Prolyte Campus blog: Yes we can (do better slinging)!
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 below 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.2).
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.3) 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.
Read more about slinging in the Prolyte BlackBook