How do I know whether a bookcase will rack/sag/twist if hung using butt hinges as a (non-secret) "door"?

Question

I want to take a bookcase and install it as a door, to hold near 100kg max of heavy food items (but probably less than that most of the time). The bookcase itself is 36kg.

The bookcase carcass and central panel use 3 cm solid pine and the bookcase has a 3 mm pine hardboard backing, and its dimensions are 207 cm x 84 cm x 17 cm overall.

The question is, if I support the bookcase on only its one side, and assuming the hinges and screws I use are sufficient, how can I know whether the bookcase itself will hold its shape being supported only from one side, and especially once fully loaded? (Without adding a wheel.)

(Why butt hinges? Because unlike most bookcase-as-door projects, this one isn't meant to be a secret. It's not even meant to be particularly beautiful, just...a really solid support in a really handy spot by the kitchen. I actually want a gap above, below, and fairly generously on the non-hinged side of the door. As for pivot hinges, I don't want to go into the floor and ceiling in this particular spot, but I think they're unnecessary anyway given that I don't care about the gap or the visibility.)

Update/narrowing

Sounds like one of the main factors, perhaps even if butt hinges were not used, would be the backing not being strong enough to withstand racking and twisting. How can I know how thick a backing (13 mm was suggested) would actually be strong enough? (And joking aside, if a steel backing would be the most effective and last the longest, are there downsides to that approach besides cost?)

Answer 1

Do you really think that will work?

Imagine you took the same model of bookcase, and screwed it to the wall edge-on* by driving screws through one sidewall of the bookcase into a vertical stud/structural member of the building. Mounted 100-200mm high so the other sidewall is dangling in air and has zero support.

"You must be mad, Harper! That's a sure way to get injured if not killed!" EXACTLY. Both you and I fully expect this rig to violently explode - either the bookshelf will rack (parallelogram) as the unsupported sidewall drops violently to the floor, and/or the 90 degree joints where shelf meets sidewall will fail altogether, causing the outer sidewall to fall away entirely in an explosive rain of lumber and now-ruined kitchen goods.

You are building a very heavy door.

And it will need to be built from the ground up to be a very heavy door (that looks like a bookcase).

Its primary job will be to resist racking - and the heavy lifting there will be done by the back sheet. That is why that 3mm veneer material isn't going to cut it (unless it's made of mild steel lol).

Its secondary job will be to resist twisting. Consider the view straight-down. The center of support will be at one corner. However the center of load will be in the center - offset in two dimensions from the center of support. The side-by-side forces we've already discussed - however the weight will also be trying to twist the back, bending it forward and down on the outside.

So this is not a "can this bookshelf do this" but rather "how do I blank-sheet design a new thing that does this, whose aesthetics resembles this bookshelf".

Answer 2

My recommendation, even though you don't intend to go whole hog on this, would be to look at the plans folks have drawn up for bookcase doors. That way you'll find solutions which have actually been tested, rather than trying to reinvent this de novo. There are some good videos and articles on the web.

If you don't want to source the more obscure parts yourself, there are kits available ranging from just the specialized hardware and a plan to a fully pre-built and pre-framed bookcase door ready to drop into place. Cost on the latter is high but it does have the advantage of saving the time involved in building and fine-tuning the beast.

Pivot hinges made for large metal doors, with pins coming into the bookcase top and bottom so the weight is resting on the threshold rather than pulling at the side of the door frame, are generally considered best of breed for swinging bookcase doors.

Remember that the bookcase itself must be built to withstand diagonal forces when being opened, and ideally should be supported on both sides when closed so it isn't carrying stress more often than it must.

I'm planning at least one bookcase passageway in my library, just because I've promised it to myself for a very long time and there's a place where I think it would be particularly likely to go unnoticed by visitors. That's not going to be a door-based version, though, at least not in the current plan. And I'm considering using it as a display cabinet rather than bookcase to reduce the weight it would be carrying; my bookcases tend to rapidly overflow.

Answer 3

I'd suggest that you get (or use) an actual door, and not an inexpensive hollow-core "cardboard" one - an actual solid wood door - and hang your bookcase on it.
Mount some brackets at the bottom edge of the door for the bookcase to rest on. These will carry the bulk of the weight, so don't skimp on the hardware here.
Position the bookcase on the brackets against the door and then mount another pair of brackets at the top.
Get a set of hinges made to hang a heavy entrance door, and use 3 or maybe even 4 of them. Ensure that the screws attaching the hinges to the door are at least 2" (50mm) long, and when attaching the hinges to the door frame make very sure that the screws go all the through the door frame and well into whatever rough framing is behind inside the wall.
Since you're somewhere in metric-land I'm not sure what your walls are constructed of. You may need to drill through the door frame and into brick or other masonry in the wall to fit at least one or two anchors per hinge so as not to be relying on just the strength (or lack thereof) of the door frame alone.

Answer 4

Consider using barn door hinge stile

or continuous hinge stile for heavy door

or use 3/4 inch plywood cut to size of the door, hang it with hinges and mount the book case onto it

Answer 5

You have two structural problems

First problem

The vertical stud that supports the shelf is subjected to lateral forces, but was designed to resist vertical forces.

If the arm of the hinges were ~2m and the shelf were very large, say 1.2 m, with a full mass, DL+LL, of 150 kg the horizontal forces are in the range of 0.5÷0.6 kN, that the stud should be able to support (note that, as the door turns on the hinges, the direction of the horizontal forces change wrt the plane of the wall).

To err on the safe side, use two couples of hinges (or some specialty hardware), and mount them on a longish steel plate connected to the stud — the problem with horizontal forces on the shelf side is not relevant because the hinges are connected to a 2D element, very stiff in its own plane.

Second problem

The back panel is very stiff in its own plane, but when subjected to a compressive force is prone to warping, and it is indeed subjected to a diagonal field of compressive stresses. Will it warp?

To err on the safe side, we have different options, not mutually exclusive…

1. use a thicker back panel,
2. glue/nail ridges in the direction of the principal stress of compression (approximately SW-NE in the drawing),
3. screw the shelf planks(?nmt) to the back panel.

I prefer no. 3, but you may want to move the supporting planes inside of the shelf.

---

Of course the stud is subjected also to forces perpendicular to the door plane, because it's not a door, it's a shelf and the centre of mass is some 20 cm away from the back panel, but those forces, due to lower eccentricity and the square root of the sum of squares, cannot change very much the resultant horizontal forces.

Answer 6

The traditional solution to this is turnbuckles or cross braces. That adds stability without adding as much weight.

But really your bookcase is the wrong tool for this job, it is incredibly heavy just by itself (it is about a quarter of the entire weight, yikes). Given that, I would reassess and either:

1. Move the heavy stuff to non-moving shelves, get a normal over-the-door pantry storage unit for the door and stuff it full of light-weight items.

or

2. If the heavy stuff really can't go somewhere else, then combine a solid door + heavy duty hinges + wall-mounted (door-mounted) wire shelving or baskets. Add cross-bracing or turnbuckles as necessary.

But really, put the heavy stuff somewhere else--that's a lot of weight so even if the hinges are perfectly balanced and frictionless it's still a lot of momentum every time you open/close the door.