Where do I stick 'em?

[MorrisWoodman12]

I'm about to put up a shelf on to metal brackets - yes I know they should be wooden but it's quick and dirty for my study. Do I place the brackets at 1/3 and 2/3 across so the shelf length between the brackets is equal to the length outside them. Or should I put them at the 1/4 and 3/4 points so there is twice the distance between as outside. Or ........?
Is there an optimum placement. What do others think?

 

[John Brown]

I would go for 1/3 and 2/3, but I'm sure some other people will come along in a minute and explain why that's a bad idea!

 

[TheUnicorn]

I'd lean more towards the quarter points, my thinking is that everything in between the brackets has two points of support, whereas the ends have only one. obviously lots of variables like studs, and the load you are are putting on the shelf

 

[MorrisWoodman12]

I probably should have mentioned that there are no studs as it's a brick/block wall. As far as the load, quite light. Maybe a book or two but not a whole shelf and then some oddments, pictures and the like.

 

[ChaiLatte]

It all depends on what you want to achieve.

It also depends on how the shelf is to be loaded. Let us say it is for books. If you sort them alphabetially, the load distribution is likely to be uniform*. If you sort them by size, with the smallest to the left and the largest to the right, then the brackets will be unevenly loaded no matter what their spacing.

If we assume the shelf is uniformly loaded, please have a look at this page and choose the points to achieve the outcome you desire (e.g parallel ends [Airy], maximum length [Bessel], minimum sag [0.5536 x length apart]):

https://en.wikipedia.org/wiki/Airy_points
* However, if authors ending in X, Y and Z always produce particularly weighty tomes, that theory goes right out of the window.

 

[MorrisWoodman12]

@ChaiLatte Thank you thats brilliant information. Since the spacing is 0.577 (Airy) or 0.559 (Bessel) or 0.554 (miniumum sag) it's always slightly more than the 'quarter' points. The average is 0.563 so that's what I will go for as I'm just improving the mounting rather than going for any particular target.
Martin

 

[eribaMotters]

1/4 in from each end will look about correct. The Maths will show this is not technically correct, but you can do anything with Maths, just ask a politician.
If the shelf did bow it is probably looking a bit thin, so an approx 30 x 20 mm batten underneath and set back 5mm the front edge should do the job.

Colin

 

[John Brown]

I would go for 1/3 and 2/3, but I'm sure some other people will come along in a minute and explain why that's a bad idea!

Actually, I got that wrong, I'd normally go for quarters. However, this thread has opened up a world of knowledge to me, I had never heard of Airy or Bessel points before!

 

[MorrisWoodman12]

..............However, this thread has opened up a world of knowledge to me, I had never heard of Airy or Bessel points before!

Me too. I guess that's the difference between being an electronic engineer and being a mechanical engineer - using engineer in the truest sense.

 

[John Brown]

Actually, I got that wrong, I'd normally go for quarters. However, this thread has opened up a world of knowledge to me, I had never heard of Airy or Bessel points before!

Never came up.on QI, either, unlike Brownie points...

 

[Spectric]

Was George Airy not the astromer royal who was involved with the first survey of the UK to produce the British National Grid and stick the zero point in Grenwich. I used to write software when involved with cartography and he was pre satelite history in the days we used OSGB36 but now use WSG84 .