HELP! Lateral Bracing in K, V, or inverted-V in OCBF

[Lungshen]

Ref: AISC Seismic Design Manual 3rd Print

Seismic Provision 14.2 on page 6.1-48 States:

"Bracing Members in K, V, or inverted -V configurations shall have KL/r <= 4 (square root)(E/Fy)

For HSS memberswith Fy of 46ksi and E of 29,000 ksi, this is about 100. This greatly increase the size of the HSS bracing .

My question for you fellow structural engineers are:

1. If the wind load governs your lateral design at this braced frame, Does the brace still need to comply with this "Seismic Provision"? In other words, does this provision only apply when your Seismic Load governs the lateral design?

Typically I would design the brace just like a column member with KL/r <=200 when wind load governs

2. This seismic provision excludes diagonal "D" bracing - so does that mean the bracing can use KL/r if it is a diagonal braced frame?

Thanks in advance,

 

[Andy Lin]

1. If the wind load governs your lateral design at this braced frame, Does the brace still need to comply with this "Seismic Provision"? In other words, does this provision only apply when your Seismic Load governs the lateral design?

If you are using R = 3.25 (response modification coefficient) and are calling out the seismic force-resisting system = OCBF (Ordinary concentrically braced frames), then the answer is "yes", the "seismic provision" needs to be applied even if wind load governs.

Now if you are using R = 3 (or less) per ASCE 7 Table 12.2-1 Section H: "Steel system not specifically detailed for seismic resistance, excluding cantilever column system" and are not calling out OCBF on the drawing, then "seismic provision" need not be applied. You can also read the AISC Seismic Provision Part I: 1 - Scope.

2. This seismic provision excludes diagonal "D" bracing - so does that mean the bracing can use KL/r if it is a diagonal braced frame?

I think your interpretation is correct. They sort of explained in the commentary the reasons why this requirement applies to these configurations.

Hope this helps

 

[TehMightyEngineer]

I'll second SEHQ, if you determine your seismic forces used an R > 3 with a steel LFRS then you need to design the steel LFRS to obtain the R used in the design regardless of the controlling load. Thus, using a lower R value can save a lot of money on a project if you have low seismic loads.

 

[TJM]

I'll third SEHQ, is that a thing?? You guys are on it in here.

 

[Andy Lin]

I'll third SEHQ, is that a thing?? You guys are on it in here.

SEHQ is just an abbreviation for my website

 

[TJM]

I'll third SEHQ, is that a thing?? You guys are on it in here.

SEHQ is just an abbreviation for my website

I just checked out your website. It's awesome! I especially wish I would have seen it before I took the exam, haha!

 

[darius]

1. If the wind load governs your lateral design at this braced frame, Does the brace still need to comply with this "Seismic Provision"? In other words, does this provision only apply when your Seismic Load governs the lateral design?

If you are using R = 3.25 (response modification coefficient) and are calling out the seismic force-resisting system = OCBF (Ordinary concentrically braced frames), then the answer is "yes", the "seismic provision" needs to be applied even if wind load governs.

Now if you are using R = 3 (or less) per ASCE 7 Table 12.2-1 Section H: "Steel system not specifically detailed for seismic resistance, excluding cantilever column system" and are not calling out OCBF on the drawing, then "seismic provision" need not be applied. You can also read the AISC Seismic Provision Part I: 1 - Scope.

2. This seismic provision excludes diagonal "D" bracing - so does that mean the bracing can use KL/r if it is a diagonal braced frame?

I think your interpretation is correct. They sort of explained in the commentary the reasons why this requirement applies to these configurations.

Hope this helps

SEHQ,

I just want to double check, what you said in regards to R=3, does not need to comply to seismic provisions, it is applicable to SDC: A, B, C. I am not sure if you can apply to SDC "D" or higher.

Any thoughts?

( I might be able to find it in Seismic provisions 341-10)

 

[Andy Lin]

1. If the wind load governs your lateral design at this braced frame, Does the brace still need to comply with this "Seismic Provision"? In other words, does this provision only apply when your Seismic Load governs the lateral design?

If you are using R = 3.25 (response modification coefficient) and are calling out the seismic force-resisting system = OCBF (Ordinary concentrically braced frames), then the answer is "yes", the "seismic provision" needs to be applied even if wind load governs.

Now if you are using R = 3 (or less) per ASCE 7 Table 12.2-1 Section H: "Steel system not specifically detailed for seismic resistance, excluding cantilever column system" and are not calling out OCBF on the drawing, then "seismic provision" need not be applied. You can also read the AISC Seismic Provision Part I: 1 - Scope.

2. This seismic provision excludes diagonal "D" bracing - so does that mean the bracing can use KL/r if it is a diagonal braced frame?

I think your interpretation is correct. They sort of explained in the commentary the reasons why this requirement applies to these configurations.

Hope this helps

SEHQ,

I just want to double check, what you said in regards to R=3, does not need to comply to seismic provisions, it is applicable to SDC: A, B, C. I am not sure if you can apply to SDC "D" or higher.

Any thoughts?

( I might be able to find it in Seismic provisions 341-10)

Darius you are correct, it can only be used for SDC A, B and C.

You can tell by looking at the same table (ASCE 7 Table 12.2-1 Section H) where D, E, F is noted us "NP" which stands for "not permitted" in the footnote.

Thanks for pointing that out!

 

[Lungshen]

Hi guys, thank you for all your help and input. It's been very educating and refresh my memory from studying the seismic design manual.

And Andy I think you nailed my question right on the spot.

First I thought this project was in SDC C and thus thought that I can get away from meeting the seismic provision, but Unfortunately there is a fluke in the geo-report and turned out this project is in Seismic Design Category D when I run my double check. It is the first SDC D project that I run into with inverted V bracing frame.

Architect is trying to put this braced frame in a 6" common wall between the tenant, because of the geometry, height and needing to meet Seismic Provision Section 14.2, this brace need to go to HSS8x8 and Mr. Architect don't like it at all and will not change the wall thickness to hide the brace. So that leaves me A. reconfigure the brace to limit the slenderness ratio to meet section 14.2, by reducing slenderness or go with a diagonal D bracing instead of inverted V. I am just hesitatant to do that at the very late stage of the production or B, to see if I can get away from meeting the requirement if the seismic don't control the design. But I want to make sure that my interpretation of the code is not biased and not missing anything.

I am located in SDC C and I can say that we (and other local structural engineers) never check that inverted V brace for Section 14.2 in SDC C. From some of the local construction photos I have seen, those braces look like tooth picks...and I am certain they do not comply to seismic provision 14.2