How do I calculate the strength of a design?

[Mr. Lahey]

I'm designing a wall bookcase that will go up to the ceiling, which is about 12 ft high or so. I'd like to determine the strength of my materials and design. For example, I would like to calculate the maximum weight an individual shelf can support, and how much total weight the bookcase can support.

In general, I would like to be able to determine these things without becoming an engineer lol. Anyone know a good resource for learning?

 

[gator]

See if this helps:

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

George

 

[Trent Mason]

That's a really cool link George. Thanks for posting.

One thing I remember about designing bookcases is that if you are designing it to go all the way to the ceiling, it's probably a good idea to build it in place. If you bring it in to the room as a single unit and it is as high as the ceiling, you won't be able to stand it up (unless it is a vaulted ceiling). :thumbs_up:thumbs_up

 

[mkepke]

I believe you need to be prepared to spend some days learning the physics involved, or work to emprically established values ("best practices").

You can simplify the problem a lot by specifying what the design requirements are, e.g. how much weight must the bookcase hold ? How many shelves ? Must the shelves be removable or fixed ? Must the bookcase be freestanding ? Must the bookcase be made out of wood ? MDF ? etc. Which dimensions are fixed - for example, the 'case is 12' tall, but must it be a certain width and depth or can that vary to fit other design requirements such as strength of the required materials ?

-Mark

 

[DaveD]

I think you are over thinking this. Its not a Saturn booster rocket or the space shuttle. Its just a bunch of boxes/rectangles, Probably made out of 3/4" lumber and/or 3/4" plywood with a face frame and 1/4" plywood back.

If you dado in some of the shelves in to the sides it will really firm the 'box' up. If the shelves are about 36" wide or less and made out of 3/4" solid lumber they won't sag unless you put a couple of hundred pounds of books on the one shelf. Put a 1-1/4" thick front strip on the shelf and it will hold even more.

The whole cabinet will typically hold whatever the floor will hold. The floor will hold more than anyone could practically put in the cabinet.

Here is the design for one I did
http://i180.photobucket.com/albums/x168/DJDphotos/04170001.jpg

for this cabinet
http://i180.photobucket.com/albums/x168/DJDphotos/09040001a.jpg

here is it loaded up
http://i180.photobucket.com/albums/x168/DJDphotos/10300009.jpg

The biggest challenge was how to go about making it in pieces and merely 'assembling' it in the house that was 10 miles from the shop. When done it looks like it was custom made in place one stick/board at a time. The other challenge is that some of those 'cubicles' are actually recessed in to the wall another 4" to gain depth for the stereo equipment. The existing lights in the ceiling limited how deep the cabinet come out in to the room.

Granted, if you want looooong shelves and minimal lumber thickness, or if you just want to nail it together with no rabbits/dados/etc, then it might have to be 'engineered'.

 

[HMH]

I think you are over thinking this...... If the shelves are about 36" wide or less and made out of 3/4" solid lumber they won't sag unless you put a couple of hundred pounds of books on the one shelf. Put a 1-1/4" thick front strip on the shelf and it will hold even more......The whole cabinet will typically hold whatever the floor will hold. The floor will hold more than anyone could practically put in the cabinet.

+1.

I was contemplating chiming in on this last night, but didn't want to "accidently" get too technical, (I'm a structural engineer). To calculate the exact strength, you have to enter a world of basic physics, statics, material dynamics, lumber grading, & receding hairlines. It would be much easier to work from established values like Mike suggested.

Also, with a 12' tall bookshelf, I would suggest lagging into wall studs near the top to prevent it from tipping over. 12' of books would make a pretty big mess...

Good luck!

 

[Mr. Lahey]

I think you are over thinking this. Its not a Saturn booster rocket or the space shuttle. Its just a bunch of boxes/rectangles, Probably made out of 3/4" lumber and/or 3/4" plywood with a face frame and 1/4" plywood back.

If you dado in some of the shelves in to the sides it will really firm the 'box' up. If the shelves are about 36" wide or less and made out of 3/4" solid lumber they won't sag unless you put a couple of hundred pounds of books on the one shelf. Put a 1-1/4" thick front strip on the shelf and it will hold even more.

The whole cabinet will typically hold whatever the floor will hold. The floor will hold more than anyone could practically put in the cabinet.

Here is the design for one I did
http://i180.photobucket.com/albums/x168/DJDphotos/04170001.jpg

for this cabinet
http://i180.photobucket.com/albums/x168/DJDphotos/09040001a.jpg

here is it loaded up
http://i180.photobucket.com/albums/x168/DJDphotos/10300009.jpg

The biggest challenge was how to go about making it in pieces and merely 'assembling' it in the house that was 10 miles from the shop. When done it looks like it was custom made in place one stick/board at a time. The other challenge is that some of those 'cubicles' are actually recessed in to the wall another 4" to gain depth for the stereo equipment. The existing lights in the ceiling limited how deep the cabinet come out in to the room.

Granted, if you want looooong shelves and minimal lumber thickness, or if you just want to nail it together with no rabbits/dados/etc, then it might have to be 'engineered'.

I like it! it's very versatile with all the different sized shelves.

 

[Mr. Lahey]

This is my design so far. I'm planning to keep this design, except this is only about 7 feet tall, so I plan on adding additional shelves on top. The large empty space in the center is where the fireplace is and a flat screen tv will be above the fireplace. This is planning to use 3/4 pine.

I'm concerned that with the large empty space, it may not be able to support additional shelves above that area.

 

[Gotcha6]

Are you planning on using a back in the book case? It would help reinforce the long spans of shelves if they aren't adjustable. It would also help if you faced the shelves as suggested, but used 5/4 material for the long spans. I assume you have that available. If not, 4/4 SYP stair tread should do very well.
I'd also make the case with certain strategic shelves (4', 8', etc.) fixed for rigidity and the others adjustable.
I had a book case similar in my last home that followed a vaulted roof to the peak @ ~12' off the floor. It was only good for knickknacks, photos and dust collection (not the GOOD kind). Too inaccessible without an 8' ladder. :eusa_doh: JMTCW.