"Diamonding" box truss?

[derekleffew]

An interesting (to me, anyway) discussion arose at work yesterday. A rigger stated, "No truss manufacturer approves or advocates its truss for use in a diamond configuration." So should one...

1. Never do it. Period. End of story.
2. Apply some sort of derating factor (either less load or more closely-spaced points).
3. It's perfectly fine, whether or not the mfg approves it.
4. Other (Please explain...).

Not an official poll, as I'm not looking to reach a consensus; just want discussion from experts.

BTW, the truss in question was 20.5"x20.5", 70' long, picked with three 1T Lodestar s, and held 24x 80 lb. fixture s.

 

[MNicolai]

My understanding of physics makes me think you should derate heavily or not use it in that configuration at all.

There are no cross-members internal to the box truss that go diagonally between what would be the top and bottom chords and also cross-members between the opposing chords on the horizontal axis. Without those reinforcements, it's no different than the concept of crushing or pulling apart a cardboard box -- it's much easier to do applying compression or tension on two opposing corners than on two opposing sides.

 

[derekleffew]

... There are no cross-members internal to the box truss that go diagonally between what would be the top and bottom chords and also cross-members between the opposing chords on the horizontal axis. ...

I'm pretty sure every piece of GP or MD box truss I've ever seen has at least one internal going each way.

But now that you bring up "the crushing of the cardboard box," the rigger rigged a single SpanSet to choke the bottom chord and wrap the top. I think using two slings, choking the bottom, wrapping the sides, and coming out on the opposite side of the top chord is more common.

 

[MNicolai]

http://www.jthomaseng.com/gptruss.htm

I don't see any cross-members between opposing chords in those images of GP trusses. Everything is between adjacent chords.

 

[derekleffew]

Pictures from http://www.jthomaseng.com/pdffiles/GP 20x20-99.pdf :

 

[DuckJordan]

Are we talking horizontal or vertical loading? Of horizontal no problem if verticle, is expect the points you will be using will will have not only the force of the truss its attached but the same force would be applied doubly if not greater by the truss hanging under your picked truss.

It really depends on its function to what the load is but I would rate the truss with a 10:1 ratio rather than our standard 6:1

 

[MNicolai]

I do see them now for the 20.5" stuff. If they're reinforced diagonally as much as they are in a normal configuration, I don't see a problem with using them in a diamond configuration, but I suspect that they aren't as well reinforced along that axis.

Even if they appear to be close in their quality of reinforcement along that axis, I recall a post somewhere else here on CB that may have been from you about a quote from a tradeshow. I believe it was made by Bill Sapsis, where he talked about how incredibly tight the tolerances on trusses are because he had to modify some for a job but had a very difficult time getting any information out of the manufacturer if he could safely do what he wanted to. After pulling enough teeth, he finally got the green light for his modifications but commented that if the industry really understood just how tight the tolerances are on trussing, they wouldn't go anywhere near trussing.

 

[porkchop]

I'm going to assume that by "diamonding" truss you're talking about rolling the truss 45 degrees along it's long axis so that one cord is pointed up and the one diagonally opposite is pointed straight down. I would be hesitant to use the truss in this manor because as you mentioned the truss wasn't manufactured with this kind of use in mind. Thank being said IF I was going to do it 20.5x20.5 would be a good choice in my mind because even though it's not the intended purpose the square construction should produce a more even load and prevent excessive forces in any spot on the truss. So to answer your question if it was really my only option and I could derate the truss by a comfortable margin (say 1/2 of SWL) I could be convinced to that this is allowable. For certain the pick up point would have to be from the bottom cord. Picking up from the top cord only would not be acceptable.

As a side note every piece of 20.5x20.5 I've ever seen has some diagonal bracing although usually not a huge amount.

 

[soundman]

First question is why? Second is can you get anyone to approve it? Third, how much deflection is there? A 30' of 20" has an UDL of like 3,700 LBS and max deflection of 1 1/2". 12 80 pound fixtures weigh under 1,000 LBS. If the designer refuses to have it hung the truss like normal and for what ever reason its not possible to get the maker to sign off on it I would hang it and measure the deflection. If its over 1.5" don't send it above anyone's head.

 

[headcrab]

I would care more about the maximum stress than the deflection.
If the purpose of diamonding a truss is to have a single hanging point at the bottom of the unit, one could attach a single piece of sch 40 pipe to the lower pipes with pieces of strut channel and u-bolts or half-couplers.

I am not an expert, but I had to say this anyway.

 

[MPowers]

There will be (already are) several "Opinions" on this. Here is my stance and the stance my company would take.

1 Never do it. Period. End of story. Never say Never. There can be exceptions to almost every rule.
2 Apply some sort of derating factor (either less load or more closely-spaced points). Absolutely
3 It's perfectly fine, whether or not the mfg approves it.In a word, NO!
4 Other (Please explain...). See Below

The only acceptable reason for the "Diamond" orientation would be esthetic, visual. There is no possible structural reason for the diamond hanging position.

Should an event "Designer" request such an orientation, we would evaluate the known factors, including: Total load, load distribution, Type/size of truss, truss specs, span, environmental factors - indoors/outdoors/wind/weather/water etc., do the math (we have engineers) and decide if the gag is, in our calculation, Safe: beyond all doubt. Safe: no design factor, Safe: my mother can sit under it, Simply SAFE! Then we would .... "consider" .... it.

 

[len]

Outside of the possible design aesthetic, I don't see any point.
1. Trim height would be lessened by a few inches.
2. Most people wouldn't be able to see the different angle.
3. Those that would couldn't care less, or would just wonder why it was done "wrong."
4. Most riggers likely have never done this before. So you're taking something for which there are probably no load calculations, mixing in crew that are unfamiliar with the procedure, and getting negligible aesthetic benefit. I think I'd pass.