MDF Shelving limitations

[cdatrim]

Building a paint grade bookshelf above two cabinet lowers inside an alcove. Problem is the builder wants adjustable shelving above the cabs and it is about a 59" span. The material we usually use is MDF and that is what material is waiting for me. Usually the shelves are around 30" and I usually apply a 1x2 of hard maple to the face, but at 59" I know it would sag big time. Can't do a mullion because it will be used as the media niche. I was thinking of using some angle iron hid by the maple face, but I will take some great ideas to prevent sagging.

[Lavrans]

Re: MDF Shelving limitations

Do you have to use MDF? By the time you've gone through the trouble of finding & installing angle you'd be way more expensive than just picking up some paint grade plywood. Finding angle iron strong enough with the space limitations would be difficult anyway.

How deep are the shelves? If they're just around 12" use plywood for the shelves & plan on a center run of shelf support holes.

[SolarPowered]

Re: MDF Shelving limitations

Even with plywood it's going to sag big time at 59". You might be able to get away with 1-1/8" ply, but I suspect that even that's going to be pretty iffy.

"2x" (1-1/2") Doug Fir would probably work pretty well. (You can even apply your 1x2 hard maple as edge banding. :) )

Google "Sagulator" and run some numbers through it. You can learn a great deal by playing with that gadget for a while. (Note: Sagulator is a web page that you plug numbers into to get shelf sag. It allows you to select from a rather large number of shelf materials.)

One interesting thing you learn is that Doug Fir boards make one of the best shelf materials. Which, when you think about it, actually makes sense--that stiffness is why we use Doug Fir for joists.

[davestrim]

Re: MDF Shelving limitations

You'll probably have to use solid wood. A piece of angle iron that long will sag under its own weight unless it's of large cross section. Ply would need a huge edge added. Maybe torsion-box?

[SolarPowered]

Re: MDF Shelving limitations

I got around to running this through Sagulator. With 110 pounds of uniform load (typical books), a 59"x10"x1.5" Doug Fir shelf will have 0.05" of sag. With otherwise the same specs, 3/4" MDF will sag 2.39".

[dave_k]

Re: MDF Shelving limitations

The maximum allowable deflection in shelves is usually 1/4" over the span. A span of 59" is very unusual, in most cases you can break it up with intermediate supports. 48" is the max. I've seen in published span tables for shelving. You can work out the capacity if you know the modulus of elasticity of the materials you are using.

Assuming you can break up the span into 2-30" spans a 3/4" x 12" MDF shelf spanning 30" can carry 150 lbs. A 1" thick MDF shelf can carry 356 lbs.

If you add a 3/4" X 1 1/2" hardwood drop edge to the 3/4" MDF it increases the capacity to 435 lbs.

Add .050 p lam. to both sides and one edge the load carrying capacity increases by about 200%. Add .028 p.lam. 123%.

To give you something to compare to 3/4" poplar can carry 438 lbs over 30"' 1 1/8" poplar 1368 lbs.

All these figures come from the AWI quality standards handbook.

PM me if you want the formula for load carrying capacity of shelves.

dave

[-_wmp_-]

Re: MDF Shelving limitations

why not go with a torsion box system.
hate to send you to another site, but this is the fast and easy google i found.

http://www.taunton.com/finewoodworki....aspx?id=26506

http://www.geocities.com/bawanewslet...orsionbox1.pdf

[dave_k]

Re: MDF Shelving limitations

If you decide to go with a torsion box you may still want to cut down on the span. What I do in these cases is use a 3/4" steel rod welded to a baseplate installed through the back of the case, then make a corresponding slot through the back edge of the shelf to suspend the shelf. You can make a shelf any length appear unsupported while not having to have a ridiculous amount of mass to the shelf to support the load.

You do need some structure at the back of the casework to get adequate anchorage to screw the supports to.

dave

[[email protected]]

Re: MDF Shelving limitations

What I have done in that type of situation in the past is to tell them the shelving will not hold the freight that way, but this is what I can do.

Install a fixed bottom shelf; face frame that upper box and run a stile in the middle to support the adjustables mid span but without a divider behind it. That way you still get your un-obstructed counter below, continuous shelf lines above, light airy look, without having all your shelves sag and collapse. That center stile should be attached to the (wide) top rail with a steel "T" strap from behind because half of the weight of the combined shelving will be hanging from it.

If they don't want to do it that way, then I ask that they have their architect draft up some shop drawings specifying materials and connections, and sign off on them.

Torsion boxes are a great way to reduce weight, but not such a great way to reduce thickness.

Regards,
Jimc

[SolarPowered]

Re: MDF Shelving limitations

Gads, people are making this so difficult! A 1.5 inch thick solid shelf will do just fine for a 59" span. While Doug Fir is probably the stiffest choice at 0.05" of sag, most maple and oak species are in the 0.06"-0.09" range, and should also be quite acceptable. Or edgeband Doug Fir with whatever hardwood the rest of the bookcase is. (I just noticed that the O.P. said "paint grade"; Doug Fir will work quite well for that.)

[dmnoland]

Re: MDF Shelving limitations

Assuming you can break up the span into 2-30" spans a 3/4" x 12" MDF shelf spanning 30" can carry 150 lbs. A 1" thick MDF shelf can carry 356 lbs.

PM me if you want the formula for load carrying capacity of shelves.

dave

In regards to that 30" span, is that using a cleat in the back of the shelf?

Please share your formula with us.

[Lavrans]

Re: MDF Shelving limitations

A clear 59" span is difficult. There is no reason this has to be a clear span that long. a single line of shelf supports down the center, even if it's only attached to the back edge, cuts the shelf span.

I have done shelves @ 72" x 10" deep. I just doubled up the plywood & used a 1 3/4" edge front & back. No sag. Try to make it a clear span, sure there'll be sag.

Torsion box would probably have to be 2" to span that clear (unless you get into honeycomb aluminum or some other exotic).

[cdatrim]

Re: MDF Shelving limitations

Thanks for the info guys, I'll be talking to the PM tommorrow. I don't think that drafters and architects realize what they are trying to do some times.

[dave_k]

Re: MDF Shelving limitations

In regards to that 30" span, is that using a cleat in the back of the shelf?

Please share your formula with us.

No. According to the text these figures are from results of tests done on shelves fixed at both ends and unsupported in the middle.

The formuls is: D=(0.1563wl^4)/(Ebh^3)
D = deflection in inches
w = load per lineal inch per span
l = span of the shelf
E = modulus of elasticity
b = width of the shelf
h = thickness of the shelf


^ = exponent

dave

[dmnoland]

Re: MDF Shelving limitations

Thanks and wow, I thought it might be something real simple but I'll work on that tomorrow.