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Double Shots (no chaser)

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During a recent site visit, the contractor casually mentioned that some of
1/4 inch diameter powder actuated fasteners had been shot twice per
fastener. These fasteners were only functioning to hold some small zones of
metal lath away from a concrete bond beam for support of some plaster
finishes. Much to my surprise, several other contractors subsequently told
me this practice is not uncommon. I temporarily stopped the installer and
had several of the double shots tested in tension. The measured values were
acceptable and the installer corrected his technique to use only one shot.

My limited knowledge of shot pin "theory" is that pin values are based on
friction from displaced concrete compressing the pin shaft plus a tip
fusion effect(sintering). It seems like a second shot messes up the whole
situation. Has anyone else encountered this condition?

Thanks in advance,

Michael Krakower SE
Arcadia, CA.

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