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Re: Live load / occupancy correlation

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I was walking across the Brooklyn Bridge last year during the blackout, and
the bridge was swaying from side to side like crazy with the roadways full
of pedestrians. Pedestrian/human loads can be surprisingly high, plus
lateral pedestrian induced vibration is an extremely complex and poorly
understood area.

Michael Ludvik, PE



                                                                           
             "Michael L.                                                   
             Hemstad"                                                      
             <hemstad.ml@tkda.                                          To 
             com>                      <seaint(--nospam--at)seaint.org>                 
                                                                        cc 
             10/26/2004 03:31                                              
             PM                                                    Subject 
                                       Live load / occupancy correlation   
                                                                           
             Please respond to                                             
             <seaint(--nospam--at)seaint.or                                             
                    g>                                                     
                                                                           
                                                                           




Jim,
I remember seeing photographs of studies done to arrive at live loads.
They were done using very tall thin people (typically, the basketball
team of the college where the professor worked who was doing the study)
packed together into a square.  200 pound people, per square foot, are
"fluffier" than tall thin people, who have a higher percentage of bone
and, of course, are taller.  No offense intended to either of the above
groups.

Anyhow, it seems to me that you might want to consider the structural
members carrying the load.  If you have small joists closely spaced
without much of a floor over them, it is entirely possible to have a lot
of people jammed closely together over that member--say, standing in
line to vote next week.  If the structural system is such that that
member can't reasonably distribute its load out to less loaded members,
then your live load should be big.  If, however, the floor is, say,
large beams spanning 50 feet with cross bracing and a concrete deck,
then the chances of one beam seeing that kind of load is about nil, and
the lower live load is appropriate.

If you need to make them understand, think of some pretense to have a
bunch of people (ideally, the committee which is your client) gather
tightly around some video monitor or something you bring in.  Note how
much room they take on the floor.  Give each of them a folded piece of
paper and identical black pens to write their weight on, and tally it
up.

Just a suggestion.

On a related topic, I have heard a story (I believe it's true) that a
marathon was started at the Golden Gate bridge.  Someone had the
foresight to mount a camera high up on one of the towers, pointing down,
which took pictures of the crowd.  The runners were crowded pretty
tightly together at the start.  The weight was so large that the curve
of the bridge deck was noticeably flattened.  When the photos were
studied and an estimate made of the weight of the runners, it was
several times the design load of the bridge.

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota

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