good reference for alterations of existing reinf. CMU walls?

[Engineerataltitude]

Looking for a reference for retrofitting openings into an existing CMU bearing/shear wall.

Most of my structural work has been with wood framed construction in CA. High snow load and seismic country. In my area of practice seismic loads in buildings are huge always govern the lateral design.

In the last 2 weeks I have gotten 2 inquiries about retrofitting existing buildings with exterior CMU bearing/shear walls. Both want to add significant openings into the existing exterior shear walls.

I have a bunch of reference books on designing buildings with new CMU walls, but nothing that mentions retrofitting openings into existing CMU walls.

My concern is that for new construction, there is a lot of reinforcement above and around the corners of an opening in a CMU bearing/shear wall. Is there an industry norm for handling these corner loads in a retrofit opening?

If I put in a steel OMF into the opening of an existing shear wall, isn't that introducing another kind of lateral force resisting system into that shear wall line that has a different seismic response factor, R? My recollection is that along any given shear wall line in a building, the shear is distributed in the wall by relative stiffness, G. Correct? Seems pretty analytically tricky to have CMU shear walls and and steel OMF in the same shear wall line.

Can anyone recommend a reference for this kind of work? With details, perhaps?

 

[Greenalleycat]

I'm not aware of a specific reference. I don't think one is needed though: working through this rationally is what engineers are paid for
I work in a high seismicity country so am familiar with the intricacies but not US specific stuff

Issue 1 - providing appropriate gravity support to your new opening, fairly self explanatory. Commonly done with angles.
I would get them to scan for reinforcement and ideally cut out to underneath a reinforced cell (otherwise you will have unreinforced cells 'hanging' off the bottom of the new lintel

Issue 2 - durability, how are you going to treat exposed steel ends (scabble back, cut bars back 20mm, treat with Sika and patch repair)

Issue 3 - lateral loads
Do you even need new lateral capacity or can the existing building acccommodate it? Again, I'd be cutting back to a reinforced cell so that the end of your wall isn't an unreinforced cell
If you need more capacity - your new system will need to have comparable stiffness, as you've identified
From my experience, a steel braced frame using square sections or similar should do the job
Foundations can be a headache though - you'll develop huge new loads under the ends
The response factor thing is not something I can help with as, to be honest, it seems to be a huge fudge that's not really appropriate to match reality anyway

 

[Engineerataltitude]

Thank you for the response.

Both the buildings in question were built in the late 1960's. They both look in great condition, but seismicity was not a big design consideration back then. Definitely did not have all the technicality of ASCE 7-16 Ch 12 to deal with back then (i.e. seismic response factors, providing calculations for chords and collectors, etc.) We have to deal with that now though on any alteration to an existing structure in CA. My jurisdiction uses a very rigorous plan check firm, so they will want to see everything calc'd out and detailed.

A steel braced frame wouldn't work for these since they want doors and windows in the new openings. Would probably have to be an OMF, if anything.

 

[Greenalleycat]

Braced frame could be channels with their webs against the block and bolted into it
Doubt you'll get a (reasonble) moment frame to be stiff enough to compare to a wall

Could look at building new pilasters at the ends of your openings to frame them out and create boundary elements for the wall?
Might be hard to get the composite action to work though

 

[milkshakelake]

I don't have references for how to do this. I've looked myself. I came up with my own details which is similar to what @Greenalleycat mentioned, which is a double channel sandwich. You can also put the double channel as a jamb. This method removes the need for needle beams, so it's a plus for constructability.



In terms of lateral, I wouldn't rely on a channel moment frame, because it's iffy. I mean, I guess it can work if you do a bunch of welding and try to find an equivalent channel moment frame to the whole CMU wall on that side. I'd add lateral elsewhere. In my jurisdiction, when making an opening in an old wall, the entire building has to be designed for seismic forces using new provisions of code. The old CMU walls were probably ungrouted or unrebar'd CMU, and I doubt they'd work for new code provisions. So you might have to add new shear walls with new collectors/chords to dump the seismic load into them, which can handle all the shear demand that the old walls were handling. (This is dependent on an ASCE 7 appendix provision IIb or something, I can't remember the exact reference.)

Also, you will be dealing with material incompatibility between steel and CMU. They will thermally expand at different rates. That's why steel lintels on brick or CMU lead to cracking at corners, because that's combined with a corner stress concentration. Solution: none. We do it anyway.

Maybe an idea is to use needle beams to make a larger opening, and you can make a concrete moment frame in there.

Regarding your concern about seismic response coefficient R, first of all, you have to determine if the walls can carry any seismic load at all. If it's unreinforced, it probably can't, or only nominally. And if you're making a cut, you're severely reducing the stiffness of that wall (think of large versus small moment arms).

 

[RWW0002]

Regarding lateral - If in line with other masonry shier walls (presumably) - How would an OMF ever pick up any lateral load? As Greenalleycat alludes to above, the shear walls are going to be orders of magnitude stiffer than the moment frame, and load will distribute to the lateral system(s) relative to their respective stiffness.

I think your best bet would be either to 1. Limit the amount of shear wall you are removing to within IEBC limit for lateral systems (Basically 10%), or 2. somehow demonstrate compliance with the current amount of lateral resistance (might be a tough sell with older masonry that does not meet prescriptive seismic requirements for IRMSW or SRMSW.)