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Re: Lag Screws

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Fellow engineers,

        I admit that I haven't been following this thread very closely; but
Gary's posting caused a bell to ring.  I hope it's not too far off base.

        Is it possible to use simple joist hangers combined with a strut
between adjacent trusses.  This would not only avoid drilling (and
weakening) the bottom chord, but could also reduce the actual load applied
to any given truss.

        Just a thought.  Use it if it's of any value.

Regards,

H. Daryl Richardson

Gary Hodgson & Associates wrote:

> 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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