huangyhg

simpson mst type strap installed over gypboard
does anyone know of design values or test info on
simpson mst type strap installed over gypboard.
simpson will only give a value for their nail and/or
their strap when applied over wood framing (df,syp,spf)
what reduction should be taken when a strap is applied
over sheetrock which is applied to cdx shear panel which is applied to 16"oc df framing.
the length of the nail has been increased 5/8"+ to achieve full penetration into wood.
bending of the nail will control in this case, since the drywall offers little confinement to mobilize the shear.
suppliers such as simpson do not rate such an application because of the potential to develop out of plane bending when the drywall doesn't keep the anchor flat because of localized crushing of the drywall.
why not cut out the drywall under the anchor and replace with wood blocking of the same thickness? this will keep the manufacturer involved in the event of a failure because you are using their product in accordance with their instructions.
another approach, if you want to take on the liability, is to downrate the capacity of the anchor based on the bending of the nails as compared to just shear in the nails. compute the bending stresses, compare the load required to bend to the load required to fail in shear and reduce capacity by that ratio.
thanks, ron.
one supplemental thought...if you cut out the drywall, why not just attach the anchor to the supporting sheathing/studs. you could then re-patch over the slot that you made to re-finish the wall.
the sheetrock slot to accept the strap and rock over would obviously be the engineers delight. but on a larger scale, numerous multi-story buildings have been built and stand as this construction technique has been accepted practice for years in the portland/wa area. recently red-tagged project is standing still since the inspector has requested someone answer his concern.
additionally: wall panel manufacturers are looking for non-reduction solution since they prefer to pre-fab shear-ply and gyp-over without pre-strapping (shipping/alignment problems).
this is one of my pet peeves. the common contractor lament ("we've always done it this way and we haven't had a failure yet") doesn't hold water. show by testing or calculations that it works, or revise the detail to comply with manufacturer instructions.

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