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Re: load factor question

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>Subject: Re: "[SEAOC] question"


>I am a law professor teaching builidng regulation in the Fire protection
>engineering department at the University of Maryland.  I am trying to
>understand something about the "load factor" term used in limit state
>design.  a commentary that I have indicates that the load factor is used
>"to account for those instances when the actual load may be greater than the
>Calculated load" and that it is the "safety factor" in the calculation.

>Is this correct?  


I think you're pretty close with your interpretation. The Load Factor is
intended to allow for uncertainties in loads on structures. We try (our
building codes try) to set design loads which represent upper limits on
loading that could occur during the life of a structure. Then we expect
users of the structures to limit occupancy loads to the maximum stated
values. However, we can't control what users might actually place on the
structures, and our guess about maximum climatic loads such as snow or wind
will be wrong, as likely as not. So we have an additional load factor for
that uncertainty.

There are also uncertainties in the reliability of structural materials. A
batch of steel or concrete may be weaker than it was supposed to be, and
construction workmanship may slip and result in poorer welds or misplaced
reinforcing. So we need a safety factor which allows for material
uncertainties as well as load uncertainties.

Our Canadian codes and the Uniform Building Code separate load uncertainties
from material uncertainties with two factors - one is "phi", a number less
than one, which we multiply against the specified strength of the material.
Phi is smaller for wood and for concrete than it is for steel, because those
materials are supposed to have strengths which are less predictable than
steel strength. The other factor, the "load factor", allows for the
possibility that loads on the structure during its service life may actually
be higher than our estimate of the probable maximum loads.

So, we multiply the specified design loads by a load factor, and material
strength assuptions by a Phi factor, to get a total safety factor which is
supposed to result in a pretty reliable structure. Most of the time.

If you really want to try a legal question, can you tell us what our chances
are of being sued, when we say something controversial in an e-mail message
to our structural engineers' group? Suppose we try to warn other engineers
about a what we believe to be a dangerous practice in the industry, but it
offends or hurts a person or a group of people?
 
                                          Jim Warne, Vancouver, BC