Moment Frames in Both Directions - Problem 1

[shemp]

I was planning on using steel moment frames in both directions for a one story portion of a building which is in a Seismic Design Category D (FEMA 302). For seismic design, it's important to have a strong column with a weak beam so that the plastic hinge will form in the beam rather than in the column. However, if this is applied to the situation of a beam framing into the weak axis of a column, the beam size (which is determined from gravity loads) will force the moment of inertia of the column's weak axis to be very large resulting in huge columns for a little one story portion of the building.
I also noticed that no connections for beams framing into the weak axis of a column have been prequalified by testing in FEMA 350 (seismic design for moment frames). Also, tubes (which have the same I in both directions) are not prequalified either.
Do these requirements and lack of testing make moment frames in both directions not feasible?

 

[dougantholz]

Shemp,
There are a couple of things you can do, but let me say that first off I don't think moment frames will be feasable for your use. I say this because I think that you will have a serious problem with drift.

I am assuming that if you are framing into the weak axis of a column, it is only because you have no choice as to the framing location.

Okay, having said that, there are a couple of things that you can do. First off, the procedures in FEMA 350 for Steel Moment connection design can be adapted for use in the weak column direction. They only walk you through the steps needed for design.

Secondly, look at your R value you used for design. Because this is not a pre-qualified joint it is likely that you could use what the IBC calls 'Structural steel systems not specifically detailed for seismic resistance' and an R of 3.

Third, a lot of people get caught in 'strong column/weak beam design.' I'm in a moderate seismic zone and with a spandral beam that is often a W30, a stronger column gets absolutely rediculus. What do I do then? I design the connection for FEMA 350 Loads, but I use 2R/5 as the design load instead of Mp. That way I am designing the connection for twice the worst case load I expect it to see. This will prevent the brittle connection failure, and promote failure in a member. The fact that the column fails first is irrelevant because your already 200% over the design load anyway.

And for a one story building weak-beam/strong-column doesn't matter because that is mainly for progressive collapse prevention. If either your column or your beam fails in a one story building, your building will fall down.

Hope this is helpful.
Doug

 

[JAE]

Doug - good points. Also...what about adding steel cover plates to the W columns to create a box column situation?

 

[zliazmei]

Jae
Your idea is o'k.
But what about the PRICE?

 

[zliazmei]

Jae
Your idea is o'k.
But what about the PRICE?
And What about stability of this cover plates?

 

[shchao99]

"For seismic design, it's important to have a strong column with a weak beam so that the plastic hinge will form in the beam rather than in the column. However, if this is applied to the situation of a beam framing into the weak axis of a column, the beam size (which is determined from gravity loads) will force the moment of inertia of the column's weak axis to be very large resulting in huge columns for a little one story portion of the building."

You don't have to put all the columns in the same direction.
The strong axis can be always put in the direction to resist the seismic load. Even though in the same column line, you can have different column orientations.

 

[zliazmei]

Stemp,
If there is no other decision.
Try to use section like this one.


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It was a good decision here a few ears ago.
Of course here the prices of the unions are to lees than in other countries so it was cheeper.
Hope will Help.