Cantilever Column R Determination

[akastud]

I would like to determine the R value for a steel cantilever column that supports wood beams and framing. ASCE 7-05 references special, intermediate and ordinary moment frame systems under the cantilever column section in table 12.2-1. It referances 14.1 for detailing requirements for each type which then sends me to AISC 341 which discusses moment frames with R = 3 and higher. Most of the requirements are in regards to the beam to column connection. What is special for different methods when it comes to cantilevered columns?

akastud

 

[msquared48]

According to Table 12.2-1, G.3 for ordinary steel moment frames, just detail the base connection as you would as if it were an ordinary steel moment frame. As there is no moment at the top - it is just a wood beam bolted to a column, probably resting on a simple steel bracket connection.

Don't forget about the requirement listed in table 12.3-3 either.

Mike McCann
MMC Engineering

 

[akastud]

How would you get a "special moment frame" out of a cantilever column situation?

akastud

 

[msquared48]

It's all about how you detail the connections - the base connection in the case of the cantilever column. Look at the differences in requirements between the OMF and SMF on the IBC. The SMF requirements are far more restrictive.

Mike McCann
MMC Engineering

 

[OldPaperMaker]

msquared48,
It is my understanding that the R, Omega & Cd values are defined in ASCE 7. But, to determine what an OME,SMF, etc actually is, one must go to the ACI or AISC to figure out what kind of detailing is required to meet that specific Seismic Force-Resisting system.
Is that what you meant in your second sentence, above?

 

[akastud]

I am having a hard time finding any special requirements for a base connection, unless the idea is that the moment frame is inverted and the moment connection that is normally beam to column must be provided at the column base. In which case it would be somewhat impossible to create a SMF because the strong column weak beam becomes difficult when your beam is your foundation. Does that make sense?

akastud

 

[msquared48]

OldPaperMaker:

Yes... exactly.

akastud:

I suppose that you could look at it that way, but I think you making the problem harder than it is. If you simply chose an OMF, the design forces may be higher, but there may not be the need for a special inspection, depending on the jurisdiction, due to the FS in the higher loading. Additionally, for cantilever columns what usually controls the size of the member is the allowable or tolerable lateral deflection at the applied lateral load, not the shear or bending stresses. The OMF is more conservative and simpler to detail. For high and mid rise buildings, use the SMF as it is more economical with material, but keep it simple in Residential and low rise with the OMF.

Mike McCann
MMC Engineering

 

[akastud]

Mike,

Les lateral deflection does control, but if I use a SMF (R = 2.5)the load producing that deflection is half the load in an OMF (R=1.25) and Cd is the same so the half load produces half the required I which results in a significantly smaller column and footing even if I use the SMF requirements.

akastud

 

[msquared48]

Yes, that's true, and that is what I was saying. Because the restructions are less for the OMF, the load is greater to increase the effective FS.

Mike McCann
MMC Engineering