Re: Monday answer.
Today's trivia question information was derived from a copyrighted pages at other sites. For full use go to:
http://www.woodbin.com/calcs/sagulator.htm
A number of points identified on that site, some of which follow.
�Notes
1. The eye will notice a deflection of 1/32 inch per running foot, or 3/32" (0.09") for a 3' wide bookshelf.
2. Once you have a maximum allowable sag figure in mind, you can design a shelf by tweaking the material types and dimensions.
3. A fully loaded bookshelf weighs 20-25 pounds (9-11 kg) per running foot, or 60-75 pounds (27-34 kg) for a 3' wide shelf.
7. The deflection calculations are based on average wood stiffness properties measured in controlled laboratory conditions (U.S. Forest Products Lab).
8. � As an engineering rule of thumb, wood beams/shelves will sag an additional 50% over time beyond the initial deflection induced by the load.
Tips for Managing Shelf Sag
� If shelf span is reduced by one-fifth, stiffness is roughly doubled (deflection is halved).
� If shelf span is increased by one-fourth, deflection doubles.
� If shelf span is doubled, deflection is eight times greater.
� If shelf thickness is doubled, deflection is reduced to one-eighth.
� If shelf depth is doubled, deflection is cut in half.
� Slide a support mid span under a shelf and it can support eight times more weight.
� Anchoring a shelf at both ends so that there is no (zero) movement can increase the stiffness of the shelf by up to five times. This opens cross grain problems however.
Credits
Harry A. "Steve" Stephenson for providing advice and formulas. Another very helpful resource was Bruce Hoadleys book - Understanding Wood: A Craftsman's Guide to Wood Technology.� Also helpful information can be found from �Woodworkers Essential Facts and Formulas and Short Cuts� by Ken Horner
Answer
�� glass .04
Sapele wood .06
Yellow Poplar .07
Red Oak .07
White Pine .09
�� Fir Plywood .09
Balsa .22
MDF .22
You see that anything with a sag deflection of greater than .09 is pointless without some correction. Even then, over time, with most woods all he work you put into it will be unpleasing if you under build. If fully loaded you can count on bad stuff if you lock yourself into IKEA or office catalog type MDF covered bookcases. With all those winter projects going it might be wise to plan for maximum use for that project shelf--especially all those HD TV media centers.