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.