Mechanical Engineering Filter!
May 22, 2006 6:48 PM   Subscribe

If I wanted to hang something heavy from a rafter, how should I do it?

I have a cathedral ceiling in a room, and would like to suspend a cabinet from eye bolts in some rafters. I'll have the roof off for replacement, so it's easy enough to do The Right Thing, rather than just screwing in some lag-eye bolts from the inside. I think there are two basic ways I can go:

A is easier, but B should be stronger.
My questions are:
  • How much of a forged eye bolt's strength do I lose if I load it out-of-line with the shank, as shown in A?
  • Is it worth the hassle of fabricating a steel cap for the rafter, including a cutout for the bolt (as shown in B), given the weight of something I could reasonably hang from the ceiling of my house?
posted by spacewrench to Home & Garden (14 answers total)
 
First of all, I wouldn't try hanging anything of serious weight from a single point eyebolt attachment, from a single rafter, for reasons that this guy points out, in a limit case. Far better to spread the load to at least a couple of rafters, or better yet, build a minimal bearing frame even if it is just a purlin, and attach your eyebolts to that.

Shouldered eyebolts can be used in angled applications, but shoulderless eyebolts are not recommended for such installation [link is to a small MS Word doc file, which is a simple rigging primer].
posted by paulsc at 7:10 PM on May 22, 2006


A would probably be more than enough, but I'd definitely go with B if you're aiming for the Right Thing. It keeps the load parallel to the eye bolt, taking advantage of steel's tensile strength, but more importantly it only stresses the wood in one direction. Method A exerts both tensile and shear stresses on the bolt, which in turn cause the bolt to exert a sideways force on the wood it's passing through.

Since the steel eye bolt is the stronger of the two materials -- the wood should break first if you overloaded the system -- you want it to handle as much of the force as possible.
posted by pmbuko at 7:18 PM on May 22, 2006


paulsc, did you read the second sentence of the post?
posted by pmbuko at 7:18 PM on May 22, 2006


"paulsc, did you read the second sentence of the post?
posted by pmbuko at 10:18 PM EST on May 22 [+fave] [!]
"


Yes, I did. But in a question which asks about hanging loads of such weight as to focus on advice about eyebolt applications where safety factors are apparently not going to be so great as to make load direction moot, I think it's fair to suggest alternatives. A "cathedral ceiling" that is just sheetrock screwed to the underside of rafters doesn't preclude the use of a bearing frame, and not every purlin need be internal.

What's your point?
posted by paulsc at 7:37 PM on May 22, 2006


Don't do it. You haven't given enough details to really be helpful, but here's what I know. The beam is probably a wood laminate which was designed specifically to hold up your roof and to look cool.

Here are some things that one would need to know to better help you: When and where was the structure built? What kind and what size is the mentioned beam? What kind of foundation do you have? How heavy is the object you wish to hang?
posted by snsranch at 7:40 PM on May 22, 2006


You've got to define heavy - 50lbs, 200lbs, 1000lbs?

The cut to the rafter in B will severely weaken it. You are effectively reducing the size of the rafter by the size of the divot you take out of it. Say you are starting with a 2x10, if you cut a one inch notch you have made it a 2x9. It would be better to just screw in the bottom. I'm not sure if it would be good enough to be 'right' though..

Actually, best (leaving aside paulsc's suggestion for a second) would be to buy/make something like a joist hanger, and hang whatever off the bottom of the hanger. This will allow multiple fasteners attached to the rafter in the 'web' instead of the 'flange' (those are terms borrowed from I-beams, I don't know if they are correct here).

paulsc's point sounds good too. Best to spread your load as far as you reasonably can until you are sure it isn't going to significantly effect your structure. You could make/buy a set of hangers to suspend a cross piece from 2 or more rafters, and then suspend you item from below the cross piece (that is what a perlin is, isn't it?).
posted by Chuckles at 7:45 PM on May 22, 2006


From the Rigging Primer:

Correct loading of eye:
The eye is designed for a pull along the axis of the bolt. Reduce loading for a pull 15 degrees off the axis by 45%, 25 degrees off axis by 60%, and 45 degrees off axis by 75%


That's just the kind of information I needed. Thanks! Method A loses strength fairly quickly, and I'm the kind of guy who'd lose sleep just as quickly, thinking about how my ridiculously over-spec'd cables and eye bolts might fail, causing the cabinet to swing around and take out a post in the wall, causing the wall to fail, causing the roof to fall down, which would cause my bank account to shrink precipitously.

More generally, where can I find basic information like this? There must be an Architect's Handbook or something that people who do this for a living have on their bookshelves...
posted by spacewrench at 7:47 PM on May 22, 2006


The beam is probably a wood laminate...
The cut to the rafter in B will severely weaken it...

I guess I didn't focus the question carefully enough. These things are true, but I was really trying to get at the strength loss from out-of-line loading an eye bolt.

I'm not far enough along to say how much the cabinet will weigh, but I did plan to hang it from multiple cables (at least 4 mains, with cross cables to reduce swinging and safety backup cables/chains) attached to different rafters. I would probably also double/triple up the rafters with 2x4s or 2x6s, and use a steel U-channel cap to spread the load from each eye bolt along the length of the rafter.

If I can beef up the rafters and keep the eye bolts strong, then the question becomes, how much can my ridge beam support, and (probably secondarily) how much can the wall support?

I'm a EE, but have a good enough grasp of mechanics to work out loads in a simple system like this. I just don't know where to find the numbers for "what's the maximum load an XxY wood beam, Z meters long and supported on both ends, can carry?" or "how much strength does a fitting lose if loaded out-of-line?"
posted by spacewrench at 8:05 PM on May 22, 2006


How 'bout a U shaped piece of steel bent to slip around the sides of the rafter, and screwed/bolted throught the sides. You could either have a hoop welded to it or drill it out and bolt the eye bolt through it.

(just another option you may not have considered)
posted by davey_darling at 9:34 PM on May 22, 2006


spacewrench, given that your profile says you're located in L.A., I'm going to suggest that tension mechanisms such as you are proposing for suspending your cabinet haven't been, uh (how can I say this?), the architectural way of doing things through most of building history. Mostly, that's been because it has only been in the last 100 years or so that people have begun to have a real tool set, mathematically and practically, for designing tension devices, as such, into structures; even today, the use of tension devices in building is more a matter of engineering design, than it is architectural choice. Architects generally prefer, greatly, to span a load to the nearest post that can safely carry it to the foundation, and be done with it, because building that way has been shown to be inexpensive, very durable, and a mechanism that generally allows for progressive local element failures, such that small failures don't tend to become Rube Goldberg worthy structural collapses. You jest, surely, in your responses about such cascades of ill fortune, but the fact that you do so jest means you are quite aware of what swinging even a few hundred pounds of cabinet around in an earthquake could mean for the stability of your home, if things go badly awry.

So, if you look around, you can get data for the effects of incising on lumber strength and stiffness [PDF file linked], and for $55 you can get the 2005 edition of the National Design Specification for Wood Construction, which will keep you busy for days.

But given where you live, I'd really think about this project in terms of bringing the load to ground gracefully, using compression, and not tension. No cables, no pendulum moments, no drilling into structural members whose strength in fire and quakes could mean the lives or well being of not only you and your family, but rescue personnel as well. Leave the interesting tension engineering to the pros and their deep pockets and iterative tests and series models, and enjoy continuing to live in your house.

Just sayin'...
posted by paulsc at 10:14 PM on May 22, 2006


It's not a tough problem to figure out, but you'd have to supply a lot more information to solve it properly. Eyebolts probably aren't a good solution, since they can pull out, and by screwing them into the bottom of the beam, you're reducing the tensile area. If you're determined to do it, I'd recommend some kind of hanger that clamps around the beam, rather than anything that penetrates it.
posted by electroboy at 9:42 AM on May 23, 2006


Also, link to the paper on incising isn't particularly applicable to your problem. Incising is a process where thousands of tiny cuts are made in the beam to increase penetration of preservatives. All you're dealing with is cross sectional reduction.
posted by electroboy at 9:52 AM on May 23, 2006


There is a lot of information that is missing from this post.
Before you answer this question yes or no, consult roofing
span tables for your lumber size. Correlate that with an
increased "dead load" for that section of roofing. And
remember that NO NOTCHING is allowed in the center third
of beams or rafters.

How is the blocking in your ceiling? Adding weight to rafters
will tend to induce twist unless they are stiff enough to
resist it, either by simple bulk or by good blocking.

If you're going to have the roof off for replacement, I would
sister the rafters in question with some honkin' beams
to provide ridiculous rigidity in the region where you
intend to suspend your cabinet. Does your roofing system
have a structural ridgebeam, or are there midspan rafter
supports?
posted by the Real Dan at 10:45 AM on May 23, 2006


If I can beef up the rafters and keep the eye bolts strong, then the question becomes, how much can my ridge beam support, and (probably secondarily) how much can the wall support?

A little late, but regarding your ridgepole/wall weight-bearing considerations, it seems to me whatever extra weight a bookshelf would add would, spread out among multiple rafters, be inconsequential in relation to the total weight of the roof. Even if it weighed 500 pounds, that's easily less than, say, the weight of two or three roofers reshingling the roof. Not to mention the weight of another layer of shingles up there. I think the real issue, as others have pointed out, is maintaining the integrity of the individual rafters. And I'll chime in and say that I'd really try to stay away from notching anything.
posted by heydanno at 7:49 PM on May 23, 2006


« Older Failed disk drive data recovery   |   Swordfish = Worms!? Newer »
This thread is closed to new comments.