base plate design w large hss member

huangyhg

base plate design w/ large hss member
hello all.  i've been working on base plate design using the 13th ed. for hss square tubes and round steel pipe members with axial compression loading only.  i understand the adjustments of the m and n for the cantilevered projection of the plate, but how do i check the plate stresses occurring within the hss member?
when the member is relatively small (hss6x6x1/2 w/ 14"x14" base pl), the cantilevered projection controls and the plate thickness is determined by
checking a 1鈥?strip.   my problem is when the member becomes larger
(hss12x12x1/2 w/ 20"x20" base pl), the moment from the plate bending over the center span is greater than the moment on the cantilevered projection when using a 1" strip. technically, for the correct design, i need to do a yield line analysis, but that will be difficult and very time consuming.
my questions are, what is the best way to design for this without performing a true yield line analysis? is there an easier way or simplified equation that takes into account the plate bending over the center span?
i'm sure i can use the 1" strip design over the center span and design it with one-way bending, but it will yield fairly conservative results. any help would be greatly appreciated.
well, you answered your own question: yield line analysis.  if you're fluent at it, that's about a half a page and 10-15 min.  if you're out-of-practice, then you're right: much longer.
if i only had a handful of these, i'd just use the 1" strip method like you're proposing.  it's like a little simple-span beam with cantilever on each side.  if it's 1/4" or 1/2" thicker than it needs to be, then so be it.
if i had a crapload of these, i'd probably resort to a black box fea solution like risabase.
this is not guaranteed.  i solved for it quickly just so you ahve something to compare with:
treq = b*sqrt(fp/(8*phi*fy))
where
b = hss dimension, in.
fp = bearing pressure, ksi
phi = 0.9 for bending
fy in ksi
for an hss12x12 with fp=2ksi, i get 1.05" required thickness.
again, for information and comparison use only.
you didn't say if your base plate is supported on anchor bolts or bearing on concrete, but either way if your only loads are axial compression, won't the maximum stress always be outside the hss?
mike, not necessarily.  imagine an hss12x12 with 13" square base plate.  the cantilever distance outside the hss is very small.
jake1ryan,
under the hss the plate is stiffened by the hss walls that are perpendicular to the bending line effectively giving you two t-beams.
google'rectangular hollow section connections and you should find an australian research paper on this exact thing (with yield line or fem resultas to back it up).
csd72, keeping in mind that i just withdrew my application for a search and rescue crew, do you have the name of that paper or the authors?  i googled it and some other names and didn't find it.
i will see if i can find it.
have you looked at the aisc hollow sections connections manual?
csd72,
i also couldn't find it and i would be very interested in reading it.
as far as this topic goes, there are very few sources that address it, probably due to the rarity that the column would be large and the baseplate would be slightly larger.  from what i do know about yield line theory, it would be the best solution, but as for me, i have never done a yield line analysis.  i did contact the aisc solutions center and i just received a reply.  they suggested two options.  the first was to modify the 'internal check' parameter used for bases of w-shaped columns.  i've looked into that, and from aisc design guide 1, it specifically states to ignore the lambda coefficient when dealing with hss and pipe   
from csd72: "have you looked at the aisc hollow sections connections manual?"
i just did and didn't see anything helpful.  very disappointing that nothing made it into that manual.
from jake1ryan: "the first was to modify the 'internal check' parameter used for bases of w-shaped columns.  i've looked into that, and from aisc design guide 1, it specifically states to ignore the lambda coefficient when dealing with hss and pipe   
271828,
for the dg1, see the last sentence in section 3.1.3.
also, i would be interested in knowing how to do a yla for this situation.  i really don't know where to start.  any good references?  thanks.