Large Cabinet Build

[Bryan Cramer]

I am commissioned to build 4 library bookshelves for my local school's library. Each cabinet is 54" high by 84" long and 13" deep. :eek: Are there any suggestions on building a temporary assembly table to assemble the cabinet? I have a traditional style work bench that is 30" by 60"; way too small for this project. Can you help?

Also do you have any pointers when working with a large scale project like this. I will break the sheet goods down with a track saw. The joints are but joints with biscuit renforcement. I will use screws and pocket screws to hold it together while the glue dries. I only have so many long clamps.
Tonight I will post a sketchup drawing of the cabinet.

Thanks

[Prashun Patel]

You can get a 24" or 30" x 84" luan hollow core door from HD for cheap. That on a pair of saw horses makes a wonderful assembly table.

[Richard Coers]

I just curious, and more than a little apprehensive. You have a commision to design and build bookcases, and you need help with a design for an assembly bench? I just hope you do your homework on shelf strength and sag under the huge loads that books can put on a shelf. Under designing a piece that goes into a public place could end up with you testing your liability insurance.

[Michael Dunn]

84" WIDE?!? Geeze... My brain first told me you said 84" tall. I know that's what you were told to build, however, I would try to convince them to change the design. Sure, you could reinforce the shelves, but a complete design hangs would be best IMHO.

[scott vroom]

What is the longest shelf span in your design?

You'll find this helpful: http://www.woodbin.com/calcs/sagulator.htm

[Bryan Cramer]

You'll see in the sketchup drawing I have a divider in the middle so the shelf span is about 42" instead of the 84". That is the shortest shelf span they want. It will be built with 3/4" maple hardwood and plywood. I will edge the shelves with 1 1/4" front and rear edging. I did use the sagulator; what a great tool it is! I wanted extra renforcement so I added a rear edging as well as the front. The sagulator doesn't have that as an option so adding the extra edging should give it the extra reinforcement needed.
A door was my first thought for an assembly table. I suppose I will end up doing something like that.

[scott vroom]

12" of books from my shelf weighs 34 lbs. Using this figure, and your supplied data, I get .02 in/ft sag using Sagulator. Adding the rear edging will only inprove that number....you should be fine.

[Bryan Cramer]

That is what I got too. The ends aren't fixed (per thier discresion) so I will use strip (pillister) metal shelf supports dadoed into the case sides.

[Mark Bolton]

Pretty substantial span for a school. I dont mean to sound like chicken little, the liability factor is one thing, and the "ladder effect" of some kid(s) sitting/climbing, laying, and so on would be something I would be thinking hard about in my design as well. It gets you nowhere to say "well its what they wanted". Its your responsibility to inform them of any concerns with regards to the end product.

Unfortunately in the public arena unless your covered by a pretty solid indemnification agreement you had just better make sure your covered. If something happens a parent, insurance company, or both, will try to get a piece of everyone they can and being the maker, you'll likely bear the brunt.

[Dave Zellers]

I don't care what the sagulator says, a 42" long 3/4" plywood shelf is going to sag significantly if it is loaded up with books. I would probably glue a 1/2" and 3/4" ply together and if they had a slight crown, I would align them, not oppose them to start out with that in my favor. In a situation like this, I like to make the shelf solid and symetrical e.g., 1 1/4" thick with a nosing applied and trimmed flush so even if THAT sags, it can be flipped over down the road.

Also, I don't think you will be happy with a luan door for this project. These will be fairly heavy and I would want a more substantial assembly table. Mine is a 4x8 sheet of 3/4 UL plywood screwed to 2x4's on edge that were carefully hand picked for quality, left to acclimate in the shop for a week or so then jointed straight and ripped parallel. All around the permitter and 2 down the middle lengthwise with 2" coarse thread screws about 8" apart. Having that available has made me much more productive. I actually built it as the table to build an assembly torsion box on but pushed that to the back burner since this is already so flat. I rolled on 3 really heavy coats of urethane and couldn't be happier. If you don't have room for a 4x8 table I would say the length is more valuable than the width. Or maybe something hinged that could be dropped down when not needed.

[David Weaver]

Sag. I'd split those in half and put dividers in, even though it makes for ugly shelves.

I'd use hollow doors, too, they were the gold standard for us when I was building model airplanes as a kid - cheap and flat.

[Mark Bolton]

I would probably glue a 1/2" and 3/4" ply together and if they had a slight crown, I would align them, not oppose them to start out with that in my favor

I would agree completely that a 42" shelf span using 3/4", regardless of the solid wood edging, is a guaranteed sag. A bookcase I built 20 years ago has 3/4"BB shelves, rabbeted solid wood edging both sides, less than 36"OAL, and they sag.

That said, I think you'd be guaranteed a crown with an unbalanced glue up (1/2&3/4) even though its ply. Would seem two 5/8" or bump to 1" with two 1/2" or 1 1/2 using 3/4" would be safer.

Though the sag will definitely be an issue my concern would less be the sag and more to make the shelf flush on both faces for two reasons. One, as you say its flip-able when it does sag, and two it eliminates the "hand hold" that will be built into the shelf edge with an 1 1/4" solid wood edge on a 3/4" ply shelf. You can walk into any municipal library and see signs telling children not to climb for a book but rather to ask. I would imagine those signs are effective about 6% of the time.

If I wanted 42" clear span in this application I'd be looking at a lot more than 3/4" ply, sagulator or not.

[Bryan Cramer]

I am glad I posted this thread. I thought I coved all the issues with in the design process, but you guys raise new questions I have never thought of before. Let me add more info: The library currently has 48" bookshelves with a full shelf span. They said they have had no trouble with kids climbing the shelves because they were filled with books which also helped weight it down to the floor. The original shelves were just 3/4" particle board wich sagged quite a bit. I expressed my concern with the shelf length but they still insisted on long shelf spans. The idea was to have a long unit divided out to match their existing shelf span. The longest I agreed to go was 42". All four units would be placed together along a wall. I plan to bolt the four sections together, but not to attach them to the wall. This would add to the stability of the whole unit. The school wanted the long units because there was less wasted space from multiple sides. A single unit with a divider gave the most shelf length in the smallest foot print.
In short, I had to design a multiple unit book shelf that could come apart, had shelves as long as possible, and had to fill up 28' of wall space. I should ask how would you have design it? Would it be better to build the shelves in 8 42" sections or 4 84" sections with a divider?

They agreed that I was to be liable for the construction of the units, not the misuse, but I will still restate our agreement upon completion (and put in writing). I do know about liabilities but I have had little experience with liability issues. I am also not a professional woodworker.

Thanks for all your help!

[Mark Bolton]

Are you planning on getting insurance for this project? Again, not trying to be a chicken little but just defending yourself against some outlandish allegation could bankrupt you. Its sad that in this day and age you cant get to do a little something you like (woodworking), help the school (very rewarding), and make a couple bucks while your at it (assuming you will have some profit). But unfortunately its the truth, liability is a major major issue. Absolutely anything could happen with these cabinets and even if its eventually deemed misuse or lack of supervision you will still have to defend yourself through the process. Its a very serious concern.

Im sure I, and others, could be overstating the sag issue however I know in my experience, with a shelf that long, I'm not. How I would design the shelves would depend on several things the main one being budget.

[Bryan Cramer]

I strongly encourage them to choose the option of strengthening the plywood for the shelves, but they insisted on a single layer with front and rear edging because of budget. Anything will be better than the existing particle board! I do understand the l liability concerns. I have made it clear to the school I am not liable for misuse. Maybe I should write up a sort of warranty stating what and what not I am liable for, but I have no legal experience so any lawyer could punch holes in it. At least the superintendent understands woodworking (he does some himself) and agreed to my proposal about my liability.

[Bryan Cramer]

261318
Here is the sketchup picture.

[Michael W. Clark]

Why are they not anchored to the wall? Are they sitting out in the middle? At 54" high and only 13" deep, it is a tipping hazzard. You can design around the shelf sag easy enough, but personally, I wouldn't do it unless the units are anchored to the wall or floor. Is installation part of your scope? If not, provide a means of anchoring and tell the school that the installer has to anchor them. Then it is the school's and installer's liability.

An indemnity clause will not stop a civil suit. Your contract is with the school, not the parents, not much you can do about it.

Mike

[Bryan Cramer]

261329
Here is the sketchup drawing. I did warn them that the 54" might be tippy, but they said a load of books stabilizes it. Originally they wanted 66" but I settled on 54". Looks like I have some investigating to do before I start.

Thanks for all the usefull information!

[Joe Angrisani]

....I am also not a professional woodworker.....

You are now.



.....I have made it clear to the school I am not liable for misuse....

In it's current state, I would guess this means next-to-nothing in the big picture.

[Grant Wilkinson]

Your pic is a .jpg not a sketchup file so I can't open it in sketchup. How thick is the back you are putting on it. If you go with 3/4", you can drill shelf pin holes in it like those in the gables. Doing so adds a lot of sag resistance.

[Richard Coers]

Joe says it very clearly. Once you accept money for any work you are 100% liable. They can tell you how they want it built, how wide of spacing, etc....But if you built it, it's always yours if it fails, or falls, or bends, etc....That's why you need a $1 million liability insurance policy for commercial work, maybe more. Ever seen a kid climb a cabinet by using the drawer as a step? How about the janitor standing on the top to change a light bulb? Once you design and build it, you are the professional and no signed piece of paper that even waves all liability will keep you out of court.


You are now.




In it's current state, I would guess this means next-to-nothing in the big picture.

[Phil Thien]

These threads are so funny, you really never know which way they are going to go.

To the OP, make sure the shelves are secured to the wall, or to shelves that may back them up. If you aren't installing them, make sure you inform them (in writing, at delivery, that the shelves need to be secured properly to avoid tipping, and that is the responsibility of the installer).

Don't trust the sagulator. Go to the school, and check how many books they are packing on each shelf currently. Now get some plywood, cut a test shelf, suspend it between some blocks of wood, and load it with the same # of books, distributed similarly as what you saw at the library. If it sags, redesign your shelf (add edging front/back, make it thicker, whatever). Then retest.

Go to your public libraries and examine their shelves for ideas/techniques.

You are going to be fine. Do not let fear paralyze you. You are not splitting atoms, just study what others have done, do some testing, and you'll be fine.

[Bryan Cramer]

Testing the shelves is a great idea! I would have never thought of that. Thanks Phil!

I did some asking around and here is what I will do: I will still have a written contract with the school stating I am not liable for improper installation and misuse even though that might not mean anything. I will provide a secure spot with screw holes to fasten them to the wall. On the back I will write that the cabinet needs to be fasten to the wall to avoid potential tipping. I will not install them. As stated above this is not the ideal scenario, but I am not in the business yet and don't have the insurance. I was told this will protect me from common mishaps. It is the unlikely ones (the one in a million) mishaps that could hurt me. I was told that planning for the common occurrence is better than just letting it go. This is a great example why anyone who is in the business needs insurance.

[John T Barker]

I saw the dimensions and the numerous cautions about the size of these bookcases and I wanted to throw in my two cents. I sell unfinished furniture, four of our manufacturers sell a variety of bookcases. All three have a maximum length of 60." Anything 48" and over has a center divider. Typical construction is plywood with faceframes of solid hardwood. Shelves are ply and one uses a hardwood facing on the front of the shelf, glued so the 2" width gives more support to the 3/4" shelf. When asking these manufacturers to build a case where the shelf must span more than 42" they wave off any guarantee. I would be very cautious about such a bookcase and would advise my client to change the plan. It is cheaper to build one big bookcase than two small ones and this may be in their thinking, but I would not build this.

I just read your comment on the tipping and I would remind your client that just about any manufacturer that wants to stay in business gives their client hardware to attach tall pieces to the wall that might be tipped by the user, especially children. This hardware is readily available in hardware stores which should be an indication to them of its need. Unless they are putting books only on the lower shelves simple logic will tell them that this will tip as easily loaded as it does empty...and it will tip if someone hangs on it. I move them every day.

[Richard McComas]

My day job is working in the carpentry shop for our local school District. I have built quite a few book shelve units with 46 1/2" spans. I contact cement 2 pieces of 3/4" plywood together and edge band the front and back with 3/4" by 1 1/2'' hardwood. Works well.

[Mark Bolton]

Now get some plywood, cut a test shelf, suspend it between some blocks of wood, and load it with the same # of books, distributed similarly as what you saw at the library. If it sags, redesign your shelf (add edging front/back, make it thicker, whatever). Then retest.

The problem with this is the sag never happens immediately unless you've drastically under-engineered :eek:. It happens over time. While you may be able to measure some deflection upon initial loading its what happens over the course of months/years and a few humidity cycles. Beyond that, indemnity clause or not, in a commercial application like this while you may not specifically design for it, there is usually some factoring in for the scenarios mentioned. Climbing, janitor or teacher standing atop one of these to hang decorations, and so on. I agree that one should (and likely ultimately would) never be held accountable for such misuse, but its often cheaper to slightly over engineer (safety factor) than to rely on a misuse clause in a contact.

We've all seen commercial casework, store fixtures, and so on. As a rule its usually built much more robustly than anything for the residential market. Its for a reason.

Looking at a large library installation would be helpful as long as your local libraries are not like most Ive been to that have switched to virtually 100% metal shelving (hmmm... wonder why). It likely helps with the fire marshal as well.

[Phil Thien]

The problem with this is the sag never happens immediately unless you've drastically under-engineered :eek:. It happens over time. While you may be able to measure some deflection upon initial loading its what happens over the course of months/years and a few humidity cycles. Beyond that, indemnity clause or not, in a commercial application like this while you may not specifically design for it, there is usually some factoring in for the scenarios mentioned. Climbing, janitor or teacher standing atop one of these to hang decorations, and so on. I agree that one should (and likely ultimately would) never be held accountable for such misuse, but its often cheaper to slightly over engineer (safety factor) than to rely on a misuse clause in a contact.

We've all seen commercial casework, store fixtures, and so on. As a rule its usually built much more robustly than anything for the residential market. Its for a reason.

Looking at a large library installation would be helpful as long as your local libraries are not like most Ive been to that have switched to virtually 100% metal shelving (hmmm... wonder why). It likely helps with the fire marshal as well.

From the sagulator page:

Comparison to Sagulator Estimates
For floating shelves, the theoretical sag estimates computed by the Sagulator agree reasonably well with the empirical woodshop sag measurements for shelves that have a relatively short span (under 32") and for shelves that are 12" or deeper. For wider span shelves that have a depth of 8" or less, the Sagulator values tend to overshoot the shop values - by as much as 35%.

For fixed shelves with no support edging, the Sagulator estimates considerably undershoot the shop measurements. The measurements are approximately 2.5 to 3.5 times greater than the calculated values, depending on shelf span and depth. The disparity is smallest for shelves that have a short span (under 32") or that are 12" or deeper. The disparity increases as shelf span increases and depth decreases.

For fixed shelves with a support edging, the shop measurements are approximately 1.5 to 3 times greater than the Sagulator estimates. Once again, the disparity is smallest for shelves that span 32" or less or that are 12" or greater in depth. The disparity increases as shelf span increases and depth decreases.

It's not entirely clear why the Sagulator estimates are not in closer agreement to the shop sag measurements but I suspect a big part of the answer is that some of the assumptions made by the beam deflection formulas do not hold up in the real world. One of these is that the shelf support structure is completely rigid; in reality, some degree of racking and movement will occur in a shelf, especially for one that is free-standing. Another assumption is that the shelf material has a fixed elasticity, known as the modulus of elasticity, and that the elasticity remains constant along the length of the shelf. The reality is that the elasticity of lumber can vary considerably from piece to piece and even within a piece depending on the grain of the wood, moisture content, and any internal defects in the wood.

Though disconcerting, the computed versus empirical shelf sag disparities are well under an order of magnitude (10x). For a shelf with an observed sag of 0.15", an estimated sag of 0.05" is off by a factor of three, but for all practical purposes, it's probably not a big deal given the small magnitude of the sag. However, when dealing with sag of 0.25" or more, a 3x difference could be crucial. For this reason, shelf sag correction factors have been added to the Sagulator to bring the calculations more in line with the recorded woodshop measurements.

The Sagulator shelf sag correction option is turned on by default. It is most applicable for shelves that have a uniform loading (vs center loading) for the following scenarios: 1) floating shelves, 2) fixed shelves, no edge supports, 3) fixed shelves, front support edging. Also keep in mind that the correction factors were derived from specific wood samples so they may not be as applicable to your shelf material. Even so, I think you'll find the correction factor option will improve the accuracy of the Sagulator results and encourage you to try it out. Any good or bad feedback is appreciated...



Based on that, I think the OP would be far better off doing his own tests.

My own experience is that particleboard and MDF sag more as time marches on, but plywood and hardwood not nearly as much.