AISC Column Splice Location Requirement 1

[bookowski]

341-16 D2-5a states that column splices shall be located 4ft above the beam/column connection. I am not asking why this is useful for erection, for safety, etc. - I get those. I am asking if anyone knows of any commentary or discussion that allows splices at other locations, in particular just above the floor level. This is for gravity only columns, not columns part of the lfrs. D2-5a specifically adds "including those not part of the sfrs" and the scope section of 341-16 states that it applies to components "of the sfrs & gravity column splices and bases" so it's clear that they intend this for all columns. I get why it would be a requirement in the sfrs.

I've got a client/condition where they essentially want to stack floor by floor. I know it's not typical, I know osha etc. But what I am trying to figure out is if there's any exception that would allow this. The commentary to D2 doesn't add much, it states that this location reduces the effect of flexure.

 

[JAE]

I thought the 4 ft. was totally in response to OSHA and not some steel analysis/design issue.

 

[dik]

It likely reflects the location of the point of inflection for the column as well as creating a comfortable working location for installing the splice bolts, or welding.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

Well column inflection points are mostly near the mid-height for moment frame buildings - not 4 ft. off the floor, right?

 

[dik]

yup... at maybe 6', but try to work on a splice at 6'... it's a challenge. I should have said likely in an area of small moment, not zero moment.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[bones206]

I assume you aren’t able to use any general exceptions that would permit you to avoid designing to AISC 341 in the first place.

There is exception b) to D2.5a that allows for splices just above floor level:

“(b) Column splices with webs and flanges joined by complete-joint-penetration groove welds are permitted to be located closer to the beam-to-column flange connections, but not less than the depth of the column.”

So good luck to the welder who has to lay on their belly for days at a time making these CJP splice connections…

 

[lexpatrie]

To me the four feet also has to do with the convenience of placing fall protection against them or attached (temporarily) to them so it's more than just a seismic requirement. That would be the OSHA portion of it, there are ways around that with alternative systems of course.

I'd talk to a fabricator or an erector on this, but I suspect the 4 feet is typical more or less globally.

A floor level splice would generally put it in an area of maximum moment for a sway frame and would be undesired. On taller buildings the splices are not every floor, either, more like 2 to 3 stories, as I recall.

 

[bones206]

This is a pretty excellent seminar on splice design:

https://youtu.be/aVtV6LWfoMg?feature=shared

A fabricator/erector who presented recommended 5 ft splice height to clear interference between the safety cables and the splice connection plates.

 

[bobbyboucher]

Deformation compatibility for seismic drift. See commentary D3. This is why the CJP alternate is permitted for splices <4ft.

 

[JoshPlumSE]

To me, this portion of the code is pretty clear:
For all building columns, including those not designed as part of the SFRS, column splices shll be located 4ft or more from the beam to column flange connection.

Exceptions:
(a) when the column clear height between beam to column flange connections the splice shall be at half the clear height.
(b) column splices with webs and flanges joined by CJP groove welds are permitted to be closer to the beam to column flange connection, but not less than the depth of the column.
(c) splices in composite connections.

In addition, the commentary says the following:

This requirement is not intended to apply at columns that begin at a floor level (i.e. transfer columns), or columns that are interrupted at floor levels by cantilevered beams. However, the splice connection STRENGTH requirements of D2.5 still apply.

I'd argue that for some situation that I've seen (industrial, not buildings) where you have "modular" construction and you are stacking one module on top of another. In these case, I believe it is common that some of the columns are interrupted at each level.

 

[bookowski]

I'd argue that for some situation that I've seen (industrial, not buildings) where you have "modular" construction and you are stacking one module on top of another. In these case, I believe it is common that some of the columns are interrupted at each level.

@Josh - Yes, this is what I'm asking about. When you say "I'd argue" - what is the argument? This is being done frequently in modular. There are some very big steel buildings, for example atlantic yards in brooklyn @34 stories, citizen m hotel in manhattan @20 stories, and many others. I have looked into the design of these and they are designed as steel buildings per aisc (at least that's the claim). In all cases the columns act as columns, i.e. carry vertical loads and in some cases lateral loads, and are always spliced at each floor level. I guess this is a generous reading of the exception for columns "that begin at a floor level"?

 

[JoshPlumSE]

Well, in the cases I'm thinking of, it was an industrial structure not a building, so you have some extra leeway with any code sections related to buildings. Plus, many of these were designed with an R=3 value.

For buildings, I would think this is something you might have to discuss with the jurisdiction ahead of time. A number of things could help:

a) Low ductility requirements and / or low seismic zones.

b) You make it clear that you will abide by the splice strength requirements of that section.

c) For any interior columns, where the beams are continuous, there really isn't a question. This is basically the same as the cantilever exception.

d) For exterior columns, if the building department isn't buying your argument, you could have stub columns that extend up 4 feet and do a field splice for them. But, I would point out that this would be more problematic for worker safety and that you'd want to ask for a letter from the jurisdiction saying that your erectors safety concerns have been acknowledged, but that they believe this splice location is more important and overrides your engineering judgement on this issue.

 

[Deker]

Don’t think you’ll find anything codified outside of the exception bones mentioned. But, OSHA concerns aside, I think you can satisfy the intent of the code by detailing a connection that will accommodate the expected seismic drift of the building. This might involve capping the connection demand at the strength of a ductile element.

 

[bookowski]

@Deker - Yes, that makes sense in general. I'm looking in particular at modular construction and how it satisfies this requirement. It seems clear that it does not but I'm digging around to see if it's that simple, i.e. they just ignore it on the basis that there's a lot going on here that makes the spirit of that requirement not applicable, or there was some codified exception. Until now I always thought that the 4ft was a recommendation not a requirement so I was hoping someone would note an exception. As an fyi, this is an example of the modular I am talking about. The vertical connection between stacked modules is a single bolt, very minimal. There's a slideshow you can click through, pretty neat.

https://www.skystone.com/citizenm-bowery

 

[bones206]

There are some very big steel buildings, for example atlantic yards in brooklyn @34 stories, citizen m hotel in manhattan @20 stories, and many others.

I looked up atlantic yards and it's kind of a new approach of modular construction. combined with a traditional structural steel podium and braced frames. I'm not sure what code path the design team would have followed for this, but it's pretty novel. Almost like a giant storage rack system for human dwelling units. Construction photos here:

http://fieldcondition.com/blog/2014/4/11/atlantic-yards-b2-april-10-2014

 

[bookowski]

@Bones - That was novel(ish) at the time but has since been replicated quite a bit. Often they go ahead with concrete cores and use those in place of braced frames. Either way the modular columns still act as columns, and are governed by aisc. I have quite a bit of info and drawings on that project and others. The connections between adjacent modules are typically done on top and consist of plates and various diaphragm type connectors, but vertically there is often a very minimal connection such as s single bolt.

 

[bones206]

I guess if it was my project, I’d take the R = 3 approach and hope the AHJ bought off on it. What do the drawings indicate for seismic resisting systems?

 

[Deker]

@Deker - Yes, that makes sense in general. I'm looking in particular at modular construction and how it satisfies this requirement. It seems clear that it does not but I'm digging around to see if it's that simple, i.e. they just ignore it on the basis that there's a lot going on here that makes the spirit of that requirement not applicable, or there was some codified exception. Until now I always thought that the 4ft was a recommendation not a requirement so I was hoping someone would note an exception. As an fyi, this is an example of the modular I am talking about. The vertical connection between stacked modules is a single bolt, very minimal. There's a slideshow you can click through, pretty neat.

https://www.skystone.com/citizenm-bowery

That is pretty neat. For modular construction with a single bolt at each corner, perhaps they are considering frame action of the module resolving the column base moments into T/C couples at the corners. With tight drift control on the SFRS, I'd guess that it wouldn't be too difficult to design the frame and corner connections to remain elastic at the imposed drift.

Perhaps you could also argue that these should be characterized as column bases, and with single bolt connections not providing flexural continuity of the columns, there is no engineering basis for shifting the connections away from the floor level.