Some common misconceptions

The public writ large has no idea whatever what civil engineers do for a living, which might explain why so many crazy ideas have been broadcast during the coverage of the Florida condo collapse. The two most insidious:

  1. Safety Factors are not cumulative. Suppose that one part of an assembly is rated to carry 100,000-pounds, and has a safety factor of 3.0, meaning that it will fail at a load of 300,000-pounds. A different part of the same assembly is rated to carry 100,000-pounds, and has a safety factor of 2.0.

    That does NOT mean that the assembly has a cumulative safety factor of 6.0. No. The safety factor for the assembly is 2.0. This should be obvious, but I’ve lost track of the times some contractor on a construction site has told me his jury-rigged “fix” is fine because of all the other safety factors.

  2. Estimated “life” assumes maintenance. A structure that has an estimated “life” of 30-years, for instance, is not going to make it to 30-years without routine maintenance along the way — and I’m not talking about a coat of paint every few years. I’m talking about roofing, retrofitting to accommodate evolving drainage patterns, major mechanical such as HVAC. Generally, estimated life means that the structure will continue to be useful for its original purpose for X many years, e.g., a retail storefront or a grade school; after that, it may need a retrofit in order to serve some other purpose.

The past week has made me acutely aware of how many misconceptions are widespread about these sorts of things, and I’ll be spending more time in the future trying to clear them up.

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