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Thread: Relaxation of Lag Bolt in Wood Connection

  1. #1
    Associate Engineer
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    Feb 2013
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    Relaxation of Lag Bolt in Wood Connection

    I am a structural engineer looking at lag bolt connections between a roof top solar panel support system (racking) and typical residential construction (2x joists or trusses). I have been trying to find information on the long term withdrawal capacity of a single lag bolt. I am interested in determining if over the long term the lag bolt might lose some of the clamping force it exerts on metal foot that connects the rack to the roof. Given the various cycles of temperature swings, wind downforce and uplift, and snow loading and unloading does the lag bolt tend to “relax” its grip on the wood grain?

  2. #2
    Technical Fellow
    Join Date
    Feb 2011
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    Welcome,

    In such instances it would be wise to use bolts and nuts with spreader plates top and bottom to distribute the load over a larger area of the wood.

    There may be uploading with wind lift as well as down loading. Depending on load, 3/16" to 1/4" steel plates the width of the wood and maybe 1.5 x width in length should suffice. Use a flat washer and spring washer under the nut.

  3. #3
    Project Engineer CCR5600Design's Avatar
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    Feb 2011
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    Central USA
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    Quote Originally Posted by PinkertonD View Post
    Welcome,

    In such instances it would be wise to use bolts and nuts with spreader plates top and bottom to distribute the load over a larger area of the wood.

    There may be uploading with wind lift as well as down loading. Depending on load, 3/16" to 1/4" steel plates the width of the wood and maybe 1.5 x width in length should suffice. Use a flat washer and spring washer under the nut.
    I agree with this and would also add the use of thread locker or nylock type nuts.


    Ron

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