<!DOCTYPE HTML PUBLIC "-//W3C//DTD W3 HTML//EN"> <HTML> <HEAD> <META content=text/html;charset=iso-8859-1 = http-equiv=Content-Type><!doctype html public "-//w3c//dtd html 4.0 = transitional//en"> <META content='"MSHTML 4.72.3110.7"' name=GENERATOR> </HEAD> <BODY aLink=#ff0000 bgColor=#ffffff link=#0000ee text=#000000 = vLink=#551a8b> <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> <BLOCKQUOTE style="BORDER-LEFT: #000000 solid 2px; MARGIN-LEFT: 5px; PADDING-LEFT: = 5px"> <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 </P></BLOCKQUOTE></BODY></HTML>