Side-face blowout, bolt group - App D.5.4.2

[pehiggins]

I am seeking clarification/viewpoints on App D.5.4.2, nominal strength for side-face blowout for a group of anchor bolts. I interpret the language and formulas to indicate that if Ca1 < 0.4hef and s > 6Ca1, then the nominal strength for the group is Nsbg = n*Nsb. When Ca1 < 0.4hef and s < 6Ca1, then Nsbg = (1 +s/(6Ca1))*Nsb. (s = clear spacing of anchor bolts, n = # of bolts, Nsb = strength of single anchor.)

If the above is correct, then when s=6Ca1: Nsbg = 2Nsb.
When s > 6Ca1: Nsbg =n*Nsb.

I second guess my interpretation as it seems odd that there would be such a discontinious jump in Nsbg. And, strange results can occur when you play with moving bolts toward the edge in order to increase bolt spacing, assuming you can increase the size of your base plate. Consider a design with 4 anchor bolts in a pier and "s" slightly less than 6Ca1. You have room in the pier to move the bolts toward the edge, slightly decreasing Ca1 and increasing "s" bumping its value over 6*Ca1. Nsb will decrease linearly as function of Ca1 (eq D-17): call the the new value Nsb'. So Nbsg nearly doubles, 4Nsb' versus 2Nsb, and the value of Ca1 has decreased!

 

[miecz]

Looks like a change in the newer version of the code. In ACI 318-02, "s" was "s_o" and was the total spacing of the anchors along the edge, not the centerline spacing between each bolt.

 

[Lion06]

I don't think it's changing as much as you think (or the post makes it seem). If you have the four anchor pattern as you describe (assuming one at each corner of the baseplate), then you would have a value of slightly less than 2Nsb (as you note), but for the two anchors close to the edge. You would have the same capacity for the other two anchors, essentially making your total capacity 4Nsb. This equals the capacity you calc'd when you bump up the s to just above 6ca1.

 

[miecz]

Somehow, I was picturing 4 bolts in a line.