Large Cabinet Build

[Bryan Cramer]

2.jpg
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]

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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 . 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 . 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.