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Re: embed bolts in CMU

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Andrew,

If you take a look at the commentary for the MSJC (ACI 530), you will
probably see the reason for the difference.  First of all, yes, the
equation for tension capacity of cast-in-place anchors due to masonry (EQ
2-1...ASD...seperate equation for tension capacity due to failure of the
anchor itself is eq 2-2) failure is empirical, but IT IS based upon
testing.  Thus, in theory, it should be not too much different than what
you would get from a post-installed anchor catalog, which is also based
upon testing.

When you look at section 2.1.2.2.2 of the MSJC commentary, it discusses
the fact that Eq. 2-1 is based upon testing.  It further states toward the
end of the paragraph:

"Comparison of Eq. (2-1) to test results obtained by Brown and Whitlock
(ref 2.2) show an average factor of safety of approximately eight."

Yes, it does appear according to that statement in the commentary that the
MSJC committee decided to use a factor of safety of about eight (8).  So
if you take your value of 1200 lbs (which is actually a little off for an
anchor in TOP of a 8" CMU wall...more on that below), then to really
compare with a value from either Simpson (3140 lbs for 5/8" or 3/4" and
2225 lbs for 7/8") or Hilta HIT HY 150 (1795 lbs for 1/2" anchor and 1965
lbs for 5/8" anchor assuming that anchor is placed in middle of cell) you
should be using 1200*2=2400 lbs (to get the "equivalent" factor of
safety).  Thus, will still some differences (especially with Simpson's
5/8" and 3/4" anchor values), things are much closer.

Now, first I will point out that if in fact you are placing the anchor in
the horizontal surface at the top of a 8" wall, then the tension capacity
of the masonry from Eq. 2-1 will be on the order of 885 lbs.  This is due
to have to use the edge distance (3.82") of the anchor to determine Ap
(45.7 sq in).  There could be an arguement made that in this case the Ap
should be slightly larger than that because the edge distance only applies
in one direction, but the code does not consider than (at least that is
how it appears to me).  In otherwords, in theory the failure cone may have
two "sides" that are truncated by the edge distance, but then two "sides"
that are not.

Regardless, that get's us back to the use of a factor of safety of 8.  To
be entirely honest, I don't know why they chose to use a FS of 8, but I
can speculate.  It is possible that a FS of 8 is used because masonry
construction is HIGHLY dependant on quality control during construction,
even more so than concrete.  Thus, it could be the committee's feeling
that a high FS is warranted in this case.  Again just my own theory.
Maybe someone from the committee could respond (I know at least one
committee member used to be a member of SEAINT...regardless I may try to
find out myself since it has piqued my curiousity).

One other comment...while I certainly have absolutely noting against
epoxy/adhesive anchor (used them MANY times), you do need to excercise a
little caution with them.  While it is definitely true that they allow
"precise placement with much less room for errors in the field", there is
still are places where field "errors" can really screw up such anchors.
The anchors might be able to be located very accurately, meaning that
errors in edge distance and spacing are less likely, you do have to make
sure that the hole preparation is done properly.  If the hole is not
cleaned properly before placement of the anchor or adhesive/epoxy, then
you can even have situations where a person can physically pull out the
anchor by hand (worst case scenario with a very badly cleaned hole) to
tension capacity of much less than specified in the catalog.  Thus, many
times I have specified some actual testing of in-place anchors for
critical applications.

FYI, the minimum embeddment depths in the MSJC code are not there to
produce masonry failure over failure of the anchor, but rather in
consideration of "...a practical minimum based upon typical construction
methods for embedding bolts in masonry."  (from MSJC commentary section
2.1.4.2.1)

HTH,

Scott
Ypsilanti, MI


On Thu, 12 Sep 2002, Andrew D. Kester wrote:

> 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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