removing masonry prtitons

huangyhg

removing masonry prtitons
the architect requested our opinion in removing a number of masonry partitions from the second floor of a 2-storey steel framed building built in the sixties. i reviewed the existing drawings and found no lateral resisting system. the beam/column connections were shear connections, no bracings, no shear walls except for the masonry ones.
the base plates for all columns have 2 bolts.
some of the partitions to be removed are only in the second floor.
i assumed all the columns are fixed. checked whether they can resist the lateral loads if they act as cantilevers.  analyzed the building and found that the building would conform to the current code. the base plates can resist all of the forces.
question: can you assume columns to act as cantilevers to resist lateral loads?

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you are assuming that columns are fixed which have only two bolts?  not sure i buy that one.
as it is a two storey building, i take it the base plates with two bolts are at the ground floor or footing, and the columns are two storeys high, going up to support the roof structure.  if my assumption is correct, the columns are propped cantilevers with pin supports at both floors.  whether or not the columns are adequate to resist the lateral loading is for you to analyze.  deflections will be much greater for propped cantilevers than for fixed bases.
then again, i have assumed that there is a lateral load resisting system at the ground floor level.  is that correct?
before assuming the column bases are fixed, check the anchor bolt embedment and verify the foundation can resist the moments induced by the fixed column bases.  the baseplates might be ok, but the anchors might not have adequate embedment and the footings/piers might not be reinforced or sized to resist the moments.
my initial impression would be to keep the masonry walls.  determine if all of the walls are needed and if not, how much of the walls can safely be removed?
it seems unlikely that the building would ever sway enough for the columns' cantilever action to be developed. it sounds like you have masonry shear walls whether you analyze it as such or not.
i agree with jlnj. you have rigid ,masonry walls that will take the majority of the shear up until the point that they fail.
thanks all for the help.
jae:
i assumed a 2-bolt base plate has a moment capacity equals to the force in anchor bolt/compression block times the distance between center of bolt to compression block.
hokie66:
your assumption is correct. we are not touching the structure in the first floor.
jlnj and csd72:
what i am worried about is the effect of removing the masonry walls in the second floor, which changes the configuration of the lateral resisting system. when you say that enough sway will not happen to prompt a cantilever action, is it correct even when all the masonry walls in the second floor are removed?

ttoma,
i you remove all the masonry walls on the second floor then what you say should work.
i have designed buildings similar myself.
but keep in mind that the cantilever action will amplify the reaction at second floor. you will therefore need to check the connections,diaphragm and the lower walls for the additional loads.
csd
ttoma,
i just can't believe that a whole building can be laterally braced with only two bolt connections at the base of some steel columns with no beam-column moment connections.  
yes, you can certainly use the method you describe to get some sort of max. moment capacity, but re  
jae,
from the post, i am assuming that they are double floor height columns and the two bolts are at the ground floor. the column is then a continuous cantilever supported at the second floor level.
the two bolts at ground floor then act as a pinned connection.
csd
csd72 - ya, i thought of that too, but the original post says there was "no lateral resisting system" in the entire building...thus i assumed (maybe incorrectly) that that meant there were no walls below the second floor.