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Author Topic: eye lag screws or eye bolts for anchoring to rafters?  (Read 20309 times)

Keith Shannon

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eye lag screws or eye bolts for anchoring to rafters?
« on: July 02, 2013, 01:17:23 PM »

I'm spec'ing a rigging system for some 30-lb speakers in a church. There are some worries among the congregation about flying a speaker, so I'm making sure all I's are dotted and T's crossed to provide a rigging system with a ridiculous safety factor. The actual wire rope slings (1/8" 7x7 aircraft cable), with thimbles and 3 splicing clips on each end, are not a problem at all; at up to a 30* angle on the basket sling, I've got a safety factor of more than five based only on the WLL (over 25 against the breaking strength), and there are at least two points of failure per speaker. The forged shoulder eye bolts screwed into the speakers are also the manufacturer's recommended spec, and should safely hold several times the speakers' actual weight.

The last point of concern is the anchor into the rafter. The wood itself is pretty beefy; think of the wooden rib vault style of architecture, and that's what I'm anchoring into. I'm using two forged shoulder eyes to anchor each speaker, one for the two main support cables and one for the angle adjustment cable. The ideal, which would be to bore all the way through the rafter from underneath and install a long-shank eye bolt, is a no-go; the rafter directly supports the pile roof (two or three layers of offset 1-by) and above that is shingle and sky. Boring an access hole into the side of the rafter and then drilling halfway through into that space to install a similar bolt isn't an option either, because the ideal mounting point for a straightline hang of the main support anchor is at an intersection of rafters.

So, that leaves me with two options; a lag screw into the rafter from underneath, or drilling sideways through the rafter to install an eye-bolt, and then using an extra sling to drop the angled basket sling lines down below the rafter.

The sideways bolt installation puts a perpendicular stress on the bolt itself, reducing its WLL by 75%, but it also hides said bolt up in the rafters, and a 3/8" forged shoulder bolt will still have 10 times the WLL I need (300 lbs at 90* offset). A shoulder eye lag screw would allow more or less a straightline pull, but the big unknown is the working load limit, which I understand is based on the interface between wood and metal in the threads, which in turn is based on the density and toughness of the wood.

Is there a back-of-envelope figure for a straightline WLL of shoulder eye lag screws in the average structural hardwood? Should I even be considering that installation option?

Thanks in advance.
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Tom Young

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Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #1 on: July 02, 2013, 03:58:44 PM »

I'm spec'ing a rigging system for some 30-lb speakers in a church......... Is there a back-of-envelope figure for a straightline WLL of shoulder eye lag screws in the average structural hardwood? Should I even be considering that installation option?

I doubt you will get a an opinion telling you how to do this here.

I find your question interesting and I do not know the answer. But, even if I did know, I would tell you that you have to pay for a licensed structural engineer to advise you, review the final installation and then provide stamped documents for your files.  The issues are both legal and moral liability.

God forbid that something happens and I/we are included in any legal action. And who needs the burden of responsibility if someone gets hurt (or worse) ?

Practicing what I preach, I have found that having the review and getting stamped drawings is not terribly expensive. Maybe there's a qualified engineer in the congregation.

HTH
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Tom Young
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Scott Hofmann

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Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #2 on: July 02, 2013, 04:27:12 PM »

I think the general rule of thumb is that anything that is flown should use bolt assemblies and not screw fasteners.
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Lee Buckalew

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Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #3 on: July 02, 2013, 04:34:47 PM »

I'm spec'ing a rigging system for some 30-lb speakers in a church. There are some worries among the congregation about flying a speaker, so I'm making sure all I's are dotted and T's crossed to provide a rigging system with a ridiculous safety factor. The actual wire rope slings (1/8" 7x7 aircraft cable), with thimbles and 3 splicing clips on each end, are not a problem at all; at up to a 30* angle on the basket sling, I've got a safety factor of more than five based only on the WLL (over 25 against the breaking strength), and there are at least two points of failure per speaker. The forged shoulder eye bolts screwed into the speakers are also the manufacturer's recommended spec, and should safely hold several times the speakers' actual weight.

The last point of concern is the anchor into the rafter. The wood itself is pretty beefy; think of the wooden rib vault style of architecture, and that's what I'm anchoring into. I'm using two forged shoulder eyes to anchor each speaker, one for the two main support cables and one for the angle adjustment cable. The ideal, which would be to bore all the way through the rafter from underneath and install a long-shank eye bolt, is a no-go; the rafter directly supports the pile roof (two or three layers of offset 1-by) and above that is shingle and sky. Boring an access hole into the side of the rafter and then drilling halfway through into that space to install a similar bolt isn't an option either, because the ideal mounting point for a straightline hang of the main support anchor is at an intersection of rafters.

So, that leaves me with two options; a lag screw into the rafter from underneath, or drilling sideways through the rafter to install an eye-bolt, and then using an extra sling to drop the angled basket sling lines down below the rafter.

The sideways bolt installation puts a perpendicular stress on the bolt itself, reducing its WLL by 75%, but it also hides said bolt up in the rafters, and a 3/8" forged shoulder bolt will still have 10 times the WLL I need (300 lbs at 90* offset). A shoulder eye lag screw would allow more or less a straightline pull, but the big unknown is the working load limit, which I understand is based on the interface between wood and metal in the threads, which in turn is based on the density and toughness of the wood.

Is there a back-of-envelope figure for a straightline WLL of shoulder eye lag screws in the average structural hardwood? Should I even be considering that installation option?

Thanks in advance.

Have a look here:

http://www.polarfocus.com/catalog/family/custom-beam-attachment-44/#family-header

This is the type of mount that Polar Focus will design for beam attachment.  They will certify the mount.  As Tom said it is best to get a certified engineer to stamp your configuration based upon their layout.  Possibly if there is an engineer in the congregation they would review a design and stamp it.  It has happened that way for some of my designs.

Lee
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Tom Bourke

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Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #4 on: July 02, 2013, 04:42:06 PM »

Take some pictures with measurements of the structure and then send them to places like JR Clancy and Polar Focus.  Also let them know what speakers you have and what angle and direction.  They can take care of the engineering or refer you to some one who can.

I rarely see eye-bolts being good enough in theses situations.  Usually some sort of bracket that distributes the load is used.
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Jason Lavoie

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Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #5 on: July 02, 2013, 07:55:30 PM »


Is there a back-of-envelope figure for a straightline WLL of shoulder eye lag screws in the average structural hardwood? Should I even be considering that installation option?

Thanks in advance.

Same as everyone else.. answer #1 is to consult an Engineer.

My comments though:
I don't know if I've ever seen a shoulder eye lag screw..

the problem with lag screws is that the pullout rating is dependent on the density and construction of the wood, the size of drillbit that you use, how much you wiggled the drill while drilling the hole, how straight you held the eye while screwing it in, etc... things that nobody can confirm (even you)

depending on the load required, you'll probably be looking at a flat plate on the side of the beam with several large screws into the beam (whose shear load ratings don't depend nearly as much on how well you drilled the hole) with the load suspended from the bottom of that plate, or a yoke type bracket as mentioned above where you bolt all the way through the beam (note: drilling through the beam and coming out the other side spot on is a LOT harder than it looks)

And at the end of the day, if the people that have to sit under it are showing concern, a piece of paper certifying its safety from someone whose job it is to do so is worth every penny.

Jason

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David Allred

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Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #6 on: July 17, 2013, 01:51:16 PM »

What are the physical dimensions of the beam (H & W)?
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Keith Shannon

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Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #7 on: November 18, 2013, 08:05:02 PM »

What are the physical dimensions of the beam (H & W)?

The two main rafters are approximately 24"Hx12"W, and as it turns out they're wood cladding over a steel rafter, so no dice drilling into/through them.

Smaller supporting rafters nearby are closer to 12"x6" and are laminated wood construction.

Since I first asked, I did consult with some engineers, and the plan has changed; we're going to build custom mounting rigs for the cabinets out of plate steel. A member of the congregation, a member of our media team, does racing mods on Porches for a living (from engine tuning to rollcages) so he's got a full metal shop, CAD tools and the raw materials needed. The design will have a mounting base that will attach to the back side of the rafters, secured with a half-dozen 1/2" lag screws each giving them a shear load so they won't pull out, and this bracket will support a pole that drops down below the rafter to a T-bracket with shoulder eye bolts for the fly cables. The design's been worked up and stamped, so we just need to built it to spec.
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Cailen Waddell

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Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #8 on: November 18, 2013, 09:44:39 PM »

As far as I know, there is no such thing as a rated lag screw.  Thus i would not recommend depending on the shear strength of the lag screws.  Of course if the licensed engineer provided you with a stamped drawing, then he is accepting liability for the design and thus you should be fine....
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Jim McKeveny

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Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #9 on: November 19, 2013, 09:09:36 AM »

Check out www.SammySuperScrew.com. for the threaded rod anchors. With the proper installation tool these anchors are load rated into wood, for tension & shear, with historic engineering data available. I have used thousands over decades without issue.

They were a "Eureka!" product for us.

And buy a proper swaging tool for the wire rope.  Faster, safer, better looking results. It is a gift that keeps giving.
« Last Edit: November 19, 2013, 09:24:39 AM by Jim McKeveny »
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ProSoundWeb Community

Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #9 on: November 19, 2013, 09:09:36 AM »


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