Replies

1. 
   DonaldCBrown | Oct 19, 2007 06:32pm | #1

   http://www.woodbin.com/calcs/sagulator.htm

1. 
   ToddSliver | Oct 19, 2007 06:43pm | #2

   Thank you. Right after I posted this, I googled shelf calculator and found the same one. Thanks again. Todd

1. 
   dougmeyer | Oct 20, 2007 01:13am | #3

   Yeah but....

   Alwas a but.  First off it does not seam to take into acount if you put a front or back edge on the shelf (at least the version I did) and frankly I am not sure it does a great job. I ran the numbers on a shelf I built (to fit existing cases I was given) and with out a thick front and back (the shelfs I built have said fronts and backs) for the depth of the shelf using the numbers they give for books, the shelfs should work, so with the front and the backs added (and a pin in the center of the rear) I figured I was pretty good, but they sagged something nasty. (I now have a support on the bottom 2 shelfs).

   I will admit that size is a bit extreem in both depth and widths but the numbers did work in the caculator but not in real life. So keep that in mind when desiding how much to trust the sagulator.

   Doug

1. 
   HowardAcheson | Oct 20, 2007 07:21pm | #4

   I have found the Sagulator to be fairly good whenever I have used it. That said, there is nothing like building a prototype of the shelf and actually testing it. In the shop I was involved with, we would build a mock up of the shelf, put the ends on a couple of 2x4 blocks and load the shelf with the items expected to be used. We would then measure the sag. If there was any sag, we would consider and test options like building the shelf thicker, putting a support lip on the front, back or both or permanently affixing the shelf in the rear.A mock up is the only sure way of determining sag resistance.Howie.........

1. 
   dougmeyer | Oct 21, 2007 11:57pm | #5

   True, a prototype is not a bad thing, and I know a lot of peaple like the sagulator I just wanted peaple to be aware that sometimes it is not perfect.

   Doug

2. 
   kjm | Oct 22, 2007 09:02pm | #6

   The sag calculation isn't that complex. Basically, you need to calculate the moment of inertia for the shelf's cross-section, determine how it's loaded (even load over the length, or concentrated in the middle), and whether it's fixed on the ends or free to move. The standard equations are available from a variety of web sites -- look for "beam deflection."Even if you understand everything I just wrote, you can still get into trouble. The single biggest difference between reality and the equations is the contribution of creep, or deformation over time. Plywood tends to creep under load and shelves will develop a permanent "set" that persists even when the load is removed (hint: make plywood shelves removable so you can flip them if this happens). MDF is worse, and particleboard is even worse. In fact, for long term book storage, my opinion is that MDF and particleboard are essentially useless unless the shelf is supported on three edges. Too many of us have seen what happens to plywood, even 3/4-in thick, when it's not supported right during storage -- if it can creep under its own weight, it's definitely going to creep in use unless you design around this.Creep resistance for engineered materials (plywood, etc.) is going to depend strongly on the glue used to hold everything together.One of the reasons that I like wide edgings on plywood is that the edging is both stiffer than plywood (less deflection) as well as largely creep-resistant. We have about 7000 books in the house, so I've built a few bookshelves, and I've seen what happens over time. The latest shelf project has 30" spacing, fixed ends, and 1-1/2" wide edging in white oak. Fully loaded with hardcover books, there's little or no measurable deflection.The best edging is wide and attached on edge. If you look at the moment of inertia equations, it's clear why: resistance to deflection is proportional to the cube of the distance from the center of the cross-section. This is why I-beams work so well, and why you can get a substantially stiffer shelf by using 1" thick shelves instead of 3/4".The other tricky part is the load and length of the shelf. If you're storing books, the weight is proportional to the length, so going from 30" to 36" increases the weight by 20%. Unfortunately, the beam deflection is proportional to the cube of the length as well, and the resulting deflection is proportional to the 4th power -- a 20% increase in length means that the sag will more than double.Calculations get even more complicated, of course, but this should help. The short, mostly math-free version:* For creep (permanent set) resistance: Solid wood is vastly better than engineered wood of the same thickness, and the farther you get from solid wood, the worse it gets.* Length really does matter. A workable bookshelf design at 30" will probably perform like crud at 36" wide, and don't even think about 40" without additional support.* Thickness also matters. That cube relationship means that going from 3/4" to 1" thick means that the sag decreases by a factor of 2.3. A 1x2 used as edging vertically is 4 times as effective than if it's attached horizontally; if I can believe my calculations, a 1x3 would be 11 times better.* As always, your mileage may vary. Check sag (deflection under load) and creep (permanent deformation under load) with the materials and design you plan to use. Sag is definitely less if the shelf ends are attached to the case, so if the shelf works ok supported by a couple of bricks or paint cans, it should work even better if it's attached to the case sides or back.Ken (I was a real engineer once upon a time) M

3. 
   dougmeyer | Oct 23, 2007 04:25am | #7

   And I thought my 2000 plus books was bad, but then again I am a single guy in my thirtys so I have to buy them all myself (although of late finding anything to buy is harder then the buying is)

   Doug

2. 
   frenchy | Oct 24, 2007 05:18pm | #9

   DonaldCBrown

    What a rip off<Grin> here I thought I could put a 9x12 white oak timber in and calculate if it would be adequate across a 22 foot span..

2. 
   WillGeorge | Oct 24, 2007 06:02am | #8

   There is a new post in here about using metal in your woodworking projects. I am NOT stating that this is the best way but worked for me..

   My Son-In-Law is a Dr. of Education and he has more books than I could ever open much less read and understand and he had these expensive bookshelves that the SAGGED ALOT!

   I said "self what do we do".. And I said to myself.. "I was a tank mechanic in the Army and we used METAL!"

   So I purchased some 1/2X1/2 angle iron.. Take a hunk of that about 8 foot long and put it between two supports and try to push it down to the ground! OK if ya can bend it... IF ya' drive drive a truck over it or you into that World Wide Wrestlin' thing...

   You can sink that hunk of 'steel' into the back of a 3/4 inch shelf with a groove and dado and never notice it. Clean all oil off and glue into place down the center of the shelf.. A face frame on the shelf would help..