety

Greetings. I?m trying to learn about factors of safety in general terms. In levees/floodwalls, documents show the standard FOS to be 1.3. We?ve had some engineers saying this was too low, at least for New Orleans, given the extreme risks involved with a breach during a hurricane. But I am trying to set this issue in context. What are typical safety factors for other kinds of structures ? dams, highway overpasses, hospital buildings, etc. Are they prescribed by code or statute in some cases, or by some other means? How are extreme risks, loss of life, property, etc. taken into account? I won?t publish comments generated on this list. Just seeking input/perspective at this point.

Thanks,

 

John McQuaid

The Times Picayune, New Orleans

Washington bureau

1101 Connecticut Ave. NW s. 300

Washington, DC 20036

202-383-7889

john.mcquaid(--nospam--at)newhouse.com