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Mechanical Engineer / Structural question

 
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Sparky015



Joined: 12 May 2009
Posts: 1159
Location: Cleveland / Akron, OH


PostLink    Posted: Mon Apr 04, 2011 6:11 pm    Post subject: Mechanical Engineer / Structural question Reply with quote


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Hey guys, looking for some mathmatical expertise around the pull-out strength of a 1/2-13 hangar bolt. The wood portion is 3" in length and going into softwood. I am changing out my ceiling mount for an electric lift Chief with a finished ceiling bracket, so I was looking to use quantity 4, 1/2-13 hangar bolts into 2x8 joist supports. Thanks for the help!

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ecrabb



Joined: 13 Mar 2006
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Location: Iowa & Michigan

TV/Projector: JVC RS45


PostLink    Posted: Mon Apr 04, 2011 6:49 pm    Post subject: Reply with quote

A half-inch lag screw is way, way overkill and you unnecessarily compromise the joist - for no good reason.

The pull-out value for a 5/16" lag screw in most lumber is something over 100 pounds per inch of thread. Increasing to 3/8" puts the value over 200 pounds per inch of thread. So, if you use even a 5/16" screw with two inches of thread in, you could hang the entire projector off one lag.

SC
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ecrabb



Joined: 13 Mar 2006
Posts: 14385
Location: Iowa & Michigan

TV/Projector: JVC RS45


PostLink    Posted: Mon Apr 04, 2011 6:59 pm    Post subject: Reply with quote

Looks like a 5/16" lag with 2-1/2" of embedment is good for 368 pounds according to the table in this document:
http://www.kineticsnoise.com/SeismicBook/pdfs/a7/a7.3%20-%20seismic%20design%20data%20for%20lag%20screws.pdf

The same table says a 1/4" screw with 2" of embedment is good for 272 pounds... Four 1/4" screws would put your mount capacity over 1000 pounds - probably over 5x the weight of your projector...

See why a 1/2" lag is way overkill?

SC
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dturco



Joined: 06 Feb 2009
Posts: 3779
Location: Eastern Shore Maryland

TV/Projector: Runco DLP VX-3000i Marquee 9500 parts doner


PostLink    Posted: Mon Apr 04, 2011 7:40 pm    Post subject: Reply with quote

SC covered it well. All of the charts always show the pull out from a wide flat board, think 2x8 on it side. Now the pullout changes somewhat once you only have 1-1/2 inches of material, that by putting the bolt in, you have decreased the amount of wood available for strength.

This is one of the few times where bigger is not better, in fact it is detrimental.

Do not use anything over 3/8. 5/16th is actually the best choice as it reduces the chance of splitting the wood.

Also drill a pilot hole and use soap on the threads when putting the lags in.

[Actually now that I reviewed those charts , there great and covered it better than I originally thought.]

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Sparky015



Joined: 12 May 2009
Posts: 1159
Location: Cleveland / Akron, OH


PostLink    Posted: Mon Apr 04, 2011 10:07 pm    Post subject: Reply with quote

thanks guys, this is the info I was looking for. I couldn't find a definitive answer, and I wanted to do the math before starting. The nice thing is that the throw puts the pj in between ceiling joists, so I am lagging into 14.5" long 2x8s going in between two joist cavities, so not only am I not lagging into house joists, but the weight is distributed over 3 joists. Anyway, thanks for helping me out with this. I was concerned with a 1/2" lag into a 2" width of wood, and you guys confirmed that concern.
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greg_mitch



Joined: 03 May 2006
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PostLink    Posted: Mon Apr 04, 2011 10:29 pm    Post subject: Reply with quote

My only advice....GO SLOW!
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Sparky015



Joined: 12 May 2009
Posts: 1159
Location: Cleveland / Akron, OH


PostLink    Posted: Mon Apr 04, 2011 10:34 pm    Post subject: Reply with quote

greg_mitch wrote:
My only advice....GO SLOW!



haha, way ahead of you! I have been pondering this for a few weeks, and making sure everything is laid out and thought of before I re-hang my investment on the ceiling. Aside from personal safety, I really like my coffee table Laughing Not really, but I do like my PJ that I've put money sweat and time into. Laughing

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dturco



Joined: 06 Feb 2009
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Location: Eastern Shore Maryland

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PostLink    Posted: Mon Apr 04, 2011 11:52 pm    Post subject: Reply with quote

Sparky015 wrote:
thanks guys, this is the info I was looking for. I couldn't find a definitive answer, and I wanted to do the math before starting. The nice thing is that the throw puts the pj in between ceiling joists, so I am lagging into 14.5" long 2x8s going in between two joist cavities, so not only am I not lagging into house joists, but the weight is distributed over 3 joists. Anyway, thanks for helping me out with this. I was concerned with a 1/2" lag into a 2" width of wood, and you guys confirmed that concern.


Ok, so don't overlook what's holding the 2x8 up or in place. If they're only nailed in with a few nails....you have a weak link to deal with. Wink

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greg_mitch



Joined: 03 May 2006
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PostLink    Posted: Mon Apr 04, 2011 11:59 pm    Post subject: Reply with quote

Sparky015 wrote:
greg_mitch wrote:
My only advice....GO SLOW!



haha, way ahead of you! I have been pondering this for a few weeks, and making sure everything is laid out and thought of before I re-hang my investment on the ceiling. Aside from personal safety, I really like my coffee table Laughing Not really, but I do like my PJ that I've put money sweat and time into. Laughing


I was more referencing to actually installing the lag screws.

GO SLOW!

I broke off two after I already had them about 3/4 of the way in because I got hasty and overheated the screw and it snapped off at the head.

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Sparky015



Joined: 12 May 2009
Posts: 1159
Location: Cleveland / Akron, OH


PostLink    Posted: Tue Apr 05, 2011 12:20 am    Post subject: Reply with quote

dturco wrote:
Ok, so don't overlook what's holding the 2x8 up or in place. If they're only nailed in with a few nails....you have a weak link to deal with. Wink


I am planning to use 2x6 joist hangers, so not only will they be nailed horizontally to the house joints, but they will be sitting on the house 2x8 joists and nailed on top as well via the bracket. They say 2x6 joist hangers or ridge rafter hangers, but I would think they would work for 2x8s or 2x10s for that matter. They are ridge rafter hangers that work from 0 to 30 degrees. Since I am butting two braces back to back in two joist cavities, I didn't see where I had any other choice but to use hangers since I can't nail horizontally through the house joists into my 2x8s. Let me know if you guys see it differently.


greg_mitch wrote:
I was more referencing to actually installing the lag screws.

GO SLOW!

I broke off two after I already had them about 3/4 of the way in because I got hasty and overheated the screw and it snapped off at the head.


Oh! That's good info to know. Did you use soap on the threads? I would be seriously upset if I broke one off.

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greg_mitch



Joined: 03 May 2006
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PostLink    Posted: Tue Apr 05, 2011 12:33 am    Post subject: Reply with quote

I pre-drilled and used only a bit of soap. I think more soap and more patience would have suited me a bit better.

Do NOT be tempted to use a power tool. Just do it by hand.

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Sparky015



Joined: 12 May 2009
Posts: 1159
Location: Cleveland / Akron, OH


PostLink    Posted: Tue Apr 05, 2011 12:59 am    Post subject: Reply with quote

Thanks Greg. I planned to double up two nuts and put them in by hand with a wrench. If they snapped like that on you, I certainly don't want to use any power tools.
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ecrabb



Joined: 13 Mar 2006
Posts: 14385
Location: Iowa & Michigan

TV/Projector: JVC RS45


PostLink    Posted: Tue Apr 05, 2011 3:13 am    Post subject: Reply with quote

Greg, did you pre-drill according to manufacturer spec, or did you guess? The heads shouldn't have twisted off the bolt unless the pilot hole was too small!

SC
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CIR Engineering



Joined: 25 Aug 2008
Posts: 3031
Location: Chicago USA & Berlin Germany


PostLink    Posted: Tue Apr 05, 2011 12:53 pm    Post subject: Reply with quote

ecrabb wrote:
Greg, did you pre-drill according to manufacturer spec, or did you guess? The heads shouldn't have twisted off the bolt unless the pilot hole was too small!

SC

Heads can snap off if you install the lags too quickly, especially if you use a power tool to drive the lag bolt into the hole.

As mentioned, the holes should be drilled to spec for the diameter of the bolt, and then hard soap should be applied to the entire length of the lag bolt threads before it is driven into the hole. The soap lubricates the bolt so that there is less sheer stress. When you drive in the bolts, use a hand held socket wrench and do drive them in slowly to avoid sheer stress.

Also, when thinking about bolt size I often find that folks really want to over engineer their bolts for the purpose. I like to suggest a mental exercise... Imagine a single screw-eye of say a 5/16 diameter and 4" into the joist. Now imagine a rope hanging from the screw-eye that is 3 yards long. Now imagine, do you think you could hold onto that rope and hang there... I sure do. Do you think you could even swing from this same screw-eye... I do. You weigh more than half what your projector does (at least) and one screw-eye with you swinging from it can hold. I think four longer lags holding a static projector will be just fine Wink

as long as your joist is good

craigr

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Sparky015



Joined: 12 May 2009
Posts: 1159
Location: Cleveland / Akron, OH


PostLink    Posted: Tue Apr 05, 2011 3:58 pm    Post subject: Reply with quote

Thanks Craig. Sounds like 5/16-18 hangar bolts are more than enough for the application based on others experience and the numbers themselves. I'll post a picture of what my 2x8 joist support insert looks like with the ridge rafter hangars installed. I think that is more than enough for the application too.
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dturco



Joined: 06 Feb 2009
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Location: Eastern Shore Maryland

TV/Projector: Runco DLP VX-3000i Marquee 9500 parts doner


PostLink    Posted: Wed Apr 06, 2011 3:27 pm    Post subject: Reply with quote

Sparky015 wrote:
dturco wrote:
Ok, so don't overlook what's holding the 2x8 up or in place. If they're only nailed in with a few nails....you have a weak link to deal with. Wink


I am planning to use 2x6 joist hangers, so not only will they be nailed horizontally to the house joints, but they will be sitting on the house 2x8 joists and nailed on top as well via the bracket. They say 2x6 joist hangers or ridge rafter hangers, but I would think they would work for 2x8s or 2x10s for that matter. They are ridge rafter hangers that work from 0 to 30 degrees. Since I am butting two braces back to back in two joist cavities, I didn't see where I had any other choice but to use hangers since I can't nail horizontally through the house joists into my 2x8s. Let me know if you guys see it differently.


greg_mitch wrote:
I was more referencing to actually installing the lag screws.

GO SLOW!

I broke off two after I already had them about 3/4 of the way in because I got hasty and overheated the screw and it snapped off at the head.


Oh! That's good info to know. Did you use soap on the threads? I would be seriously upset if I broke one off.


Your fine with the hangers. I could write two long paragraphs, but nevermind, joist hangers are perfect for what your doing.

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