Shelf Sag

[Aidex]

Hi Everyone

I have to make some shelves and ideally they'll be as thin as possible 18-25mm thick (torsion box has been shot down). The span is approximately 1600mm and the shelf depth will be approx 300mm. I thought to use two sheets of either 9 or 12 mm mdf, route a 10x5mm groove (or 2) on each sheet, glue a mild steel 10x10mm rod 1600mm long and glue the two sheets together. The mild steel would be about 50mm from the shelf front with the back and sides of the shelf being totally supported.

The shelves will be used as bookshelves. Do you think this will work and if so will 2x9mm do the job or am I better off using 2x12mm glued together? Also, should I add a second metal rod along the centre of the shelf (ie @ 150mm)?

As always many many thanks for all your help!!!
Aidan

 

[undergroundhunter]

Mdf is rubbish for this kind of load, you would be much better off with ply. Try looking at this http://www.woodbin.com/calcs/sagulator/ it should help.

Matt

 

[Jacob]

Mdf is rubbish for this kind of load, you would be much better off with ply. Try looking at this http://www.woodbin.com/calcs/sagulator/ it should help.

Matt

MDF is useless for shelves.
Ply is a bit better but pine is much better by far. For that span I guess you'd need the full 25 mm thickness plus a bit if you can get away with it.. 300 width means edge joining 2 or 3 boards, though you can get very wide whitewood boards.

NB your metal rod insert would have very little effect on the stiffness. If you want to use metal it's be better in the form of 25mm angle iron for the edges, or similar.

 

[Aidex]

Thank you both for your prompt replies!

Looking at the sagulator, is the "redwood" we generally get over here , pine (white eastern), pine red, spruce or redwood young growth?

On the metal rod, purely out of learning and not being difficult, I thought to use it as it's used in stringed instruments to ensure that instrument necks like on guitars do not bend too much or snap under the torsion created by the strings. The strings cause the necks to be under very high stresses and the rods work to counter the effects that would have on the wooden neck. I this thought to apply that logic to shelves....I'm guessing there's a reason why you can't just apply that logic to shelves?

Many thanks again!
Aidan

 

[Jacob]

Thank you both for your prompt replies!

Looking at the sagulator, is the "redwood" we generally get over here , pine (white eastern), pine red, spruce or redwood young growth?

On the metal rod, purely out of learning and not being difficult, I thought to use it as it's used in stringed instruments to ensure that instrument necks like on guitars do not bend too much or snap under the torsion created by the strings. The strings cause the necks to be under very high stresses and the rods work to counter the effects that would have on the wooden neck. I this thought to apply that logic to shelves....I'm guessing there's a reason why you can't just apply that logic to shelves?

Many thanks again!
Aidan

European redwood (timber) is scots pine (tree species) and no close relative of american redwood (timber and tree species)

 

[rafezetter]

I've used central metals rods to strengthen OSB shelving, however in order for it to work it needs to be under tension, as it is in the neck of stringed instruments. It's the tension that provides the stiffness. My shelves are freestanding and the rods go out both sides and large washers and nuts used.

For 2 joining shelves between them I put a strip of iron plate 25mm deep and 1.3 metres long on the back screwed every 200mm and this has supported it with books etc for 9 years without issue.

 

[Aidex]

Thanks Rafezetter

How thick are your chipboard shelves? Did you route a channel for the rod? What thickness rod did you use M8,10,etc? Did you use one rod per shelf?

I'm assuming that the rods go through the carcasse sides with washes and nuts there? How do you know when the tension is sufficient when tightening the nuts?

I'm sorry but I'm not totally sure I understand the part on joining the shelves...is this to increase the span?

Many thanks again!!!

 

[custard]

The sagulator is your friend. And it shows that a 25mm thick, 300mm deep, 1600mm long shelf of a stiff softwood (Douglas Fir) will support an evenly distributed 28 kilo load with a 2.9mm deflection. Any deflection under 3mm is generally considered acceptable for commercial work, personally I aim for about 1.5mm or less but I recognise that's a bit excessive.

What that tells me is that you're skating on thin ice with this project. 28 kilos isn't all that many books across a 1600mm wide span. You'd get away with it provided he's mainly a paperback kind of guy, but with big hardback books it's borderline. Switching to Oak won't help, because Douglas Fir is stiffer than Oak. Personally I don't think an MDF steel sandwich is the way to go either. MDF is as floppy as a blancmange so all the stiffness will come from the metal and the fact is mild steel just isn't particularly stiff. Picture in your mind a 1600mm mild steel rod suspended at either end, you know it'll sag under its own weight, so loaded with books it'll look like a banana.

Personally I'd go back to the client and explain 25mm thick only gets him 28 kilo of load before the deflection becomes completely objectionable. But if he accepts 30mm thick Douglas Fir he can have a 48 kilo load with the same 2.9mm deflection, and a 28 kilo load will only produce a 1.7mm deflection. That's the sensible solution here, 30mm or thicker Douglas Fir.

 

[custard]

Incidentally, when I was making fitted bookcases for my own home I went for maximum 800mm and 1000mm spans, 30mm thick MDF shelves but with massive hardwood lippings front and back. Net result was zero visible sag even under heavy load.

 

[matthewwh]

Your torsion box idea was a good one, the closest you can get through material selection would be flat sawn oak, the older the better, where the hard medullary rays will act in a similar way to the ribs of a torsion box.

You could add a very flat chamfer, over say 1/3 of the width, to the underside at the front, giving the appearance of lightness but sufficient substance in the centre to bear the weight; you can take this almost to a knife edge without affecting strength and from three feet away you'd never know.

It will cost a bit more, but your client needs to decide how they want the job done: well, quickly or cheaply - pick any two.

 

[Aidex]

Hi Custard

Thanks a lot for your input!

Excuse the lack of knowledge but is redwood comparable or the same as Douglas fir? Also on the sagulator, the weights you mention are they per foot or total weight? Unless I'm misunderstanding it 30kg/foot on 25mm Douglas fir gives a little over 3mm deflection which it says is acceptable. On this span that would allow approx 150kg on the shelf...this seems like a lot so I'm not sure that I'm understanding the sagulator properly.

On your own shelves (incidentally they're very nice!) what size hardwood did you use for lipping?Would the meranti you can get from places like Selco and TimberStore in similar dimensions to yours work as well?

Many thanks again!!
Aidan

 

[Aidex]

H again

I'm not sure the client will go for it but knowing that budget is a concern would a bracket made out of redwood, mdf etc curved to look a little prettier in the middle of the span help to keep it with a 25mm thickness?

Or even a metal bracket if I could find a pretty one?

 

[thetyreman]

150KG is very heavy, surely you'd need industrial strength shelving for that? are they putting an engine on the bookcase?

 

[petermillard]

I've done this with Ply shelves over a span of 1100mm & 300mm deep. I used a pair of 13x6mm steel bars on edge, glued with PU adhesive into a slot in 18mm Birch Ply, then skinned over (when set obviously) with 6mm BP, plus a hardwood lip front and rear. Worked well, but there's no way I'd try and do it over a 1600mm span.

 

[Aidex]

Hi Tyreman

I sure hope they don't plan on loading them that much. It was just the weight that the "sagulator " was working with (ie across our span 30kg/linear ft approx).

Hi Peter thanks for your post. I've dropped the metal idea! I'm looking at thicker and different timbers like fir or even redwood. I was hoping 225mmx25mm redwood with 50mmx25mm hardwood ("meranti") might do it...still a little concerned tbh! Granted that's 270 depth or having slightly 200mm redwood and 50mm lipping front and back. The back in las worried about as it's totally supported (glued and screwed along its whole length).

Thanks

 

[rafezetter]

Thanks Rafezetter

How thick are your chipboard shelves? Did you route a channel for the rod? What thickness rod did you use M8,10,etc? Did you use one rod per shelf?

I'm assuming that the rods go through the carcasse sides with washes and nuts there? How do you know when the tension is sufficient when tightening the nuts?

I'm sorry but I'm not totally sure I understand the part on joining the shelves...is this to increase the span?

Many thanks again!!!

20mm thick OSB, no, m8 and yes

Yes the rods go through the sides with washers and nuts - I used OSB and the visible rods for the "industrial" feel of them; For the tensioning I "plucked" them in the same way as tuning a guitar, once they actually started giving off a "note" sound as opposed to nothing, I just gave it a couple more turns. I didn't want them so tight that it was putting undue pressure of the shelving thus forcing them to bow to accomodate the reduced gap between the sides compared to the shelf length.

The joining shelves, that was my bad description. I made 2 of the heavy duty shelving units to go across one wall with my PC desk and TV between, but as they are so much higher I thought I'd make use of the available gap above the TV and span it with "joining shelves" but because of the way they are placed I couldn't use the same method, so as I said I just used 2 flat bars screwed onto the back to keep them stiff and it's worked a treat, they are as flat / straight today as when I made them.

Edit..... you never mentioned the back was going to have a full length strip screwed to the wall! This removes a good portion of the possible sag issue as now it'll only really apply to the front and it would take a significant point load on just the front edge to be able to cause sag - and that's unlikely.

If you REALLY wanted to go for it, belt and braces, a central decorative bracket would look nice with deep chamfers (rather than bullnose) both sides so from the direct front it'll only be noticable by the gap in the books. This would help transfer the load onto the rear of the shelving supported by the strips screwed to the wall. I would personally make my own and make them a feature depending on the style of the bookcase.

 

[custard]

Hi Custard

Thanks a lot for your input!

Excuse the lack of knowledge but is redwood comparable or the same as Douglas fir? Also on the sagulator, the weights you mention are they per foot or total weight? Unless I'm misunderstanding it 30kg/foot on 25mm Douglas fir gives a little over 3mm deflection which it says is acceptable. On this span that would allow approx 150kg on the shelf...this seems like a lot so I'm not sure that I'm understanding the sagulator properly.

On your own shelves (incidentally they're very nice!) what size hardwood did you use for lipping?Would the meranti you can get from places like Selco and TimberStore in similar dimensions to yours work as well?

Many thanks again!!
Aidan

- I quoted Douglas Fir for several reasons, it's exceptionally stiff and because of its special properties it's both widely available and generally sold as a separate species from other types of softwood, Redwood is a generic name for decent quality softwood but it's different to Douglas Fir.

-The weights I quoted were total, and furthermore they were spread evenly across the shelf.

-I used a hardwood lipping front and back that from memory was about 60 or 70mm wide, Meranti would work fine. The sagulator has the facility to include lippings in the calculation, it's worth experimenting with it until you're clear how it operates, the results seem to correlate well with real life experience which makes it a valuable tool.

Good luck!

 

[cornishjoinery]

Can i ask how long you have been in the trade? I think knowing about basic shelf building and spans etc is a basic carpentry knowledge. Plus not knowing the different type of timbers etc. Mean that in no offence but i think this job is a little bit over your head at this moment.

 

[Jacob]

Can i ask how long you have been in the trade? I think knowing about basic shelf building and spans etc is a basic carpentry knowledge. Plus not knowing the different type of timbers etc. Mean that in no offence but i think this job is a little bit over your head at this moment.

Every body has to start somewhere. I did loads of my first jobs in a state of total ignorance!

Useful tip - whatever it is - have a look at examples around you. The maker probably knew more than you and me about how to do things .

 

[Sgian Dubh]

Unless I'm misunderstanding it 30kg/foot on 25mm Douglas fir gives a little over 3mm deflection which it says is acceptable. On this span that would allow approx 150kg on the shelf...this seems like a lot so I'm not sure that I'm understanding the sagulator properly. Aidan

The evenly spread load of 28 kg given by custard means a total load of 28 kg evenly spread across the whole width and length of the span. Count on a load of ~11 kg per linear 300 mm shelf length (or ~25 lb per linear foot) as typical for books. Allow more per linear length if you're dealing with large format books. So, typically, if you're making a bookshelf, let's say 900 mm long, it needs to be strong enough to carry ~33 kg [(900/300) X 11 kg = 33 kg) whilst also deflecting (sagging) less than 3 mm to generally be considered acceptable. Slainte.