From your photo, it does not look as if it is broken off. It is a self
threading screw ..... either the hole it went into is stripped or the installer missed the joist altogether. If the hole is stripped, find the same length screw in 5/16 and that should thread in just fine. If there is nothing there to thread into, I would do without it as drilling to find the joist may cause larger problems.
JM2¢, Hank
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Hank & Lynn
2007 Cougar 290RKS, E-Z Flex, 16" XPS RIBs ( SOLD
.. Gonna miss her ... looking for new 5r)
2004.5 Dodge 2500 QC, LB, 5.9HO, WestTach gauges, Ride-Rite