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Cantilevered steel column design

Cantilevered steel column design

Cantilevered steel column design

(OP)
I have a question regarding cantilevered steel columns as the seismic force resisting system.  I have a steel framed platform on the interior of a building.  I am using cantilevered column system G.3 from Table 12.2-1 in ASCE-05 with R=1.25 for the platform.  Also, section 12.2.5.2 limits the axial load on the cantilevered column elements to 15% of the column capacity.  My question is, when calculating the axial capacity of the columns, should I be using K=2.1 for a cantilevered condition, or can I use K=1?  The columns are obviously cantilevered under horizontal loads, but for axial loads are they braced by the platform floor framing (i.e. essentially by the other columns, i guess).  If you combine the 15% limit on axial loads and K=2.1 the columns get really conservative in my opinion.

RE: Cantilevered steel column design

but all the columns are cantilevered..  thus, you must use k=2.1..  If you'd like to use K=1.0, you must use the direct analysis method..

RE: Cantilevered steel column design

agree with frv.

RE: Cantilevered steel column design

(OP)
Thanks guys.  That's what I thought, but was hoping I was wrong.  For my current project, for 10.5 ft columns supporting approximately 20 kips and only 500 lbs lateral load, this means the difference between HSS6x6x1/2 and the traditional HSS4x4x1/4.  I was not aware of this 15% limit on axial load before now.  In hindsight, I have seen a lot of canopies, carports, etc. that were apparently way underdesigned.  In my mind, this basically eliminates cantilevered columns as a viable option for "cheapo" architecture projects.

RE: Cantilevered steel column design

terminology?
cantilevered columns? is this a free standing structure with the moment resistance provided by the platform framing?

Dik

RE: Cantilevered steel column design

(OP)
dik, this is a free standing structure with lateral resistance provided by columns with fixed bases.  All connections to the elevated platform are pinned.  Columns with fixed bases and pinned tops = cantilevered columns.

RE: Cantilevered steel column design

Thanks... I was thinking of an upside down cantilever (Australian?) thinking that it would be easier to create a moment resistance at the platform level than at the slab?.

Dik

RE: Cantilevered steel column design

Why does the top have to be pinned?  Provide flexural restraint top and bottom and you can reduce the factor 'k'.

BA

RE: Cantilevered steel column design

(OP)
BA, the tops are simple shear connections.  The architecture will not accomodate bracing, and I am avoiding moment connections for several reasons.  So, at this point I'm going with the larger columns.  The Arch has OK'd it.  I'm sure the contractor will complain.

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