Campus blog: Raking a truss, which point takes the load?
Not on Wind Ups!
First and foremost let’s get one thing clear, do not attempt to rake trusses using wind up stands. That type of operation invites just too much variables, all of them with ‘Murphy’ on the front seat!
But back to truss raking. We all know that lighting designers sometimes increase the dynamics of their design by raking the lighting trusses. Some designs have moving pods or fingers, to create stunning effects, like with synchronized motion control. Clearly, this increases the complexity of your truss and loading point calculations and of course the control actions and safety procedures.
Single simply supported spans in UDL*:
Let’s start simple; one single truss suspended in a level plane from two hoists. Just like what you’ll see in the manufacturers loading table specifications for their truss types, in UDL* and CPL*. The question that often causes misunderstanding is how that new (raked) situation distributes the loads over the supporting hoists, as most of us will have a gut feeling that it is not the original (ca. 50-50%) division, that was present in the hoists with the truss in a level situation in UDL loading.
Is it the top or the bottom hoists that will get more load, and as a consequence becoming more hazardous with the risk of overloading in mind?
How does this work at home?
It’s always helpful to think of your own experience in similar situations.Like carrying household gear, such as a fridge or a washing machine up the stairs. We all know that the guy at the bottom is doing ‘the real job’ - weight wise, and the guy at the top only might loose some finger skin between the wall and the load.
In this case the Centre of Gravity is pretty much straight over the shoulder of the lower guy. Where for the top guy, this is straight, under the fingers of the out-stretched arms. And we also know that fingertips on out-stretched arms are not the preferred limb parts used by weightlifters. One can compare this type of guided load lifting somewhat to the static situation with wind up stands.
Raking; move with care!
In chain hoist suspension the (unguided) load (the truss pod with lighting fixtures) has its Centre of Gravity positioned below the lifting machinery. In the case of raking your truss, the load will increase towards the two hoists in the highest position.
That load will even increase more when the combined centre of gravity of the truss and fixtures move into a position deeper below the pick up points on the truss.
Rake angles of about 15 degrees normally shouldn’t be alarming, but raking your truss or truss pods steeper than a 15 degrees angle needs a careful calculation before you move it into position. This however is not so simply calculated, because there are lots of parameters to take into account. Time to get an engineer in and adding a load monitoring system might not be such a bad idea as well.
Oh yeah, just to be clear: Do NOT rake trusses when using wind up stands….
* UDL – Uniformly Distributed Load. For example a curtain tied in repeating identical distances, or a series of identical fixtures with identical distances in between.
* CPL – Centre Point Load. This is one (heavy) load in the middle of a span, between two supporting points. E.g. a vertical array of LED-screen modules, or a Line Array PA system.