Rinus Bakker, a respected man in the market, wrote two blogs about the uniqueness of lifting shackles and about the identification of lifting shackles. These blogs are shown below. The latests blog will also appear on the Prolyte Campus Facebook page. Do you have any comments or do you want to discuss something? Let us know at the Prolyte Campus Facebook or send us an email.
Just one single round tube…
“What is the capacity of just a single round tube?” This question is generally ignored by truss manufacturers, as it has little to do with their business. And it’s too vague for a structural engineer to answer just like that. If he answers the question as it is asked, the invoice could become quite high when all the possibilities are tackled…
The issue: Many rental companies have a range of additional single round tubes that are used in an endless amount of variations. Crossing between two trusses, short outriggers, boom arms, vertical extenders up or down etc etc. So any of these particular situations can only be answered after some counter-questioning.
1) What type of metal? Both steel and aluminium can have tensile strength qualities that range from “toilet-to-top”. [Or “sh@t-to-summit” as others might say].
2) What is the alloy, the tensile and/or yield strength or that round tube material?
3) Is they’re welding done to that tube? A lug or an eye-plate perhaps, or a ‘splice butt-weld’?
4) What outer diameter (OD)? Trusses and truss clamps range in OD of 48 to 51mm, and that can already mean some difference.
5) And what wall thickness (t). This ranges from only 1,6mm up to 10mm.
6) What is the round tube orientation?
6a) Is it used horizontal in a simply supported span, or as an outrigger?
6b) Can the support point rotate, or is it fully restrained?
6c) Or is the tube orientation perhaps vertical?
6d) Is the tube used upright on a single small base plate, or on a heavy big one?
6e) Or is it fully restrained + clamped to a truss perhaps?
6f) Or is the round tube used ‘hanging’ down vertically, like an ‘extender’?
7) What kind of loading is to be applied? UDL? CPL?
8) Is the applied load static or dynamic?
And whatever more variations one could think of for one single tube only. A lot of questions…
The answer: Let’s at least pick out one of all those questions to be answered.
CPL in static loading on a simply supported single round tube 48x3mm without any welding area (alloy EN AW 6082T6).
Span â–²---â–² (m)
CPL â–¼ (kg)
The verdict: Everything in rigging and staging structures shall be adequately safe. Make sure you know that all components and details are safe as well. Consult the manufacturer, a qualified engineer or otherwise competent person to remove any uncertainty. Maybe this also explains that no truss table in the world can ever be covering all types and variations of the structures that users of the truss might think of assembling and loading.
Yes we can …. (do better slinging)! (Part 3)
For the fans, one more blog on (scary) slinging found on the World Wide Web. Although at first impression one might say the guys rigging that sound system took extra care in covering the rear of the cabinets with plastic, thus reducing electrical risk. Question could be: why leave the chain motor un-covered? And where are the horizontal restraints for the line array system to prevent it from swaying in bad weather.
The issue: The real concern however is the basket method of slinging the chain motor point the truss. This one looks like nobody did notice the deformation in the lower chords. Being too focussed on the sound system perhaps? A more detailed look at this deformation shows a standard basket method, however applied to a truss type that cannot deal with it. Trying to establish the sling route in the truss module gives the situation shown in figure 2. -->
The answer: Application of this B024 basket hitch to this in this position is totally wrong. The internal cross strongly increases the force that compresses the lower chords inwards. No horizontal bracing is present at that point so the chords deform drastically.
The verdict: At this position the truss module bracing structure is accepting a type B011 basket with ease. The sling could be much shorter as well. For stability reasons one (B013) or each (B012) of the chords could get an additional full wrap. This increases friction thus reduces risk of movement of the sling and keeps the motor centred, and the truss load balanced. Even the B001 method is to be preferred here over the internal cross-basket (B024). Each type of truss requires a different method of slinging, also depending on the position in the span.
Rinus Bakker, February 4, 2014
Yes we can …. (do better slinging)! (Part 2)
A closer analysis of the previous blog on truss slinging that almost went wrong.
The issue: Any method of slinging needs a good transfer of the sling forces into the truss structure. First, let’s have a look at the basket method (fig.1) where the sling goes around the truss, as if itwould be a solid beam, classified as B001.
The force in the sling – pulling down from the shackle - is mainly trying to pull down the top chords. Therefore the sling must be on the node point, where diagonals, or a vertical, transfer the load to the lower chords as well. The part of the basket that runs horizontally over the top chords has the same amount of force in the vertical direction. The node points in the top must have a brace between them that prevents them from being pulled together. The lower chords are loaded much less and in an in- and upward direction.
The answer: Looking back at the example, we can observe that the method is identical but the bracing is not. In the picture (fig.2) it shows that the sling is almost in the centre of the righthand top chords in between node points. No braces go down in a vertical or in a horizontal direction, (see fig.3).
An internal diagonal to the node point on the right side connects the top chord on the left down. This diagonal brace absorbs the resultant forces V+H of the ‘sleeved steel’ sling on that chord.
In this case the sling should have moved to the node points at the top chords, and go down inside the lower cords. The horizontal part is brought down and included in the shackle, in what sometimes is called a butterfly basket (fig.4) This takes away most of the horizontal force on the chords. An alternative would be to use twoseparate slings choked to the top chord nodes.
The verdict: Each type of truss will have its own best slinging method, all depending on the bracing positions. Slinging primarily has to do with the transfer of forces from the truss into the sling or visa versa. No brace present at the slinging position (slinging outside of the node points) is bad slinging to start with.
In a next blog (part 3): Yet another tricky one.
Rinus Bakker, January 10, 2014
Yes we can (do better slinging)! (Part 1)
Recently I got some pictures of bad deformation of a master grid truss at the slinging position. A number of 50 cm size trusses were used as lifting beam for serious loads from sound and screen points. It was not a Prolyte truss but looked very similar.
The trusses were used as master trusses (individual lifting beams), rather than as one master grid. This was done to avoid making bridles to the lateral relatively weak structural beams of the venue.
The issue: The trusses were suspended from 2 tonne motors, and subsequently 1 and 2 tonne motors were to be rigged from the master trusses. Slinging of the truss was done in very straightforward baskets. The basket methods were the variations B 001 and B 011 from the basket slinging classification *. The trusses were sling-supported close to the end of the modules, and from the span inverted down to the chain motors hanging of them.
The answer: Know what you are doing. Manufacturers (like Prolyte) always state that the support should be at the ends of a span and close to or in a node point. Thus being next to the end bracing that has vertical and horizontal braces in each side, plus the internal diagonal or stabilizing corner plates in that area. Slinging next to this is never a real problem. On the body of the truss the word is different. Slinging must be at the node points, with braces present in each direction of applied force. In a basket that will always be two directions in a 90 degree - or less - angle to each other.
The verdict: Each type of truss in cross section or in bracing pattern has its own number of preferred and discouraged or even not allowed methods. Too often people think this is the best method and we should apply that to all truss types.
For example if method B024 had been applied the risk would have been considerably higher for a complete failure of the chord and subsequently the whole truss. Similar accidents have already been recorded, where mistakes in slinging were (partly) to blame for a collapse.
* Over 200 basket variations have been described with a single sling on a square truss.
Next blog (part 2): How to sling this better
Rinus Bakker, 5 December 2013
Believer, Faithful or Fanatic? The real story.
We all make mistakes, even when it’s our job to prevent those are made. In that process we sometimes hurt people, which have tried to do a job to their best knowledge.
Here’s the real story about this picture, which we received and did not check well enough.
The issue: The truss in the front is a B-100 from Prolyte. A strong type of module for sure - to use as temporary stage roof beams or mother grid. But are they temporary stand-alone towers as well?
The answer: To see is to believe? Not always.
There’s more to this set up than you can see. To start with, a structural report was made, where this structure is calculated for its intended use, including all wind forces, etc. The structural report advices precautions to make the structure safe. These precautions were followed. In the ground – where you can’t see it, there’s a big steel plate attached to the tower and on top of that an enormous amount of ballast is placed. This ballast is sufficient to be compliant to all applicable regulations. So this project was done following all the rules and offering a safe solution.
The verdict: We never can exclude all risks from human life. Not even the risk of making mistakes that harm another person’s reputation. However, can learn from our mistakes and we can amend. Hence, this is the real story behind a blog on PA towers.
Marina Prak, January 2014
Believer, Faithful or Fanatic?
We all have our favourites. Whether it's a performing artist, sportsman, team, or even politician. Some people are not just fans, they can be fanatic.
At least that idea came to my mind after receiving this picture.
The issue: The truss in the front looks very much like the B-100 from Prolyte. A strong type of module for sure - to use as temporary stage roof beams or mother grid. But are they temporary stand-alone towers as well?
The other two towers in the picture are made of (stronger and rustier!) steel. And these still are fitted with stabilizers.
The answer: To see is to believe? In this case I want to see a structural report. And a convincing base module. And stabilizers. Or a cut through picture, where we
might see a complete 3m section poured in 10 tons (or so) of concrete all resting under the ground. And I would like to know what the calculations assume as a limit
to the wind speed. And my believe will grow when seeing the insurance policy of the company that put it up, and the insurance of the organizers.
The verdict: We never can exclude all risks from human life. But this kind of believe in B100 truss is one that we should be able to verify and control. Proper stabilizers, ballast and base sections should be part of any type of tower system. The shown type of truss-faith should not accidentally change into truss-terrorism.
Rinus Bakker, 10 October 2013
Soft steels - use on structural beams?
Since 1997/8 the entertainment industry has been graced with a 'new' product. A kind of round sling but not filled with thousands of polyester yarns but instead having a 2-3mm diameter wire rope going around for 25 to 30 times.
In the Netherlands the inventing company (Roodenberg Staalkabels) named them 'Soft Steel'. In the USA another company (Lift-all) completely independent developed the same type of product and introduced it as 'Steel Flex'.
The issue: Is this product – originally designed to cope with high temperatures of lighting instruments on the trusses – suitable to be used for other purposes? Can you use it to wrap a beam and make a basket or choke with it?
The answer: “Mais oui, bien sur!” meaning so much as “Yes most certainly!” (and that’s enough French for today). But the answer is correct. Nothing is against slinging structural steel beams with soft steels.
The D/d ratio is better, so the cutting hazard of a sharp edge is less. The distribution of the force is along a wider surface, so less local friction stress and better longevity of beam metal coating. Wooden beams will also be happy with these types of slings.
Further it has no bulky terminations like a mechanical splice or Flemish eye that reduce the effective length of the sling body allowed being bend.
Another great feature is the possibility to push or pull the soft steel through narrow slots on top of beams or narrow holes in walking grids as there are no unnecessary thimbles that beef up the termination’s eyes.
The verdict: Yes you can use soft steels for slinging of structural beams. If you want to be fast, efficient and safe buy them as ‘basket soft steels’ with a special sling hook already connected to it.
Rinus Bakker, 24 September 2013
As stated in a previous blog: don’t waste your money on some inspection company to check your shackles if you can easy learn to do it yourself.
The issue: Notified Body inspectors barely do a serious job in checking your shackles, because they don’t need to. But they still send you an invoice, so you might be better off to do the job yourself.
The reply: Any user should know the company that manufactured his shackles. Most manufacturers have a distinguished colour of their shackle pins; so strange colours or no colours at all could be a warning that the item might be strange inventory. The manufacturer will showcase drawings with specific dimensions for the specific load rating sizes.
You don’t need a set of callipers all the time, but a strange shaped shackle of a well-known brand is a warning sign. Any change in dimensions of over 5% should be a reason to investigate what happened, and any change of 10% or more is reason for discard and still do the investigation.
One of the most common mistakes – and a potential hazard – is using the wrong bolt in the shackle. BMW is BMW and never use a Mack-part in a Mercedes, or a GM-part in a Volkswagen.
It just is not safe!
The picture on the right is no fiction, this could happen!
Wear can happen more easily in our business, since people drag bridles across a concrete arena floor, that will act as sandpaper or even a grinder. The most obvious positions that could show wear are indicated on the right. In our business the internal wear might be less likely, but nevertheless a risk.
The verdict: Always check the dimensions, easy rotating of the pin, wear of the shackle and the WLL mark.
When a grinder is used to remove the brand name (or any other identification), there should be serious checking on other aspects as well, to start with the mind-set
of the guy who did that.
Rinus Bakker, September 6, 2013
Slinging methods: do we know ‘m all?
The topic of slinging entertainment trusses has been hovering over the market for about three decades. Mainly because the open character of the truss structure allows for so much more ‘routes’ for a sling compared to the ‘classic’ solid beams…
The issue: Slinging of trusses is of course not the most common cause of accidents that involve truss structures. Beyond any doubt that would be overloading of the truss span itself or overloading of a chain motor. Negligence in planning or maintenance does play a role. And in outdoor situations it is undoubtedly wind, wind or wind. But slinging does play a role for the internal forces in the truss module members. And at one point this can lead to stresses causing failure, either as a direct collapse or in a fatigue induced manner.
The answer: When categorizing the pro’s and con’s of method variations it must be clear which one is discussed, so a name (or rather a …number) is needed. 15 years of collecting slinging methods still has not reached a ‘saturation’ point: every year of few ‘new variations’ keep popping up. Below some examples are shown of over 200 slinging variations on a square truss, with one sling only in a ‘basket’ set-up.
The verdict: It looks like imagination has no limits and some should really be in a Museum of Modern Art. To be continued.
Rinus Bakker , August 19, 2013
Identification of lifting shackles
Yes – a bit more about these things. Real cash-cows for inspection companies if you want to buy their annual paperwork that is often confused with an inspection.
The issue: All lifting components that can be affected in their safety as a result of use, abuse, wear and so forth shall be inspected at least once a year. Thus no sense in denying shackles shouldn’t be.
The reply: Nowhere in EU-Directives, Act’s, Laws or Regulations is specifically stated that the annual inspection of shackles must be done by a government recognized inspection company or a Notified Body. Buy the right shackles and be less worried on the subsequent annual paperwork.
First thing is to make sure using shackles that are designed an manufactured for lifting purposes. Most nations require a minimum amount of identification on the shackle. The picture + table on the right compares the situation of the EU with that in the USA and Russia. The position of these markings is free.
A) Manufacturer name or name-code
B) WLL = Working load limit – the rated safe working load for general us.
C) CE marking. Referring to a type IIA declaration by the
D) Diameter of the body or leg – in mm’s or inch.
E) Material alloy, in a number or code
Do you find all these mandatory ID’s then use them. An important thing that should be considered is that shackles are not the weakest parts in our systems, in most cases the wire ropes are.
The last one is an overall risk assessment: in the range of rigging accidents that have hit our industry in the last couple of decades, failure of a lifting shackle has never been the cause.
The verdict: Inspect them yourself, as will be explained about in a next blog. Put the accredited (expensive) attention to where it is more (cost-) effective…
Rinus Bakker, August 7, 2013
Uniqueness of lifting shackles
The first one that comes to mind, reading shackles will be the common type used in the entertainment business, known as: screw pin anchor shackle, and also referred to as screw pin bow shackle. The following remark however is just as valid for this one: the screw pin chain shackle, or D-shackle.
The issue: Often the remark is heard that the body (the Ω omega or U-shaped part) of the shackle, and the shackle pin (or bolt) are a unique combination, that should always stay together.
The reply: That type of remark is absolutely false.
In the manufacturing plant the two parts are produced complete separate of each other in different machines and processes. Only at the very end the two will meet, in order to be shipped out.
Somewhat similar to the tapered steel pins and bi-conical connectors of Prolyte that only will meet the trusses at the first trail build at the plant or – more often - the first production the user sets the into action.
The verdict: You can mix the parts of one manufacturer in one capacity-range, but don’t mix shackle parts of different manufacturers, even if they have the same load rating. Just as you should not mix components of truss brands that make bi-conical connector systems.
Rinus Bakker, July 31, 2013.