1940's Unreinforced Masonry Building

[msquared48]

Just talked to a client who wants me to determine the feasibility (he really wants to save the structure) of bringing up to code this structure to support two additional stories of apartments in the future. The structure is currently 2500 square foot, 8" unreinforced masonry with a wood roof. Adding footings is no prob.

Obvious problem is future code changes, but I can deal with that. What my son has suggested though is using the existing 8" CMU as a veneer architecturally with the proper lateral anchorage, and building an interior bearing/shear resisting system inside the exterior CMU wall shell. This sounds more cost effective than reinforcing the wall vertically, grouting, and coating with an FRP finish.

Anyone have any experience with the two options cost-wise that they can share?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[a2mfk]

Couple of thoughts:

Where is this project? (wind/seismic)

8" URM is still pretty stiff, so you would have to have compatibility with your backing system to ensure proper lateral load transfer. I wonder if using brick institute type support standards would apply here. Are you thinking steel stud and steel frame structure inside the CMU walls?

The other thing is even though its cladding, you are buying it now with your modification. So any poorly done window or door openings, cracks in the wall, deteriorated mortar, etc. may have to be addressed. You may end up needing to do some repairs and retrofitting even to make it work as a cladding and be sure everything is OK to current codes.

I am curious as to what is more economical: using it as a cladding vs tearing it down (if you are not going to use it as a structural system).

I have had lots of projects where they have had to add dowels and grout via slot cutting holes in the CMU. One was an old warehouse (several buildings hodge-podged together) that was converted into a print shop and it went from URM circa 1930s to current FBC requirements. We did not do an economic analysis of other choices as the roof stayed, but nobody batted an eye at this method as far as cost goes. But this was for 110 mph wind loads only in Florida. If you have special seismic criteria the reinforcing and grouting can get pretty heavy, and the shear walls and mass of the walls become an issue...

 

[msquared48]

Are you thinking steel stud and steel frame structure inside the CMU walls? YES, for now anyway.

I have not been to the site yet, but will be going tomorrow. it is in North Seattle from what I understand from the client, near where he had me design another building for him a few years ago that was never built due to the economic downturn - that was new construction and this is adjacent.

Teardown is not an option at this point as he wants to save it to preserve the local architectural flavor of the area.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

If the roof is coming off, you may be able to reinforce and fill the wall from the top. Just provide cleanouts at the base, and away you go.

 

[Brad805]

Any chance you could add some pilasters at whatever spacing you deem the existing block can span unreinforced? It should be easy to add a bond beam at the top of the existing if none exist.

Brad

 

[msquared48]

Hokie:

Yea, I thought of that, but then there is the problem with developing the horizontal shear, and having to use an internal/external FRP wrap, or some other means. We are in a high seismic area, so this is a real concern.

There is the thought that if no further weight is added to the walls via grout infill, undoubtedly a major contributor the Seismic component of the design, then the lateral forces will be reduced. Just have to see I guess.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Why wouldn't the added vertical reinforcement provide the shear resistance you need?

 

[msquared48]

Still need horizontal bond beams at 4' minimum vertically with horizontal steel - usually (2) #4 bars as a minimum.

In years past, as you might remember, "K-web" was used instead of the horizontal steel bars, usually every third course in the mortar joint. This is the steel that would be impossible to install, but necessary to emulate some other way.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

Sounds like the kind of project to avoid like the plague.

BA

 

[hokie66]

No, I don't know what you mean by K-web, unless it is the useless (in my opinion) wire joint reinforcement. What I was suggesting is designing the wall with a truss analogy, ties being the new vertical bars and struts being the grouted masonry. I don't know why horizontal reinforcement would be required if the V/bd stress on the grouted masonry is below the allowable.

 

[msquared48]

Hokie:

To quote from page 203, section 4.2.3 of the third edition of the Masonry Design Manual by Amrhein and Kesler:

"Masonry has a capability to resist shear forces, however, many times it is necessary to exceed the shear capability of masonry. When this occurs, the beam or shear wall must be reinforced with stirrups or horizontal steel to resist these high shear forces."

These horizontal bond beams with reinforcing, or the "K-Web" of years ago, serve just that purpose for shear walls. Due to the high seismicity of our area, it is just common practice here to always use bond beams. I always do, regardless of the stress level seen. That's the way I was trained years ago by the then President of the State Board.

By the way, I guess the K-Webbing was placed every other course, not every three. On page 136 of the third edition of Amrheins "Reinforced Masonry Engineering Handbook", it depicts the two types of interstitial joint reinforcing. The one with the diagonal elements is the K-Web. I do not use either any more - only 8" reinforced bond beams at 4' centers.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

Fair enough, but I was just suggesting there may be more than one way to analyze how the wall sees the horizontal forces. I doubt that Amrhein considered strut and tie analysis in his work. You can't insert bond beams, but you can reinforce vertically and fully grout the wall.

Merry Christmas to you and yours!

 

[msquared48]

Yes, and thanks.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask