More on Base Plates
More on Base Plates
(OP)
I was looking at this old thread:
http://ww w.eng-tips .com/viewt hread.cfm? qid=88803& amp;page=1
and I'm wondering if anyone has seen any more follow up on it?
J1D notes:
"From the guide, the force and bending moment equilibrium equations at the plate-to-base interface are (page 21):
T+P=fp*A*B
P*A'+M=0.5*fp*A*B*(N'-A/3)
The dilemma is that negative (compression) values of T (anchor force) were obtained when loads (P, M) have large eccentricities.
Let's take Example 16 (page 23) as the example, but change Mu to be 480kip.in. Then Pu=88kips, Mu=480k.in --> e=5.45in >N/6 = 2.33in (limit of full compression)
Therefore, part of the base plate will be separated from the base and the anchor bolts must be in tension.
However, the outcome is: A=4.04 --> T = -0.16kips (compression)
This can be proved more easily by a Spreadsheet or MathCAD. Keep changing Mu, you'll find there is a theoretically tragic zone. The results fall apart and are contradictory."
I have looked at the paper "Inconsistencies in Column Base Plate Design in the New AISC ASD Manual" and it presents an alternative equation, which I will be working through. I'm just wondering if others have run into this and what you may be doing about it.
http://ww
and I'm wondering if anyone has seen any more follow up on it?
J1D notes:
"From the guide, the force and bending moment equilibrium equations at the plate-to-base interface are (page 21):
T+P=fp*A*B
P*A'+M=0.5*fp*A*B*(N'-A/3)
The dilemma is that negative (compression) values of T (anchor force) were obtained when loads (P, M) have large eccentricities.
Let's take Example 16 (page 23) as the example, but change Mu to be 480kip.in. Then Pu=88kips, Mu=480k.in --> e=5.45in >N/6 = 2.33in (limit of full compression)
Therefore, part of the base plate will be separated from the base and the anchor bolts must be in tension.
However, the outcome is: A=4.04 --> T = -0.16kips (compression)
This can be proved more easily by a Spreadsheet or MathCAD. Keep changing Mu, you'll find there is a theoretically tragic zone. The results fall apart and are contradictory."
I have looked at the paper "Inconsistencies in Column Base Plate Design in the New AISC ASD Manual" and it presents an alternative equation, which I will be working through. I'm just wondering if others have run into this and what you may be doing about it.






RE: More on Base Plates
Mike McCann
MMC Engineering
RE: More on Base Plates
Mike McCann
MMC Engineering
RE: More on Base Plates
Mike McCann
MMC Engineering
RE: More on Base Plates
Mike McCann
MMC Engineering
RE: More on Base Plates
Sum F = 0, P = T+C
Sum M = 0, M = C*d = T*d
As long as the e (M/P) falls within the plate, there is a unique solution, which can be obtained through a few iterations. (Note: the last responder on the referred link has pointed out correctly)
RE: More on Base Plates
RE: More on Base Plates
Thanks Mike, very nice concise two page summary. From 1983! You're a pack-rat like me, it's hard to throw stuff away - you never know when it will come in handy again.
I'm seeing part of my problem, the columns have small vertical loads compared to their moment loads and the e=M/P
eccentricity lands outside the length of the plate. I'm wondering now about adding a horizontal brace to the frame to take the lateral load out of the base plate design.
RE: More on Base Plates
Using Limit States Design (LSD), P and M would be factored loads with M = P*e.
If e < 1/6 the load is inside the kern of the baseplate and there will be no tension in the anchor bolts.
If e > 1/6, the problem may be solved by moving the load, P to align with the anchor bolts and adding a moment P*f where f is the same as in the Blodgett solution, i.e. centerline of baseplate to bolts. The moment about the anchor bolts is then P(e + f).
Estimate a uniform stress block and, on that basis, calculate T' (tension in anchor bolts due to P(e + f)). Adjust the stress block as required to comply with the code and recalculate T'.
Finally, T = T' - P where T is the ultimate tension in the anchor bolts due to factored loads.
I think this may be the same procedure as kslee was talking about.
BA
RE: More on Base Plates
RE: More on Base Plates
RE: More on Base Plates
RE: More on Base Plates
Thanks for all the comments guys, very helpful.
RE: More on Base Plates
And yes, I hope to be around for a few years and hopefully learn enough to pass around the knowledge when I get old.
Thanks to all you guys for being around and teaching "us" the physics and mechanics of problems that we encounter.
PS: I did not mean to be offensive to anyone about their age, and no I am not an ageist. I was only trying to put things into perspective about how far I have to go and how little I know.
RE: More on Base Plates
RE: More on Base Plates
No worries.
Mike McCann
MMC Engineering
RE: More on Base Plates
And jheidt2543, yes, from this solution you can see the interplay of the moment with the resisting load and how the resisting force changes location. The solution on one extreme is pure tension, and pure compression in the other.
Mike McCann
MMC Engineering