Permanent truss install

[Dave]

I have to hang a 6m length of trilite (apex up) off three points for an install. Just wondering what would be the best way to do it. Total UDL is 150kg (generics and cabling), so not much.

 

I could get some plastic coated steels made up and use them in the usual "basket" fashion with ends crossed over under the top chord. But this isn't the most attractive option and I believe the aluminium ferrules have a fairly low max working temperature. I'm worried that the client could stick a few pars underneath them at a future date and, whilst I'm sure it won't get that hot, I'm a little uncomfortable with the idea. The clearance to the ceiling isn't that great and there won't be much air movement.

 

So I'm thinking of Doughty slimline hanging clamps (see here), on the top chord, hung from steels made up using grips so to avoid the aluminium ferrules. I was concerned about the truss just being hung from the top but I have spoken to Opti and they seem quite happy with the idea.

 

But I was then thinking I could use M12 studding and the Doughty slimline half-coupler (see here) instead of steels. An SWL of 250kg per point has been suggested for this (see this topic), which is more than enough. This would look better than the wire rope. Can anyone see any issues here? My concern is primarily that most studding is untested and of "commercial" quality. Flints do a high-tensile version of the studding that I am tempted to use - has anyone tried this and if so, how hard is it to cut? Out of curiousity, can anyone (Brian?) estimate the SWL for this - I guess it depends on the grade of HT steel, though.

 

At the top of each piece of studding (inside a false ceiling) I would put a rated M12 eye-nut. This would have two 1/2 tonne shackles in it which would connect to the ends of a length of hardened chain that would go over structural steelwork that the client has had OK'd for the weight loading. The steelwork is an unusual shape and can't easily be clamped onto.

 

I'd have no hesitation in rigging this lot temporarily using spansets etc, but as it's going in to stay and I will not have any control over what happens to it once it's up, I want to do the best job possible. I'm probably being over-cautious (especially given what I've seen in many venues over the years) so would appreciate any suggestions. And before anyone asks, my PLI does cover this.

[jonathanhill]

Can you put a basket over the beam?

 

If so I would use badger clamps on the truss, eye up, shackle to a short stinger to a sleeved steel basket over the roof beam.

 

I would avoid dogs and studding as I do not rate dogs and studding may mean that people may view the truss as 'fixed' and try to rest ladders against it.

 

By the way, aluminium has a melting point of 660 degrees Centigrade (~1200 degrees F), about the temperature that a SpanSet will fail.

[Seano]

But I was then thinking I could use M12 studding...

This would look better than the wire rope.

 

That's a matter of opinion, I guess. My first thought when you mentioned studding was "yuk!"

Likewise the chain basket, though there's prolly nothing wrong with that if its a suitable short-link chain (with a sleeve, or something to 'soften' the beam).

I don't much like the idea of two shackles in the eye-nut either, will they even fit? It certainly isn't good practice to use an eyebolt (or nut) as a master link, given the low loads you're talking about that might not be too much of a worry structurally, but it gets another big "yuk!" from me I'm afraid.

 

The working temperature of aluminium ferrules really isn't a worry: using grips instead would be a step backwards imo, regarding both reliability and aesthetics.

 

As its a permanent install, it'd be a good idea to mouse the shackles (if they're screw-pin) or you could use bolt types, which have a split pin to retain the nut. This is also something to bear in mind if you really want to use studding (especially at the eye-nut, where a lock-nut up behind it wouldn't really be good enough imo).

 

hth

[jonathanhill]

But I was then thinking I could use M12 studding...

This would look better than the wire rope.

 

Likewise the chain basket, though there's prolly nothing wrong with that if its a suitable short-link chain (with a sleeve, or something to 'soften' the beam).

 

 

I would not use chain such as STAC for a primary point, only for secondary safeties.

[Seano]

I would not use chain such as STAC for a primary point, only for secondary safeties.

 

The OP talked about using a hardened chain to go over structural steelwork - I assumed he meant using it to make a basket, and since he seems to know what he's talking about I've assumed he means a chain suitable for slinging. That means a short-link chain suitable for use as a sling - STAC is a long-link chain, and as such should never be used for slinging (ie: to make a basket or a choke hitch).

 

Whether it should be used in rigging generally is the subject of several other threads, so lets not derail this one. (Though your position does seem very odd - if you'll use it for one thing, why not the other? Or vice versa.)

[jonathanhill]

I would never use chain for a basket, long or short lnk. I would use STAC or a shortening clutch from the safety basket to the safety stinger to make it as tight as possible.

 

Back to topic...

[Dave]

Can you put a basket over the beam?

 

If so I would use badger clamps on the truss, eye up, shackle to a short stinger to a sleeved steel basket over the roof beam.

 

I would avoid dogs and studding as I do not rate dogs and studding may mean that people may view the truss as 'fixed' and try to rest ladders against it.

 

By the way, aluminium has a melting point of 660 degrees Centigrade (~1200 degrees F), about the temperature that a SpanSet will fail.

Sorry, I'm not familiar with badger clamps. It's not the melting point of aluminium that matters - it expands and softens at much lower temps.

 

Likewise the chain basket, though there's prolly nothing wrong with that if its a suitable short-link chain (with a sleeve, or something to 'soften' the beam).

I don't much like the idea of two shackles in the eye-nut either, will they even fit? It certainly isn't good practice to use an eyebolt (or nut) as a master link, given the low loads you're talking about that might not be too much of a worry structurally, but it gets another big "yuk!" from me I'm afraid.

 

As its a permanent install, it'd be a good idea to mouse the shackles (if they're screw-pin) or you could use bolt types, which have a split pin to retain the nut. This is also something to bear in mind if you really want to use studding (especially at the eye-nut, where a lock-nut up behind it wouldn't really be good enough imo).

Yes, hardened short-link chain. I hadn't planned to sleeve it given the low loads, but I suppose I could use some PVC tubing - I'm not sure it would necessarily achieve anything, though.

 

Good point about the eye-nuts - I think two shackles will fit, though. They're not very big ones as they need to go through the chain. I could just use a bow shackle instead of a masterlink, but I was thinking the eyenut is basically the same thing. If I go for this option I'll check with whoever makes them first and if they don't like it I'll put an extra shackle in.

 

Was already planning to mouse regular shackles as bolt ones are a bit dearer. Was planning to use locktite and a locknut on the eyenut. Why don't you think this would be adequate?

 

I would not use chain such as STAC for a primary point, only for secondary safeties.

 

The OP talked about using a hardened chain to go over structural steelwork - I assumed he meant using it to make a basket, and since he seems to know what he's talking about I've assumed he means a chain suitable for slinging. That means a short-link chain suitable for use as a sling - STAC is a long-link chain, and as such should never be used for slinging (ie: to make a basket or a choke hitch).

 

Whether it should be used in rigging generally is the subject of several other threads, so lets not derail this one. (Though your position does seem very odd - if you'll use it for one thing, why not the other? Or vice versa.)

Yes, I think of it as an inverted basket and as mentioned before the chain will be hardened short-link stuff. Wouldn't dream of using STAC for this.

 

I'm also thinking I could use the chain for the vertical suspensions, if they are all the same length. It's even less pretty than studding, though, but it's certified for lifting so gives me a warm fuzzy feeling.

 

The sad thing is that many companies wouldn't worry about all this and would just use any old rubbish - unsiezed commercial shackles, long link untested chain, gripples, etc. Believe me, I've seen it.

[Seano]

Sorry, I'm not familiar with badger clamps. It's not the melting point of aluminium that matters - it expands and softens at much lower temps.

Colloquial name for the same kind of hanging clamps you're already planning to use.

True - the tensile strength of aluminium, for example, is halved somewhere between 200 - 250 degrees. Steel ropes terminated with aluminium ferrules generally shouldn't be used in ambient temperatures above about 80 - 100 degrees, and if you're seriously expecting that you'd also need to think about whether aluminium truss is appropriate. Just can't see it myself, unless the venue is on fire.

 

Yes, hardened short-link chain. I hadn't planned to sleeve it given the low loads, but I suppose I could use some PVC tubing - I'm not sure it would necessarily achieve anything, though.

Maybe it is just aesthetic, though it also depends on the shape and size of the beam in question. And whether it's painted, as its nice to leave the coating undamaged if so.

 

Was planning to use locktite and a locknut on the eyenut. Why don't you think this would be adequate?

Visceral reaction, and experience of how locknuts never seem to stop turnbuckles (rigging screws) from slackening over time.

This is because loosening the threaded rod also loosens the locknut - both are released by the same action, turning them anticlockwise. I think its better for a locking mechanism to be either unaffected by the action that loosens the thing its locking, or to actually be tightened by it.

The clamps you're looking at consist of a half-coupler with a bolt and an eyenut - usually in their case the eyenut is seized with a bit of weld on the end of the rod (inside the 'eye').

 

I'm also thinking I could use the chain for the vertical suspensions, if they are all the same length. It's even less pretty than studding, though

Again, matter of opinion, the studding would be (marginally) less pretty to my eye. Same length shouldn't really be an issue if you have the means to cut off the odd link on site - one link should be a small enough increment of adjustment.

 

Really though: steel wire rope! If you just can't get over the aluminium ferrule thing, how about stainless steel (with copper ferrules)?

 

...any old rubbish... Believe me, I've seen it.

Yup. I've seen it too.

[Dave]

Colloquial name for the same kind of hanging clamps you're already planning to use.

Sounds like an americanism!

 

True - the tensile strength of aluminium, for example, is halved somewhere between 200 - 250 degrees. Steel ropes terminated with aluminium ferrules generally shouldn't be used in ambient temperatures above about 80 - 100 degrees, and if you're seriously expecting that you'd also need to think about whether aluminium truss is appropriate. Just can't see it myself, unless the venue is on fire.

I guess I'm just worried if someone puts 4 x 1kw PARs around the suspension point. The truss is just below a suspended ceiling and I'm worried about all the hot air rising!

 

Visceral reaction, and experience of how locknuts never seem to stop turnbuckles (rigging screws) from slackening over time.

This is because loosening the threaded rod also loosens the locknut - both are released by the same action, turning them anticlockwise. I think its better for a locking mechanism to be either unaffected by the action that loosens the thing its locking, or to actually be tightened by it.

I think thermal effects - expansion/contraction - can cause things to loosen over time, and it's something worth guarding against as you say. I try to mouse turnbuckles whenever possible, but I'm not particularly convinced by it.

[Chris Higgs]

aluminium has a melting point of 660 degrees Centigrade (~1200 degrees F), about the temperature that a SpanSet will fail.

 

I think I understand you to mean that a roundsling will fail at 660 degrees C???? This is very wrong.

Heat will start to affect the tensile strength of polyester at around 180 degrees C, maximum operating temperature usually 80-85 degrees C.

Polyester will melt at 250 degrees C.