What size angle to span gap for desk

[bradpac]

I am building a new desk with tool box bases and an angle iron frame on top to span the gap with a 3/4" plywood top surface. This will be for normal desk duties, but will have a 77" open span.

Biggest load would be if a person or two decides to stand/sit on it, so lets say 500lbs spread across the span.

I was thinking 1.5" x 1/8" or 3/16" angle would work for the top frame, but if I can get away with a smaller angle I would prefer it. Would 1" be sufficient?


Here is the illustration of my design.

 

[mike93lx]

If you attach it to the top, it will increase strength considerably.

I think 1"x1/8 will be plenty

 

[bradpac]

Sounds good. Using 1" I won't have to cut out for the cabinet locks and it will give me just a little more leg room under the desk.

 

[kbs2244]

I just bridged it front and back with bed frame stuff.
30 inch hole

I am 300 lbs and have not bent it when going for stuff on a high shelf.

 

[bradpac]

I think if I move the joint of the plywood from in the middle and have two over each pedestal it will strengthen it up quite a bit as well. If the overall height wasn't an issue I would use butcher block or a solid core door and not have any concerns at all.

 

[mike93lx]

I think if I move the joint of the plywood from in the middle and have two over each pedestal it will strengthen it up quite a bit as well. If the overall height wasn't an issue I would use butcher block or a solid core door and not have any concerns at all.

Can you do two layers of 1/2 with the joints staggered?

 

[BD1]

Weld spacers in between like a ladder. IF you can't use angle than flat stock. Make a mock up and test it first to see how it feels. It won't cost too much more to go larger.

 

[LXCam]

If you want to use 1" I wouldn't go less then 1/4" thick. If you do center load it that heavy the angle iron will want to twist as it deflects/sags . I'd suggest 1" x 1/8" wall square tubing.

 

[Jeff95TA]

I ran some calcs on your layout. Some conservative assumptions:

- the steel members are just sitting on the boxes (pinned end conditions)

- ignore the plywood stiffness (I was trying to make this a quick calc)

- people would sit on the front edge, and all the load would go into just the front steel member

I ran two loading conditions: a 300 lb point load at the center, and a 500 pound load over 48". The 500 pound load is the worst case.

The attached pdf shows the results. I assumed the steel would be A36 which has a yield strength of 36,000 psi. You want to compare the Von Mises stress to the yield strength.

You can make the decision if you want to design for these conditions or not!

 

[mike93lx]

If you want to use 1" I wouldn't go less then 1/4" thick. If you do center load it that heavy the angle iron will want to twist as it deflects/sags . I'd suggest 1" x 1/8" wall square tubing.

attaching the angle to the plywood top should control the twisting movement, no?

 

[bradpac]

Jeff,
Those are some interesting numbers there, too bad I don't know what any of it means, other than the yield strength. Paying attention to the Von Mises as you said, it looks like 2x2 angle would be the choice. I was planning on attaching the top to the frame and also attaching the frame to the toolbox pedestals can be done although I don't see that that would make much difference.

I wonder how much the plywood top would help distribute the weight better as I really want a max of 1.5" angle.

Maybe adding a third beam in the middle, then some of your sitting on the edge of the desk should get distributed to that.

Decisions.... I'm thinking of using 1.5" 3/16" angle as that was my initial gut thought and adding a third beam in the middle with a cross member as well to keep it from twisting. Although, I still may try 1" angle with this beefed up design and see what it does as some of you are thinking it will be good with the plywood, which provides some strength on its own.

Revised design...

The one thing I have learned for sure though is that there are a lot of great people on here who are happy to take their time and help others out and I really appreciate that. As metal roofing and 70s Dodges are about my only points of real knowledge, I don't chime in much, but hopefully when I am able it helps others out as much as you guys have helped me.

 

[Spareparts]

What about deleting the center crossmember on the last drawing and putting a 1" X 6" deck board across the opening glued and screwed to the bottom side of the plywood top, both front and rear. Would be completely out of sight and would strengthen it up considerabley. What thickness will the top be. Maybe an oak trim board around the top and round the edge over, be a lot easier on the arms when using as a desk.