Column Splice in a steel moment frame

[jaytweed]

I am designing a 4-story building with special moment frames of steel. I can obtain lighter columns if I introduce a pinned connection (column splice) at the third floor level; however, I am having a tough time wrapping my mind around whether this is a feasible approach. It seems proper to me to model the columns as being continuous and capable of transferring moment across the floor levels, but if I can save the client money on steel, then I will do it.

Has anyone here done something similar?

 

[JAE]

So you are suggesting using a lighter column from third level up to the roof? That can be done and has been done using lighter columns but with a moment splice, not a pin.

Not sure how you actually would fabricate a pinned column splice that still takes full axial load through the splice....I'm sure it can be done but aren't most column splices detailed as moment resisting?

Also, your splice should be modeled above the third floor (i.e. 30" plus) for erection purposes.

 

[Lion06]

Don't the column splices shown in the steel manual (pg 14-22) behave as a pin? They certainly don't seem like they would transfer much moment? In what instance should these splices be used compared to a full moment splice? Obviously, you want a full moment splice anywhere you have a large moment to transfer. Are these splices (pg 14-22 in the 13th edition manual) limited to gravity columns only and/or certain locations in the gravity column?
Should they not be used at any location in a frame column? Does this answer change if it is located at or near the zero moment location? Although (just thinking out loud here) the zero moment location for the lateral case is the point of higheset P-delta moment for gravity/buckling case, correct?
Can anyone elaborate as to the appropriateness of these "typical" column splices on pg 14-22 in the 13th edition manual?

 

[Taro]

I don't think you will end up with a lighter section by using a pinned connection. A fixed connection will certainly result in a lighter section because it will resist moment at each end instead of at one end only.

In the U.S., the AISC code requires that moment frame column splices must have a minimum flexural stength so pinned connections are not allowed. Also, the splice must be located at least 4 feet above or below the beam.

 

[twinnell]

Design the column you want to lighten for the member required member loads. Then design the connection to handle the end forces of the column. If you want to release all the moments, then the lower column will have to handle the lateral reactions of the column above. Of course this can be done. Don't think I have seen it though. We reduce columns as we go up all the time, but we don't consider the splice as a pin.

 

[jaytweed]

Thanks, Twinnell. What I meant to say was that I wanted to release the moment at the third floor and essentially have a "moment frame on top of a moment frame". In reality, though, I know these guys will erect the column in one piece (the building is about 40'-0" tall), and there wouldn't be a release at the third floor.

I am squared away now with how to proceed, it is just that I was hoping that I had an idea that could reduce some of these sizes.

 

[haynewp]

I am not sure what OSHA would think. They require a 4 bolt connection at the base of columns to give some fixity for safety, so someone might have to prove the "pinned" splice is as strong as a 4 bolt base connection just in terms of erection safety.

 

[twinnell]

You can do a cap/base plate connection. Place cap plate on bottom section and base plate on top section. Place 4 bolts through plates.

 

[haynewp]

I bet there are still minimum flexural continuity requirements by AISC like mentioned above, especially since it is a moment frame. I can't seem to put my finger on it.

 

[haynewp]

I found it.

9.9. Column Splices

Column splices shall comply with the requirements of Section 8.4a. Where groove welds are used to make the splice, they shall be complete-joint-penetration groove welds that meet the requirements of Section 7.3b. Weld tabs shall
be removed. When column splices are not made with groove welds, they shall have a required flexural strength that is at least equal to RyFyZx (LRFD) or RyFyZx / 1.5 (ASD), as appropriate, of the smaller column.

 

[twinnell]

Haynewp,

Which manual did you find this in?

 

[haynewp]

seismic provisions for special moment frames