TomLyall Posted June 4, 2007 Share Posted June 4, 2007 Hi All, We're looking at the idea of putting a truss arch in-front of a proscenium for a performance next year. The proscenium is 24ft (7.2m) wide, and around 14ft (4.2m) high. If possible, we'd like to hang a 3 or 4 movers off the inside of the truss, and some blinders on the outside. Right now, the initial thoughts are to go with half of a 4.5m radius box truss circle free-standing just in-front of the prosc. This would be attached to the floor though the base plates, and anchored to the brick-wall of the pros. The first FOH LX bar would be just a few feet away from this too, so a solid bar between the top of the truss circle and this LX bar would be possible too, I'm just not sure it'd be an advantage (or even safe) to do so. The other suggestion is to attempt a four legged structure. If we were to use two half-circles, one in front of the other, joined together to do this, how far apart would they have to be to be stable when freestanding? We'll probably be taking the design to a professional rigger to do the calculations eventually, but I'd like to get some initial ideas on feasibility. Tom Link to comment Share on other sites More sharing options...
AndrewR Posted June 5, 2007 Share Posted June 5, 2007 The first obvious concern is that the arch structure you are considering will need bracing in the same plane as the curve of the arch. As you put load on the curved section of truss it will push the base apart where it joins the legs. (The recent Take-That tour included an extreme example with a huge steel cable holding the ends of the huge curved truss together) No reason why it can't be done though, either with something under tension between the ends of the curve or possibly some form of bracing from the outside. Either way you need someone suitably experienced to work out the best way forward and the SWL of such a structure. Link to comment Share on other sites More sharing options...
Jivemaster Posted June 5, 2007 Share Posted June 5, 2007 Engineering wise an arch translates the downward force into a sideways force, so internal tensile bracing or external comressive bracing is called for. Lighting wise as a support it is great but all the existing FOH positions will be at risk of causing great shaddows of the truss. Link to comment Share on other sites More sharing options...
timmath Posted June 5, 2007 Share Posted June 5, 2007 This is something that has been done many times before... A decent pair of base plates and depending on the weight your hanging some ballast. For a circle that size I would shy away from trilite and the like so 12" or 20" Truss the way to go, although you will have fun getting it into position without some sort of lifting equipment. www.trusscircle.co.uk may be a good place to start looking for the circle... Regards Tim Link to comment Share on other sites More sharing options...
TomLyall Posted June 5, 2007 Author Share Posted June 5, 2007 I'm going to start my reply by pointing out again that this is something that will be calculated and checked by someone competent to do so, and that I'm just in the process of getting some initial ideas to ask said competent person about. The first obvious concern is that the arch structure you are considering will need bracing in the same plane as the curve of the arch. As you put load on the curved section of truss it will push the base apart where it joins the legs.I was thinking that as it's half a circle, and not just a minor arc, there wouldn't be that much force exerted outwards. I was hoping that this could be helped further by keeping the weight towards the bottom, more vertical bits. I guess it's possible that attaching it to the floor through base plates would be sufficient. I'd be a bit cautious as there'd be all sorts of nasty lateral forces on whatever fixings are used there, right? Lighting wise as a support it is great but all the existing FOH positions will be at risk of causing great shaddows of the truss.I'll make the point to the LD, but I don't think he'll see this as a problem. Lighting wise, the performance is We Will Rock You, so it's going to be pretty heavy on back/side/top light. There's also a fair bit of throw FOH to play with. A decent pair of base plates and depending on the weight your hanging some ballast. For a circle that size I would shy away from trilite and the like so 12" or 20" Truss the way to go, although you will have fun getting it into position without some sort of lifting equipment.It'll be square prolyte stuff, I should imagine. Though this is going to be up to whoever ends up doing the final calculations on the job. Link to comment Share on other sites More sharing options...
ianl Posted June 5, 2007 Share Posted June 5, 2007 bear in mind that not all truss circles were built to be used vertically and therefore would not be suitable for an arch Link to comment Share on other sites More sharing options...
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