Guidance on shelves......

[dannykaye]

Do you mean that they are cantilevered out from the back edge? If so the issue is simple bending, not torque.
<snip>
You just need a firm fixing or stiffer shelves, whether solid or box construction.

shelves supported at the ends and along the back weight in the middle pushing the front down, sounds like torsion to me

I need stiffer shelves and so the torsion box (or whatever you call it) - its stiffer

 

[paininthe]

The plane takes off

 

[jlawrence]

Personally, I couldn't care less what the things are actually called.
I've to make some small shelfs for some speakers next week. They'll be invisibly fixed to the wall. By using a box (Torsion or beam or whatever) construction it allows me to make the actual shelf itself a lot lighter than it otherwise would be - obviously this is all comparative as in a big shelf the weight difference would be a lot more than in my small ones. At the end of the day I can use thinner wood (ie offcuts) to make my shelf - less hassle all round imo.

It is good to get educated as to what something should really be called though so please keep up the discussion - though I'm still not sure what a torsion force actually is.

 

[mr grimsdale]

A box construction for a shelf makes perfect sense. To call it a "torsion" box doesn't tell you anything extra. The expression is redundant, unless you are designing for peculiar circumstances involving torsion.

"Torsion" is twisting - the forces which would bend a plank into a propellor shape e.g. if you fixed one end and twisted the other. This is a real problem with aircraft wings and has to be designed for, but not a problem with bookshelves as a rule.

 

[johnjin]

Tiddles
Why do you feel the need to be so rude to a well respected woodworker.
If you could produce even one tenth of the quality of the work of this man I might have a little respect for you as well. Maybe a little read through the forum rules might be in order.

John

 

[dchallender]

quite interesting watching this debate actually :lol:

As an engineer I don't believe it matters what you call it as we're all talking about semantics as the area of engineering diciplines this comes from is Mechanics.

Again the area of mechanics that looks at forces in 2 axes (x&y) is Beam theory (refers to cantilevers, etc. - think bridges) and which discusses the resulting stresses & strains on a 2 dimensional object. Whereas torsion is a result of the stresses and strains where you have forces in 3 axes (x, y and z), however we still use beam theory.

Conclusion it doesn't matter what you call it as its beam theory with the intention of reducing torsion which is causesd by stresses within the structure.

Therefore is we take this argument to its logical conclusion then the resulting outcome will be the prevention of STRESS!!! :shock:

Could I take a pragmatic approach and suggest we call it a shelf from now on

 

[TrimTheKing]

quite interesting watching this debate actually :lol:

As an engineer I don't believe it matters what you call it as we're all talking about semantics as the area of engineering diciplines this comes from is Mechanics.

Again the area of mechanics that looks at forces in 2 axes (x&y) is Beam theory (refers to cantilevers, etc. - think bridges) and which discusses the resulting stresses & strains on a 2 dimensional object. Whereas torsion is a result of the stresses and strains where you have forces in 3 axes (x, y and z), however we still use beam theory.

Conclusion it doesn't matter what you call it as its beam theory with the intention of reducing torsion which is causesd by stresses within the structure.

Therefore is we take this argument to its logical conclusion then the resulting outcome will be the prevention of STRESS!!! :shock:

Could I take a pragmatic approach and suggest we call it a shelf from now on

Excellent explanation, thanks for that.

Finally, and purely for my own understanding, what if it's not a shelf?

If you make a hollow table top with 2 faces (top & bottom) let's say 8x4 for ease, then a 6" gap between faces and the edges clad with 6"x1" stock. Next fill the middle bit with half lapped ribs (effectively what The WoodWhisperer and other have built as an outfeed table and called it a torsion box), what is that called in the engineering world?

Ta

 

[mailee]

OH MY GOD! PLEASE, DOES IT REALLY MATTER?? :roll:

 

[jlawrence]

I can see the problem with calling things by the wrong name.
In the end it causes confusion between those who work in that sphere and those who use the incorrect term.
There are numerous other terms in life which have over time taken on an alternative meaning - it can and does causes masses of confusion at times.
A bit of education is good - I've learnt things in this thread that I didn't know before so that can only be a good thing.

 

[mr grimsdale]

OH MY GOD! PLEASE, DOES IT REALLY MATTER?? :roll:

Only that innocent woodworkers might be lead into thinking that there is a "torsion" boxy way of making a shelf etc, differetn from an ordinary box.

 

[TrimTheKing]

OH MY GOD! PLEASE, DOES IT REALLY MATTER?? :roll:

Necessary?

I wasn't being antagonistic, as I thought I made clear in the way I wrote it.

In way yes it does matter because if I want to ask a question about something I nothing about then I need to know WTF I am asking. Don't get so bloody riled, it was a simple and innocently asked question.

Christ it's getting f&^king chippy round here lately!