How to calculate number of bays according to ASCE? 3

[Rosebay]

Hi guys, a fresh Structual engineer. I am confused to define the number of bays in ASCE 7-05 section 12.3.4.2.

A concrete shear wall structure. If the story height is 12 ft and a continue concrete shear wall length is 24ft, May I look it as a 2 bays shear wall since the ratio of length/height equals 24’/12’=2? Thanks,

 

[HTURKAK]

I copy and pasted the subject requirement ;


In your case if the length of perimeter shear wall is 24 ft and storey ht is 12 ft, yes .. You may assume two bays..
In order to avoid confusion, you may post the structural plan and get better responds..

 

[Rosebay]

Thank you HTURKAK!

If the structure is steel brace whose length is also 24 ft. May I also think it’s a 2 bays steel brace? This confused me because in my opinion, a steel brace bay is the space between two columns with one beam. Thus in this steel brace case, the bay should always be one as long as there are only two columns, no matter how long the steel brace length is.

 

[jayrod12]

I would agree with that assessment regarding braced bays. As indicated in the requirement posted by HTURKAK, that clause is only pertaining to shear walls.

 

[Rosebay]

Got it! The ratio of length to height is only applied to calculate shear wall. Thank you Jayrod12!

 

[KootK]

You've already gotten the correct answer on this but you may enjoy a little additional commentary.

The provision regarding the number of bays is intended to reflect the degree of redundancy along the framing line I believe. If you have a single steel braced bay, then there's no redundancy regardless of the size of that braced bay. You don't get any more redundancy until you add additional, physical braces. The difficulty then comes in how to apply that same concept to a shear wall structure fairly. Is a 60' long shear wall still non-redundant? Probably not. So we do this length / story height thing in an attempt to capture the redundancy aspect in shear walls in a way that is -- hopefully -- consistent with what we do with braced frames. I sometimes speculate that the form of the calculation for concrete walls is meant to be reflective of anticipated, 45 degree shear crack formation in the walls but, again, that's just speculation on my part.

 

[JoshPlumSE]

I sometimes speculate that the form of the calculation for concrete walls is meant to be reflective of anticipated, 45 degree shear crack formation in the walls but, again, that's just speculation on my part.

I had a conversation with my old boss (from my days at Fluor Daniel) about this some years ago. He was the chairman of the SEAoC Seismology committee back in the 1997 UBC days when they came up with this redundancy concept for shear walls.

You're essentially correct. They tried to come up with an idea to capture the "redundancy" of a single long shear wall. And, the 1997 UBC rules are what they came up with. It's not inherently analytical, but what happens when a group of engineers gets together and says, "We know what to do for braced frames and moment frames, but what makes sense for shear walls?" They gave it there best shot (essentially based on a 45 degree angle shear crack), and decided that the future code writers could improve upon it if they had the time and money to do more research. The fact that we're 25 years in the future and we've got essentially the same formulation is a testament to how wise (or lucky) the SEAoC seismology committee was back in the 1990s.

 

[JoshPlumSE]

I should point out that he was a little "sheepish" at the time about it. "That's what we came up with and it seems to work".... Big smile and a shoulder shrug.

 

[KootK]

@JP: thank you very much the commentary that you added with regard to the genesis of these provisions. It feels great to have a hunch validated and to learn something new. I hope that Santa takes extra good care of you this year.