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Old 08-20-2010, 09:41 AM   #1 (permalink)
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Lag screw torque

I am making a half ton ovearhead crane for my garage and searching for information regarding the I beam to wood joists connection.

So far I have found the lag screw size, spacing, depth, and pilot hole size I should use for maximum connection strenght. I have a hard time finding information regarding how much I should torque the screws though. Anyone as experience with that?

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Old 08-20-2010, 12:52 PM   #2 (permalink)
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The strength of a lagged connection is very variable due to differences in the wood species, moisture content, bolts, design of the connection, etc. The best way is to make a test connection and tighten it with a torque wrench until the bolt breaks. Then use 50-75% of the breaking torque value on the actual connection.

Or, tighten just until the lags touch the wood (or metal) surface, then just a little more by feel. If you feel the torque decrease, you have stripped the wood and the strength of that connection will be lower - add another bolt.

If possible, I would highly recommend throughbolts (for maximum strength) instead of lags unless the I-beam is going on top of the joists.
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Old 08-20-2010, 06:54 PM   #3 (permalink)
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lag screws should NEVER be used under tinsel load, they should only be used when they are under sheer load.
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Old 08-20-2010, 07:17 PM   #4 (permalink)
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tensile, tinsel is for trees. And there is no way that I would lag bolt a steel beam to the wood in my garage. If you have trusses in your garage, they were not meant for that anyway. If you have joists, you really need to check the span/height and you may have to add some (double them up) to get the appropriate strength.
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Old 08-20-2010, 08:31 PM   #5 (permalink)
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The I-beam will be right below the 2x10 SPF joists on 16 inch centers which are spanning 14 feet.

I am planning on fastening the I-beam to 7 joists using 2 lag screws per joist. The load will only be able to travel the middle 96 inches of the 10 foot beam.

Thanks for your link Old Tele man. According to that, the maximum
direct withdrawal load of a 3/8 lag screw penetrating 3 inches into the side grain of SPF would be:

8100*0.42^1.5*0.325^0.75*3 = 2847 pounds

So I don't need to worry much about the 14 lag screws not being able to handle half a ton. Even with the load 12 inches from the end of the beam I'm looking at a safety factor over 10.

Each joist is able to support over a thousand pounds of live load. I know this is for a distributed load, but since I span seven joists with the beam and the trolley will not travel to the very end of the beam, I'm not worried either.

I was just wondering about the torque. I will do as suggested, make a test connection and go with 75% of stripping or breaking torque, whichever comes first.

Ryland, lag screws can support greater tensile loads than shear loads.
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Old 08-20-2010, 09:09 PM   #6 (permalink)
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consider using a plate above the joists and running threaded rod to that. It may cost more, but I am sure that your safety factor will be raised. The other thing to remember is the tensile vs shear. Use lags in shear if possible. I have had boards splinter off the portion that I was screwed into without pulling the threads out. Running threaded rod to a piece of 2x10 laying flat across the joists with fender washers distributing the load on the wood would be significantly safer (IMHO, although I am sure that it could be proven).

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