ot, need some help with thresholds

im having a problem with office thresholds, here is the setup, the main hallway is tile, the realstuff and its about 3/4 inch thick, the offices are carpet, never had thresholds before ,the ladys are tired of stepping on the carpet tack strip tacks as they run around sholess in there ,[i know just put yer shoes back on, but...] so i put in some low thresholds, they didnt want the doors trimed on the bottom and there is very little space under them, they work clearance wise but after a week or 2 the screws pull out, the "good" hardware store here has limited stock and the other one has very little stock, im using plastic inserts placed in holes i drilled into the office slab which is concrete, the way the door jamb, the carpet and the tile all come togeather leaves about a 3/4 inch valley between the tack strip and the tile right where the threshold has to be, its 3/4 on one side and about1/2 on th eother, ive used longer screws, and ive used bigger screws[ number 8's yesterday] but cant get them to stay, this is a lawyers office complex and the foot traffic is high any ideas? or a pic of anything that will hold these things in there, getting tired of doing the same job over and for free

Epoxy then in but don't use plastic inserts...get anchor bolts and with apoxy...it'll be there forever.

Long Tapcon screws.

I never did like those plastic inserts.
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was in the floor business 20 yrs.remove tackstrip,install a piece of snap-track for vynil moulding,this goes under the tack strip.hold it down with a 1 inch concrete nail drove into a 1/8 inch hole.any good flooring store should have this stuff.then the rug is cut to expose the groove in the track.next a piece of vynil molding with a upside down t on the bottom is cut to lenghth then tapped into the track.the molding is made of various lengths to cover both the tile and carpet.ive put down miles of this in commercial buildings over the years and its as close to bulletproof as you can get..

now thats what i need to find! the place didnt have any but i have another town close, [ 70 miles] i will head down there and find some fo those thanks!

i'll look at that too, its agravating as this should be a simple no brainer job, but this one isnt

im guessin this is a low pile cut loop commercial carpet,by far the easiest is to simply pound the pins in the tackstrip down,they are currently leaning toward the tile.pound em down in that direction from the top.dont disturb anything else

Glue (epoxy) Thread certs #8X32 into the holes. Use Locktite on the screws. Jim

We sometimes drill 3/8 holes and pound 7/16 wood dowels in. You can then nail or screw the carpet bar down over the tackless and it is covered up.

Transition Strip...