ASCE/SEI 7-10 Table 12.2.1

[Barbaneagra]

Does anyone know what seismic force resisting system would be knee braced wood columns (for patio covers, decks, trellises and so on) (ASCE/SEI 7-10).I need to get an R. Or shall I just use lowest R=1.25 and I'll be safe ?
Even in ASCE 7-05 wasn't clear about this system. Am I missing something ?

 

[KootK]

If your knee braces form triangles that are relatively small compared to the overall structure, I'd call the system a moment frame. Really, the knee braces are nothing more than obtrusive -- and convenient -- ways to moment connect beams and columns.

I would only use this system if designed for low ductility (conventional construction). In a high seismic, high ductility application, the system would be rife with difficult to address buckling potential.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Barbaneagra]

Thank you.
My problem is that these are wood beams and columns. There is no wood moment frame in Table 12.2.1. I just spoke to a plancheck engineer who says "use R = 1, you can't go wrong" but then I get seismic loads that are too big for my wood connections. The wood becomes too beefy, then my structure looks ridiculous, and so do I.

 

[jvvse]

If you use R=1 then you don't have to follow any seismic code. I've used that a few times for nonstructural stuff.

There is no such thing as a wood moment frame. Wood members cannot transfer moments like wood or steel. Can each member be considered pinned and so it acts like a truss? The knee braces can transfer the lateral load into the column and put bending on the column, but as a point load and not a moment connection.

 

[Barbaneagra]

Of course. Thank you, for small things does not matter, and for heavier structures I'll just use steel most likely. but thank you for taking up the question.

 

[Bagman2524]

I don't think a seismic analysis is required for a structure this small. See the exceptions in ASCE 7section 9.5.2.5 (Analysis Procedures. Buildings for light framed construction of 3 stories or less and buildings of any construction 2 stories or less are exempted from seismic analysis.
We recently designed a stair for a 1 story building and used R=1.0. Client requested seismic analysis.

"Look for 3 things in a person intelligence, energy and integrity. If they don't have the last one, don't even bother with the first 2. W. Buffet

 

[JoshPlumSE]

Why not use item G6 (Timber Frames) from that table and follow the detailing requirements of section 14.5? That's still got a low R (1.5). But, that's significantly better than 1.0.

 

[Barbaneagra]

That's what I used to use, but now it appears that ASCE dropped it... we are in ASCE/SEI7-10.

Thank you for looking into it.

 

[JoshPlumSE]

It's still in the version of ASCE7-10 that I'm looking at. . . . . I suppose it could have been removed in a supplement or something.

 

[msquared48]

Me too - ASCE/SEI 7-10, table 12.2-1 on page 77. Item G6. R = 1.5.

However, it is not listed in table 12.14-1 in the Simplified Design Procedure section on page s 101 and 102.

Mike McCann
MMC Engineering

 

[Barbaneagra]

Of course it's there (now that I'm looking at it). I must have fallen from bed the other day.
Sometimes I think, I work too much.
Mike's comment is interesting. I wonder why that is. But either way, at least I have something to point to and 1.5 is somewhat better than 1.

Thank you.

 

[msquared48]

Well, the way I read it is that Simplified is allowed under 12.14 for simple bearing wall or building frame systems, what would be normal for a residence or very light commercial. The knee brace system could be used if computed under this section.

However, under section 12.6, all other structures must be designed by one of the methods outlined in Table 12.6-1. which use Table 12.2-1 for the building structural system description.



Mike McCann
MMC Engineering