hanging lights outside span

[adamharman]

When (many moons ago) I did my Civil Engineering Degree course we did indeed talk about Uniformly Distributed Load when designing structures like bridges. So maybe your "classic texts" are about as old as me :).

 

It was UDL when I did my degree 5 years ago (as a mature student). Maybe EDL originates from American manufacturers?

 

In terms of the bending moment, the cantilever is fine as long as the load is distributed reasonably evenly (so no massive movers on the ends). Bringing the supports in from the end will actually reduce the bending moment in the centre. It will obviously cause some bending moment at the hanging points so with triangular truss you're effectively loading it apex up/down in different places.

 

Unless you're getting close to manufacturers figure for allowable load on the entire length of the truss (rather than between the points) you can't go too far wrong. Seano's 1/6 span rule is a good one too. It should be possible to push it further in many cases, but I'd want to do some maths. Wind and dynamic loads (e.g. movers) make a difference too as the more you bring the points in, the more likely it is to swing around.

[brainwave-generator]

...EVENLY DISTRIBUTED LOAD (EDL) ...

I've always known this, and seen it referred to in all the classic texts, as UDL (as in Uniformly Distributed Load). Is it the same thing?

 

Aye, same thing, different word.

[Simon Lewis]

Aye, same thing, different word.

Then why not use the correct, universally understood word?!

[brainwave-generator]

Because I say tomato and you say tomato and everyone just uses the word they've grown up with.

 

Everything in the rigging world typically has about 5 different names anyway.

 

If you can't translate that EVENLY DISTRIBUTED LOAD is probably referring to the same thing as UNIFORMLY DISTRIBUTED LOAD then you need to think more, but I think you're just making a fuss out of nothing.

[Seano]

It was UDL when I did my degree 5 years ago (as a mature student). Maybe EDL originates from American manufacturers?

As David so um.. graciously points out, its fairly obvious that the phrase "evenly distributed load" means pretty much the same as "uniformly distributed load".

As an acronym though, "EDL" doesn't seem to originate anywhere, certainly not the USA it just doesn't exist. "UDL" it is.

 

It will obviously cause some bending moment at the hanging points so with triangular truss you're effectively loading it apex up/down in different places.

True. Also the case to some degree when hanging longer trusses on three or more points.

 

Seano's 1/6 span rule is a good one too. It should be possible to push it further in many cases, but I'd want to do some maths.

Not a rule, a rule of thumb - not a big difference, but a crucial one. ;)

 

Regarding the maths, few riggers are capable of a detailed mathematical analysis of this kind of thing (certainly not me). But many make these kinds of decisions fairly routinely in a 'qualitative' (as opposed to 'quantitative') kind of a way.

The bending moments imposed at the lifting point by the cantilever are either: a) relatively trivial compared to those the truss is designed to handle or b) significant - too much so to say "it'll be fine" without getting a proper engineer to do hard sums.

 

In case a) you go ahead and do it. In case b) you consult a structural engineer and/or the truss manufacturer, or (the more common approach) add another point or two so that case a) applies after all.

 

Incidentally, this looks like an interesting approach to dealing with this kind of thing in load tables: Prolyte proprietory truss calculator thingy

(Slightly annoyingly, I can't try it at the mo because I'm not using Windoze. I'll definitely have a play when I'm using a different computer.)

Perhaps in the future more truss manufacturers will release data in "app" form.

[brainwave-generator]

Regarding the maths, few riggers are capable of a detailed mathematical analysis of this kind of thing (certainly not me). But many make these kinds of decisions fairly routinely in a 'qualitative' (as opposed to 'quantitative') kind of a way.

The bending moments imposed at the lifting point by the cantilever are either: a) relatively trivial compared to those the truss is designed to handle or b) significant - too much so to say "it'll be fine" without getting a proper engineer to do hard sums.

 

In case a) you go ahead and do it. In case b) you consult a structural engineer and/or the truss manufacturer, or (the more common approach) add another point or two so that case a) applies after all.

 

 

That is what, had I not been running out the door, I pretty much would have said regarding countering bend in the truss.

 

If you've got something in the middle which is bending the truss so much that it's going to become unsafe, you should not do it regardless of what is countering it.

If you've got something in the middle which isn't bending the truss so much that it's going to become unsafe, it's fine anyway, regardless of what is countering it.

 

If you wanted to be scientific, you could see that the amount of dip in the centre of truss concerns far more than the truss alone - it will draw the motors closer together, angling the chains differently, and thus increasing the load on the chains (50kg load on a 0º chain becomes 100kg at 30º I think). Increased load on the chains = increased load on the point / bridle, etc etc. It goes on. And yes, no rigger is really going to be sat with the calculator working out exactly how much, you're just going to make an assessment on whether it's safe, and if not, how you can make it so.

[mumbles]

That is what, had I not been running out the door, I pretty much would have said regarding countering bend in the truss.

 

If you've got something in the middle which is bending the truss so much that it's going to become unsafe, you should not do it regardless of what is countering it.

If you've got something in the middle which isn't bending the truss so much that it's going to become unsafe, it's fine anyway, regardless of what is countering it.

 

If you wanted to be scientific, you could see that the amount of dip in the centre of truss concerns far more than the truss alone - it will draw the motors closer together, angling the chains differently, and thus increasing the load on the chains (50kg load on a 0º chain becomes 100kg at 30º I think). Increased load on the chains = increased load on the point / bridle, etc etc. It goes on. And yes, no rigger is really going to be sat with the calculator working out exactly how much, you're just going to make an assessment on whether it's safe, and if not, how you can make it so.

just being pedantic, though I agree with the general gist of what you're saying, but it would actually be closer to 60kg not 100, actual value will be a little less but can't divide 50 by 0.866 in my head this early after a loadout. Sideways loading will be then be half of the chain load, is slightly less than 30kg. And while I appreciate you're exaggerrating to prove a point but long before your chain has toed by anything like this much you'll have exceeded the stress/strain point or maximum deflection and the truss will have snapped. I'd say that normally you'll achieve more of a toe by the point not being perfectly where you want it, than the deflection on a piece of truss could toe the motors together.

 

As I say I agree entirely with you're general sentiment, rather think you've over hyped the issue....

 

M

[Seano]

If you wanted to be scientific, you could see that the amount of dip in the centre of truss concerns far more than the truss alone - it will draw the motors closer together, angling the chains differently, and thus increasing the load on the chains

Sorry, this is just complete rubbish.

Its like those who say you should never coil the 'slack' in a mic cable because of the inductance of the coil - it has a basis in (poorly understood) theory but in practice, even in the worst case imaginable, it's totally irrelevant.

 

And yes, no rigger is really going to be sat with the calculator working out exactly how much, you're just going to make an assessment on..

I didn't say that. I said "detailed mathematical analysis", by which I mean the kind of analysis a chartered structural engineer would do.

Many rigging situations are statically indeterminate so no amount of tapping the keys of a calculator will produce "exact" figures.

 

Sitting down with a calculator and working things out is very much a part of the process of making decisions about such things - but what you get that way is still a "rule of thumb" estimate.

The trick with using a "rule of thumb" is to have some qualitative idea of the errors that are being made, and to ensure that the errors are on the side of caution (ie: that you're looking at the 'worst case'). Experience helps.

 

Edit to add:

This thread started out being about Slick Litebeam.

Loaded to capacity the manufacturer states that a maximum 12m span of Litebeam will deflect by 10cm.

 

Just out of interest, some "rule of thumb" calculations:

The shape of a 'deflected' truss is something a bit like a parabola - lets just assume its more 'curvy' than a straight line but 'straighter' than a circle.

Take a straight line between two points, 12m long and 'deflect' it so that the centre passes through a point 10cm off the 'straight' line.

 

If you assume the shape of the 'deflected' line is a triangle, the ends move a bit under 0.2mm closer together.

If you assume the new shape is a circular arc the radius of curvature is about 360m and again the ends are a bit under 0.2mm closer together

 

Mildly interesting side result: If you had a big empty field, a *lot* of ratchet straps and 2.3km of straight Litebeam, you could assemble the truss into a circle without damaging it. ;)

[Smiffy]

<snip> If you had a big empty field, a *lot* of ratchet straps and 2.3km of straight Litebeam, you could assemble the truss into a circle without damaging it

 

Now THAT I would like to see... Although where to get the projection screen from and that many VL4's? http://www.blue-room.org.uk/public/style_emoticons/default/huh.gif

[Simon Lewis]

Now THAT I would like to see... Although where to get the projection screen from and that many VL4's? http://www.blue-room.org.uk/public/style_emoticons/default/huh.gif

 

Blow the lighting - let's add some magnets and that spare liquid nitrogen tank, and make a Slick Cyclotron !!

[Pete McCrea]

Another Civil Engineering Degree graduate here (back in 2004...) who was mildly confused by EDL. I did spend a minute trying to work out why video editing had join the discussion.

 

The problem comes when your trying to talk it up. If you use the wrong terms you look a numpty amongst those that do know what they're talking about. And there's quite a few people here who do know what they're talking about at the basic and more advanced levels. Personally I don't post in a lot of threads, as I'm not confident to go sticking my balls on line in a public place where I know my input will be left for all to see indefinitely, though I take a similar approach in public too.

 

"Better to be thought a fool than open your mouth and remove all doubt..."

 

Back on topic - as Seano has said, dealing with relatively small cantilevers is going to be done more so through the qualitative process than sitting down and dusting off my Degree texts and working back through the bending moments etc. Even 7 years out from my degree I'm not confident enough now to deal with things over the it's 6kg on a 1m cantilever on 30x30 medium duty box truss.

 

I'd also like to see a tangental truss circle like that Seano. I'll provide the surface if someone can find enough slick....

[Smiffy]

Better to be thought a fool than open your mouth and remove all doubt...

 

I only ever open my mouth to change feet these days anyway :)

[brainwave-generator]

If you wanted to be scientific, you could see that the amount of dip in the centre of truss concerns far more than the truss alone - it will draw the motors closer together, angling the chains differently, and thus increasing the load on the chains

Sorry, this is just complete rubbish.

Its like those who say you should never coil the 'slack' in a mic cable because of the inductance of the coil - it has a basis in (poorly understood) theory but in practice, even in the worst case imaginable, it's totally irrelevant.

 

That's why I said 'if you really wanted to be scientific' and then went on to explain that in real world terms it was irrelevant.

 

Then you said 'this is complete rubbish, in real world terms it's irrelevant'

 

Confused.com

 

The problem comes when your trying to talk it up. If you use the wrong terms you look a numpty amongst those that do know what they're talking about. And there's quite a few people here who do know what they're talking about at the basic and more advanced levels.

 

So I don't know what I'm talking about in rigging, purely on the grounds that I had 'EDL' in my head rather than 'UDL'. The fact I was out rigging arena stuff today is irrelevant... EDL not UDL proves I know nothing.

 

I love forums.

[Pete McCrea]

Not necessarily. And I didn't say that. What I said (certainly meant to say if it didn't come across) is that using non standard terms in conversation/down the pub/ on the Internet can lead to those in the know dismissing your input because it doesn't seem to match with your claimed experience.

 

As I'm sure you also know from your experience of working as a rigger, that the time pressured and often hectic world of a load in requires clear consistent communication that doesnt confuse. Something I also believe should be carried into other communications.

 

Plus, if you don't like it here, then you know where the door is. But your more than welcome to stay and join the conversations.

[Seano]

Now THAT I would like to see... Although where to get the projection screen from and that many VL4's? http://www.blue-room.org.uk/public/style_emoticons/default/huh.gif

Oh, sorry, I don't think you can't hang anything on it. Already up to max deflection y'see. ;)

 

If you use the wrong terms you look a numpty amongst those that do know what they're talking about.

I think everyone who posts here gets things wrong now and then, it takes more than a simple mistake like getting an acronym wrong to make you look a numpty.

Making a simple mistake and then getting all huffy and defensive if someone politely pulls you up on it though, that'll do it pretty much every time..

 

That's why I said 'if you really wanted to be scientific' and then went on to explain that in real world terms it was irrelevant.

Then you said 'this is complete rubbish, in real world terms it's irrelevant'

Eh? If you scroll up a bit your post and mine are both still visible right there, so stop digging.

 

You didn't say anything about it being irrelevant in "real world terms" in your post.

Neither did I in mine. I said it was "totally irrelevant" - its rubbish both in practice and theory.

 

So I don't know what I'm talking about in rigging

No one has said that. You've posted some stuff that was wrong, and some people have responded to those posts to say "this is wrong". If you find that difficult to deal with you probably shouldn't be posting stuff you're not completely certain of.

You should also probably be aware that seeking to exaggerate your knowledge or experience also carries some risk of a bruised ego.

 

The fact I was out rigging arena stuff today is irrelevant...

Completely.

None of the techs backstage care if you're David Bowie or the milkman.