Glue Lam shelving Question

[Peter Tucker]

Hi there, I'm making some heavy duty shelves and plan to just glue up some dimensional lumber then joint and plane it down (I have a 20" planer). I need the shelves to be as stable and flat as possible, and wondering what you would advise using. Cheapest would be just kiln-dried 2x4's, but I could also see using 4x4's if that will be more stable long term. The shelves will be about 3" thick, and roughly 80"x 20". I'll be coating with lacquer primer (Zinsser) then epoxy paint to keep out moisture (this will be holding up fish tanks), but not sure what people advise using 2x4's (~14 glued up per shelf) or 4x4's (~6 glued up per shelf)?

Thanks for the help - appreciate it.

Best, Peter

[Bruce King]

Where are the supports going to be?
Sounds like several are needed since fish tanks are going on there. How many gallons of water total?
Sawn lumber will sag unless lots of support underneath.
Plywood standing on end (plys visible on top
and bottom) would be very strong and stable. You would cap and wrap these laminations with some 3/4 plywood. You would need a thickness or height based on the load and space between supports. I think it would be thicker than 6 inches. The ply would be glued and lots of nails or structural type screws.
Big tanks will really cause you to need an engineer to specify materials and support spans.
The big fish tank installation companies have probably done this but might be leery of giving you information due to major liability.

[Jamie Buxton]

Here in San Francisco, I don't see kiln-dried 4x4 construction lumber. Kiln-dried 2x4s yes, but not 4x4s. That said, in green construction lumber, 4x4s generally are closer to clear than 2x4s.

Also, the spec for "kiln-dried" softwood is different from kiln-dried hardwood. Hardwood is dried down to 8-9% EMC. Softwood is kiln-dried only to 18% EMC. Wood indoors in California will dry down to 9% or so. Wood at 18% still has a long way to go. It isn't as stiff against bending as fully dried wood, and it may warp as it dries in service.

You might consider poplar. Around here it is the least expensive hardwood, and it is kiln-dried. I buy rough-sawn 4/4 for about $2/bdft. Thicker stock is available, at a higher bdft price.

[Mike Wilkins]

Sounds like metal working is in your future. Wood can be made to work, but a strong species and bomb-proof joinery would be needed. Not a project for biscuits or pocket holes.
Welded steel will provide the needed strength for a lot of water weight.

[Jamie Buxton]

You might also consider LVLs -- Laminated Veneer Lumber. You get flat straight dry lumber in big construction sizes -- like 4x12. Two of those side by side would do your job. You won't find LVLs at Home Depot and the like, but real lumber yards either stock them or will order them in. https://www.weyerhaeuser.com/woodpro...lam-lvl-beams/

[Mark Gibney]

Those LVL beams are expensive. An (actual) 3 1/2" x 14" PSL beam is about $25 / foot here locally to me.

I make tables from select 2x4s that I dry fully - a shelf of the size you write, Peter, will be like a workbench top in weight. But I suppose you already know this.
The tables I make stay flat, and of course there is normal expansion and contraction across the width.

My curiosity is more about what you will use to hold up these shelves and a full water tank. That's going to weigh a ton! Would love to see a drawing.

Mark

[Richard Coers]

Just the weight of the shelf will be approx 105 pounds. I've never seen a load of 4x4s or 2x4s that are ready for glue up. A lot of 4x4s are cut from the center of the log and have a tendency to crack. Kiln dried construction lumber can be as high as 19%. That really in the category of air dried and still moves a ton as it dries. It's actually tough to find enough straight 2x4s to build a wall sometimes. It will take jointing and planing to remove the twist and cupping. I would build torsion boxes for the shelves, if for nothing else than not having to shove 100 pound boards through my thickness planer. 3/4" plywood skins, and poplar grid inside. Zinser primer is either water based or shellac based. It's not lacquer.

[Dave Mount]

I don't think LVLs are made to take big lateral loads when laid flat like a shelf. When laying them flat while laminating them or preparing them for installation, they're pretty floppy, despite their significant strength when installed on edge.

[Roger Bull]

Are these glass or acrylic tanks? Glass tanks do not need center support but acrylic tanks do. If glass you can save yourself a lot of time and materials by building rails and spanning it with plywood.

[Jamie Buxton]

I don't think LVLs are made to take big lateral loads when laid flat like a shelf. When laying them flat while laminating them or preparing them for installation, they're pretty floppy, despite their significant strength when installed on edge.

Beam stiffness goes up as the cube beam height. So a 4x12 is 27 times stiffer upright than it is laying down. But it is still pretty stiff laying down flat. I’d send the OP off to sagulator, but he seems to have asked-and-run.

[Dave Mount]

Beam stiffness goes up as the cube beam height. So a 4x12 is 27 times stiffer upright than it is laying down. But it is still pretty stiff laying down flat. I’d send the OP off to sagulator, but he seems to have asked-and-run.

Absolutely true for solid wood, but LVLs are not solid wood. So I went off to do a little reading. Turns out I might not have been imagining things, the orientation of the plies does affect stiffness over and beyond the effects of beam depth. This article measured bending under impact rather than a static load, but did so with square LVLs (in cross section) so the depth and width of the beam were equal regardless of veneer orientation. LVLs with the veneers laying flat bent about twice as much under load as did those with the veneers oriented on edge (as in a typical LVL beam). https://ojs.cnr.ncsu.edu/index.php/B...icle/view/3320

However, someone else made the excellent point that what likely matters more in this case is the flexibility of the bottom of tank compared to the flexibility of the supporting structure and both in relation to the points of shelf support.

I learned something today!

Best,

Dave

[Roger Bull]

However, someone else made the excellent point that what likely matters more in this case is the flexibility of the bottom of tank compared to the flexibility of the supporting structure and both in relation to the points of shelf support.

The bottoms of glass tanks do not contact the supporting surface so no support is needed. All of the weight is distributed to the bottom edges of the sides which should be uniformly supported. A small area that sags is not normally a big concern because as it sags the pressure on it is reduced because there is essentially zero sag in a glass tank. If enough of the support changes the tank can crack. Most important is a flat surface with support that can not change significantly such as evenly spaced vertical support.

The OP did not give a height so can't calculate the volume and weight but he is likely going to be in the 135-150 gallon range. He will obviously not be supporting a tank weighing well over half a ton on a shelf without vertical supports underneath. The weight bearing edge of a glass tank is normally less than 1/2" wide. A rail of 2x4s (or 2x6 if you like overkill) on edge supported every foot or so by 2x4s directly underneath works very well. All that need to be attached to a base that spreads the load on the floor. No matter what, keep the support in a vertical line from tank to floor. Plywood attached to the sides to prevent racking is critical as well. With that basic design you can pretty it up with nicer woods, trim, etc. Of course seal everything to prevent swelling from the inevitable splashes.

[Bruce King]

I agree, never use flat, they will bow just like sawn lumber.