steel shear connection modification 3

[a2mfk]

Sorry its a bit long, trying to answer all the questions I would be asking... I am not the EOR, just designing the steel shear connections.

-Standard AISC steel shear connection, single shear plate bolted to the web of a supported beam and welded to an embed plate support in a concrete tilt-panel.

-Existing building, finishes up and this will have to be a weekend job probably.

-Third floor beam, temp shoring and jacking pretty much out of the question.

-Dead loads and some live loads (obviously more or less depending on the time of day) in place, building occupied.

-It was determined the existing shear connection was about one bolt short or about 6 kip.

1. Does AISC allow you to add the strength of an additional weld from the shear plate to the web of the now-bolted beam?

2. To maintain a connection that allows some rotation to maintain a pin-pin assumption, the weld could be placed only near the bottom. I'm winging this email so pardon my mechanics of materials memory, can't remember where exactly the least amount of movement at the end of the beam would occur..

3. The bolts should be in full bearing condition and local
deformations for the most part should have taken place inside the connection, correct? I know it depends on % of dead and live load, size of the holes, etc.

4. Alternatively, what about adding a shear plate to the opposite side of the web and turn this into a double shear connection? They could take the nuts off of the existing connection, slide another shear plate on, weld it to the embed plate, then tighten up the nuts....

The problem I see with that is making sure the bolts are bearing on the new shear plate once it is welded up there , so it is not just there for peace of mind. I am worried the connection would have to yield slightly to begin bearing on the new plate, but maybe that is acceptable? Would I have to use a slip critical connection to ensure load transfer?

5. Other options include possibly a angle seat below the beam, or drilling the holes larger and replacing the bolts with 7/8" dia.

Any references to AISC or other advice would be greatly appreciated.


Thanks in advance,

Andrew Kester, PE

Florida

 

[nutte]

In the double angle connection, there is eccentricity in the weld, but in this case, the weld is on the edge of the plate (angle leg), so it is more in-line with the loaded element. This is different than the shear tab, where the fillet weld is on the wide face of the plate instead of on the edge. Does that make sense?

 

[connectegr]

Welding the outstanding legs of angles in axial loaded beam connections is not allowed. This is an excellent example of the prying and rotation about the weld root that dhengr mentioned. Rotation at the root of a fillet weld is a stress concentration that can create a crack. A crack in a fillet weld can significantly decrease the strength significantly.

The welding of the outstanding leg with a return is for shear loaded connections only. The limited return allows for simple end rotation and connection flexibility. But rotation is not at the root of the weld.

http://www.FerrellEngineering.com

 

[HassanAstaneh]

Since 1989, I have done research on shear tab and had some contributions to development of original shear tab design methods in the AISC Manual. I like to contribute my 2-cents to the discussion by first providing some solutions to Andrew Kester, PE's questions and then commenting on some items by others.

Best wishes to all,
"Hassan"
Abolhassan Astaneh-Asl, Ph.D., P.E., Professor
University of California, Berkeley
======================
Here are Andrew Kester, PE’s Questions and my comments following each question:

1. Does AISC allow you to add the strength of an additional weld from the shear plate to the web of the now-bolted beam?

Astaneh’s Comment: I cannot speak for AISC, but, as far as I know, the only shear tab connections AISC promotes are those in the AISC Manual. I am not aware of any test results or even analytical studies of a shear tab where the shear tab is connected to the beam web by bolts and welds. Such connection, in my opinion not only will not have sufficient rotational ductility nor adequate rotational flexibility needed for a shear connection.
Welding an existing connection , where dead load stresses are already in the connection is something that is not very desirable. Under such high temperatures, you do not know where the existing stresses in the connection will shift.

2. To maintain a connection that allows some rotation to maintain a pin-pin assumption, the weld could be placed only near the bottom. I'm winging this email so pardon my mechanics of materials memory, can't remember where exactly the least amount of movement at the end of the beam would occur..

Astaneh’s Comment: The bottom half of the shear tab as well as the web of the beam connected to it are in compression due to dead load. It may not be a good idea to weld a plate that is under compressive existing stresses. You may cause local buckling of the web or shear tab. But, your idea of welding the bottom of shear tab to avoid restraining it against rotation is correct. It is just those dead load stresses in your connection that you do not want to disturb!


3. The bolts should be in full bearing condition and local deformations for the most part should have taken place inside the connection, correct? I know it depends on % of dead and live load, size of the holes, etc.

Astaneh’s Comments: Correct. Shear tabs usually slip into position under dead load although not all bolts will carry the same amount of shear. The tests show that bolts near the bottom carry more shear first , then , as they cause bearing yielding in the plate, other bolts pick up more shear loads.

4. Alternatively, what about adding a shear plate to the opposite side of the web and turn this into a double shear connection? They could take the nuts off of the existing connection, slide another shear plate on, weld it to the embed plate, then tighten up the nuts....

The problem I see with that is making sure the bolts are bearing on the new shear plate once it is welded up there , so it is not just there for peace of mind. I am worried the connection would have to yield slightly to begin bearing on the new plate, but maybe that is acceptable? Would I have to use a slip critical connection to ensure load transfer?

Astaneh’s Comment: Adding a second shear tab is in right direction to increase strength of a shear tab, but, for your case where you just need 6 kips additional strength it is overkill. Also, you may not have sufficient bolt grip with existing bolts to be able to accommodate new plate as well, ending up in replacing the bolts with new and longer bolts as well. Again this is too much work 3 floors above the ground to gain 6 kips!


5. Other options include possibly a angle seat below the beam, or drilling the holes larger and replacing the bolts with 7/8" dia.

Astaneh’s Comment: Adding a small seat angle seems reasonable, but, the embedded plate may not have enough room below the bottom flange. Most likely, the embedded plate even does not extend below the shear tab much, may be 2 inches or so?
Drilling holes and replacing bolts with larger diameter bolts is doable but again requires filed fabrication. You also need to check the edge distance now to make sure it is OK. The bolt spacing if it is 3” standard will be OK even for 7/8”.


(AK Any references to AISC or other advice would be greatly appreciated.

Astaneh’s Comments: Not to promote my own publications , but, you may want to download 2 Steel TIPS reports that I have done on Shear Tabs from

http://www.steeltips.org.

Registration and a nominal fee is required for download (free download for California and Nevada residents).


(AK Thanks in advance,

Astaneh’s Comment: You are welcome and I suggest the following:
If bolts are A325N, can’t you change them to A325X? Or, even change them to A490N or A490X. 6 kips short of your target strength, should be easily done by this. If you choose to replace the bolts, I suggest that you first un-tie all bolts to finger tight condition, starting from the top bolt, but, don’t take any nuts out. One at a time, starting from top bolt, take the bolt out and replace it with the new and stronger bolt bolts and finger tight the new bolts. When all bolts are replaced, start tightening the bolts from the bottom bolt moving up to snug tight position, then , to full tightness. You notice that in shear tabs, designed as bearing bolt condition (not SC) you can actually leave the bolts snug tight provided that the stability bracing provided to the column is not jeopardized. However, your EOR might want the bolts tightened if they are tightened now, which is fine and you tighten them. I think if you follow the procedures, based on what I have seen in numerous tests that we have done , you can end up disturbing the bearing stresses the least while having a safe connection at all times with no need for shoring.

========================
Here are some comments on other items of discussion posted for this thread:

A2mfk- “I think rigidity in the connection to the supported beam is a GOOD thing. It's taking demand off of the embed plate. If the connection to the supported beam is a pin, then the shear tab is a cantilever off of the embed plate and the plate is taking shear and moment. If the connection to the supported beam is rigid, then the load to the embed plate is a pure shear (not shear and moment). This is beneficial to the embed plated.”

Astaneh’s Comment: Based on our past experience with studying shear tabs, in this case, if you weld the shear tab to the beam web the part of sheer tan welded to the beam web will stay elastic and will not yield in shear or bearing. The embedded plate side is also very rigid and is expected to stay elastic. As a result, ALL the rotation demand of the connection which is more than 0.03 radians ( see AISC Spec, Commentary), should be supplied by the ¾ or 1” length of shear tab between the embedded plate and end of the beam. Depending on the depth of shear tab, the 0.03 radian rotation in the shear tab can get the top fibers to reach fracture strains.
Another problem of course is strength issue. By welding the shear tab to beam web, the connection will be very rigid and will develop large moments both in the shear tab (the ¾” or 1” length gap area) as well as in the embedded plates and shear studs behind it. This moment combined with shear can be too much for the shear tab , the weld on the embedded plate, even for the welded-bolted connection on the beam web. Added to the complexity is that we have really no idea on how a connection as such behaves based on reliable tests!

 

[connectegr]

Dr Astaneh
You understate you involvement in the development of shear tab connections. Your design method replaced the "Richards" method in the 1989 9th Edition Manual. There are frequently questions on this site with regard to single plate or shear tab connections. Thanks for your input.

http://www.FerrellEngineering.com

 

[a2mfk]

Thank you very much sir for your input!

As it turns out, I studied the fabricator drawings and the in-field connection would be a threads excluded condition regardless of how it was installed. Lucky for the fabricator and for me...

For the record i did not state the last part about welding the plate to the embed plate. I thought this would prevent the beam from rotating and induce undesired/unplanned moment into the embed plate.

Thanks again!