Acceptable deflection in jarrah bookcase shelves

[Knight_of_Ni]

Using the "sagulator" (http://www.woodbin.com/calcs/sagulator.htm) I calculate the deflection on 19mm x 900 mm jarrah to be between 1.5mm and 3mm (depending on width and loading weight).

Is this an acceptable amount of deflection? Or do I need to think about adding support to the shelves?

The reason that I am trying to avoid support is that the bookcase I am building is matching an existing old bookcase that has 900mm lengths in 15mm jarrah with very heavy loadings (set of old encyclopaedia for example) with minimal to NO deflection.

Is older jarrah so much better than the stuff we get today?

I would prefer to stick with the 19mm and no shelf support - but I do not want to be coming back here to ask for a fix for sagging shelves...

Cheers,

Slade

[Pusser]

Does the bookcase have a back on it and are the shelves fixed. If so you would not have any sag. I don't know about movable shelves. If you have a back you could put a centre support in the back if it is a problem. The books would cover the holes?! You could also use shelves which bow opwards by a few mm and use the deflection to level them out! If none of these are suitable you could rebate a piece of aluminium or steel bar into the back of the shelf.

I would test a shelf of the size you want with a test load and see if it bothers you first.

Pusser

[echnidna]

19mm Jarra would be quite ok.

The proof is in the bookcase you are comparing too.

[Knight_of_Ni]

Thanks Pusser and Echnidna,

No back, but deep rebates on the sides (15mm).

The existing case also has no back support, and much smaller rebates - so in principle it should work.

Worth the risk, and retrofit some support if neccessary....

Cheers,
Slade

[Harry72]

Slade, using that calc you need a load of 1000kg on a 900x200/19mm shelf(even loaded) to get a 1mm deflection!

I would like to actually see that tho!

Test it... get the jarrah and put it between to bricks then stand on on it, it should take your weight with a little sag/flex.

[Frank&Earnest]

[quote=Harry72;606274]Slade, using that calc you need a load of 1000kg on a 900x200/19mm shelf(even loaded) to get a 1mm deflection!

:? As I read it, 30 kg will result in 1.88 mm sag, according to the notes undetectable by the eye on 900 wide. IMHO you mixed measurements.

If the defection is unacceptable, we'll court martial the shelf.:D

[eddie the eagle]

Hi Knight,

I'd make the trenches that the bookshelves fit into about 1/3 of the thickness of the side - without looking at your drawing, if it's a 2" side, then the trenches are about 15mm deep, a 25mm side has a trench of about 10mm or so.


The deflection of 3mm is undetectable to the eye - if you need more support, use the metal cleat or build up the front or back lip of the shelf.

Cheers,

eddie

[Chesand]

I have made 2 bookshelves 1 metre wide & 2.1 metres high from 19mm recycled jarrah. They do not have backs and there is no noticable deflection.

I used sliding dovetails for top, bottom and middle shelves and trenches for the others. They have a narrow face frame as well

[Knight_of_Ni]

Thanks all for your responses.

Harry72 - I have put in Jarrah - 30kg Uniform load - 900 x 160 x 19 mm and get a deflection reading of 2.36mm.

I was considering having some narrow to shelves of 125 mm wide at the top, which is where the 3mm deflection figure came from. Then again, narrow shelves are for small paperbacks - so 900 mm of paperbacks would probbaly only weight <12kg which results in a 1.2 mm deflection.

I may increase the width of the higher shelves for reasons of practicality, rather than deflection. Most books in the shops are oversized these days, and the need for the smaller width shelves may not exist.

Cheers,
Slade

[Harry72]

[quote=Harry72;606274]Slade, using that calc you need a load of 1000kg on a 900x200/19mm shelf(even loaded) to get a 1mm deflection!

:? As I read it, 30 kg will result in 1.88 mm sag, according to the notes undetectable by the eye on 900 wide. IMHO you mixed measurements.

If the defection is unacceptable, we'll court martial the shelf.:D
Yep looks like I did :doh:
I just played with the calc again it seems to have a little quirk... in mm it =1.88 and in cm it =1.9mm:??

[Frank&Earnest]

I just played with the calc again it seems to have a little quirk... in mm it =1.88 and in cm it =1.9mm :??
The little quirk is called rounding...:) Cheer up, the accountant that audited my SMSF made a similar mistake, we are all human.:wink:

[Harry72]

Yes rounding... but using metric there is no need for rounding, its not like you need to convert it from imp to met... its met to met!

[Frank&Earnest]

Yes rounding... but using metric there is no need for rounding, its not like you need to convert it from imp to met... its met to met!

Number of after the point digits. If 2, mm1.88 = cm0.19

[BobL]

I used a 1000 x 140 x 19 mm jarrah board with a 17.5 kg semi-central load and got a deflection of 2.65 mm

Sagulator give between 2.15mm to 3.45mm so it seems to be working correctly.

Is it acceptable? That's about what we have for our jarrah bookcases and you don't notice it unless you really look down the sides or compare a lightly with a heavily ladened shelf.

Cheers

[Harry72]

Number of after the point digits. If 2, mm1.88 = cm0.19

Nah mate... 1.88mm=.188cm surely a calculator can compute a 3rd number after a decimal point!
Its a quirk!(and yes I realise in this shelf situation it doesnt mean anything:q)

[Frank&Earnest]

Nah mate... 1.88mm=.188cm surely a calculator can compute a 3rd number after a decimal point!
Its a quirk!(and yes I realise in this shelf situation it doesnt mean anything:q)
I give up...:U If it really concerns you, you might write to them to say that accuracy to 1/100th of a millimetre is not good enough for you and they should present the results with more digits.:wink:

[Harry72]

:D:D:D:D:doh: