Size of base vs height of structure - any rules of thumb for jacks?

[Uncle Dirtnap]

A few months back the high school I volunteer at put on Forever Plaid. We built these clever LED lighting towers - 15 feet high, 2 feet wide,1/2 ply backing, built very light - they are maybe 60lbs a piece. We overbuilt the bases: we had the wood and space, and the guy building them made 5ft x 2 ft 'sleds' out of 3/4 ply, mounted to the backs with 10' 2x4 jacks linking them. Complete overkill, but we had the room.

The play has been selected to play again at the state Thespian conference, but the theatre we are going to be in is smaller - mainly, we have about 1/2 as much depth to the stage and I need to figure out the minimum amount of space I need for safety and stability. They need to be free standing, and actors go between them.

Are there any rules of thumb for those calculations? It would propably be the same for traditional jacks, but I can't find any of the info in my ancient college theatre textbooks.

Thanks!
-rj

 

[BillConnerFASTC]

I can't quite picture what you built but it seems it has more to do with weight at base than dimension of base. A 2' x 2' base with a couple of sacks of tube sand probably is fine - just be sure stand will stay on the base.

Strictly dimensions - base should probably project in all directions about 1/3 of height - based on scaffold and ladders - but it sure seems you needed weighted base like a side light boom. Obviously the base should be nearly under the center of gravity of the tower.

 

[Van]

Assuming that the units bases are 24"x24"... I would make sure the base of the jack is 24". I'd want the jack 8 - 10' tall. You'll want 40# of either sand bags or stage weights on the bottom of the jack; One jack per tower. Make sure the jacks are attached by using loose pin hinges installed on each side of the upright so that it can't fold!
Most importantly, you need to add a Stretcher between the towers either across the tops or on the backs at the top. the stretcher can attach either via screws or you could rig it in place with loose pin hinges so it is easily removable but there need to be at least three per unit. The stretcher will provide the lateral bracing necessary and the wider apart you place the hinges the greater resistance to the shear forces if something were to start to lean.

I would make these so that the stretcher and the jacks can be installed while the units are on their faces then the whole thing walked up as a unit and then the weights added.
Having done more than my fair share of Thespian one acts and ACTF's I understand set up can be a pain and needs to be fast but it also needs to be safe. The only other thing might be to put ceiling plates or d rings and keepers on the tops of each unit and then rig them to a battens.

BTW a good "rule of thumb" for dynamic structures is 1:3 Base to Height. So if your base is 24x24 and that dimension remains constant for the height of the piece then 1:3 would 6' tall. By doubling that base dimension in one direction to 48" would mean a max height of 12'. However, the weight on the base of the jack changes the center of gravity thus the force necessary for it to tip forward and the jack being static on the back makes it difficult to tip back, therefore your biggest foe is the side to side that you haven't overcome yet, therefore the stretcher at the top. If you were to attach the stretcher at a single point on the back of each tower you'd basically just make sure they stayed in a pretty arrangement as they tipped over. By adding a point another point of connection on each piece you help overcome the lateral force. You could, theoretically attach the stretcher to the center tower in a rigid, multi-point fashion and have a single point on each tower and accomplish much the same result but the safety factor is increase by having multiple connections on each tower.

 

[Uncle Dirtnap]

Hmm. I will definitely need to give this some more thought. Last time we had the towers up they were free standing, with 3 rows. (first 2 were 35 feet apart, 10' back were 2 25' apart, and last 2 were 20' apart. The base was 24' wide, but the towers were only 8" deep, hence the 'sled' they were attached to. The idea of attaching the towers to each other seems very problematic, but I may need to give it some thought.

There are decorative 'wings' that increase the width of the towers up to 8 feet. There are 1x2 and luan, but perhaps that is a good avenue to explore as well.

The theatre we are setting up in has no battens - it used to be a silent movie theatre, so we are essentially on the apron.

I found a clip of the towers from the last show in the 'real' theatre-

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[Van]

Are those "wings" on each side also 'pointing' upstage at an angle? from that clip it looks as if they may be acting as a jack for the units.

 

[egilson1]

ANSI E1.15 - 2006 (R2016) Entertainment Technology--Recommended Practices and Guidelines for the Assembly and Use of Theatrical Boom & Base Assemblies can be a good resource for this. it's available for free from http://tsp.esta.org/tsp/documents/published_docs.php

Even though you might be using truss, wood, or other materials the methods in the standard for determining if the structure will remain upright still apply.

Ethan

 

[Uncle Dirtnap]

Not really - they have a slight bit of dimension to them, but they are essentially a 8" wide piece of luan with a 1x2 on the outside edge. Add another 8" luan with 1x2 on that, and repeat. From above you get ~ 6" of dimension. They are flimsy as anything. The major structure not shown is a 2' x 5' box framed out of 2x4s with a 3/4 ply bottom. (the 'sled'). Each tower has one, and we would throw ~60# of weights on each of them. These would also have 12' 2x4 diagonals on both side of of the sled, hidden by the wings.

 

[BillConnerFASTC]

Its about the center of gravity. Fill that base - looks like it could be a 2' x 2' x 2' cube - with sand - or partially - as full would be in the 450-500 pound range - which would be OK and sure to keep them upright.

 

[Van]

Not really - they have a slight bit of dimension to them, but they are essentially a 8" wide piece of luan with a 1x2 on the outside edge. Add another 8" luan with 1x2 on that, and repeat. From above you get ~ 6" of dimension. They are flimsy as anything. The major structure not shown is a 2' x 5' box framed out of 2x4s with a 3/4 ply bottom. (the 'sled'). Each tower has one, and we would throw ~60# of weights on each of them. These would also have 12' 2x4 diagonals on both side of of the sled, hidden by the wings.

Sorry, I interpreted you statement to mean the wigs were framed like a flat and they increased the width of the tower base to an effective 8' not that they were 8".

I feel like I'm in Spinal Tap now.

 

[bobgaggle]

I'll chime in with the easy answer. Build 1 jack to the size requirements, stand up the wall, then try to knock it over. It sounds stupid, but if you need a sanity check that's the way to go. You'll give yourself a lot of reassurance.