From: Charles Espenlaub <Cespenlaub(--nospam--at)martinaia.com>
Date: Thu, 18 Jan 2001 20:07:18 -0500
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#>
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
<50# <50# <50# <50# <50# <50# <50# <50# <50# <50# 500#>
The forces net out to zero externally, but create the internal tension
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
middle bay <1000# 1000#>
end bays <500# 500#>
Charles F. Espenlaub, III, P.E.
From: Bill Polhemus [mailto:bpolhem(--nospam--at)swbell.net]
Sent: Thursday, January 18, 2001 6:36 PM
Subject: QUERY: How Would You Model This?
How do you typically model the "chord force" induced in a structural
forming part of a perimeter shear resisting system by diaphragm action,
are using a general-purpose analysis program?
The problem of course is that these are really INTERNAL forces and
readily induced in the members by external loading methods. The
program I am using does not have a "prestress force" option. I was
MIGHT be able to do it with thermal differential, but I haven't thought
the way through. Anybody have any other ideas?