Special Moment Frames 1

[Gillespie]

Does AISC358 require all moment frame connections (from the beam to the column) to occur on the column flange. ie... the beam can not be continuous over an interior frame column of a lower frame column.

 

[JoshPlumSE]

That's an interesting question. I'm surprised no one has answered.

I have done moment connections like this at a roof before. That was with the older code and with an Ordinary Frame. However, I don't recall anything in the code that would specifically prevent you from doing this with a SMRF.

I'm looking through the AISC seismic detailing specs this afternoon (for a totally un-related reason). If I find anything more I will let you know.

 

[haynewp]

Section 9.2b of AISC 341 states the 3 requirements for connections for SMF which is also referenced in section 10.2b for IMF. The 3 requirements as I read them are:
a) Using one of the prequalified connections designated in AISC 358, there is also a new supplement to 358 you can download from AISC.
b) Testing by AISC 341 Appendix P
c) Testing by AISC 341 Appendix S

 

[Gillespie]

Thank you for all your responses.
Upon further reading of AISC 341, 358 and Supplement with the newer approved connections, it is so clear that the code is demanding beam hinging to occur. Which would rationally desire the beam to be spliced at each column.

However, the inability to use more (numerous) columns over a continuous beam for the first story of a steel frame(in lightly loaded structures) is a code that essentially weakens my steel frame.

I wonder if I could detail the frame with rigid knees on
the interior columns and then neglect the columns during analysis?????

Note on AISC 358 and Supplement, the difficulty in using certain column and beams for each of the preapproved SMF connections.

 

[JoshPlumSE]

I agree that the code is requiring beam hinging.... But, why does that REQUIRE a splice.

To me, all it requires is that the connection between column and beam (and the panel zone) be strong enough to force the hinging to occur back in the beam. Haynewp's comment is a good one though. How different can your connection get from the pre-qualified connections before you're forbidden from using it without rigorous testing.

At my old company we used a non-prequalified connection with beam haunches to force the hinge point away from the beam-column connection. It took a lot of over design to get through plan check without the extra testing (which schedule would not allow).

 

[Gillespie]

In response to JoshPlum:
I would be required to splice so that the 'frame beam' connects to the 'column flange'. Let's also realize that
we are creating a connection which also needs to service loads (dead and live).

I wonder if any Engineers was have been required by Building Dept's. or plan checkers to detail a column so it is not part of the Seismic Load Resisting System.

I agree with Haynewp that only Preapproved connections can be used (without doing your own testing...not reasonable for a building without numerous use.) But none of the preapproved connections relate to a ground floor interior colun????

 

[ishvaaag]

I will add to my most times discordant viewpoint by again adding that by enforcing mandatory details the public bodies are repeating the error that showed in practice at Northridge, by mandating details already shown flawed in the technical literature. I can understand many may feel this may put public interest at risk and may be partly true but I think not as much as enforcing details. Plus, that is my view, this is the fair position if the structural designers are to (they) warrant designs; otherwise, they (public bodies) warrant designs, I mean, with their insurance and wealth.

 

[JoshPlumSE]

Detailing a column so that it is not part of the lateral load resisting system should be pretty easy.

Run the beam over the top and then use a bolted end plate connection, but with only two bolts on the inside of the column flanges. It's a pinned connection and cannot take any moment.

Now, if you force it to be a moment connection then that's a differnt story. But, as long as it's a pinned connection then you should be good.

 

[Gumpmaster]

ishvaaag,

Which of the current pre-approved AISC 358 details have been shown to be flawed in the technical literature? In what way are they flawed?

 

[ishvaaag]

I am not saying that, gumpmaster, I do not despise hardwork by many hardworking much illustrious colleagues, of many of whom I don't know not even 10% of what they know in the matter. However, some will be more illustrious than others, and Nature (no respecter of other codes than its own rules) eventually will show who they were.

Some may be surprised to know that to some extent we enjoy here, Spain, such standard, and we can design to suit our technical opinion as long we "prove" -normally analytically- this goes along proper treatment of the subject and allows for the same basic "target" standards addressed by the code. Normally it will be rare the approving body to make much fuss about as long it understands it has in front a competent designer. This might change, but I think must not.

And the point is clear, once licensed to practice, one officially knows what to do for what licensed, and if not, shouldn't be. And he must have the public interest as that of his client in mind, and if not, shouldn't be allowed to practice. Then he practices and warrants, but to simply fulfill standards is not design, is just submission to other minds that curiously then don't appear to take responsibility when the standard shows to be inadequate.

Many here understand that what Northridge meant to moment frames with mandatory details is somewhat what the past september meant to financial institutions: some very severe undermining on the faith of mandatory technical guide. To some, surely, this was not a surprise, for there are always those who, technically and not technically, are able to think either beyond or out of the box.

 

[sandman21]

Honestly I am having a hard time understanding what you are trying to say. First the details are flawed then they are no, then they are??

The prequalified SMF details came about after the Northridge earthquake. They were developed after examining the behavior of moment frames, and there failure at loads much less than they were expected to withstand. They have also been tested to demonstrate proper performance under loading. I don't see how these new tested connections are as likely to fail as pre-Northridge. A major way to improve a profession is to have it challenged, if approving agencies and other engineer are unwilling to challenge design ideas that is a sad state of affair in Spain.

 

[ishvaaag]

You really seem not to understand at least partly what I am saying, may be I am not expressing clearly or it is just my modest english language ability to show the intent of my entries. Or maybe you are not willing, I must also consider. First, I am not criticizing at all ANY technical guide for doing things well, be it past or present. Second, the challenge to ways of bad practice was not made with Nothridge cause in the moment frame connections, but prior to that by people thinking out of the box. The rest is, and not less than, hard work in their wake. So, if you say that we better follow what technical guide is available, I am in accord with you, say AISC 358. If you say this must be mandatory, I am not in accord with you, for I think more variety of practicable technical solutions sprang (and are challenged) at an ambient of technical freedom that at one of technical enforcement. In such light, having the freedom to diverge from the precise practical ways some code state as guide whilst meeting the true intent of the same code (safety, confort) I can never think is a sad state of affairs, but a much desirable one, and so have been thinking the legislative bodies here for at least decades that I know. I out of necessity am aware of many many things needed to be changed for the better where I live, but I can assure you this is nor one.

Furthermore, and this is very important, this acknowledgement is in fact necessary to reckon the limitations of code guide and the actual need of resource to the whole body of technical information when we do our work. Such is the case, we daily are presented with problems, solutions and design that are not properly portraited in the codes and yet we must advance (and warrant) our designs. So we resource of what everybody, mainly ourselves, know, and in doing so, we are not being incompetent or illegal, we are exercising the true core of our profssion, ensuring practical designs are delivered to wanted use. So to properly practice our professions we need to resource to that whole body of information that is NOT the code.

If you want some examples, this same morning I have been watching the online presentation of one NASCC conference in Phoenix where to engineers of Utah clearly showed how the written guide for base plates is not enough, and even if their solutions were restrained to biased elastic analysis came closer to the actual performance. Or you can think of small P-delta, what to do following the codes when in excess of the fabrication tolerances of 1/1500 or 1/1000, must we restrict ourselves to the amplified elastic calculation, must we restrain of designing then any arches for which the elements are most times to stay at bigger curvatures? In my view, mandatory technical codes must be the exception, and never be thought as the solution to our duties.