Existing Masonry Wall Extension

[erikuofo]

I've got an existing house constructed of cmu walls with #4 verts at 48" o.c. at the exterior up to about 10' to the plate height. The architect wants to extend the existing walls up to a new top of wall plate height to 12' and 14' in some cases. There is not any space to brace these walls at the new "hinge" that will be created. I will drill and epoxy dowels each face of wall as required to add up to 4' of vertical cmu, but wanted to know if any of you have had problems with cracking at the seam of the new interface or have had other problems with this issue.

Thanks,

 

[MiketheEngineer]

What kind of wind, seismic, shear loads are we working with here??

 

[hokie66]

You would have to drill down a long way to lap with the existing reinforcement. Are the unreinforced cores grouted? If not, I would look at reinforcing them.

 

[a2mfk]

It depends on your answers to the other guys' questions, and what the bond beam situation is. You may have enough bond beam to develop the tension of the extended rebar using epoxy dowels, you will have to run the calcs. I have done this before but I think I had a concrete tie beam. I think the more conservative but better procedure is to go in between the existing cells and sawcut slots in the wall and through the bond beam to add rebar and solid grout. Now you have new full-height, filled cells at 48" o.c. and partial-height filled cells in-between. Third option is to do both which will give you full height grouted cells at 24" o.c., but this will reduce your steel stress- which may make it more feasible to epoxy dowel into the bond beam.

Your other question is about the cold joint between the old bond beam and the new CMU extension, and you may expect that to crack if you are covering it with stucco. I'd have the architect address that with lathe or some other reinforcement, or perhaps make it into an architectural reveal or add some banding over top of it. I do forensics on residential structures all of the time, mostly CMU with stucco, and see stucco cracks along construction joints constantly.

 

[hawkaz]

To follow up on a2mfk's suggestion- Do you have bond beams throughout the wall (seismic construction), or just at the top course?

If you just have a bond beam at the top, remove the grout in the bond beam at 48" o.c. (or whatever spacing you need), drop in full height rebar and grout the cells with new rebar solid. Use a clean out at the bottom to make sure the bar is positioned properly.

 

[a2mfk]

That's what I said hawkaz, just more wordy and buried in there

"I think the more conservative but better procedure is to go in between the existing cells and sawcut slots in the wall and through the bond beam to add rebar and solid grout."

 

[erikuofo]

The walls are reinforced and grouted at 48" o.c. I've got a governing wind force of about 25 psf on the wall. The building is just a box shape. There is a run of wall that the wall will only be extended up one course, so I think I can drill and epoxy rebar into the existing top course bond beam to extend the wall up 8" there. Then on two sides the wall top plate gradually increases to requiring a 4' extension at the opposite side of the building. I like the idea of adding in verticals in cells adjacent to the already existing ones and grouting full height.

Thanks,

 

[a2mfk]

Yep, adding one course I would just dowel in 90 degree hooked bars and add another bond beam on top of the existing... Uplift will determine your spacing.