huangyhg

atrium column unbraced length question
i am designing a 70 foot tall glass atrium, which is basically the shape of a football in plan view, with straight vertical walls and flat roof. the "football" passes through an office building, with 5 stories on one side, and 2 stories on the other. the framing sizes are all preliminary right now, but i am having some problems with the columns. i am modeling it in staad in full 3d. see attached file for plan view at one end of the atrium.
my columns are hss12x12x1/2, spaced about 10 feet apart. the columns will be rigidly connected to the foundation and to hss roof trusses, forming a portal frame, in effect, across the width of the atrium.
i have horizontal "ribs" every 14 feet, which are hss20x12x3/8. the ribs are fully welded to the columns, to make a continuous rib. due to the shape, the ribs form a peaked arch in plan view, with the base of the arches tying into the floor slabs at the first two levels on both sides, and on one side only for the full height of the atrium.
so my question is, what do you think the unbraced length of the columns is for the strong axis (out of plane of the wall) direction? full height of the atrium (70 feet)? or do you think the rigid "ribs" will contribute to bracing the columns against strong axis buckling? weak axis is obviously braced by the ribs every 14 feet.
i know that the ribs will contribute to some extent, and that full height is probably too conservative, but i am not sure how to justify a rational approach to reducing the unbraced length.
if you imagine the arch that the ribs form, that is a very stable structure that could withstand an out of plane buckling load from the columns. so can i figure on using 2% of axial compression to calculate a horizontal oop point load at each column-rib intersection, and then check the model with those additional loads? if the arch can handle that additional lateral load, i think i am good to consider the ribs as bracing the columns out of plane, due to the stiffness of the arch. what do you think?
i told the architect that hss12x12 would work (based on some prelim hand calcs), and it does in most cases using 70 foot unbraced length. but where i have columns tied to vertical x-bracing at the ends of the atrium, the axial forces due to overturning are exceeding my column capacity (based on 70ft). so if i can justify a reduced unbraced length, then i can make the 12x12 columns work.
thanks very much.

here is an elevation of one end of the atrium, so you can see the bracing layout.
and here is a section cut through the atrium.

i could be wrong. but hss itself and all wall elements are compact, is there a problem with buckling?
for bending, i afraid you have to use the 70' unbraced length in direction of your concern.
i agree, i would use 70 foot unbraced length.
however, you may have bigger problems, but maybe you have already addressed it.....
won't you have deflection problems with your columns spanning that far trying to resist wind load or are you creating a rigid gridwork spanning 2 directions?
for propped beam, the deflection may not be overly severe.
concrete fill may help to some degree for both compression & deflection. however, the foundation design could possess the biggest challenge, especially if it is located in moderate earthquake zone or above, as well as the wind effect. have fun, and good luck.
the horizontal ribs on gridline aa are spanning about 60' from end to end. the orientation of the rib is difficult to ascertain from your section. if the horizontal dimension is 20" the rib is a lot stiffer than any column. even if the horizontal dimension is only 12", it is still stiffer, so provided the ends of the rib are restrained against translation, the rib offers significant restraint to the columns.
perhaps you could treat each column as an axially loaded
these columns are certainly braced at each level in the plane of the gridwork, by the braced frames.
i am not sure they are braced perpendicular to the gridwork, however.
daveatkins
ok--i apologize for restating the obvious. everything i just said was in the original post.
this is one of those cases where i would err on the side of conservatism. i would assume the columns are unbraced full height. i understand how you are trying to justify this, by treating the entire gridwork like some sort of arch (in plan) that resists buckling, but how would the reactions at the end of the arch be resisted? by the braced frames?
if you can get comfortable with the forces induced, and with the load paths, go for it.
another way of making the 12 x 12 column work--are you assuming k < 1 for your buckling check? you said the columns are fixed top and bottom. or are these columns stabilizing themselves in the strong direction (you mentioned they are part of portal frames)? if so, k > 1.
daveatkins
thanks for your input. i agree with most of what you recommend. i started off being conservative and using full height unbraced. but with
structuresguy:
if you are using a 2 way rigid structure to resist wind loads, i would suggest using the same size

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