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strengthening of poor-quality historic CMU walls

strengthening of poor-quality historic CMU walls

strengthening of poor-quality historic CMU walls

Hello all,

I've worked up a basic plan for the retrofit of two-story tall CMU exterior walls in a 1905 building (it suffered an interior fire with damage only to the second floor joists). The head joints in the CMU interior face were never filled, the CMU is chipped and gouged in lots of places, and it's unreinforced two stories high with the 2nd floor joists bearing into the CMU (or on a very questionable ledger) and the original roof trusses bearing on the top of the CMU walls with no visible connections.

Given the fact that this is in a seismic zone that is also occasionally hit by high winds, I'm just not comfortable signing off on the building until I know that those CMU walls can be counted on to carry the required lateral loading. I've recommended retrofitting rebar and grout but the contractor is balking at the cost. Access is available to the inside face of the CMU but not the outside - someone added a very questionable brick facade in the 70s or 80s (that's a whole separate issue). The building is seriously nothing special (and that's coming from a dedicated historic preservationist, but a realist). I'm not comfortable with carbon fiber strips because the CMU is in such questionable condition. I did think about building a steel frame interior to the CMU walls and using that to support the floor and roof, and perhaps providing lateral stability to the CMU, but that's crazy expensive too. The contractor asked me to consider using light gauge metal studs to support the floor and roof, and to support the CMU walls laterally, but I don't know that light gauge could take that kind of load. And, in the end, it's my stamp on the line.

Does anyone have any other suggestions of how to strengthen these old (and in poor condition) walls?


Please remember: we're not all guys!

RE: strengthening of poor-quality historic CMU walls

Those CMUs could very well have been made on site. Sears-Roebuck was one of the major sellers of concrete block machines to produce block.

In 1904, the units had very thick webs and face shells (2"+ was common on 10" and 12" units)and the strength were notorious high as some testing laboratories have found out when a 200,000 Tinus Olsen testing machine was inadequate.

It may be worthwhile for the effort if you can remove several block for testing. As in modern masonry structures the mortar strength has a minor effect on the wall strength.


Engineer and international traveler interested in construction techniques, problems and proper design.

RE: strengthening of poor-quality historic CMU walls

How about a new reinforced CMU wall to the inside, stitched periodically to the existing CMU wall?

Mike McCann, PE, SE (WA)

RE: strengthening of poor-quality historic CMU walls

If the following are true...

1) you're high seismic.
2) You're confident there's enough capacity and you just have to prove it.
3) you can talk your owner into some extra design fees.

...there may be value for the owner in pursuing a performance based design evaluation based on some material testing and the NEHRP stuff. "Do nothing" is always a cost savings. That said, it sounds as though, at the very least, your diaphragm connections would need remediation.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: strengthening of poor-quality historic CMU walls

I like the light gage steel idea. 8" studs should be able to handle that. Just need to figure a gage and some connections.
May also need a footing to support them. (footer if you want to upset Ron).

RE: strengthening of poor-quality historic CMU walls

Definitely don't want to upset Ron!

Dick, thanks for that suggestion. I didn't realize quite the extent that Sears offered - although I have seen a few Sears houses around. I'll talk to them about the idea of testing a few blocks.

Thanks for the responses, everyone!

Please remember: we're not all guys!

RE: strengthening of poor-quality historic CMU walls

I have seen walls strengthened by attaching wire mesh reinforcing and shot-creting. This method was used on an historic Navy 1903 building about 10 years ago as well as a paper mill some 30 years ago.

RE: strengthening of poor-quality historic CMU walls

SLTA....Sawbux approach will work. Difficulty is finding a true structural shotcrete sub, not a sloppy pool builder.

Light gage could work, but you will likely be using 12 ga. studs and heavy track, neither of which are commonly available locally. Will likely be special order. That method will require attachment to the block at regular intervals.....can the block take mechanical fastening?

The shotcrete approach can be bonded to the block face as well as provide some mechanical fastening of the wire mesh, but the mechanical fastening doesn't have to be done as closely as the metal stud approach.

XR250.....Maybe we should have a new word....."footinger" or "footering"? Do we have to call foundation subs "footerers"?

RE: strengthening of poor-quality historic CMU walls


What is your wind pressure? Does wind control?

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