anchor force of column base plate by aisc, youll find this

huangyhg

anchor force of column base plate by aisc, you'll find this
it鈥檚 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鈥?m=0.5*fp*a*b*(n鈥?a/3)
the dilemma is that negative (compression) values of t (anchor force) were obtained when loads (p, m) have large eccentricities.  
let鈥檚 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鈥檒l 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鈥檒l get a higher tensile anchor force t.
j1d,
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.  (
pmkpe
is the magazine published by aisc, chicago, il? i couldn't get into the right page for the article search. help.
thanks
j1d
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:
i ddn't have the time to fully dive into your problem, but in the example that you modified above, e=5.45, which is > n/6=2.33 but < n/2=7.00.  according to the design guide, this is a moderate eccentricity that could be solved according to the method on page 19 of the guide (2nd column).
don't know if that helps or not, but you may want to give it a try.  good luck.