torsion box shelf strength

[dirtydeeds]

the weight of library shelves is 35kg per linear metre

they should not deflect more than 2mm over this distance or it can be seen



ive recently made 4 torsion box shelves, 1080 x 270 x 30

the frame was made out of timber merchant pine ripped into 20x20 strips

top and bottom made of 6mm oak faced mdf, fixed with pva and 18 guage pins

on the leading edge 20x38mm oak



strength test

BEFORE i fitted the bottom, the customer and me both stood on the shelf, a combined weight of over 120kg


with this strength i just pinned the bottom on




hopefully ill have photos tonight

[wizer]

yes photos would be good. What brackets did you use?

[dirtydeeds]

none.............. the client didnt want to see brackets OR fixings

the reason for leaving the bottom off was so the fixings (plugs and screws) could installed then hidden.






if i did the job again id build the frame insitu (fixed in an alcove) using my domino for the joints

the frame i made had no joints, the pine was just glued and pinned to the mdf

[wizer]

wow 120kg just with Rawl plugs and screws? Impressive

[paulchapman]

In The Workbench Book by Scott Landis, there is a piece about a workbench/veneer press made by Ian Kirby using the torsion box method because of its strength and resistance to flexing under heavy pressure. Ian Kirby achieved quite impressive results using fibre board and honeycomb paper in the voids between the internal cores.

Cheers

Paul

[engineerone]

i have seen a number of ian kirby articles about torsion boxes and he advocates using a beam fitted to the wall, which is one you have sized to the inside of the box. you fit this first, and then screw up from underneath, and plug if necessary.

he's a big bugger and stands on his, and the thing to remember is that most american walls are studding covered by plaster board so more impressive i think.

as scrit has said, the whole gives the strength.

when i was first introduced to them about 15 years ago, the advice was to pin or staple the internal ribs together, no need to glue these ribs. gluing and clamping the top with a few pins was all that was required.

paul

[mrgrimsdale]

paulchapman said:

In The Workbench Book by Scott Landis, there is a piece about a workbench/veneer press made by Ian Kirby using the torsion box method because of its strength and resistance to flexing under heavy pressure. Ian Kirby achieved quite impressive results using fibre board and honeycomb paper in the voids between the internal cores.

Cheers

Paul

Thats how cheapo doors are made. Not a new idea at all. "Torsion box" a bit of a fiction IMHO

cheers
Jacob

[9fingers]

Hmm! I can see how the torsion box construction will be inherently strong but attaching it to the beam fitting on the wall I'm struggling with. Surely the pocket in the torsion box to allow for the beam on the wall will need to be pinned and glued very securely to the beam so it can only be a permanent fixture and needs to be finished in situ.

Presumably cascamite type glues (UF?) are best for this application as creep would be no good in a laden shelf.

I'm still learning lots here - thanks in advance.

Bob

[paulchapman]

mrgrimsdale said:

paulchapman said:

In The Workbench Book by Scott Landis, there is a piece about a workbench/veneer press made by Ian Kirby using the torsion box method because of its strength and resistance to flexing under heavy pressure. Ian Kirby achieved quite impressive results using fibre board and honeycomb paper in the voids between the internal cores.

Cheers

Paul

Thats how cheapo doors are made. Not a new idea at all. "Torsion box" a bit of a fiction IMHO

I don't think anyone is claiming that it is a new idea. Yes, doors have been made that way for a long time but that's just one application. It's a useful construction method for lots of things where light weight and strength are needed - like the wings of aircraft, as Scrit mentioned, where the method has been used for many, many years.

Cheers

Paul

[engineerone]

ikea make a kind of torsion box which involves having a metal bar fixed to the wall, then the round bars stick into the shelf.
they are held by a couple of screws in the underneath.
not perfect, but it works.

tonight i will check what other references i have to explain it better.

jacob is only partially right, the importance of a proper so called torsion box is that it does rely on a number of internal spars or ribs. i would think that modern doors only used this technology post second world war, and post the mosquito development.

doors tend to have fewer ribs than torsion boxes, and as most of us know because they are made in production terms, down to a price, they tend to use cardboard as internal strengthener.
but then an aircraft product called mallite used to use honeycombed cardboard internally, and was very strong.

paul

[mrgrimsdale]

Ancient in fact - any box is a "torsion box" it would seem, but some resist torsion more than others, depending on design details.
Similarly "stressed skin" construction is as old as the first tent or coracle i.e. prehistoric, probably skin and sticks.

cheers
Jacob

[paulchapman]

engineerone said:

ikea make a kind of torsion box which involves having a metal bar fixed to the wall, then the round bars stick into the shelf.
they are held by a couple of screws in the underneath.
not perfect, but it works.

There was an interesting thread on the other forum a year or so ago about invisible fixing to walls. Where the wall is strong enough, I've sometimes used Rawlbolts and threaded rod to fix beams and shelves that have to take a heavy weight but you often need to be able to hide the fitting.

Cheers

Paul

[Scrit]

wizer said:

wow 120kg just with Rawl plugs and screws? Impressive

I believe a Fisher S7 nylon plug inserted into a correctly drilled hole and using a #10 x 2-3/4in screw has a shear load of over a quarter of a tonne.........

engineerone said:

as scrit has said, the whole gives the strength.

I recently built a torsion beam in situ in a house where the beam was being buried inside an existing plasterboard. More correctly the top of an existing Victorian lath and plaster on stud wall was removed and a 5.5 metre long x 450mm high framework was made-up from clear 3 x 2in PAR pine. The birch plywood surface "skins" were glued and pinned in place and the whole was fixed in place using four M10 x 200mm Rawlbolts. The strength comes from using (as far as possible) a continuous "skin" of defect-free material (we used a particularly good B/BB-grade Finnish birch plywood) and maaximising the glue surface area. Prior to glueing and pinning the skins the framework was just a tad floppy. All the pins did was to hold the glue joint under enough pressure to make a glue joint. After glueing we had a very rigid structural element which once bolted in place and the joists above ring-nailed into it made for an extremely rigid attic floor which passed muster with probably the pickiest BCO you'd ever want to meet.

Scrit

[nickw]

Think of an I beam. If you used just the top and bottom flanges they'd be pretty useless. Ditto for the vertical web on its own. Use all three together, but not joined, and you get a better result, but, because the parts can slide relative to each other it's still not great. Now weld the pieces together and you finish up with a much stronger structure.

A torsion box is just a 3-dimensional I beam.

In both cases you need top and bottom surfaces, you need webs between them, and you need all of them to be thoroughly stuck together for best results.

[mrgrimsdale]

No it aint. A box is a box, a beam is a beam. A box may be a beam, or a shelf etc etc
A torsion beam is something else altogether, see here for example:
en.wikipedia.org/wiki/Torsion_bar
Non of the examples in these threads involve torsion in any way.

cheers
Jacob

[dirtydeeds]

nickw

think of a formwork shutter, it also is a 3 dimensional I beam but has no second face

the point im making is that torsion boxes dont ALWAYS need a second face (flange)

[mrgrimsdale]

Er, what? And what's torsion got to do with it? Nothing as far as I can see.

cheers
Jacob

[dirtydeeds]

im not an engineer

BUT surely this form of constrction because of its relativly wide width to depth ratio ratio also resists torsion

surely even a steel I beam has a torsional strength





this post is realy a question not a statement

[robo]

A beam, simply supported, is in compression on the top and tension on the bottom. That's how I beams work - the "bit " in the middle is not doing a lot except resisting shear and the wider the two layers the stronger the beam.
Torsion is really a twisting force and I suppose the cellular construction helps in this and reduces weight.
In your shelves there will not be much torsion but the cellular construction will allow a wider separation of the skins without introducing more weight - a thin skin of ply etc. is adequate in tension and the skin of ply at the top, prevented from buckling by the cellular infill, will resist the compression.

R.

[engineerone]

at the risk of jumping into a mucky pond deeds, lets think further.

you are right an i beam has torsional strength, but you might think that a square tube would have more torsional strength than a round or oval tube, when in fact it does not because of the possibility of the square edges being pushed out of shape, whereas the round/oval tubes have no edges on which you can push.

the thing about what we are talking about is that the top, bottom and sides are all co-joined and each reacts with and against each other to constrain themselves.

although many modern doors have a similar style of design, to a torsion box, in fact the loadings are different, since the doors are constrained by the two or three hinges, and to an extent, you kind of expect them to warp a little.

the torsion box as a shelf is constrained at the wall edge, but everywhere else is undergoing a downward, and maybe bending moment. the width has nothing to do with it, it is the combination of joining the verticals with the outer skin that imparts the strength.

paul

[dirtydeeds]

so "torsion box" is not "engineeingly" correct

i once called it a box beam shelf. the customer was horrified "i dont want anything that big" she said




enginnerone, give me something short snappy and "engineeringly" correct (without it sounding too big OR too complicated)

[jmk89]

If you make it just like the Wellington bomber (not to mention the R100 airship), you could use Barnes Wallis's terms, geodetic or geodesic (depending on whether it is straight or curved)

[dom]

Welcome jmk89, do you build 'em the other way up down under ;D

[jmk89]

dom said:

Welcome jmk89, do you build 'em the other way up down under ;D

If we didn't the books would fall off ;D

[mrgrimsdale]

dirtydeeds said:

so "torsion box" is not "engineeingly" correct

i once called it a box beam shelf. the customer was horrified "i dont want anything that big" she said




enginnerone, give me something short snappy and "engineeringly" correct (without it sounding too big OR too complicated)

Box construction, structural box, box beam, etc are all well known and well used terms. Torsion box is a new one and is redundant; it has nothing to do with torsion in the way it is used. Athough a box beam etc could be constructed specifically to resist torsion in some peculiar circumstances e.g. anywhere where diagonal braces would be needed. You still wouldn't call it a torsion box or construction.
What seems to have happened is that some bright spark in a magazine somewhere has kicked off this term in describing something or other, and it has spread like a virus.

cheers
Jacob
PS if you google torsion box you find it only in woodwork. It seems to be one Ian Kirby who kicked it off. It's his fault.