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
dmitri@cascade-structural.com
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