View Full Version : Straightening a 18ft wide grid

12-29-2016, 10:43 PM
Looking for some construction advise.

We have a large (18 ft x8 ft) grid that is mounted about 18 feet in the air. Using Aircraft cable and Galvanized pipe, we have a very sturdy frame, but I've struggled to find the right "top bar that won't bend. This year, i used angle bracket, but still had to put a strap in the middle to straighten it out. I'd prefer not to do that, as I'd like to be able to raise and lower the grid for repairs with the pulley system as I don't have the lift for the full season. My options are either to build a stronger frame that can support a pulley in the middle or find a "cross bar" for that top that will support all the weight of that heavy matrix without bowing.

Anybody any suggestions for a cross bar that won't bow?


(Note: Picture on the ground is from testing, before it gets hoisted)


01-08-2017, 10:29 PM
I would probably use aluminum channel. The same stuff you'd use to make the screen enclosure for a pool. Good luck! :)

01-08-2017, 11:22 PM
What about some kind of truss?

01-09-2017, 01:02 AM
I agree with scott4864.

A truss can cover that distance without any noticeable swag.

Jon C. Can you comment on this?

01-09-2017, 01:42 AM
Yeah, that's definitely a job for truss. 12" aluminum box truss would be perfect for that. But that's almost surely out of your budget range. At about $600 per stick, and you'd need 6, that'll be well outside of the budget of most DIY displays. Plus you'd need the two climber blocks, headstocks, winches, and assuming it's freestanding, steel 4' baseplates with concrete counterweights. (or outriggers).
You might consider using radio tower as a DIY alternative. It's not rated for horizontal use, but it'll be much stronger than any single member framing. I bought 40' (4 10' sections) for $200 this year for my flying star project. I've got about 1800 pounds of tension on it perpendicular to the tower section. It does deflect somewhat at that tension, but I have it pulled back in the other direction with additional cables and it balances out fine.

In the pro world, Most pixel curtain is suspended by a rigging bar that is 2x4" 1/4" steel rectangular tubing with the long dimension vertical. But for just the 8' height, you could probably get away with a regular 2" sch40 black iron pipe. (which is standard in the lighting business for lighting grids).

I must say, your vertical support system looks rather sketchy. Very creative, and probably functional, but it's above a doorway, and probably well under-engineered for the application. Never use anything under 1-1/2" sch40 steel pipe for any structural application. Have you considered just installing proper rigging points into the eave? This could be done with simple forged eyebolts installed thru the eave to some steel support inside. You'd want 1/2" or 5/8" size with machine screw thread so you can install it to the steel with bolts behind it. Menards carries 1/2"x 14" galvanized forged eyes that would be perfect for this. You could use 1/4" angle, or unistrut across the tops of the ceiling joists near the eave tails. Then drop the forged eyes down thru the sofit and use that as your lifting point. You can attach your winches straight to those points. If you choose to leave them all the time, you can use sprinkler head bezels as trim rings for them. Note that if you only support them at a single point on the high side (above the ceiling joist) and then leave them hanging thru the eave with no lower support, you can only pull downward on them. You won't be able to use them to pull sideways for a lift/winch mechanism toward the side. You'd need a steel support brace touching the shoulder of the eye to be able to do that.

01-09-2017, 06:54 PM
Um.... Never really thought about installing rigging into the eves. That is an idea I should probably pursue fruther... Thanks for the great idea.


01-09-2017, 08:55 PM
sometimes another set of eyes is all it takes to get an all new way to look at things.

01-09-2017, 09:19 PM
So that you don't waste a whole bunch of money and time you need to do some engineering calcs.

Typical codes allow for a certain amount of bending. Like floor joists in a house it is L/360 (Length in inches divided by 360 degrees). In your case 18' = 216" therefore 216"/360=.6" of allowable deflection. This may be too much for what you are trying to achieve.

This site might help you Beam Bending Calculator (http://www.engineeringcalculator.net/beam_calculator.html) You can try different channels, beams etc to see if you can get an amount of deflection that is acceptable. The size you need may end up being too large, 18' is a big span.You'll need to know the weight of your assembly.

01-09-2017, 09:20 PM
If I may. Pipe, if you can support hanging/mounting, which should be easy, will accomplish your frame. You could do square channel, round, whatever, but round is easier. A thick walled pvc in heavier diameter may work, depending on total weight of matrix. Anyway, we have hung canvas banners (different business) in this fashion. The cable in the middle with turn buckle allows you to adjust tension and keep the sides from bowing out under pressure.