AISC Design Guide 1 - Base Plate Deisgn 1st edition vs 2nd edition

[Jerehmy]

For checking the base plates for anchor bolt tension:

1st edition - 45 degree angle of force distribution to critical section from center of anchor bolt, so:

M = T*L/(2*L) moment per inch due to anchor bolt tensile force

t = (Ω*4*M/(Fy))^0.5

substituting M into t gets > t = ((Ω*4*T*L/(n*2*L*Fy))^0.5

2nd edition:

t = 2.58*(T*L/(B*Fy))^0.5
2.58 = (6.68)^0.5 = (1.67*4)^0.5 = (Ω*4)^0.5

so

t = (Ω*4*T*L/(B*Fy))^0.5

where
L = distance to critical section
t = minimum base plate thickness
T = total anchor tension force
n = number of tensile bolts

What the? They went from utilizing a very small portion of the base plate to the entire thing. Why?
(meant to preview before posting, accidentally posted with misspelled title )

 

[JoshPlumSE]

Personally, I think the 1st edition of the design guide was more clearly written than the 2nd edition. I use the 2nd edition mostly for an explanation on the concept of an assumed constant/ rectangular bearing stress block.

And, the section that you're referring to about design for the tension side is still valid... if you have multiple closely spaced anchor bolts on the tension side.

However, I agree with you that if the tension bolts are widely spaced that you're not going to be able to engage the full width of the plate.

Note:
They still have the "critical width" of plate calculation for the tension side in the design guide. It's just been relegated to Appendix B which covers the old triangular bearing pressure method. But, in my opinion, it is still a perfectly valid concept and I would use it in cases where I think it would control.

 

[Jerehmy]

In the intro they say that the second edition "supersedes" the first edition based on new research. They relegated the 1st edition to appendix B like you said.

I'm just curious what research made them change from 45degree lines to full enactment of the base plate. And they don't even mention anything about it like you said, kind of just gloss over it and have a catch-all equation.

Like you said if the bolts are spaced quite far apart, can we really have full enactment? Where's the cutoff between full enactment and partial? For a 22" base plate with bolts spaced 19 inches apart, they still use full enactment (Example 4.7). The bolts are 3.6" from the "critical section", but are 1.5" from the edge. So we have 2*(1.5" + 3.6") = 10.2" using the 45° method which is significantly less than 22".

 

[DaveAtkins]

I have always thought the 45 degree method is way too conservative. Check out an article in the Second Quarter 2013 issue of AISC Engineering Journal--"Flange Bending in Single Curvature." It is meant to be applied to beams which support monorail cranes, but I think the logic applies whenever a steel plate is bent by a concentrated load. The author uses yield line theory to show the failure line is more like an elliptical arc.

DaveAtkins