Steel tube deflection over span

Question

I am installing 16' desktop to a concrete block wall. The back wall and one side will sit on a 2x4 ledger. The opposite short side will be a solid plywood panel to help support weight.

The top will likely be 3 hollow-core doors topped with laminate.

As this will be used like a desk for multiple people, I want the front of the unit to have no support legs to be in the way of chairs/legs. I do plan to have some angle supports cut from plywood to help distribute the load (think right triangles, but with a with a significant crescent removed from the hypotenuse to help for knees clear)

At the front, I could use a 2x4 on edge to make a rail to help stiffen the edge, but I thought that perhaps a piece of box steel or angle aluminum would do the job with less deflection and a lower profile.

What size angle or box tube would accommodate the 16' length with the smallest vertical profile and least deflection?

Answer 1

Given the length and aspect ratio that you've proposed, it will be very difficult to meaningfully support the desk the long way. 16' is just way too long to get any support across. Certainly a 2x4 will be basically useless, even "on edge". Also, consider the ergonomics of having a thick tabletop surface: if this is going to be a computer workstation area you do not want to have a very thick top since that will force people to sit too low and have their keyboard too high.

I suggest ignoring the ends and building the desk to be supported the short way, i.e. cantilevered out from the wall. If you look around you will find various floating / triangle desk bracket designs. You might also consider whether adding a few legs would really be a problem; that would make the desk substantially stronger (and/or easier to build).

Whatever you do, make sure the desk is sturdy. People will invariable pile a desk with books, computer equipment, etc. and even sit on & lean against them.

Answer 2

I think the only viable low-profile solution considering hollow-core doors is full bracketing on 24-32" centers. You can get away with brackets that extend 1/2 to 2/3 the depth of the top if they're well-anchored to the wall. There will be considerable tension at the upper anchor points.

That said, you'll still have flex, and the brackets that fall on the inner (mostly hollow) portion of the door could eventually press through the veneer sheet. I'd want a plate on top of the brackets significantly wider than the brackets themselves.

Were I doing it, I'd get 24" max brackets and top them with 2x4s running lengthwise (parallel to the desktop) front and rear to distribute the load. Like so (end view):

_________________________________________
|_______________________________________|
|____|_____________|____|
|             /
|        /
|    /
|/
|

Note that the rear 2x4 here would supplant your rear ledger. A ledger at the end of the desktop is optional depending on the final bracket position. With this approach your support is nearly invisible, does not interfere with legroom, and provides a channel for cable management.

1-3/4" solid-core doors are extremely stiff and would possibly accommodate your original plan. I have a similar setup in my office, with a span of about 12'. I do use several max brackets in that case, though.

Answer 3

According to http://metalgeek.com/static/deflection.php, a 2" schedule 80 steel pipe with your dimensions would deflect less than an inch in the middle under the weight of 1000 lbs if both ends are fixed.

In your case, one end would be fixed to the wall but you'll need to prevent the other from moving laterally under the weight of the table, e.g. by running additional bracing under the table at the 45 degree angle between the end of the pipe that sits on the opposite short side and the rear long wall. (And said bracing may need to be even more massive than the pipe. An I-beam, perhaps?) You'll also need some very serious fasteners at points where pipe and bracing attach to concrete and plywood to keep this construction together.

Answer 4

Since it seems you would want a profile height of a 2x4 on edge, I would recommend a 4x4 square steel tube with a wall thickness of at 1/8"-1/4" to minimize deflection. If multiple people are using it, it needs to have even less deflection to reduce bounce/shake from people moving.