Cantilever Column LFRS

[BadgerPE]

I have a project in which the precast designer is requesting changing from precast shear walls to precast cantilever columns for various reasons. The columns will terminate at the second level precast podium slab. Above the podium will be 4 levels of wood shear walls resisting lateral loads. The building is in SDC B. Based upon ASCE 7-05, the maximum height for the cantilever column system is 35 feet in SDC B. Would that 35' apply to the cantilever system only or should it apply to the overall building height? I haven't found anything in ASCE to confirm that it is acceptable to exceed the 35' with SW framing. If anyone has any input, it would be greatly appreciated. I have submitted this to an ICC contact for a code interpretation, but haven't heard back yet.

 

[KootK]

I'm not at all certain but my first inclination would definitely have been to consider the height limit the overall height of the building.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DETstru]

I would say you cannot use cantilevered columns for this building.

 

[MrHershey]

I'd vote no as well. I'd consider it if you were doing say four floors of shear wall building and then a cantilever column roof because there's no structure above the cantilever column system.

But a couple reminders in the event they give you the all clear:
- You probably won't meet qualifications for doing a two-stage analysis, need to consider the whole thing together with appropriate R/Cd/Omega factors.
- Your columns will need to be a lot bigger than they would normally need to be. ASCE 7-05 12.2.5.2 requires you to keep the axial load on your cantilever columns under 15% of the columns' design axial strength.
- I would think you'd need to check columns for overstrength as well. Don't think that's in conjunction with the point above.

 

[slickdeals]

Agree with the above. Do you have a basement with precast walls?

You won't likely meet the stiffness requirements for a two stage analysis at the top of your precast level. The wood shear wall will be way stiffer than cantilevered columns.

 

[BadgerPE]

Thanks all for the information!

The basement has CIP walls with piers for the PC cantilever columns to attach to. I am still waiting for an official code interpretation, but have decided due to timeline, to reject this lateral framing method based upon the 10x stiffness factor per ASCE 7-05 12.2.3.1.a. We will be utilizing PC shear walls or CIP shear walls in lieu of cantilever columns. One other item that came up while I was reviewing this is that the design base shear for the cantilever columns was nearly 4x (0.079W) higher than the PC shear walls (0.02W). This should have precluded the idea from the start....