Anchor force of column base plate by AISC, you'll find this interestin
Anchor force of column base plate by AISC, you'll find this interestin
(OP)
It’s about 3 years ago, in a steel column base plate design in accordance with AISC design guideline (AISC Steel Design Guide Series 1, COLUMN BASE PLATES), I encountered a dilemma with the calculation results. This remains a wonder to be since then. Thanks for the forum.
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.
Did anyone experience this? Welcome your comments!
J1D
PS, I further examined the two equations, and believe the root is that the compressive stress fc was fixed to be a constant fp. Ironically, for the given P and M, using a smaller stress (less than fp), you’ll get a higher tensile anchor force T.
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.
Did anyone experience this? Welcome your comments!
J1D
PS, I further examined the two equations, and believe the root is that the compressive stress fc was fixed to be a constant fp. Ironically, for the given P and M, using a smaller stress (less than fp), you’ll get a higher tensile anchor force T.






RE: Anchor force of column base plate by AISC, you'll find this interestin
I believe this issue was addressed in the magazine "Modern Steel Construction". You can search their archives or write into them with your question. I am not at my office so I can not do a search for you. (www.google.com)
RE: Anchor force of column base plate by AISC, you'll find this interestin
Is the magazine published by AISC, Chicago, IL? I couldn't get into the right page for the article search. Help.
Thanks
RE: Anchor force of column base plate by AISC, you'll find this interestin
I found the following reference in the AISC web site regarding a similar topic, it states: "Inconsistencies in Column Base Plate Design in the New AISC ASD Manual"
Engineering Journal
Third Quarter 1990
2 pgs, it continues:
The new AISC steel design manual (ninth edition)1 suggests a new procedure for computing the thicknesses of column base plates to rectify problems associated with the somewhat conservative design approach adapted in its earlier version. However, a close scrutiny of the suggested method reveals that the new approach is sometimes overly conservative and even inconsistent."
They want $10.00 for the two page report. However, you may have access to the journal listed. Good Luck....I will look through my files to see if I kept a copy - but I'm not making any promises. If you want to search their lists try the following link: http://www.aisc.org/template.cfm?template=/Ecommerce/ProductDisplay.cfm&ProductID=832. If that doesn't work go to the www.aisc..org web site and do a search under advanced search using the title of the documant that you used above (i.e. AISC Steel Design Guide Series 1, COLUMN BASE PLATES)
RE: Anchor force of column base plate by AISC, you'll find this interestin
Don't know if that helps or not, but you may want to give it a try. Good luck.