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RE: Elastic "shortening" of structural members.

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There was no mention of which material was being used - this post assumes
structural steel is the material.  Anybody have a project in concrete where
shortening was a concern?
 
In my experience, elastic shortening affects steel columns subjected to
heavy loads more than beams.  The biggest concern is the differential
shortening of adjacent columns, especially when a facade/curtainwall is
attached directly to the columns.  Glass curtainwalls are designed with
expansion/contraction joints and can be more forgiving to the movement of
the structural support members.  However, masonry is not quite as flexible
and is prone to cracking if not detailed correctly for expansion and/or
deflections of the structural frame.  
 
My strategy to reduce these effects is to try to equate the compression
stresses of adjacent columns.  Engineering mechanics yields the formula:
delta L = PL/AE
Usually all columns in a building are of the same material and length on a
given floor so the terms L and E can be eliminated when comparing members.
This yields:  P/A  which is the compression stress in the column.  A column
that is highly stressed will shorten more than a lightly stressed column.
So, adjacent columns should have compressive stresses relatively equal to
minimize the differential shortening.  
 
In addition, an analysis should be made for the structure when the large SDL
is removed to compare the compressive stresses and column "growth".  
 

William J. Keil, P.E. 

-----Original Message-----
From: Solutions Research Centre (Malaysia) [mailto:ltb(--nospam--at)pc.jaring.my]
Sent: Monday, August 23, 1999 9:27 PM
To: seaint(--nospam--at)seaint.org
Subject: Elastic "shortening" of structural members.



Hello,
Have anyone of you ever consider about this thing called "Elastic
'shortening' of structural members"?
There is a concern on the fact that when we design say a beam or column to
take a massive superimposed deadload, SDL and this beams or columns in turn
would have an effect of either 'squash' or 'shrunk' to take this
'heavyweight' of SDL and would really like to understand what are the
effects, if any, on these structural members if that 'heavyweight' SDL would
be removed after a period of time say 2 yrs, 5 yrs??

Thanks

cc choo