Prolyte Campus Blog: ‘Basket Up’ - on a Truss Structure

Prolyte Campus Blog: ‘Basket Up’ - on a Truss Structure
In the previous blog: 'Basket Down' on a Strucutural Beam', different possibilities were discussed on setting-up a Basket type of hitch on a beam. Which, from an entertainment rigging perspective, means that this Basket faces downward from that structural component. If we have to lift a truss structure using a Basket hitch, the slinging method will face upward, away from the direction of gravity.

What about a ‘Basket Down’ with a single shackle?

Before discussing that ‘Basket Up’ one more additional remark to the previous blog. Not shown in that particular diagram was a Basket, where just one single shackle is used, as shown in figure 1.

Fig. 1: Beam Basket variation with a single shackle.

A) Increased Hazard of falling parts;

B) Waste of precious Load-in Time.

When using just one shackle, it is a little bit cheaper in investment on equipment. However, the actual use of such a set-up, during each application will result for a particular moment in time that all four parts are loose from each other: -sling, -hoist chain, -shackle body and -shackle pin. It is resulting in an increased falling hazard of multiple objects, where Murphy’s Law is still present. A loose object might be ‘coming in’ from heights ranging from 10m to 25m and with technicians, working on the floor underneath, only having a hard hat as protection, while a ‘body armour’ would not be a luxury.

Also, this ‘single shackle’ Basket method takes a lot more time than needed in applying and closing of the Basket.

Higher risk and waste of time, not the preferred circumstances in our business

Baskets Up: applied to a Truss. What about shackles?

However, when we are working at the venue floor level, the whole Basket Slinging Game is changing. We work on floor level when slinging the trusses. Open structures, like truss structures, allow for much more routings of a sling than solid beam cross sections, but for now, let’s stick to that Basket method. Any shackle pin falling by accident will ‘travel’ about 1m, in worst case scenario hitting our own feet. Apart from flip-flops, almost any type of shoe will give enough protection. Using no shackle at all is close to madness, but just using one single shackle here is the best option. In fact, it makes it easier in getting the sling centred exactly over the Centre of Gravity (CoG) of that particular truss. Only when the pick-up point is over that CoG, the truss loading tables will be valid, provided that the load is balanced as well.

‘Good Baskets vs Bad Baskets’

To be able to tell if the combination of Truss and Basket is a proper and a safe one, there could be about a dozen of things that must be taken into account. The following list is showing the most important ones:

1. Support points (slinging points) must right In or Against the Node points. It must be avoided that supports are on the chord centres between the Nodes.

2. The shackle shall close the Basket, establishing the connection of both sling ends to the lifting machine.

3. The included angle α between sling parts shall not exceed 90°. Meaning that β, the external angle with the vertical is not exceeding 45°. This prevents excessive forces in the sling and the truss cross-section. Included angles of 120° cause each part of the sling to be loaded at the same amount of force as the load itself causes.

4. The sling routes shall be avoiding an increase of forces caused by internal angles inside the truss cross-section.

Fig. 2: Avoid Baskets variations with internal crossing of the sling routes.

5. The load on the truss shall be balanced, meaning each side of the cross section carries half of the total load.

6. Additional wraps on the chords mainly help to stabilise the truss but do not contribute to an increase of the truss capacity.

7. Avoid letting the sling rest against the diagonals, putting a side load on them.