HSS column splice

[Veer007]

Hey guys, do you have any best idea about field weld splice connection for HSS column, also advise how to calculate its full splice capacity, suggestions that may conclude CISC would be best.

Thanks in advance!!

 

[sandman21]

Is the number of cycles in seismic territorium really that high to cause such a concern?

Yes, the Northridge earthquake while not a design level event nearly had building failures from non-brittle failure modes in modern, for the time, moment frame buildings. After the earthquake the failures led to changes in the type, size and detailing for moment frames.

 

[kingnero]

Thanks, didn't know it was such a concern. I should read up on it then, as my work sometimes overlaps with structural engineers, and I work (but don't live) in regions where earthquakes can happen. Haven't had the pleasure to detail connections for seismic work, though.

 

[r13]

kingnero,

Very well. Glad you come back with valuable information. I leave it to our practicing guys to judge.

The excerption I quoted was from a tubular steel research/promotion institute, who on the paper simply pointing out the benefits of PJP weld over CJP weld on structural tubes, from strength and economy point of view, and was coherent with our codes/standards, also adherent to the topic in discussion.

 

[Veer007]

Hey guys thanks for your valuable info, finally my design team choose to provide pjp+fillet weld around.

Thanks in advance!!

 

[KootK]

How do you add a reinforcing fillet weld in this situation? The columns are the same size and there are no mated cap/base plates at the connection, correct?

 

[kingnero]

Backing bars can create areas with stress risers. This can become a problem with fatigue type (seismic) loading, causing crack initiation.

I have no experience with seismic detailing. Is the number of cycles in seismic territorium really that high to cause such a concern? I'm talking properly welded connections with backing bars, not shoddy work that shouldn't have been executed in the first place.

Yes, the Northridge earthquake while not a design level event nearly had building failures from non-brittle failure modes in modern, for the time, moment frame buildings. After the earthquake the failures led to changes in the type, size and detailing for moment frames.

I can hardly imagine that a similar weld will cause problems in seismic conditions. I understand that there are stress risers due to the geometry of the assembly, however the backing bar will not (hardly) transfer any forces, so all of the forces (or the vast majority of them) will go through the main member. This causes no reason to initiate cracking at the (low-stress) interface between main members and backing bar.
For such a weld, I would say that corrosion is the biggest issue, or fatigue if applicable. Can somebody positively confirm that this weld type is problematic in seismic territorium? Or at least more problematic than the same weld without metal backing and with a correct weld reinforcement? Because I am not convinced that this is the case.

I have to admit I am not familiar with the mechanics and failure mode(s) of seismic, I'm asking this to learn something, not just to stir things up.

 

[r13]

kingnero,

3.3.2.6 Weld Backing, Weld Tabs, and Other Welding Details
Weld backing and runoff tabs should be removed from CJP flange welds, unless otherwise noted in the connection prequalification or demonstrated as not required by project-specific qualification testing. Refer to FEMA-353, Recommended Specifications and Quality Assurance Guidelines for Steel Moment-Frame Construction for Seismic Applications, for special requirements for weld backing, weld tabs and other welding details for moment-frame joints.

Commentary: It was originally hypothesized, following the 1994 Northridge earthquake that weld backing created an effective crack equal to the thickness of the backing and that this phenomena was responsible for many of the fractures that had occurred. Finite element analyses of welded joints (Chi, et. al., 1997) have shown that although the backing does create some notch effect, far more significant is the fact that when backing is left in place, it obscures effective detection of significant flaws that may exist at the weld root. These flaws represent a significantly more severe notch condition than does the backing itself.

It is recommended that backing be removed from beam bottom-flange joints, to allow identification and correction of weld root flaws. This is not required for top-flange joints because the stress condition at the root of the top flange weld is less critical and less likely to result in initiation of fracture, even if some weld root flaws are present. Also, as a result of the more favorable position, it is far less likely that significant flaws will be incorporated in top-flange joints. Special welding of backing for top-flange welds is recommended.

Weld tabs represent another source of potential discontinuity at the critical weld location. Additionally, the weld within the weld tab length is likely to be of lower quality and more prone to flaws than the body of the weld. Flaws in the runoff tab area can create stress concentrations and crack starters and for this reason their removal is recommended. It is important that the process of removal of the weld tabs not be, of itself, a cause of further stress concentrations, and therefore, FEMA-353 requires that the workmanship result in smooth surfaces, free of defects.

 

[wannabeSE]

Kingnero,
To learn about the problem with the backing and special (highly ductile) moment frame connections, do a quick search with keyword "Pre Northridge Moment Frames." Moment frame cracking discovered after the Northridge earthquake was not anticipated. This prompted extensive research and testing to find the cause and prevent future failures.

The current requirements for the USA are found in AISC 341, Seismic Provisions for Structural Steel Buildings, and AISC 358, Prequalified Connections for Special and . . . Both can be downloaded for free on AISCs website. In some place, the backing can remain (column splices). In other places, if the backing remains, it must be welded to in place with a 5/16" fillet weld (special moment frame top flanges). And in some places, the backing must be removed and the root pass backgouged and backwelded with a reinforcing fillet (special moment frame bottom flanges). Here is a article that provides an overview of the code requirements

https://www.aisc.org/globalassets/m...takeamomenttoconsiderthismomentconnection.pdf

 

[kingnero]

Thanks for the additional information. I'll read up on this.
On first glance, it does appear that the backing itself is not as detrimental as I was lead to believe. It does make it harder to find flaws, but other sources also clearly mention sub-par welds in the Northridge disaster... I guess I have some reading to do.

 

[Veer007]

How do you add a reinforcing fillet weld in this situation? The columns are the same size and there are no mated cap/base plates at the connection, correct?

No, i have a butt plate which welded on top of HSS..1/2" larger than HSS size

Thanks in advance!!

 

[dik]

you may want to consider overdesigning the column a tad so you don't require CJP welds. and you may want to change column size just above the second floor.

Dik