steel shear connection modification
steel shear connection modification
(OP)
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
-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






RE: steel shear connection modification
2) I think the connection to the supported beam is the place where you DO want moment transfer - this will allow the connection to deliver a pure shear to the embed plate and not an eccentric shear (or shear and moment).
4) This would require temporary shoring, no?
RE: steel shear connection modification
RE: steel shear connection modification
Welding WAS my first choice, my concerns are #1 and #2... That was the reference I was looking for. The in-place connection is bearing, non-slip critical. According to Chapter J, I'd have to size the weld to take the entire weld, in which case it would be too rigid of a connection IMO. But I need to do more research, I understand what you are saying about pure shear, but as the beam deflects and rotates, it will transfer some moment into the embed plate which we do not want...
I think that makes these the two most logical:
a) Add another shear plate and make this a double shear connection
b) Drill larger holes and replace 3/4" bolts with 7/8"
I do not know if these would require shoring, that would be a huge problem.
RE: steel shear connection modification
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: steel shear connection modification
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.
RE: steel shear connection modification
I could ignore the existing bolts and just weld the existing plate to satisfy the required strength of the connection.
However, I still have concern with this now becoming a stiff connection and moment being transferred into the embed plate. But there was moment before as a simple shear bolted connection because of the eccentricity, so the EOR would just have to OK our fix.
RE: steel shear connection modification
Adding a second shear tab and getting the bolts in double shear is a good idea.
In the original connection design, were the bolts designed as N (threads included in the shear plane) or X (threads excluded from the shear plane)? If they were N, you could check them or change them to X, gaining more strength. You could also go to a higher bolt grade, or to larger bolts, although this might pose edge distance problems.
Again, welding the shear tab to the supported beam is not recommended by AISC.
RE: steel shear connection modification
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: steel shear connection modification
Dead load is in place, portions of the live load will be in place. But if this is done in an empty building on the weekend, then additional live load will go into the newly welded connection, so additional beam rotation will occur. I just don't know if this is negligible or otherwise OK, as nutte indicated its not in my knowledge prequalified....
Trust me, I want to do this if I can justify it.
No seismic or fatigue issues.
RE: steel shear connection modification
I understand your point exactly. Welding the shear plate will reduce the moment (already partially loaded) caused by the cantilevered bolted shear plate, and replace it with a "partially"-rigid moment connection. Thereafter, additional loading will produce moment into the PRM connection... Sorta robbing Peter to pay Paul in terms of the embed plate.
I am going to look into some on Nutte's suggestions about N vs X, and using higher strength bolts. We may be able to milk an additional 6 kips out of one of these techniques. OR, I like adding the shear plate and making this a double shear connection.
I'd rather not change the loading into the embed plate as this is by the EOR, who can reject any of my proposed repairs.
Thanks for everybody's suggestions!
RE: steel shear connection modification
I would have recommended knife angles or single angle welded to the embed. This is a flexible connection and therefore the embed can be designed for shear only.
http://www.FerrellEngineering.com
RE: steel shear connection modification
These are considerations that should have been made in the original connection and other similar locations.
http://www.FerrellEngineering.com
RE: steel shear connection modification
Thanks for the input. This is a fab bust, I am trying to fix it for the fab, I am not the EOR. As it is, I am about 6 kips short. I can only assume the EOR originally considered the moment from the cantilevered shear plate.
As stated above, I am concerned that the moment I am adding due to fixing the connection would be more than that from the effects of the bolted shear plate that I am eliminating...
Pretty much have my answers though, I am keeping it simple and adding another plate to the other side of the web. That way I have not affected the original connection design intent and fixidity condition.
RE: steel shear connection modification
http://www.FerrellEngineering.com
RE: steel shear connection modification
Third floor beam- temp shoring and jacking are off the table...
RE: steel shear connection modification
RE: steel shear connection modification
AWS prohibits the use of one sided fillets in tension. The eccentricity of this connection applies shear and moment on the weld. A partial pen provides weld more concentrically about the centerline of the plate.
http://www.FerrellEngineering.com
RE: steel shear connection modification
Could you weld a small seat angle below the beam to pick up the 6 kips? I would jack this up to be sure it was seated before welding, but you don't have to unload the whole beam, and you won't change the stiffness of the existing connection conditions appreciably.
RE: "AWS prohibits," Connectegr's last statement: it's the prying action and multi-directional stress conditions at the root of the fillet weld that's the killer. Loading parallel to the root works fine, but loading across the root works less well, it is the weak link in the weld when the stress field is perpendicular to the axis of the fillet. Although, the partial penetration weld root can be just as problematic under like loading and stress fields. So, as Connectegr says, it's a matter of degree for you to puzzle over.
RE: steel shear connection modification
And since that is allowed by AISC, I would think that would be a solution if I have room on the embed PL.
I will obviously check to see if the threads are in the shear plane first. Thanks to both of you...
RE: steel shear connection modification
RE: steel shear connection modification
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
RE: steel shear connection modification
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 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!
RE: steel shear connection modification
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
RE: steel shear connection modification
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!