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Existing CMU building reinforcement

Existing CMU building reinforcement

Existing CMU building reinforcement

Hi All,

I am working on a project where an existing building is being modified. The mezzanine is being removed. This causes larger span of the 12" CMU(likely unreinforced) wall (for out of plane) and also puts the higher force on the upper roof diaphragm. For strengthening the CMU wall for out-of-plane, I am thinking of adding L-angles @ 4' O.C. to reinforce the CMU wall. However, I am not sure how I can reinforce the upper roof diaphragm as re-roofing is not an option. The roof is TPO(Thermoplastic polyolefin) over metal roofing. I would love to know how this community have been dealing (or would deal) with the following challenge. I have attached a pdf with the details for your convenience.
1. Strengthening the CMU wall
2. Strengthening the Roof Diaphragm

Thank you.

RE: Existing CMU building reinforcement

I would have to reinforce the walls and roof like you mentioned. I would have probably used steel wide flanges or tubes but if you think an angle will do it then I’m not going to argue. As for the roof diaphragm I would not try to reinforce it since it would be a lot of work. Instead replace it with horizontal steel bracing.

Also, I would be considering the option of installing a compact horizontal truss (like a strong back) to replace the mezzanine but still distribute the loads to the original load paths. This would be in lieu of all the other work.

RE: Existing CMU building reinforcement

I'd verify that the wall isn't reinforced. You know your local construction better than I do, but a lot of buildings like that where I am (east coast of the US) are reinforced even going back a few decades. Smaller ones less so, but that's a pretty good sized building.

I have some serious doubts about your angle idea. That's a really long span for a 4" angle to work. Keep in mind that wind pressures go both ways. So when it's suction pressure, the block won't be contributing at all if you assume it's not reinforced and cracked. So you'd be looking at a 4" angle spanning almost 30'.

If the diaphragm loads are increased such that the connections need to be strengthened, then that's what needs to happen. They may need to cut a 3' strip out of the roof to do the work and patch it after the structural work is done if they aren't willing to replace the entire roof.

RE: Existing CMU building reinforcement

We had a project a few years back where we had to add reinforcement to the CMU wall. 1970s construction, CMU wall had horizontal durawall mesh, but zero vertical reinforcement. We added fully grouted vertical rebar every 3ft:
3 or 4 CMU faces were cut out along each vertical line, mortar droppings were cleared out, overlapping rebar was inserted through the cutouts and doweled with epoxy into foundation. Lowest ungrouted cutout was covered up and then grout was pumped in through the next one up etc.
Sure it is a little more work than external reinforcement but you don't need to rely on any anchors holding CMU to the reinforcement and the owner was pleased with how it turned out aesthetically, as there was a concern with angles, channels etc taking up space and being a constant eye sore.

I also heard of some polymer mesh being epoxied to the outside of a CMU instead of steel, and used as seismic reinforcement, but it was hard to find in the Midwest back then

RE: Existing CMU building reinforcement

I was thinking carbon fiber strips on both faces of the wall, but slicing and adding rebar might be cheaper and equally as effective.
Not sure what to do about the roof deck other than what Pham has suggested. What about flag poling some columns or pilasters up the to the current mezanine level? Not ideal but another option.

RE: Existing CMU building reinforcement

Thank you all for your feedback. This is gold for me as I am trying to learn about the common construction practices.
@DayRooster: I am looking to add L-angle/ C channel/ Wide Flange depending on the capacity I need. I would like to avoid using tubes as I could not directly anchor the tubes into CMU. The client is removing the joist underneath the mezzanine. But we are still putting the girders under the mezzanine. The floor and roof joist of mezzanine are perpendicular to each other.

@phamENG: I just got information that the building was constructed around 1989. I could not confirm if the existing CMU has reinforcement or not. What do you think would be the minimum reinforcement judgement wise. The plan of the building is around 190 ft x 200 ft with the taller wall to be 28ft and the building is in east coast of the US with high wind (145mph). My idea is that the L angle is adding extra strength on top of the CMU wall strength. I will definitely put forward the idea of cutting 3' strip of roof and reworking the connections as they are not going to re-roof the entire structure.

@RabitPete: You idea was something I was interested in. Could you provide me more insight or point me to the direction where I can get more information on inserting overlapping rebar through cutouts and dowelling into foundation. Will this idea still work if the rebars are not doweled into the foundation or slab? The client would not want to deal with the foundation. Sorry for all the constrains.

Thank you.

RE: Existing CMU building reinforcement

NPL101 - I never guess at the reinforcement. If I'm going to use it, I find out what it is. Based on your pictures, it should be easy. The two best options: GPR scan to see if they can detect bars in the wall or 2) drill a hole in every cell for a distance of up to 6' to find out where the grouted cells are.

Don't let your client set constraints so tight that you can't do your job. You need information, and certain things may need to be done to the structure to make it safe. If it's not worth doing those things, then it isn't worth removing the mezzanine and using the space for what they're intending.

RE: Existing CMU building reinforcement

Quote (NPL101)

Will this idea still work if the rebars are not doweled into the foundation or slab? The client would not want to deal with the foundation
Regardless of reinforcement method, wall has to transmit lateral load to the foundation. In our case bottom cutout was a little larger to allow for a hammer drill, so holes could be drilled into the wall footing under the CMU. Deformed bars were installed into those holes using adhesive anchors.

28ft wall in 145 mph zone??? Doubt it is unreinforced, have you tried finding original prints for the building? That is the first thing we usually try to do. It also looks like there is no insulation on either side of the wall. Anyone ran into issues with meeting COMCHECK on projects like this? Around here they started requiring a full envelop compliance on most remodels, unless wall is furred and finished and not touched at all.

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