retrofitting new opening into an existing CMU exterior bearing/shear wall?

[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 CMU 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 a steel OMF in the same shear wall line.

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

 

[driftLimiter]

I can't really think of a specific reference document that covers your case.

You have to use your judgement, and for lateral I feel you are on the right track.

You cant expect a moment frame to take any load away from the existing CMU walls because of relative stiffness.
So you are left with evaluating the remaining cmu wall for lateral load capacity.
In my experience, unless very slender, CMU bearing walls often have quite a bit of additional lateral capacity.

For gravity you need to develop a method to support the vertical loads above the opening, commonly done with a steel beam.

If you can add new posts and center the beam under the wall thats awesome, you can also use a channel or angle on 1 or both faces.

The next challenge is to address the wall out of plane loading, you have disrupted that load path in the opening region and now it needs to be carried somewhere else.
Either the masonry above the opening can span horizontally to some jambs, or perhaps vertically to the the header, and then from the header to the jambs.

Since the wall wasn't reinforced for added out of plane loading in the 'jamb' region. You will likely want a steel column to resist additional out of plane bending. This could be either full or partial height. Full height is perhaps easier to analyze and more likely to work with simple details.

I would recommend the SEAOCC design manuals masonry and tiltup books, although they wont specifically address the situation, they have good examples of out of plane and jambs.

If your still needing more help I am in the area and have a lot of experience with this type of thing.

 

[Engineerataltitude]

Thank you for the response.

I just got an updated Reinforced Masonry Engineering Handbook from the MIA and it's corresponding Solutions Manual. Those should help.

I told both prospective clients that their projects would have to be modeled with RISA-2D to do the relative stiffness analysis in the modified shear wall line. Just pricing the analysis and detailing to retrofit the SS OMF's into two wall bays between CMU pilasters as well as projected pricing of the likely structure, I suspect the residential client will back away, but the commercial client will probably move forward.

 

[driftLimiter]

You could use a dual system or a horizontal combination system. I don't see special masonry walls combined with omfs in the dual systems section of Table 12.2-1

 

[Engineerataltitude]

Per ASCE 12.2.3, I was going to model the entire shear wall line in RISA 2D with the two systems (existing ordinary reinforced masonry shear/bearing walls + new SS OMF's in two bays) using the most limiting R value (R=2 for the CMU walls) for loading. The OMF member sizes will most likely be governed by drift, so I will have to see how much load the OMF's will end up carrying in the shear wall line because of their stiffness. I hope it's not all of it.

 

[driftLimiter]

I think you have this backwards a bit. Your procedure from 12.2.3 seems fine but.... OMF's are normally governed by drift but they will be compatible with the deflection of the CMU shear walls. The challenge will be to get the OMF's to resist any meaning full load because the shear walls are not going to drift to the 2% limit.

You will need to make the OMF's very very stiff if you want them to take any load away from the shear walls.