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What is 2t plastic hinge gusset plate design?

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Dear Mr. Venkatesh: Gusset plates were a major part of my doctoral dissertation at University of Michigan in Ann Arbor about 25 years ago and since then I have continued to do research on them, design them and teach their behavior and design in my courses. Prior to that since 1968, I have been designing gusset plates in my practice of structural engineering (full time for 10 years and then as part time designer of structures of buildings and bridges. In fact the 2t-4t and double plastic hinge design of gusset plate resulted from my doctoral dissertation done under supervision and guidance of two of Professor Subhash Goel and Professor Robert D. Hanson, two of the most knowledgeable people about structural behavior and design that I have been lucky to ever come in contact with, to learn from and have them as my role model.

Reading your e-mail prompted me to set aside my reconnaissance work here in Minnesota on the DeSoto bridge damage (http://minnesota.publicradio.org/display/web/2008/03/26/desoto/) and try to provide you with as much information and help as I can. If you still have questions or comments, please feel free to contact me. In the following, I have made comments on your question and the response by Mr. Madden, and Mr. Matchos which are very good but may be my clarification will make their comments even more useful. My comments are inside brackets [AA:.....] instead of separately responding to each.
If you like to send me your questions directly , I always welcome such questions as well and try to help.
Best wishes Abolhassan Astaneh-Asl, Ph.D., P.E., Professor Department of Civil and Env. Engineering and Center for Information Technology in the Service of Society
781 Davis Hall
University of California
Berkeley, CA 94720-1710

======================================

From: "Gerard Madden, SE" <gmse4603(--nospam--at)gmail.com>
To: seaint(--nospam--at)seaint.org
Subject: Re: What is 2t plastic hinge gusset plate design?


Vish,

If you can get your hands on the SEAONC seismic design manual vol III, it
has an example for SCBF's.

the 2T offset is for HSS braces. For WF braces, you may be able to avoid
this depending on which way your brace is rotated about it's axis.

[ AA:  Dear Mr. Maddn: Thank you for your very valuable comments.  I would say that 2t offset gusset plates are for braces that buckle out of plane of gusset plate. Since K value for out of plane buckling is 1.0 and for in plane buckling is 0.65 (in buildings it is more or less 0.5) when you use brace sections that are square, with same rx and ry (radius of gyration) they end up buckling out of plane resulting in formation of plastic hinges in the gusset requiring 2t-4t offset to allow the plastic hinge to rotate freely without fracture for many cycles. So, "2t offset is for HSS braces" is not necessarily the case for all HSS sections.  I agree totally with you on WF sections since for these section, if you place the section such that the web is perpendicular to the gusset plate and calculate KL/R for x-x and y-y axis, for most WF sections ( may be for all since I have not checked this) the governing KL/r will be with respect to y-y axis and the brace will buckle in the plane of the frame (the same as the plane of the gusset) and as a result, plastic hinges will form in the member and not in the gusset making the 2t-4t offset unnecessary since there is no plastic hinge in the gusset. Just FYI, if you use double angle bracing, if the long legs are back -to-back, the buckling will be out-of plane and if the short legs are back-to-back, the buckling will be in-plane.
For channel sections in the AISC Manual, almost always buckling is out-of plane unless you place the two channels far apart (say 6-10" apart and batten plate them) forcing buckling in plane. So, for all sections, it the question of In-Plane or OUT-of-Plane Buckling? The 2t-4t requirement is only for out of plane case . In fact for economical design we should not have 2t offset in gusset plates of in-plane buckling braces since the gusset will be long the welds , bolts connectors, will be longer , more numerous with nothing gained. In fact long gusset plates for in-plane buckling braces can result in introducing end rotational flexibility , increasing the K factor in KL/r thus decreasing the brace capacity an undesirable effect. You are paying more money for material, fabrication and erection and getting less capacity! End of AA's comment]

The two papers Michelle linked for you are good references. The 2T is now actually a range of 2T to 4T where T is the thickness of the gusset plate and the yield/buckling line is offset from the edge of the brace and a projection along the gusset from the frame column to the frame beam.

[AA: I am humbled to see that you have found our publications "good" For one of them, co-authored by me, Mr. Cochran  and Mr. Sabelli (the co-winner of this year's prestigious AISC T.R. Higgins Award) I have to give all the credit on what you found to be "good" to these good friend of mine and accept the responsibility for parts that are not as good. Best wishes and thank you for your input and very valuable comments. End of AA's comment ]

-gm

On Wed, Mar 26, 2008 at 4:57 AM, Michelle Motchos <mmotchos(--nospam--at)sw-sc.com>
wrote:


>  Vish,
>
>
>
> This provision is to allow for inelastic rotation at the joint in
> accordance with AISC-341 section 13.3b and is discussed further in the
> commentary with the recommendation of a 2T to 3T distance
>
> I also found the following two publications very helpful:
>
> http://www.steeltips.org/steeltips/tip_details.php?id=96
>
> http://www.steeltips.org/steeltips/tip_details.php?id=73
>
>
>
> Michelle Motchos, PE
> Stevens & Wilkinson of South Carolina, Inc.
>
> Columbia, SC
>
> [ Dear Mr. Motchos: Thank you for your comment regarding finding our reports ongusset plate helpful specially for the use of the word "helpful". My motto in my professional life specially in writitng design-oriented reports such as Steel TIPS is that if my work "helps" even only one enineer, I have been paid several times over for what I have done and if my works "helps saves one life or one injury" I have been paid for all the work that I can do for rest of my carrier.  Your use of word "helpful" particularly was very much appreciated and gave me such a feelingof satisfaction, Best wishes. ENd of AA's Comment.
>
>
>   ------------------------------
>
> *From:* G Vishwanath [mailto:gvshwnth(--nospam--at)yahoo.com]
> *Sent:* Wednesday, March 26, 2008 2:20 AM
> *To:* steel steel; seaint(--nospam--at)seaint.org
> *Subject:* What is 2t plastic hinge gusset plate design?
>
>
>
> We are detailing  a 600 ton  commercial 7 storyed building.
>
> Columns are braced (normal X bracing) with Tubular sections (HSS 7x7x3/8,
> 6x6x1/4,  5x5x1/4, 3 1/2x3 1/2x1/4 typically)
>
> No bracing forces are shown for us to detail the connections.
>
> We asked if we could use half the tension capacity of the HSS members.
>
>
>
> The engiener in his response has asked us to submit our proposals with
> calcs and he wil then decide.
>
> He also writes
>
> "connection of braces needs to show 2t plastic hinge gusset plate design"
>
>
> What does this mean?
>
> We normally  detail gussets using  3/8", 1/2",  5/8" or 3/4" thick plates
> depending on the forces unless the thickness is specified by the engineer.
>>
> Thanks in advance for any advice or oinions
>
> Regards
>
> Vish
>
[ AA: Dear Mr. Venkatesh: I hope the above comments and what you can find in the two Steel TIPS reports will be helpful. Again , if you have any othr question that you fel can help others as well, please post it, otherwise, if your question is very specific or if you will not comfortable to ask the question in public domain (which I am sure is not the case for you since I have seen a lot of good questions ( any questions on any subject on this page is a good question!) please e-mail me your questions on this topic or other topics that i feel qulified to answer. beleive me I will tell you right away if I am not qualified to answer and will try to refer you to some else who might have the answer. Best wishes.  End of AA's commnet]


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