Thoughts on unusual beam-beam connection - original bottom flange to new top flange 2

[OffshoreWindStructures]

Hello Folks,

I was wondering what your general thoughts might be on an unusual connection that I've been asked to assess. I've attached an image and can clarify if anything is unclear. The section I am considering is a 20 year old extension to 50 year old structure. The extension consists of a propped beam/truss supporting vertical and horizontal loads.

The internal connections joining the members of the truss extension together are pretty standard but the connections joining the extension to the original steel (purple flanges in image) are unusual as they are an after-thought that have to fit around other elements. The bottom prop connection has an eccentric work point which seems unnecessary to me but is not as unusual as the hanger connection.

The top beam/cantilever has channel extensions to fit around existing objects before meeting the original steel. The hanger connection to the original steel consists of 4 no. bolts with no stiffners, blacking plates or additional support so it's not a full moment connection but I am wondering if it should be considered pinned or partially rigid. The rigidity of the connection will obviously effect how much it will be loaded so I don't think just assuming it as either pin or full moment connection is particularly useful.

I'm also unsure what the critical failure modes might be as there is a lack of guidance on similar connections in any literature I've read. Presumably the lack of similar connections is because it's not a great design, which is clear from first glance. I've been looking a yield line analysis guidance in AISC DG04 as I thought the unstiffened column flange yield line pattern in table 3.4 might be similar but I could be widely off base with that as I don't have significant experience in steel connection design.

Beyond that potential failure mode, I was also wondering if considering the connection as a t-stub in tension might be more valid (such as

https://www.ideastatica.com/support-center/bolted-connection-t-stub-in-tension).

I looked at eqn J10.1 of AISC 360-10 for local flange bending but I think that might only be suitable for single point loads rather than a series of point loads in a bolt group. There may be an applicable section of AISC 360 that I have missed.

Any thoughts or opinions on failure modes or how best to model the connection are welcome. Thanks in advance.

 

[dik]

...maybe I should have read the thread.

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

-Dik

 

[OffshoreWindStructures]

rb1957 : The new owner has different operational needs than the previous one and so the original analysis and design is not useful for this scenario. They want to determine a safe maximum load. your suggested alterations are sensible but beyond the scope of topic of the post. There will be no structural changes in the primary structure or extension truss. This information and more is available in earlier posts.

dik: I think with each comment the discussion has been drifting away from my original limited focus on potential failure modes and stiffness of the unusual top connection so it is understandable if the topic appears to be about improving a potential overall design rather than re-assessing a single connection of an existing structure.

 

[dik]

Concur...

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

-Dik

 

[ProgrammingPE]

Since you're transferring the tension load in the channel to the wide flange using a connection that only includes one of the channel's elements (its top flange), you're going to have some serious shear lag issues so will only be able to consider a fraction of the channel's area in tension. See AISC 360's Table D3.1 (Shear Lag Factors for Connections to Tension Members) along with the commentary for Section D3 (Effective Net Area).

The connection design itself would be similar to a bolted stiffened seated connection. Just imagine the connection rotated 90 degrees.

This means you need to design it for the eccentricity from the center of the channel to the face of the wide-flange's bottom flange. This will result in some of the bolts being loaded in tension which may result in prying action occurring. This will be like the t-stub example you linked to from IDEA StatiCa. AISC also covers this in detail in Part 9 of the Steel Construction Manual.

Like you said, the connection will be partially rigid but you'll have a hard time quantifying this without doing Finite Element Analysis, but you could always model it as fixed and design for the additional moment that comes from that. (Be sure to also check the entire system using a pinned connection as well.)

Structural Engineering Software:

http://www.structuralcentral.com/tools/steel-connection

Structural Engineering Videos:

http://www.youtube.com/structuralcentral

 

[rb1957]

I don't think you can analyze pt B as a pinned connection. I would model as fixed or (as many round these parts would say) partially fixed.
You could analyze pt B as part of a beam, the lower beam BC extending to the right, to some other support ... now you can consider the lower beam SS, and the beam will carry some internal moment.
will DE behave like a beam on three supports ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[BAretired]

rb1957,
There is no longer a pin at pt B. That was my original assumption and could only be justified by a rigid connection at pt C. With the existing beam continuous to pt E, the connection at pt C can be assumed pinned.

OffshoreWindStructures,
I think you will need to resort to a frame analysis program to solve this problem. I suggest you analyze the entire structure above the tower, as the stiffness of the existing 50 year old structure is not known.

 

[rb1957]

going "full scale" and doing FE on this may be needed, but first, what is the extend of the change ?

is there now offset loading of the original tower ? how significant, compared to the previously analyzed loading ??

I think the lower beam (CBX) can be analyzed as a simply supported beam with an overhang.

The upper beam carries a small amount of bending, but mostly axial tension.

A big question is that small vertical brace (between the diagonal and uppper beam). Is it a support for the upper beam (so a beam on three supports) ? or is it "floating" on the diagonal, and so not supporting the upper beam ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[OffshoreWindStructures]

ProgrammingPE: Thank you very much for sharing your thoughts on the connection. It is appreciated. I was conscious that the tensile load would concentrate around the connection and not be distributed throughout the channel cross-section but I had not found the shear lag guidance you refer to. It is very helpful as I was uncertain how it could be quantified. The prying action I had originally considered would be due to vertical load and any moment load the connection develops. I see now that it is possible that the prying action due to the eccentric axial load could exceed that and I might have underestimated.

BAretired: I have already modelled the entire superstructure in STAAD, which is why I tried to communicate previously that the global analysis was not an issue and that I was instead focused on assessing the connections (using loads from the frame analysis).