Adding reinforcement to under-reinforced CMU

[shaneelliss]

I have a situation where a CMU foundation stemwall for a residential structure was constructed with one horizontal #3 bar in the top bond beam instead of 2 #4 bars. Correct vertical rebar was placed. The stemwall is only 4' tall. This is seismic zone D.

The question is, can a steel band bolted to one side of the bond beam at the top of the CMU be used to make up the deficient rebar in the bond beam?

 

[TXStructural]

Only for bending in one direction, toward the side with the steel bar. The strap/bar will have to be attached properly, so that it develops composite action with the CMU. If you put one inside and one out, and thru-bolt at some close interval, you can probably get sufficient composite action.

Alternatively, sawcut into the top or side of the bond beam and use an appropriate adhesive or grout to attach rebar into the bond beam. If the bond beam is filled block, it may be easier to demo all or part of the bond beam, place the reinforcement, form, and cast-in-place a new bond beam.

Or, cap the wall with a steel shape (usually a channel) which is anchored to the bond beam with closely-spaced adhesive anchors.

 

[SteelPE]

I'm not sure this will work in your case..... and I have only proposed the idea but never used it for construction.

When faced with similar structures with under reinforced wall I have proposed studding out the inside of the wall with metal studs and then designing these stud walls to take the necessary lateral forces in shear and bending. Then I basically was going to say that the exterior wall was a very thick veneer.

Came close to implementing this in the field on the rehab of a 1950 URM building but never did so I never had to finish the details or fully investigate the system with the building code.

 

[shaneelliss]

SteelPE, you and I think alike. When the issue first came up, I told the client that a wood framed shear wall just inside the masonry might be a possible fix. He has built on top of the wall already, so it is not feasible to remove the bond beam and replace it or to cap it with a steel channel.

But as far as a steel plate bolted to the side of the bond beam on top(which is filled block), do others agree that it can be done if the plate is on both sides of the block and bolted at some reasonable interval?

If I go with the wood shearwall inside the block, I was thinking it would have to be a very stiff shearwall so ll the lateral force transferred into it instead of into the block wall. Does that also seem like a reasonable solution to others?

 

[SteelPE]

shaneelliss,

I probably stole the idea from you.

My thought process was to use steel studs and fasten the flange of the stud to the back side of the masonry at regular intervals. I'm not sure how you would attach the block to wood studs (never had to think about it).

With regards to the stiffness. I'm not sure about this. I always thought the system in the code were based upon their ability to dissipate the earthquake load (ductility). I would think that as long as you had a system that was capable of properly resisting the loads then it wouldn't matter if one system was more stiff than the other. The more stiff item would take the load and then fail because it was not ductile enough leaving the system that was properly designed (accounting for ductility) to take the required load.

As long as the two system were properly tied together then the failure of the more stiff system shouldn't matter.

This is my thought process. I'm sure someone with much more seismic experience will give you a different opinion and we can both learn from the process.

 

[dcarr82775]

I don't know how much steel you need, but it is possible to install some small #2 'bar' in the mortar joints. It doesn't get you much, but it might be enough.

 

[a2mfk]

What's your bond beam really doing? Supporting wood framing that is transferring uplift loads into the bond beam and then into the vertical cells?

Is there a prescriptive code minimum in your area? And is adding another course architecturally out of the question?

If you are stuck with this I like Mr. Texas' ideas. Bolting a channel could be pretty cheap.

 

[shaneelliss]

A bond beam with 2 #4 bars is actually the IRC prescriptive requirement for a 4' stemwall in seismic zone D. I assume it is required to give the wall some ductility in a seismic event. The framing that is on top of the masonry stemwall has anchors that are embedded enough to fully develop the strength of the vertical rebar at the ends of the walls so the bond beam isn't really required for that, in my mind.

Also, as I said above, the framing above the stemwall is already in place and adding a second course above the top one that is there or bolting a channel on top isn't possible at this point. Bolting a channel onto the side could be done, but not the top.

 

[a2mfk]

Sorry I missed a couple of your details, I should have read more thoroughly. Can you overrule prescriptive requirement with engineered solutions in your jurisdiction?

Is your stem wall fully grouted?

In partially grouted CMU, my understanding has always been the main job of a bond beam is as a diaphragm chord, and also to transfer uplift forces applied to the top of the wall into your filled cells where uplift connections don't align with filled cells. You may have the diaphragm chord handled in a different way, or a #3 may be OK. A wall with filled cells @ 4' o.c. for example, you may have a situation where that bond beam must span 4 feet in uplift loading with a point load in the center. I have had high truss uplift situations where I had to use a double bond beam because of this action.

I am not sure if you have this issue and cannot make an engineering judgment on this. If your forces get transferred from the roof to the foundation properly, and your chord is designed properly, the bond beam requirement may just be what you have said it is - prescriptive.

Also, I am not familiar with the specifics of seismic design, only wind, so I may be missing something in that arena.