Cantilevered steel column design

[gte447f]

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.

 

[frv]

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..

 

[JAE]

agree with frv.

 

[gte447f]

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.

 

[dik]

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

Dik

 

[gte447f]

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.

 

[dik]

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

 

[BAretired]

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

BA

 

[gte447f]

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.