Despite the fact that many engineers treat them deterministically,
every load (with the possible exception of dead load) is strongly
dependent on what happened in the past and on statistics. Humans are
notoriously poor at predicting the future and even where rigorous math
is not used most people make decisions based on what seems likely to
happen. (Thus we don't carry eight spare tires in our cars.)
If we based our designs on what is possible (given the past) rather
than what is likely the design loads for all locations on earth would
be something like the following:
human live load = density of people in the black hole of calcutta
snow load = based on glacial cover during an ice age
wind load = several hundred miles per hour for direct tornado
volcano load = hundreds of feet of ash, mud; high thermal loads from
electrical load = direct lightning strike
earthquake load = perhaps M10 at ground zero
Once we decide to base loads on a specific service life or on a level
that is less than the maximum possible we should consider the
statistics of the situation. Once we decide that a given transient
loading is appropriate, we have set our risk tolerance. When
combining multiple transient loads, it makes sense for the combination
to have the same likelihood of occurrence as we previously accepted.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley, P.E., S.E. E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc. Tel:(206)292-1200
1301 Fifth Ave, #3200, Seattle WA 98101-2699 Fax: -1201
From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
Sent: Thursday, June 07, 2001 6:14 AM
Subject: Re: Snow load in combination with seismi
Ralph Hueston Kratz wrote:
. > I'm no statistician, but I doubt that the probability of this
. > is "none."
. > About 30 years ago a floating bridge in the Seattle area was sunk
. > combination of wind speed, wind direction (straight down the very
. > and narrow Hood Canal), state of the tide, etc. If any of these
. > had not occurred in the manner they did the bridge would not have
. > flooded and sunk. The probability of their combination in the
. > manner is "probably" very small, but it happened.
. > I'd be wary of considering unlikely events impossible.
. > Remember that kid who threw the basketball the length of the court
. > won $1 million? What were the chances of that?
Probability is for gamblers (including insurance companies) and
mathematicians, not for engineers.
Statistics always represents what happened in the past, not what will
happen in the future.
A. Roger Turk, P.E.(Structural)
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