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Re: Lag Screws

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Mike
I get involved with a monorails occasionally.  A lot of
engineers do not want holes in or welds on beam or truss
bottoms.  Consequently, we design a lot of clamped hang
-ers.  I suggest something which can over the truss bott
-om chord and then clamp the monorail top flange. You might
have to reinforce the truss bottom chord locally for bear-
ing compressive stresses, or to get around truss nailer
plates.  I hope this helps.
Gary


On 22 Oct 2004 at 15:06, Michael L. Hemstad wrote:

> List,
> I have a project where I need to hang a small steel beam from the
> bottom of some prefab wood (2x) trusses to carry the track for a
> moveable wall. The beam will cross about 7 trusses.  It's stiff enough
> to spread out the load to the trusses.
> 
> The simplest idea I have come up with is to use lag screws through
> holes in the top flange of the beam, into the bottom chord.
> 
> Simpson, for example, makes a 1/4 inch diameter lag screw, with
> catalog values for strength in shear.  However, they corporately
> refuse to give me a withdrawal value for it.
> 
> The NDS has values--rather large values--tabulated for generic lag
> screws.  I could just use those.  However, in my earlier days (say, 20
> years ago) I occasionally fastened pieces of wood together with lag
> screws.  We shied away from the smaller ones because they often broke
> during installation.  We drilled pilot holes, we waxed threads, we
> sacrificed black chickens before we screwed them in.  5/16 screws
> broke maybe one time out of four; the 1/4 inch ones broke more often
> than not.
> 
> Relating this fascinating story to the Simpson engineer, she said that
> her understanding was that current lag screws were better than those
> of twenty years ago, and theirs, being of hardened steel, were better
> than the ones we bought at the hardware store.  She simply was not
> allowed to give me a withdrawal value.
> 
> It would be easy to just use a bigger screw, but the truss
> manufacturer is going to be excited enough as it is with the 1/8 inch
> pilot hole I'm calling for in his bottom chord.  And I could bring
> straps up the side of the truss chord, but it's at about midspan and
> there is probably a metal truss plate there that doesn't want holes
> drilled in it, and vertical or diagonal members that preclude
> strapping over the top of the chord.
> 
> So, finally, the question:
> Can anyone comment on the quality and viability of 1/4 inch lag
> screws? Or does anyone have a better way to solve this simple problem?
> 
> Thanks,
> 
> Mike Hemstad, P.E.
> TKDA
> 
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