Cantilevered Columns - Special vs. Ordinary
Cantilevered Columns - Special vs. Ordinary
(OP)
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?
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?






RE: Cantilevered Columns - Special vs. Ordinary
BA
RE: Cantilevered Columns - Special vs. Ordinary
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?
RE: Cantilevered Columns - Special vs. Ordinary
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.
RE: Cantilevered Columns - Special vs. Ordinary
RE: Cantilevered Columns - Special vs. Ordinary
RE: Cantilevered Columns - Special vs. Ordinary
RE: Cantilevered Columns - Special vs. Ordinary
RE: Cantilevered Columns - Special vs. Ordinary