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RE: QUERY: How Would You Model This?

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In a three bay frame, the center bay has the most tension at midspan,
and a lesser tension at each end.  You can model this with a point load
(parallel to the member length) each end to create  the minimum member
tension at the end.  Say 500#. Add a uniform load (parallel to the
member length) along the member to gradually increase the tension until
you get to the center.  Say 100 plf.  So for a 10 ft member you get 500#
tension at the ends, and 1000# tension at midlength.

<500#  <100#  <100# <100# <100# <100#  100#> 100#> 100#> 100#> 100#>
500#>

Similarly at the first and last bay, you can adjust the uniform and
point loads so the tension is 500# one end, and decreases to 0# at the
other end.  For a 10ft beam, a 500# force one end, and 50plf along the
beam

<50# <50# <50# <50# <50# <50# <50# <50# <50# <50# 500#>

The forces net out to zero externally, but create the internal tension
you want.

This is a little bit of work, depending on how many bays you have.  You
could simplify it by just putting a point load each end of each beam
that equals the maximum tension in that beam.  To follow the example
above:

middle bay   <1000#                  1000#>
end bays     <500#                    500#>


Charles F. Espenlaub, III, P.E.
Martin-Espenlaub Engineering



-----Original Message-----
From: Bill Polhemus [mailto:bpolhem(--nospam--at)swbell.net]
Sent: Thursday, January 18, 2001 6:36 PM
To: seaint(--nospam--at)seaint.org
Subject: QUERY: How Would You Model This?


How do you typically model the "chord force" induced in a structural
steel frame
forming part of a perimeter shear resisting system by diaphragm action,
when you
are using a general-purpose analysis program?

The problem of course is that these are really INTERNAL forces and
aren't
readily induced in the members by external loading methods. The
particular
program I am using does not have a "prestress force" option. I was
thinking I
MIGHT be able to do it with thermal differential, but I haven't thought
this all
the way through. Anybody have any other ideas?