ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: 1 [2]  All   Go Down

Author Topic: eye lag screws or eye bolts for anchoring to rafters?  (Read 19849 times)

Jonathan Kok

  • Full Member
  • ***
  • Offline Offline
  • Posts: 205
  • Toronto
Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #10 on: November 19, 2013, 04:14:11 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....
Rated lag screw:
http://www.grainger.com/product/Hex-Lag-Screw-1LB71?s_pp=false
Meets ASME B18.2.1, and can be referenced to the NDS for wood construction (and give you your tensile/shear strength).
Can you get them at Home Depot? Probably not. But they're certainly available, from plenty of sources, in plenty of sizes, and not remotely expensive.
Logged

g'bye, Dick Rees

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 7424
  • Duluth
Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #11 on: November 19, 2013, 04:20:04 pm »

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.

Keith...


I may have missed it, but what are the exact speakers you're flying?  Inquisitive minds...
Logged
Pay no attention to the man behind the curtain...

Cailen Waddell

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1428
eye lag screws or eye bolts for anchoring to rafters?
« Reply #12 on: November 19, 2013, 04:26:48 pm »

The fastener you linked to is low carbon steel and asme b18.2.1 does not appear to help give strength data from what I read. Again, it seems to me that it is not a rated assembly unless a structural engineer evaluates it.  I have not seen a lag screw listed with shear strength, tensile strength etc. To me that would make it unrated  A graded bolt on the otherhand, is rated.

Separately using low carbon steel seems asking for trouble to me. If you aren't exceedingly careful setting the screws you can easily over torque the head and start to twist it off. It's like a paper clip that has been bent once. It's still there but the structural integrity has been compromised.
« Last Edit: November 19, 2013, 04:31:10 pm by Cailen Waddell »
Logged

Jonathan Kok

  • Full Member
  • ***
  • Offline Offline
  • Posts: 205
  • Toronto
Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #13 on: November 19, 2013, 06:29:27 pm »

The fastener you linked to is low carbon steel and asme b18.2.1 does not appear to help give strength data from what I read. Again, it seems to me that it is not a rated assembly unless a structural engineer evaluates it.  I have not seen a lag screw listed with shear strength, tensile strength etc. To me that would make it unrated  A graded bolt on the otherhand, is rated.

Separately using low carbon steel seems asking for trouble to me. If you aren't exceedingly careful setting the screws you can easily over torque the head and start to twist it off. It's like a paper clip that has been bent once. It's still there but the structural integrity has been compromised.
My apologies; I should have looked over that closer before I posted it. Interrupted by a co-worker, so I just posted. Stupid of me. I was just surprised by your statement that 'there is no such thing as a structural lag screw', and the inference that somehow, they shouldn't be used. Especially seeing as there is an entire section, complete with tables, in the NDS detailing lag screw requirements, both for wood-to-wood connections, and steel-to-wood connections. I rushed it. I shouldn't have.
The NDS does does have specific bend yield strength minimums if you use their quick-reference tables, though it references no specific 'Grade' or ASTM standard. But I'm pretty sure Grade 5 meets those requirements.  So here's a few pages of them:
Grainger Grade 5 Lag Screws
A little more research says that there once was a reference to standard A307, but that was dropped and the onus left up to the engineer to list specific screw requirements.
EDIT: Someone at Purdue was kind enough to scan and post the NDS section applicable here. It hasn't been OCR'd, but it's all there.
https://engineering.purdue.edu/~ramirez/CE479/FALL05/CE479WoodDesignNDS01Connections.pdf

EDIT2: I should also note, I understand your hesitance in using them. These are supposed to be installed with a wrench, reducing the twisting you're concerned about. That being said...if your house was built in the last 20 years, and it has a deck attached to it, there's a pretty good chance it's being held on by lag screws, installed with a hammer drill. Good practice? Probably not. Common practice? Heck, yea.
« Last Edit: November 19, 2013, 06:56:34 pm by Jonathan Kok »
Logged

Cailen Waddell

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1428
Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #14 on: November 20, 2013, 09:19:07 am »

Jonathan, thanks for the reply that does clarify things, I appreciate the detailed explanation and time you took to talk about the engineering involved.
Logged

Jamin Lynch

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1968
  • Corpus Christi, TX.
Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #15 on: December 07, 2013, 12:20:32 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.

You're way over engineering something simple. Use some common since, make sure it's done right and hang the damn things and get it over with. If it's taken you this long to engineer suspension points for 30lb speakers then you shouldn't attempt to do it yourself. Call an expert.

I would never use lag bolts to hang speakers though.
« Last Edit: December 07, 2013, 12:34:10 pm by Jamin Lynch »
Logged
MAGA-Make Audio Great Again

Glen Kelley

  • Full Member
  • ***
  • Offline Offline
  • Posts: 115
  • El Paso, TX
    • The University of Texas at El Paso
Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #16 on: December 10, 2013, 01:52:26 pm »

And buy a proper swaging tool for the wire rope.  Faster, safer, better looking results. It is a gift that keeps giving.

Best practice is to check and re-torque wire rope clips regularly, according to the Crosby Applications and Warnings document. For this reason, a wire rope clip is not recommended for a permanent installation. Unless you really will get on a Genie lift every 12 months or so....
Logged

Jamin Lynch

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1968
  • Corpus Christi, TX.
Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #17 on: December 14, 2013, 01:53:02 pm »

Best practice is to check and re-torque wire rope clips regularly, according to the Crosby Applications and Warnings document. For this reason, a wire rope clip is not recommended for a permanent installation. Unless you really will get on a Genie lift every 12 months or so....

That's why I rarely use wire rope to hang speakers. Too much can go wrong.

If I do, I have a local heavy industrial wire sling company make them for me. They are tested and certified. I sleep better at night that way.
Logged
MAGA-Make Audio Great Again

Len Zenith Jr

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 347
Logged

ProSoundWeb Community

Re: eye lag screws or eye bolts for anchoring to rafters?
« Reply #18 on: January 09, 2014, 05:55:51 pm »


Pages: 1 [2]  All   Go Up
 



Page created in 0.082 seconds with 23 queries.