Truss

[stevieboi]

Hi All

 

We have hung a length of truss (12m) in a school hall from an I-Beam - via 2 manual chain hoists and the relevant slings / shackles etc.

 

We have done this several times before for various shows without the need to have it certified to be safe.

 

The school are now asking that it is in some way certified / installed by a trained or competent person.

 

We are all competent having done this before, but do not want to put our names to it as we have no information on the integrity of the I-beam.

 

On the truss is 6x 575 washes and 3x 700 spots. We have hung more than this previously.

 

 

Any advice greatly appreciated.

[Brian]

The starting point has to be getting the I-beam looked at. A structural engineer will be needed to look at it and decide what additional loads it can carry. He can then sign that off. The beam can then be marked up to indicate where the rigging points are and what load they'll carry.

[Seano]

We have done this several times before for various shows without the need to have it certified to be safe.

Whether the school want some kind of written sign-off sheet is up to them, but hanging that truss is a lifting operation and under LOLER they (and you) had a clear legal duty to ensure that the work was planned, organised and carried out by competent people when you did it before.

 

We are all competent having done this before

What else makes you competent? (There's more, right?)

How did you know you were doing it right the first time? How do you know you've been doing it right ever since?

 

but do not want to put our names to it as we have no information on the integrity of the I-beam.

If you're not willing to put your name to it, why did you do it?

If you're not sure the beam is sufficiently strong and stable to support the load, why did you hang the truss from it?

 

You can't have it both ways. Either you're competent to do the job, in which case you also take responsibility for the job, or you're not.

If you're not, you shouldn't be doing the job. There's no grey area here; you just shouldn't do it.

 

Any advice greatly appreciated.

Don't do it again.

[dosxuk]

We are all competent having done this before, but do not want to put our names to it as we have no information on the integrity of the I-beam.

 

If you're not confident in the integrity of the I-beam, walk away from the computer, go to the school, and take the equipment down - immediately - and while taking extreme caution when working with this potentially lethal situation that you have caused.

 

You can not be both competant to do this work, and be unaware of the integrity of the beam.

[law99]

from an I-Beam - via 2 manual chain hoists and the relevant slings / shackles etc.

Is this equipment hired in or owned by the school? If owned by the school does it have the relevent LOLER cert and yearly inspection?

 

As stated if you're asking this question then you should be getting a compentent person (no disrespect, but you are not a competent person at this stage just because you have done it a few times) in to oversee and sign off.

 

Its all about being safe, if it would to fall down and hurt someone, just because you didnt put your name to it doesnt mean you cant get the blame, you put it up there so the blame would fall with YOU.

 

Not to say its not safe, I'd presume you safety it off with steels once its at trim height etc?

 

I'm sure theres a local company that has a rigging dept. that would come and sign it off, assuming theres the structual engineers report of the I-beam loadings as previously stated.

 

Good luck!

Law

[The Seedage]

Pretty balanced picture of the situation here... If there is no backup documentation, dont do it!

 

One other factor to consider is the type of truss and the duty charts for it.. 12m is a bit of a long span for just 2 points, and I'm guessing that if it is in a school environment, its going to be the lower end of the equipment spectrum...

 

Roughly, looking at your kit list, you are looking at around 250kg including cabling and guff, which across 12m is a fair weight uniformly distributed...

 

If you do get the calc for the point loading in the roof, your next task is to search out the specs for the truss and check that you can actually hang that load in the way you are currently...

[Jivemaster]

You need the rigging points approving testing and marking by a (likely Chartered ) structural engineer probably with reference to the original architects drawings.

 

Then you need the truss beam checked for integrity

 

Then you need the proposed hanging arrangement checked by a competent rigger

 

Then you need to look at the truss manufacturer's charts and tables to check the load rating of your truss in the situation that you have.

 

Then you have to have the weights of all the kit you want to hang and the positions that you want them at.

 

Then you have a professionally competent design, A good design can be built regularly by people of some but lesser skill than needed to design the rigging.

 

A rigging collapse over the heads of a hall full of children may not make you a popular person, The venue may well get prosecuted under HSWA, yes I can see that they want documentary evidence that the rigging is proven safe.

[gyro_gearloose]

Not to say its not safe, I'd presume you safety it off with steels once its at trim height etc?

 

But what is the use of doing that if the I-beam itself isn't strong enough?

[law99]

from an I-Beam - via 2 manual chain hoists and the relevant slings / shackles etc.

assuming theres the structual engineers report of the I-beam loadings as previously stated.

 

 

Not to say its not safe, I'd presume you safety it off with steels once its at trim height etc?

 

But what is the use of doing that if the I-beam itself isn't strong enough?

 

I was just trying to point out to the OP that there are other things to consider once the truss is up. Thats why the consistent message through the thread is get a professional in!

[Jivemaster]

Once you HAVE the full engineering design and methods confirmed from load bearing beam to hoists to strops to truss and load, and you have paid for the design, then it should be possible for people with some but less skill to arrange to repeat the same method often.

[Smiffy]

http://images.jimonlight.com/wp-content/uploads/2008/11/timberlake1.jpg

 

Luckily nobody was hurt (to my understanding)

 

Reason - Allegedly the structural engineer got things wrong. At least that was the gen at the time it happened. It may have subsequently been proven to be some other cause.

 

Nevertheless... Even if you do everything in your power to make sure all is well. Others screw ups can ensure that your day is not good.

 

Moral: Don't take unreasonable chances. I'm no rigger, and when designing a show I use my intuition and experience to decide on the structures I use. When the concept is nailed down, I start talking to Riggers, who rig things for a living and know way more than I ever will, or indeed care to know, about such things, and then we work on the finer points of the structure(s)..... Take the time to do it correctly from a Moral, Legal, and Personal Peace of mind standpoint.

 

Cheers

 

Smiffy

[Dmills]

Unless specified as being used to support additional load at design time, roof trussing (In the architectural sense of something built as part of the structure) is normally only minimally in excess of what is required for whatever the worst case wind/snow loading is. It varies by jurisdiction but I have seen factors as low as 1.2 used.

 

Sure in a theatre you would HOPE that exposed steel that just begs for a gravlock would be designed with some suitable spare capacity, but without the engineer telling you, how do you KNOW (And ten tonnes, just in case, would surprise the hell out of me just about anywhere, you might get 10kN if you are lucky)?

 

Just because there is big steel up there does not in any way mean that you can assume there is spare load capacity, unless a structural engineer tells you that there is spare capacity.

 

On a job I have just done we had an extensive array of secondary steel put in, the steel in the roof will handle stupid loads, but the vertical concrete encased columns that hold the whole thing up mean that we have to restrict the loading to ~6.5KN per beam if all beams are loaded due to the possibility of buckling the vertical structure, it is NOT just about the steel in the grid, you have to think about the entire support system.

 

Regards, Dan.

[livelifetotheverymax]

In the UK you'll find that any permanent loads have a factor of safety of 1.4 * any dead load (the weight of the structure) and 1.6 for any transient loads (people, wind, snow etc.) but this still is not to allow for extra loading, for example from a truss. Adding such a point load in the wrong place could massively effect the load cases present in the structure which could, in the worst case, lead to collapse of the structure.

 

Engineers DO NOT allow extra "to add bits on here and there" as this costs money and unless specified by the client isn't needed, any extra that is allowed for is a FACTOR OF SAFETY used only to allow for defects in materials, construction quality and ageing of the structure.

 

Morel to the story, consult a qualified and experienced engineer.

 

Cheers, John.

(MEng. Civil and Structural Engineering)

[Brett]

Steveboi

After reading all the above I think this covers everthing.

if you not a rigger then you should not be rigging go to professionals. From my experience not knowing the trussing used (manufacture, type, size, and load bearing charts) I would in most cases say you need more than 2 rigging points previously covered by The Seedage

 

I would be more than happy to supply a few contacts you could use. I wont list on here PM me

[Jivemaster]

As you have already done this rigging a few times, and there may be several future requirements for the job, either get a professional rigging company to do the job, or purchase a instalation specific design from a suitable rigging company.

 

 

Either way you have the whole thing designed by competent and insured professionals from the architectural supports to the final hook clamps with all the safeties.

 

Once you have the design you still need the rigging method. Some truss collapses have occured during the assembly process rather that when fully assembled.