inverted tripod as a cantilevered column

huangyhg

inverted tripod as a cantilevered column
folks,
i am analyzing an inverted tripod as a column (you gotta love the architects). the column is significantly large.
however, this column is also subjected to moments in both directions. in one direction, this column is a cantilever, while in the other direction, it is part of a moment frame.
how would one design the three legs of the tripod? how does one calculate the effective length factors?
it is a cantilever in one direction, so k=2.0? is that a conservative way of estimating? are there accurate ways to determine the k factor for analysis?
first, how are the three legs attached at the top?  i am picturing three legs springing from the ground that get farther away as it goes up.  
in addition to looking at the column as a whole, you will need to make sure that none of the three legs buckle before the global section buckles.
also, if there are shears and moments, what provisions are being made to transfer shear between the three to make it act compositely?  are you even attempting to make this happen?
the 3 legs form a cradle to support a box girder. one of the legs stops short and the other 2 legs extend higher. a thick diaphragm will connect across the 2 extending columns, and the box girder will bear on the lower column.
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the legs go up about 40'. yes, the idea is to make a diaphragm on the top connecting the three legs together. this helps a better load distribution (almost keeps the load on the centroid of the tripod).
for true cantilever (tip are free to translate and rotate), the theoretical k value is 2.0, but 2.1 is suggested. please read nto steel design text books to find methods to determine proper k values when framing is involved.
other than that, i think the design can be carried out by any of the conventional methods (computer modelling/analysis is more practical though).
watch out for ot of the footing.

i must mention that this is a rather huge concrete column. i see it the way streit sees it. i have 3 columns on a tall pedestal which forms a tall single column.
i could be missing something, but still don't see any significant problems (other than time consuming).
i evision a big old (non-composite) bold tree, it has a huge trunk with 3 diverging braches that supporting some structures above, which is/are linked to the branches by eith moment connections or else. analytically, i would first assume the branches are fixed at the top of the trunk, then each branch will get its own share of loads, derived from typical frame analysis, or static method. now the loads will pass down to the top of the trunk, calculate the resultant forces as usual using static method, here you go. (provide aboundant/closely spaced shear reinforcement at the root of the branches, and the top of the trunk, for which stress induced spliting is very likely to occur)
still, watch out for global stability, especially in areas with high wind, seismic events. settlement could be a big problem too.
the coefficient "k" which defines the length of buckling of the bar can be obtained through the relationship between the rigidity of the bars that join the column. first you need to assess whether the structure is moved or not. then, simply relate the stiffness of the bars with their length using the following formula: g = (iv1/l1 + iv2/l2 +...)/( ih3/l3 + ih4/l4 +...). where "i" is the inertia of the bar seen and "l" the length. on the value of g can enter the chart in the iacs, the section that discusses design of columns, and get the value of "k". if you model this structure in a program of structural analysis such as sap2000, get active efforts can easily tailor your column. but, as observed by you, the coefficient "k" is important and needs to be assessed in this case really well.
sorry for my english mistakes.
i have not seen your third post and so i thought it was a steel column.disregard my comments above.  
following the description of the structure you made, i understand that two of the tripod bars belong to the same plan and are linked directly to the upper beam, while the third is a cantilever and it make part of another plan. with this description, i understand that only two legs of the tripod are receiving the load of the beam and therefore should be designed to flexo compression. in that case you will design a column in "y" form. if you put more details, perhaps i can help better.
4ntunes
4ntunes has very good grab on this topic. listen on.  
kslee1000,
sorry, but i dont understand what you mean with the word "grab". could you explain? thanks.
"handle", or "understanding". anytime, english is not my native language too.