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Wind load and pile depths for wooden fences (San Jose)

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Being a  mechanical engineer, I know just enough be to dangerous, when
it comes to S.E. codes.
  * Is there a typical factor of safety designed into building code
     equations (1.5? 2.0?)?<

This is a favorite topic of mine. I have been asked this question by many clients, and nearly all of my mechanical engineer clients have asked me about it at one time or another. The simple answer is that there is no simple answer. We structurals like to hide our safety factors, and make them so inscrutable that we totally loose track of them and loose track of how they combine and add up as we work through our calculations. So when we arrive at our final design, what is the safety factor? Who knows. It depends a lot on how you define it. On the load side, what load are we comparing to? A 10 year return interval event? 100 year? 2500 year? It makes a huge difference. And do we pick the same standard for every load type? Of course not! On the material side, how do we define failure? Yielding? Excessive deformation - how much is excessive? Fracture minus one standard deviation? If we are discussing safety factor, and we use fracture minus one standard deviation, there will be no visible sign (if we were conducting a lab test) that failure had been reached, so is this really the "factor of safety" as it is commonly understood? When clients are asking the question, they are usually thinking about the margin of safety relative to the structure falling down or becoming unusable. Redundancy and load path also play a roll. In a redundant system, if one element nears failure and begins to deform, it may transfer load to another element that has plenty of reserve strength. So although as engineers we would consider this a "failure", the safety factor for the system is a long way from being reached. The problem with this whole complex mess, is that we loose sight of the reality of what we are designing and evaluating. I have worked with many young engineers and even a few older ones that did not have a grasp of the factor of safety. They could tell you if the allowable stresses were exceeded, but beyond that, the structure entered the black void of "failure". As far as they were concerned, if another pound of load was added, the structure would crash to the ground. While this is a useful point of view for verifying code compliance, it does little to further our understanding of how a structure responds to a spectrum of loading conditions. I advocate a radical shift in the way building codes are integrated into our design process. The basic design equation should be rewritten from: phi*material strength >= load factor*load to: phi*material strength >= Safety Factor*load factor*load. Currently, the material standards do a good job of defining the phi and material strength for a given material. ASCE7 (in the States) defines the environmental loads. The Building Code then injects the currently accepted arbitrary balance between safety and economy. It does this by tinkering with the phi and load factors and adding a bunch of prescriptive rules. It would be so much simpler and more clear if the sole purpose of Chapter 16 was to define the Safety Factor for a given situation. 2.5 for steel beams in bending, 5.0 for seismic drag struts, etc. This would enable a much better understanding of the material, load and code elements of design for both engineers and non engineers.

Well, hopefully this stirs up a hornet's nest of engineering debate. If not, I can always bash the conservatives or the liberals, or some ethnic group. That always seems to produce a lengthy, if nonproductive debate!

Dmitri Wright, PE
Cascade Engineering, Inc.
245 SE 4th Ave, Suite A
Hillsboro, OR  97123-4033

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