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Re: Cold-Formed Joist Lap Splice Question

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> From: "Ritter, Mike" <mritter(--nospam--at)lgt.lg.com>

>       Suppose we have a two span condition with a 3' long lap splice
> located over the center support.  At each end of the lap, there is a
> cluster of self-drilling screws connecting the two members.  A couple
> more screws are used directly over the center support to hold the
> members together.  Due to the typical gravity loading, there is negative
> moment over the center support.  The clusters of screws at each end of
> the lap are not at inflection points, so there will still be negative
> moment in the continuous members at those locations.  Now my question...
> 
>       Do we design the cluster of screws for :
> 
>       a)  the magnitude of negative moment at that exact location; or

>       b)  only vertical shear forces due to the resolution of the
> negative moment into a couple about the center support?  ie, each
> cluster of bolts would have only shear and no moment.  The moment at
> these locations would be resisted by the capacity of the single
> continuous member at that point.

For this lapping condition, if the end connection is truly a shear
connection, then you only have to be concerned about the shear loads on
the bolts. This is true even if the connection does not occur at a point
of inflection.
 
Fasteners in the web can be assumed to transfer zero moment in the same
way that web connectors on a W section are assumed to be ineffective in
moment transfer. The fasteners essentially force the two members into a
deflection compatibility condition at each end and at the support. The
rotations/moments are not shared equally at any point except the support
under a completely symmetric condition (including loads).

Moment capacity and shear/web crippling above the support should achieve
approx. double a single joist, especially with the added fasteners at
the support location.

If you want to be really precise, sketch shear and moment diagrams for
each member.

If this is a two span continuous condition, be prepared for the 25%
increase in shear at the support.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project
Burlington, Ontario, Canada
<ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>