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<DIV><FONT color=#000000><FONT size=3>Frank:</FONT></FONT></DIV>
<DIV><FONT color=#000000><FONT size=3></FONT></FONT><FONT
size=3></FONT> </DIV>
<DIV><FONT size=3>I have not participated in this discussion simply because it
is over my head. I'm learning, not teaching. </FONT></DIV>
<DIV><FONT size=3></FONT> </DIV>
<DIV><FONT size=3>However, if you think if compression as the state where
something is smaller than it wants to be, and tension as something being larger
than it wants to be, that can simplify the engineering definitions. If a
traditionally crowned board is held smaller than it wants to be, it is under
compression. On this board that is constrained by the rib, the bottom is
smaller than it wants to be because of the rib. The top is smaller than it
wants to be because of the bottom - and the fact that it is one piece, this
panel. If the top, although it is larger than the bottom, it's smaller
than it wants to be, so it is under compression.</FONT></DIV>
<DIV><FONT size=3></FONT> </DIV>
<DIV><FONT size=3>Does this make sense???</FONT></DIV>
<DIV><FONT size=3></FONT> </DIV>
<DIV><FONT size=3>dave<BR></FONT><FONT face=Arial size=2>*********** REPLY
SEPARATOR ***********<BR><BR>On 9/27/99 at 12:01 PM Frank Weston
wrote:</FONT></DIV>
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<DIV><FONT color=#000000 size=2>Richard,</FONT></DIV>
<DIV><FONT color=#000000 size=2></FONT> </DIV>
<DIV><FONT size=2>Forget humidity. Forget made up numbers. Do
this. Tape a flexible steel rule onto the end of one side of a spruce
stick (a wooden yard stick will do for demo purposes). Mark the
opposite end of the steel rule on the spruce stick. Bend the spruce
stick. Notice that as you bend it, the mark on the spruce changes
position relative to the steel rule. If the rule is on the outside of
the bend, the spruce gets longer. If it is on the inside of the bend,
the spruce gets shorter.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>If the spruce is elongated it is in tension, if shortened,
in compression. Now, bend the spruce stick and glue another piece on
the bottom. Nothing has changed. When the glue dries, the
assembly will hold a curve. The outside curve of the spruce is still
longer than it was originally, and it is still in tension.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>I hope this is simple enough for you.</FONT></DIV>
<DIV><FONT size=2></FONT> </DIV>
<DIV><FONT size=2>Frank Weston </FONT></DIV>
<DIV><FONT color=#000000 size=2></FONT> </DIV>
<DIV> </DIV>
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<P>Lets say we have a panel exactly 1 meter across grain, and a rib that
is 998 cm long in a room with 50% relative humidity. The panel is put
into the oven and dried to the point that it is also 998 mm long. Now if
we take the panel out and simply let it re-adjust to the room humidity
it will "grow" back to 1 meter in length. The top and the
bottom of the panel will both shrink the same, and grow back the same
amounts... ok so far ?? But if we quickly attach the ribs while
the panel is at 998 and then allow it to re-grow to what ever length the
room humidity and this constraint from the rib allows for, its length
will be less the 1 meter. (This following what I think I got from Dels
description.) Ok.. assuming this is a correct picture so far, stretching
a measuring line across the <I>top</I> of this ribbed panel after
its re-adjusted to room humidity shows that it measures less then 1
meter.
<P>If, (Frank) you accept that this is correct so far, I would
appreciate it very much to know the reasoning behind why this top half
of the panel is not (if it is not) to be considered in compression. As
initially stated, please follow Dels kind "laymans language"
approach to demonstrating the reasoning.
<P>Richard Brekne <BR>I.C.P.T.G. N.P.T.F. <BR>Bergen, Norway
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<PRE>
_____________________________
David M. Porritt
dporritt@swbell.net
Meadows School of the Arts
Southern Methodist University
Dallas, TX 75275
_____________________________</PRE>