Cantilevered Columns - Special vs. Ordinary

[JAE]

Designing a carport type of canopy at an entrance where cantilevered columns are a possibility.

Design is under ASCE 7-02, which references AISC Seismic Provisions - 1997 with Supplement No. 2 from 2000.

ASCE 7 indicates, for cantilevered columns, two choices for steel
1. Special steel moment frames R = 2.5
2. Ordinary steel moment frames R = 1.25

For a cantilevered column, you really don't have a moment frame - simply a vertical column bending under lateral load so we are not sure why the reference to "moment frames" except that the inverted pendulum system is also included in the title which suggests cross-T beams...unsure.

When you go to AISC Seismic - the difference between "special" and "ordinary" moment frames is ALL about the beam-column relationship. The requirements for column compactness criteria are the same for all.

So why would I ever design for "ordinary" when the "special" is the same column, and gives me a larger R and thus smaller base shear?

And in addition to this, we'd like to use HSS square columns for this and for modern (AISC 2005 spec) special moment frames - tube columns aren't allowed.

Any ideas? Seems very confusing - and/or - really not quite researched and developed?

 

[BAretired]

Don't know, JAE but I suspect the latter, i.e. not well researched.

BA

 

[haynewp]

I don't have 1997 but I see first off that AISC 2005 requires columns to be seismically compact by 8.2b for special moment frames. Looks like there are add'l splice requirments for special frame columns as well.

Plastic hinge dissipation is probably not well defined in tests at the base for cantilevered HSS columns. You might could justify using R=1 for an elastic response if you really want to use cantilevered HSS columns. A building official might not buy using R=1 though. Where does it say HSS are specicially not permitted?

 

[JAE]

haynewp - we got that from AISC 358 where they indicate, for SMRF frames certain types of columns that are accepted....WF, built-up box columns, star columns, etc. It appears that the column limitation really is rooted in the beam-column prequalification process and not specifically just for column behavior on their own.

But it just seemed strange that cantilevered column R values were listed with a reference to "special" systems when there really isn't anything special about them.

 

[haynewp]

There are ordinary and special cantilever column systems defined in the AISC 341-10 draft. There are additional compact criteria and bracing for the special cantilever columns and protected zones at the base. I think the 'special' is in the anticipated inelastic straining ability at the base of the special columns versus ordinary cantilever column systems...

 

[wannabeSE]

Can you classify the canopy as a non-building structure and use chapter 15? The inverted pendulumn in table 15.4-2 does not references AISC 341.

 

[sandman21]

Hopefully they finally fix this confusion in 341-10. They we have been doing it is to provide a full pen. weld at the base and to satisfy the compactness requirements. There has been debate on base plate design, one for overstrength loads or max moment of the column. We have gotten it through many different building departments and state agencies.

 

[JAE]

sandman21 - I assume you use an R = 2.5 for them right?

 

[sandman21]

Yes.