Is there a "BUST" in Appendix D of ACI 318-08 ??? 2

[esenef74]

Something doesn't add up in section D.5.2 (concrete breakout strength). If I work through the example shown in Figure RD.5.2.3, I actually get a HIGHER capaciy using the h_ef restriction in section D.5.2.3. Here's what I'm coming up with:

Using limit on h_ef:
h_ef = 4 in
A_NC = 200 in²
A_NCO = 144 in²
N_b = 12.1 kips
psi_ed,N = 0.90
other psi factors = 1
----------------
N_cb = 15.1 kips


Neglecting limit on h_ef:
h_ef = 5.5 in
A_NC = 223 in²
A_NCO = 272 in²
N_b = 19.6 kips
psi_ed,N = 0.85
other psi factors = 1
----------------
N_cb = 13.7 kips

The commentary says that the limit on h_ef is because the results are too conservative using the full embedment. The above example seems to indicate the opposite. The only thing I can think of is that the new h_ef should not be used in D-6 (even though the code says that equation is included). Am I missing something here, or is there a "BUST" in the code???

 

[frv]

Since the commentary is telling you that the full embed depth was too conservative, then you'd expect higher results with the alternate embed depth. The results are consistent with the commentary.

 

[PicoStruc]

@Frv, I think you misread OP.

@esenef74, The problem with your analysis is that you supposed that an anchor group can be less effective that a single bolt... That make no sense !

Thus, limiting the depth.

Explanation : By increasing hef, you area ratio is going from 1.39 to 0.82... From Amplification due to anchor group to reduction due to edges effect

With 3 edge condition the area ratio because very sensitive and can lead to either result (conservative or not), thus limiting the depth.

Because the area ration is less than 1.0 means that your anchor group is going to be less effective to less than a single rod.

It's like cheating equation by generating higher Anc with a bolt group but getting something less effective that a single anchor rod. That make no sense !

I would like to have others opinion on that... that a good findind !

 

[frv]

PicoStruct.. How so? The op mentioned that he thought there was an error in the equation because he got a higher result with a shallower embedment depth. I pointed out that the commentary actually reflects this. Not sure how I misread that.

 

[abusementpark]

The whole appendix is a bust.