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More of an engineering question

Discussion in 'Scenery, Props, and Rigging' started by Crewguy7, Nov 28, 2003.

  1. Crewguy7

    Crewguy7 Member

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    I'm trying to design an A-frame that will lean over the opening for the catwalk ladder to help hoist some heavy blacklights. The A-Frame will be supported at its base by blocks, from the top by aircraft cable. My problem comes from the fact the i need to find a way to attach the wooden blocks to steel grating. They need to be strong. The only idea i have thought of would be to take some wood stock about 6 inches long and make half of the wood the exact size of the slots in the grating and the other half larger, pretty much creating a wedge. Do you think this would be strong enought to hold about 60lbs of lighting instruments along with a great deal of pressure from hoisting?
     
  2. ship

    ship Senior Team Emeritus Premium Member

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    Nope.
    Granted I'm not there to comment on the A-Frame idea but as for the question of attaching the blocks to grating, how about using some unistrut nuts that will lock into the grating and bolt the blocks down. Otherwise, I would go with steel plates equal in size to the block and thru bolt.

    I assume you are not talking about 1/4" 2x4 C-Channel grid decking but some type of actual floor grade such as a perferated, wire mesh or 1" parallel strip.

    The reason you probably don't want to use wedges besides the sheer strength of the lumber grain splitting is that by wedging you are prying apart the structure that's supporting you. If you both twist and force a downward pressure on a item it's going to fail. By bolting, you are clamping. Compression but preventing that prying action will be stronger and prevent movement by the friction of the clamping pressure as opposed to the wedge moving as it spreads the for all intensive purposes beams you are wedging apart and relying on for strength. The unistrut nut should grip into normal 1" parallel grating. Disperse a few of them across a large amount of the grating so you don't over stress any few individual members. If you are talking about some type of mesh grating, it might or might not have it's notches line up and you would need to distribute the clamping pressure better anyway.

    A plate that encompasses all the bolt holes will even distribute the clamping pressure around the area you are supporting. It do this with a little less compression necessary than with a unistrut nut and thus potentially cause less of a problem but for a 60# load it shoud not be. Make sure all hardware is at least and at bare minimum 3/8" and grade 5. You need a minimum of four per plate and a minimum of 3/16" steel plate. Such a plate should not bend between the pulling from the weight and torque from the bolt.

    Here is the next problem. The lumber atop the clamp that your A-Frame is attached to will want to compress since the bolt and plate below will not. Remember that when working in the theater we do a 5:1 safety ratio. In other words, you should be able to support given two legs, 150#. You should probably use some steel for this purpose also. Say some 3" x at least 3/16" washers at a minimum for plywood to prevent too much compression of the lumber at the bolts by distributing the pressure on it. Unistrut Washers might be a fair alternative given 7 layer 3/4" plywood plates.

    After that, your plates need to be larger than one square foot. The grating if even rated for a 50# live load will need a 1' square block to distribute the load. Given two blocks at about 16" that's 30# for each thus fairly safe given a 50% safety factor already calculated into the live load. This would especially be true if it's less than a 50# live load bearing grate as it probably is. Otherwise and also at bare minimum and if across the parallel grading, two 6"x24" plates might be suffficient given with any larger plate you should have bolts at least 6" on center.

    It's probably over engineering on my side in the blind but also a safety thing.

    All this said, what about a block and fall clamped to the steel supporting structure for the fly system? This would be the normal way of doing this and the fly system's steel is made to carry the point loads not the deck. Another option might be to clamp some steel beams or heavy wall pipes to the steel supports for the grating for your structure.
     
  3. Crewguy7

    Crewguy7 Member

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    Another idea i just thought of was using some 2" square tubing and just using some 1/4" aircraft cable to secure it to the grating. But if you don't think this would be adequate, please tell me.
     
  4. ship

    ship Senior Team Emeritus Premium Member

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    Again, this is a bunch of assumptions on my part because I'm not where you are or in knowing your exact details or plans. I would also be interested in what others think. My assumptions and ideas are very wide on the safety factor especially since I don't do this kind of thing for a living anymore and have not for at least 6 years now.

    2" square tubing could or could not be sufficient to span the grading both above and below. Depends upon the type and application of the grading assuming it's going perpendicular to it, or spanning between supports for it. It also depends upon what type of box tubing is used. Box or structural square tubing is primarially designed as posts so figuring out it's bending and crumple strength is a bit more difficult. "Welding for the Theater" if I remember right has an excellent section on this. Thru bolting will be different than welding, where the holes are will also be of interest. What gauge the tubing is will also determine what kind of load it's good for. Now granted one length with an expected load of 30# on it will probably be fine for even 16 ga tubing, at least hopefully given any bolts in it span the tubing to it's edges such as if 2" Unistrut Washers are used to prevent too much force on the web. 14GA Tubing would be safer of course, but still insufficient for spanning say a 48" gap between load bearing members given a 5:1 safety factor.

    As for the 1/4" wire rope, depends upon what type it is. Bronze or Iron wire rope is far different than say Galvanized wire rope. Plus there is the stranding and core fill materials that factor in. In general, even a bronze wire rope should be more than sufficient but it's minimum bending dia. of 1/4" wire rope is 7.1/2" around a pulley. Kind of hard to lash a bar down with 1/4" wire rope without doing bad things to the wire rope when stretched around the box, much less in getting it tight enough. Granted at swage fittings you are bending it tighter but it's also supported by a thimble. Not really sure what the intent of the wire rope is. Are you dead hanging the pipe or something or intending to lash the bar to the grading with it? In either case, 1/8" wire rope might be a little better at it. It's still going to have the challange of getting tight enough but at least will take corners a little easier. 1/2" Dracon braided rope should work best of all given no sharp edges to cut into it. Such sharp edges would also cause problems with the wire rope anyway. I suspect you are planning something like a clove hitch around the bar and grading in making them as one? Could use some slit PVC tubing or burlap to prevent sharp grading edges from digging into the rope of which ever type.

    I'm afraid that in not understanding the application, I don't fully understand where you are going with it. Certainly the ladder opening will have sufficient steel around it to support a bar or pipe across it's span. Given say a 24" opening, it's just a question of finding pipe or steel that will span that opening with a point load of 60# and tying the pipe off to the grading or opening frame so it does not move since the steel around the ladder well is supporting the weight. You could even use I-Beam clamps and tie wire or plumbers tabe between bolt holes to prevent it from moving about while at the same time using the position of the beam clamps to prevent it from moving. A 48" normal 1.1/2" Sch. 40 water pipe batten or fly system batten pipe will very likely be able to do this given you are not too much worried about engineering in a safety factor. A Sch. 80 pipe of the same size would definately do it. A 2" batten of any of the above types would probably also not have a problem with it. 14ga Box tubing like the 1.1/2 Sch. 40 pipe might work satisfactory for this also, I would prefer a 3" box or better yet at least 3/16" structural tubing in 2" or more if available. A double 2x4 could probably do this also but would need floor plates so as to disperse some of the pressure from the ladder well frame's opening across the grid and prevent it from tipping over which would put pressure a bit too close to it's stress limit given a safety factor. Its force is still directly downward and not lateral so it does not need a huge amount for clamping. Could just use a I beam clamp or use drywall screws to sandwich the deck between sheets. This all assumes that you don't have to bring the lights all the way up to the grid.

    If you do, than your A-Frame I assume is braced and going thru the rung of a ladder in a cantiliever fashion. Your intended blocking is as opposed to pushing down on the grid, pulling up on it with the force. If that's the case, I don't think I would use wire rope because dynamic loading on the pipe or plate you secure could cause movement or sliding the wire rope will not be able to compensate for. Bolting to or thru something would be preferred. This includes if it's too hard to bolt a plate thru the deck, putting a support up to the ceiling and wedging it between the flange and web of a steel supporting member, than roping it off or securing it with I-Beam clamps. Otherwise thru bolting it and going with flange spanning I-Beam clamps such as on a underhung block. An A-Frame will give it sway bracing, but in going to the ceiling to counteract the upward force on the catniliever or bolting to the grid, there won't be a lot of movement happening, but a A-frame will still be helpful. It's only a 60# expected load, if well tied off to the ladder it's not going far.

    My quick easy thoughts, your pipe if 14ga or some batten pipe tied off to the grid and ladder should be sufficient given a block and fall can't be attached to some steel. The longer the lever, the more leverage it will have. Say a A-Frame or pipe put at 30 degrees is going to need a lot less force to keep it in place than one at 45 degrees. Find a pipe that will extend into the corner of some steel between wedge and flange, and rope it off into place. Perhaps a timber hitch around the pipe and a clove hitch around the steel to prevent movement. Should be good enough given it's 1/2" Spectrum grade. The main force is at the joint in the beam. Perhaps a pipe running to the corner where the grid touches a wall or something otherwise, than tying it off. Seem simpler? My answers so far were in direct response to your questions and very general.

    You might get a rigger or Pro level TD up there to safely sugguest how to do it and teach you some stuff. Real life training and supervision when in question is always safer. I learned half what I know about rigging from classroom and books, the other half was from being trained in person, there is no substitute for OJT.
     
  5. ship

    ship Senior Team Emeritus Premium Member

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    By the way, Structural Steel Design, 3rd Ed. by Jack C. McCormic; Harper & Row - N.Y. 1981 ISBN: 0-06-044344-8 is a fairly easy to read and understand text on engineering. If you can find a copy at a library, it might be worth a read or two to understand things you are attempting better. Otherwise here is more on the subject that might be of use which could also teach you stuff. Since you are reading about some of these terms and attempting to use them it might ease your learning and understanding of them. Understand the terms stresses etc. and understand how the formulas for engineering work and the rest is easy. Just a question of experience and training after that. Good to ask for for X-Mas.

    -Applied Structural Steel Design, 3rd. ed. by Lenard Spiegel; Construction Savvy - Dist. #AF3906
    -Engineering Design Graphics, 5th Ed. by James H Earle; Addison Wesley Publishing Co. - Reading MA. 1987 ISBN: 0-201-11641-3
    -Engineering Drawing and Design by Jensen
    -Manual of Steel Construction, AISC pub.
    -Simplified Design of Structural Steel, 5th Ed. Harry Parker, M.S.; Interscience Pub. John Wiley & Sons - N.Y. 1983
    -Simplified Engineering For Architects & Builders, 5th ed. by Harry Parker M.S.; Wiley -Interscience Publication - John Wiley & Sons. N.Y. 1975
    -Standard Handbook of Architectural Engineering, by Robert Brown Butler 1998; Construction Savvy Dist. #AF6716-98
    -Standard Handbook of Engineering Calculations, 3rd. Ed. Tyler G. Hicks 1995; Construction Savvy - Dist. #AF8426
    -Steel Design for Engineers and Architects, Rene Amon; VanNostrand Reinhold Co. - Cincinnati 1982
    -Structural Design for the Stage, by Alys Holden and Bronislaw Sammier ISBN: 0-240-80354
    -Structures for Architects, 2nd Ed. by B.S. Benjamin; Van Nostrand Co. - N.Y. 1984 ISBN: 0-442-21190-2
    -Structural Engineering Handbook,4th ed. Gaylord & Gaylord; McGraw Hill - St. Louis 1968
    -ASD Manual of Steel Construction, 9th ed.; Construction Savvy - Dist. #AF3896
    -LRFD Manual of Steel Construction, 2nd ed. by LFRD; Construction Savvy - Dist. #aF2119
    -Welding for the Theater
    -Stage Rigging Handbook, 2nd Ed. by Jay O. Glerum; Southern Illinois University Press - Carbondale Il. 1997 ISBN: 0-8093-1744-3


    From such books the followin quote from the above former Structural Steel Design book would make sense:
    Torsion occurs when the line of action of a load does not pass thru the shear center of the beam. When this happens, the resulting torque or twisting moment equals the product of load times the perpendicular distance from its line of action to the shear center.
     

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