SMRF column flange bracing

[Despy]

I am designing a single story SMRF that supports a wood framed structure above, including a plywood diaphragm at the top of the SMRF. According to AISC 341-05 section 9.7 I need to brace the column flanges at the level of both the top and bottom flanges of the beam. What type of brace is this talking about?

If the intent is to resist buckling of the column flanges, then shouldn't the SMRF continuity plates provide adequate resistance? The local building department is indicating that this is referring to a brace perpendicular to the plane of the SMRF going back into the building to the floor diaphragm. This does not make sense to me since the entire code section is referring to "column flange bracing". Any thoughts?

 

[JoshPlumSE]

Despy -

I surprised that no one has responded to your question. Could the Eng-tips group be a bit light on Seismic experts?

First off keep in mind that this bracing seems only to be required for columns that cannot be shown to remain elastic. They define a SC/WB ratio of greater than 2.0 as meaning that the column can be assumed to be elastic.

In the 2010 code, even the columns with a SC/WB ratio greater than 2.0 required bracing of the connection.... but only at the level of the beam top flange.

The 2010 code clarifies the bracing requirement (for columns which cannot be shown to be elastic). This requires bracing of the connection at the top and bottom flange levels of the beam. It is allowed to be direct or indirect.

Direct bracing is a member, deck, or slab directly attached to the column flange at this location (top or bottom flange level of the beam).

Indirect bracing is achieved through the stiffness of members and connections that are not directly attached to the column flanges, but act instead through the column web or stiffener plates.

I would interpret a pinned beam connection in the columns weak axis to satisfy the indirect bracing requirement.... especially if it is connected to continuity plates.


But, keep in mind that if you follow the provisions for an unbraced connection (L/r < 60 and such). Plus, you will need to design the column to resist the overstrength load combinations.

 

[Despy]

Thanks for the response.

In this particular case my SC/WB is less than 2.0, so I need bracing at the level of the top and bottom beam flange.

This is a wood structure with a wood nailer and plywood floor sheathing on top of the beam/column connection. There is no concrete slab on top of the connection. I wood interpret this as a Braced Connection per AISC 341-05 section 9.7. Accordingly, I need to provide bracing.

At the top of the beam level, my plywood floor diaphragm could provide the required direct bracing provided I show that I can transfer the required load (per AISC 341-05 section 9.7a (2).

At the bottom of the beam level I have no immediate direct brace present. There is no beam connection in the column's weak axis at the connection (joists are hung from the nailer on top of the frame). There are continuity plates present at the column, but apparently these do not provide sufficient flange bracing by themselves. Do I need to provide a diagonal brace from the level of the bottom beam flange (at the column) up to the roof diaphragm?

 

[JoshPlumSE]

I would interpret the code to mean that you have to brace the column at the level of the bottom beam flange in some way. So, putting in a little kicker up to the roof would probably work.... though personally, I might just choose to go with a bigger column so that my SC/WB ratio was greater than 2.0.