AISC Seismic Provisions 341-05

[ARLORD]

In the AISC Seismic Provisions-341-05, pages 3-66 to 3-69,Example 3.11 SCBF Brace-to-Beam/Column Connection Design, they are using the amplified building code loads and not the expected yield load. Where in the specification does it indicate to use amplified loads for SCBF connection design.

For OCBF, section 14.4 clearly indicates when to use the amplified loads, but I do not see a similar indication in Section 13 for SCBF.

Also, in Example 3.12, pg 3-96, Rub=4.9k, from building code Load Combination, not the expected yield load.

Why was amplified loads used in Examples 3.11 and 3.12, and not the expected yield load per section 13.3.

 

[Mcgill]

I agreed with your comments. Design should be based on brace yielding load ie-RyFyAg.

I don't have seismic design experience to confirm this ,so I am expecting some experienced person to verify it.

 

[ARLORD]

Just to clarify my question, in the above example, of a Brace & Beam connection to a column, the expected yield loads were used for the Brace connection forces, but the amplified axial load was used for the beam axial force. I would think that the expected yield axial load should have been used for the Beam force as well. But what do I know, I am just an east coast guy preparing for the SE II Exam.

 

[kevo_55]

ARLORD,

The seismic design example is incorrect in calculating the required force on the brace connection. It should be based on the brace itself, and not the amplified seismic load. I'm looking at my "2006 IBC Structural/Seismic Design Manual" volume 3 (linky) and it's an example of a SCBF. They use RyFyAg for their force.

As for your beam question in the SCBF system, I must admit that I don't have my AISC seismic design manual at home so I can't help too much about your question. But I do know that beams are basically collector elements in BF systems so they should be analyzed with an amplified seismic load. Of course, depending on the type of SCBF (zipper, V bracing, x bracing, etc) you might have special requirements. I'll take a look with respect to this questions in detail tomorrow.

I hope this helps!

 

[ARLORD]

kevo_55

For SCBF's the connection design should be based on the Expected Yield Load, per Section 13.3 of the Seismic Provisions, therefore the example is correct regarding the Brace load. However the Beam, is also in tension/compresion along with gravity loads, but the example used an Amplified load with an Overstrength Factor. I do not see anywhere in the spec for SCBF connections where it directs you to use an Amplified Overstrength Load. It does direct you to use the Overstrength Load for OCBF's in Section 14.4. At first I thought that since the Beam seismic load was combined with gravity loads that the Overstrength Load was used instead of the Expected Load, but the spec does not state this. I don't think you combine the Expected Load with any other loads. I think this is where I am confused.

 

[kevo_55]

ARLORD,

I think we're in agreement in your issue #1.

As for your issue #2, it does not say what loads to take into account for designing a beam in a SCBF system in AISC 341-05. In lateral force design you must "learn by doing." As I had said in my previous comment, beams in BF system are basically drag struts. Per ASCE 7-05: 12.10.2.1, you'll have to use the overstrength factors for most drag struts in seismic design category C or higher. The AISC 341-05 will give requirements on some portions of the frame system but will be mute on others. The best way to learn why things are done in lateral force design is to do problems and try to figure out why.

I hope this helps!

 

[ARLORD]

Yes kevo_55,

There it is. That addresses the drag forces being amplified for collector elements. The beam is not a brace, but a collector element.

Another question, I remember reading somewhere that shear loads should be amplified, but I do not remember where. In the same example, the gravity shear for the beam in the brace connection was amplified. Does that ring a bell to you.

Thanks for your help.

 

[kevo_55]

ARLORD,

Take a look at ASCE 7-05 section 12.4.3.2 or 12.4.2.3. Are you talking about the seismic component of the dead load? If so, this is never amplified. If, for some reason, a lateral force generates a shear on a beam it may certainly be amplified.

Of course, there will always be a seismic component to the dead load as you can clearly see in these load cases.

I hope that I'm getting your question right.

 

[Mcgill]

As we are discussing about seismic manual on this thread, I am adding 2 more questions that I encountered while going through this manual.

1) Weld size: Example 3.10 (AISC seismic manual)

The thickness of the brace is 0.34 in (pipe 10 std.) and they are using 3/8 weld (0.375” > t) for the brace to the gusset connection. Here the question is - can we use fillet size greater than the thickness of joined members? Or something else is going here.

2)Leaning column amplifier:

The seismic manual example (Chapter 4 – Moment frame) uses the leaning column amplifier factors for Kx (unbraced length factor for compression), which drastically increase the value of Kx. The question here is: Is that a common practice in seismic design field? Are we supposed to use that factor in SE II exam? This seems to me as an engineering judgment and depends on the designer. However, want to know others opinions on it and a ground for common agreement.

thanks

 

[kevo_55]

Mcgill,

Well, atleast I have one easy answer for you!

1.) There is no real "maximum" weld size. So, a 3/8" fillet is ok on a 0.34" wall tube. The main problem is checking the shear in the material itself. If using A36 steel in this case, you might have to do a few more side checks. But, since it is Fy=42 ksi, you should be ok.

Another problem with using huge welds is the possibility of "blowing through" the patent material. This is more of an AWS issue and if you're using AWS certified welders this shouldn't be a problem.

2.) This is the hard one. I believe that you're looking on page 4-9. Since this is a sway frame, you technically have to use the leaning column amplifier. This should have been introduced in school so I hope this isn't a new thing for you. The only thing you have to know is that MF's are sway frames and hence a few other things need to happen when calculating your K's.

If you had a computer model do most of the work for you in this example, you won't really have to do this though.

I hope this helps!!!

 

[Mcgill]

1) I am still not clear, eventhough, you considered it as easy one, Actually my concern is about specification limitation J.2b-b, where it says max weld size is t-1/16. How you can figure out it is not applicable for this case.

 

[kevo_55]

McGill,

I think you're reading into the code a bit too much. Look at the commentary on page 16.1-331 to 16.1-332 of the 13th edition steel manual.

The maximum size of the fillet weld was based on a fillet weld "holding" onto the thickness of the parent material itself. If a larger fillet weld is specified than the thickness, than the weld couldn't physically be done. If one is making a T-joint with a pipe if infinite length, than the "thickness limitation" isn't really valid.

To go on any further, is there a specific area in the seismic design manual that I should be looking at?

 

[Mcgill]

I got it. Yes, it is really easy. Thanks for elaborating clearly.

 

[ARLORD]

Leaning Columns was introduced with the first LRFD manual. In the past, it should have been used when you

use LRFD. Now that the two ASD and LRFD have been combined into one maual, I think it should be used

at all times with sway moment frames, when it applies.

As far as the max weld size, see page 8-17, 13th edition, "Excessive Welding" for an interesting take on the subject.

 

[kevo_55]

^^ Nice find!

 

[Greenaqua]

I may be missing something. My book appears to be designing example 3.11 p. 3-65 with the correct forces. The tension force used in the design is 622 kips and the compression force used is 631 kips.

I know that some of the older printings of this book were poor and possibly I have a newer printing?

My book is the second printing dated October 2006.

Regards,

Greg

 

[ARLORD]

Greg,

Yes, I also have the second printing. My initial question regarding example 3.11 was regarding the beam forces not the brace forces. True, the brace forces on page 3-67 are the expected yield, however, on the same page, Ru=21.3k, the factored beam gravity load is amplified. Also, on page 3-69, Hb =36k is amplified also.

The beam is a collector member, collector members are amplified per ASCE 7 CH 12.

 

[Greenaqua]

You are correct sir.

I didn't read the thread thoroughly.

Sorry about that.