I have little experience with concrete screws and don't know their weight-bearing capacity.
Would four (4) 5/16"-diameter ("heavy duty") tapcon anchors, each 3" in length, screwed into the mortar of a brick wall that's in good condition, be able to support a flanged metal rod with equipment hanging on it, the total supported weight being about 100 pounds?
Best Answer
Although from experience we could conclude that 4 heavy duty Tapcons will hold a 100lb weight whether in brick or its mortar, let's work through the spec and material calculations to see if we were just lucky that it never failed or that we were lucky that we never overloaded.
Tapcon's specs
According to Tapcon single 1/4in Tapcon can be loaded 250lbs in tension or 620lbs in shear if embedded 1in in lightweight block. Your's is 5/16in so expect even a bit more.
Your rod assembly puts the screws in shear load
The top screws may be under tension too, depending on your rod assembly, if the distance from rod to wall is not much less than the distance from the rod to the top screw.
Strength of Mortar
Typically, mortar has less compression strength than the bricks it holds, at perhaps 50% to 70%. If we assume brick strength of 5Mpa, then 50% of that is 2.5Mpa for the mortar.
Is 2.5Mpa for the mortar reasonable?
It matches the strength of type "O" mortar. We don't know what mortar was used, but we do know that the numbers we're working with are conservative, based on the weaker mortar.
Mortar vs Tapcon's "Lightweight Block"
"Lightweight block" is listed by Tapcon to hold 620lbs shear, and it is rated at 3.6Mpa (3.6N/mm^2) compression strength.
Calculating by ratios puts the maximum shear load in mortar (at 2.5Mpa from above) at 2.5/3.6*620lbs = 430 lbs per screw.
Your 4-screw rod
Your 100lb load will distribute over the 4 screws you plan, plus any friction between bracket and wall. Assuming zero friction as a most conservative estimate, this leaves 25lbs shear force per screw, or a mere 5% of the maximum static load capable.
As for tension (if it applies, see above), the load on the top screws could be around 50lbs each for a 1:1 lever, with the screw rated at 250lbs each.
It seems you have plenty of margin.
Static or dynamic load?
Your failure mode will not be the screw, but the unknown compression strength of the mortar at the specific but random locality where the screw penetrates it. Relying on several screws, 4 in your case, each singly capable of handling the maximum load, is prudent and provides plenty of margin.
Also note that all calculations are based on static loads. For dynamic loading, e.g. hanging up the equipment, you should include a factor of perhaps 5x. That still keeps you at 25% of the maximum load.
Brittling over time
Material fatigue is an important further consideration.
Consider that with every dynamic action some of the mortar risks cracking and brittling away due to the sharp screw thread to mortar interface.
The multi screw arrangement will somewhat mitigate this.
Usually shear strength does not require more than say an inch of penetration, but in the case of mortar, the 3in length is an advantage to help overcome fatigue or soft pockets, and so there's a good chance it will remain firm over time.
How the mortar responds to drilling and driving would affect my confidence, and for 100lbs I'd rely on that. For loads of say 1000lbs (well above what I can personally test) I would go by the spec sheet, and mortar isn't covered.
Nylon plug
If you notice brittling over time, you should consider a screw & plug arrangement rather than the present plugless screw.
A nylon plug will help "soften" the dynamic load transfer from the sharp threads into the brittle mortar and help avoid or delay fatigue of the mortar.
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