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Old 07-25-2013, 04:25 PM   #1
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Lag bolt sheer limits


I just went into my wall above my fireplace to add framing for my wall-mounted tv (see picture below). I bolted the tv mount to the three 2*6 studs with (6) 4 1/2" (5/16") lag bolts. I know that this would be strong enough for a tv that doesn't move on the wall, but I have a full motion tv mount and the tv can pulled away from the wall about 15". The tv weighs 90lbs and the mount weighs 25lbs.

So here's my question...are the (6) 5/16" lags I used strong enough OR should I replace them with 3/8"?

Thanks.
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Old 07-25-2013, 04:39 PM   #2
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Lag bolt sheer limits


well, shear strength depends on the grade of the bolt.
http://www.boltdepot.com/fastener-in...ade-Chart.aspx

for hanging any mount of TV, you should reply on the documentation of the mount for proper bolting requirements.

and I don't think I've ever seen framing line that for a TV. my mounts I've always used the wall studs and mounte according to the mount directions. and yes, I have a mount upstairs that is articulating. I think it goes about 2 feet out.

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Old 07-25-2013, 04:57 PM   #3
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Lag bolt sheer limits


I would have preferred to use the wall studs, but other than the 2*6 in the center, it's an open space, so I had to build something for the mounting bracket to attach.

The mount suggests 5/16" lags, but I don't trust Chinese steel and I couldn't find any grade 5 or 8 bolts at my HD.

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Originally Posted by bbo View Post
well, shear strength depends on the grade of the bolt.
http://www.boltdepot.com/fastener-in...ade-Chart.aspx

for hanging any mount of TV, you should reply on the documentation of the mount for proper bolting requirements.

and I don't think I've ever seen framing line that for a TV. my mounts I've always used the wall studs and mounte according to the mount directions. and yes, I have a mount upstairs that is articulating. I think it goes about 2 feet out.
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Old 07-26-2013, 09:05 AM   #4
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Lag bolt sheer limits


It should be fine. Checked a few extending TV mounts and they say up to 130 lbs. You're well under that at 90 lbs, so if you follow the installation spec, installing the size and number of bolts indicated in the instructions, it should hold. If it doesn't, you could sue them to replace your TV, but heh good luck w that.

So given that if it fails it's your loss and there is no harm in over-engineering, why not? You'll sleep better having gone one size up. I probly would. It's my 90-lb TV hanging a foot out from the wall so I'd want to know it's secure.

I'm more concerned about that wall framing... sideways? Is that because this is in front of a fireplace? And why the short 16" studs instead of full height? The main issue here is the bending force of the top bolts pulling out as the TV tries to pivot down about the bottom of the mounting bracket... without going to the top plate, those 2x6's don't provide much more support than the strength of the drywall holding them back. A 2x4 that goes clear to the top plate would be better than those short 2x6's I'd think... can you open up just a bit more of the drywall to incorporate that?
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Old 07-26-2013, 10:57 PM   #5
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Lag bolt sheer limits


I framed it this way because the area above the fireplace is mostly hollow and the top plate is another 3 feet above the opened drywall section in the photo. The 2x6 in the center (sideways) is the original builder framing. I simply added two more short studs that are bolted to 2" angle iron that is bolted to studs on each side of the fireplace and the center 2x6. I added a second 2x6 behind the front studs and secured behind the angle iron to give a 3"+ wood depth for the mounting lag bolts. The studs are 18" and the tv mounting bracket is 9" tall, installed in the middle of the studs with 4.5" inches both above and below. From what I can tell, because the studs are bolted to the angle iron, it is very strong (please tell me if I'm wrong) with the weakest point being the lag bolts, which is why I'm asking if I should put in bigger bolts.

Again, if I'm overlooking anything in my design, please tell me. I added another pic showing a bigger area of the wall.

Thanks.

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Originally Posted by SquishyBall View Post
It should be fine. Checked a few extending TV mounts and they say up to 130 lbs. You're well under that at 90 lbs, so if you follow the installation spec, installing the size and number of bolts indicated in the instructions, it should hold. If it doesn't, you could sue them to replace your TV, but heh good luck w that.

So given that if it fails it's your loss and there is no harm in over-engineering, why not? You'll sleep better having gone one size up. I probly would. It's my 90-lb TV hanging a foot out from the wall so I'd want to know it's secure.

I'm more concerned about that wall framing... sideways? Is that because this is in front of a fireplace? And why the short 16" studs instead of full height? The main issue here is the bending force of the top bolts pulling out as the TV tries to pivot down about the bottom of the mounting bracket... without going to the top plate, those 2x6's don't provide much more support than the strength of the drywall holding them back. A 2x4 that goes clear to the top plate would be better than those short 2x6's I'd think... can you open up just a bit more of the drywall to incorporate that?
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Old 07-26-2013, 11:41 PM   #6
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Lag bolt sheer limits


Quote:
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well, shear strength depends on the grade of the bolt.
No, not in the context of the original post. with a lag bolt in wood, the wood will deform long before you get anywhere close to the force required to shear a lag bolt.
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Old 07-27-2013, 08:57 AM   #7
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Lag bolt sheer limits


Quote:
Originally Posted by A Squared View Post
No, not in the context of the original post. with a lag bolt in wood, the wood will deform long before you get anywhere close to the force required to shear a lag bolt.
You are right about the wood. But the post asks about the bolts, as does the title.
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Old 07-27-2013, 10:56 AM   #8
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Lag bolt sheer limits


Quote:
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You are right about the wood. But the post asks about the bolts, as does the title.
ok, I read that as asking about the strength in shear of the connection, not of the bolt itself.
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Old 07-27-2013, 12:16 PM   #9
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Lag bolt sheer limits


Sorry for any confusion about my question. Basically, I just want opinion(s) about the design and if the bolts are the "weakest link" of the framing and design. I appreciate the help.

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Originally Posted by A Squared View Post
ok, I read that as asking about the strength in shear of the connection, not of the bolt itself.
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Old 07-27-2013, 01:15 PM   #10
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Lag bolt sheer limits


As mentioned earlier, the wood is going to give long before the steel in the screw. But if your load is 15" away from the wall, the loading is not shear anyways. With the load 15" away from the wall, you will be trying to pull out the upper screws. The pull out strength will vary due to many things, including wood type, but if you do a google search for "pull out force lag screw in wood" you will start to get a few numbers that will give you a rough idea.
One thing that you may want to consider is bolting the TV mount directly to the angle iron, rather than the wood that is bolted to the angle iron.
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Old 07-28-2013, 08:34 AM   #11
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Lag bolt sheer limits


if your framing is good. and you use washers. you have no problem with that.
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Old 07-28-2013, 08:50 AM   #12
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Lag bolt sheer limits


with that framing set up bolts and washers look like a better call than bolts bolted through into the framing. have the washer located on the block behind the metal L angle so the L angle helps keep the block from pulling forward. if those were vertical studs placed sideways in wall I would suggest lag bolts into stud for max pullout prevention but they are not. you might could lag through the center flat stud and it will most likely hold.

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