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embed bolts in CMU

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I think the other person writing about this subject interpreted ACI 530 the
same way we do. That is why we do not even bother designing using masonry
embed bolts anymore, due to their limited capacity. If you plug in 8" x 8"
(the size of one filled cell of CMU), A=64in^2 to the equation, and
fm=1500psi, you get a measly 1200lb. From what I understand this would be
the max you could get from a bolt in the top of 8" block, one per cell, no
matter how big your embed is your effective shear cone area will not exceed
this. Also, this is an empirical formula based only on the shear cone
relationship and the strength of your masonry/grout. It seems they give you
min embeds so that you are developing the strength of the bolt fully, or as
is required to get an equal strength of your shear cone, such that bond
stress failure between the bolt and the grout is not an issue. We know it is
not the A307 bolt controlling this relationship, so it must be a
conservative, empirical formula. This is all we can come up with at our
office.

So what do you do when you need a bolted connection with more then a 1200 lb
capacity? This is a common occurence, especially in low rise construction,
down here in FL. We have been exclusively using Simpson SET Epoxy, which
give you an allowable tensile value=3140lb in the center of a grouted cell
opening, w/ 5/8" diam. A307, 5" embed. My question is why is this so much
greater then the code formula for cast in place bolts? I think it goes back
to Simpson bases their results on testing, with a factor of safety of 4.0.
That makes me feel very confident. Also the flexibility of a post-pour epoxy
bolt allows precise placement with much less room for errors in the field.

And no, I am not a salesman for Simpson nor do I get a commission. I am sure
other epoxy systems work just as well. Simpson does the best job , I think,
of providing quality information through their manuals and seminars. I went
to a one day epoxy bolt seminar on their tab, and we drilled and epoxied and
tested in the workshop. It is pretty neat seeing the bolts fail in tension
(steel yielding) while the epoxy holds strong......

Give their website a try for more info. If anyone has any insight as to why
cast in place bolts for CMU are so "weak and crappy" sound off.

All IMHO...

Andrew Kester, EI
Longwood, FL



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