Long Bookcase w/ Open Front - Design Question

[PittTrack]

I have been planning a bookcase for a while now and am loosely basing it off of the free plan from FWW (link below)
Free FWW Plan

Now this looks great but the desire is to have the bookcase fit into an opening that is ~61" wide and about 13" total in depth.  To accommodate this I can do two different things:

1) Plan it with 6 posts. Basically make two compartments and have 2 adjustable shelves on each side for a total of 4 shelves, each shelf would have 4 pins and be ~29" long

     Top View (x = Post - & | = panels)
   X---------------X---------------X
   |               |               |
   |               |               |
   |               |               |
   X---------------X---------------X

2) Plan it with 5 posts (remove the one in the front) and have an 'open' feel to the unit while getting some extra support in the rear as there would be shelf pins at every post 

     Top View (x = Post - & | = panels)
   X---------------X---------------X
   |                               |
   |                               |
   |                               |
   X-------------------------------X


4 Options for shelf material (still up in the air) - Listed in the likelihood of me selecting (top being most favored)
   1) 3/4" 9 ply with a nice vertical piece (similar to this Vertical front) but instead of simply gluing a piece to the front, the piece on the front of the shelf will be rabbited so that the shelf sits in it and gives it even more support.  The vertical piece will start out as 13/16 x 1, the rabbit will be about 1/2 so there is good support on it.
   2) Do the same as in #1 on the front but add a vertical piece to the back.  Only instead of adding a piece that is thicker than the shelf go the shelf thickness, kind of Like This on the back to get even more strength
   3) I may make some cauls and try my hand at making real oak shelves and glue pieces up, I'll still do the vertical front.  I've had some issues with boards being glued up that long and getting a little twist afterwards.  
   4) Make a torsion box style shelf (least likely) similar to this: Torsion Style

Notes:
-All adjustable shelves will have the 1/4" shelf pins inserted into the metal shelf pin sockets 
-Material will most likely be red oak, the shaker panel will be 1/4" plywood as will the rear of the unit


============================

My Question is this:
If I pick the second option (wide open) I think there will be plenty of support but will it be setting myself up for failure?  I really like the openness to it and for the time being it will have a lot of books on it as the kids have an amazing collection.  Eventually I  know it will go to being something that will house picture frames etc but mentioning the now as it's the worst case scenario.  Yes I went to the sagulator site but I don't know how to figure out how the rear shelf support would play into the equation.

[Phil S.]

The sagulator might help sort that out -

https://www.woodbin.com/calcs/sagulator/

[paul2004]

do not do option 2, 61 inches is too long of a span for bookshelves.. I am assuming at some point, books might be on it.
For 3/4" plywood, the rule of thumb is that you can span 32" max, any more than that, you risk it sagging. (or at least that is my personal rule of thumb, lol)

[GeeDub]

I would consider an alternate design.  I would go with three vertical panels, sides plus a center divider.  Less desirable IMHO would be some sort of center dividers per shelf.  You could also up the thickness of the material used for the shelves if a center divider will ruin the look you want.

A 1" thick white oak shelf could be profiled at the front to appear to be 3/4".  I did this in a media cabinet that had to hold some heavier than usual electronics.  This cherry glue-up panel is 1-1/8" thick.

   

The bottom edge at the front is 'raised' like a raised panel door.  The round overs give it a visual appearance of a fat 3/4".  If plywood is your thing you could laminate 2 1/2" piece of ply and apply a solid wood similarly shaped front edge to get the thinner look.

   

[jteneyck]

Certainly 61" is too wide without support unless you use a torsion box.  You could do it with your option 1.  Whether you do it like in the link or rabbet it to the shelf is irrelevant; it's the vertical height of the cleat that determines the stiffness.  And if you want to do something at the back edge, use another cleat and not that inset hardwood edging.  I'm not sure how wide those cleats need to be but it would be pretty easy to mock up a test piece to find out.  The general formula for beam deflection is:



Deflection = load (uniformly distributed x span^4 / (384 x Young's Modulus x Moment of Inertia)  

So you can see why span is so important; if you double the span the deflection goes up 16X.  Young's Modulus is the stiffness of the material you are using; plywood is a lot lower than solid wood, and you can look up the values.  Moment of Inertia is determined by the shape of the beam you are bending, both the plywood and the cleat.  For any rectangle:

MI = Width x Height^2/12  

Both the width and height of the cleat (or plywood or hardwood shelf itself) are important, but deflection goes down with the square of the height of the cleat so a 2" high cleat will result in 4X lower deflection than one only 1" high.  

John

[DaveR1]

I like GeeDub's suggestion of the center frame and panel arrangement. I did a quick comparison here. The one on the left is to the dimensions in the article. The top on the right hand one is 61 inches long.