Existing unreinforced masonry options? 1

[SteelPE]

This problem has come up in my office from time to time and it has come up again and I am wondering if there is a better solution.

We are responsible for the renovation of an existing building including a small addition. The existing structure is about 60 years old and appears to be unreinforced masonry (I would be surprised if it wasn’t).

The modifications they are making to the structure require me to analyze the building for new loads (removing portions of the exterior load bearing shear wall system). Since unreinforced masonry is not allowed, I have to figure out a way to reinforce the wall system. One thought is to open up the top of the wall system and drop in some new bars and grout. Nobody ever likes this idea. Is there a better way to bring an unreinforced masonry building up to code other than with a wrecking ball?

Building code is to be the IBC 2009 with the IEBC 2009.

 

[ron9876]

Knock out openings top and bottom at each added bar. Also openings as required to get the new bars in. Epoxy dowels top and bottom into existing footing/bond beam. Lap new bars with dowels and grout.

 

[JAE]

Similar to above - cut vertical slots in the wall, opening up the empty cells and install vertical reinforcing bars (with dowels to footings) and fill cells with grout after forming the cells closed.

If needed based on strength, do this in alternating cells so if you need vertical bars at 32" o.c. - do a first series of cells at 64" o.c. and then come back and do the remaining cells after grout curing.

Another idea is to install vertical steel elements up the wall surface, tying into the roof system and the floor/foundation system as required. Bolt the steel elements to the wall at required intervals.

Also there is always the use of adhered carbon fiber sheets on each face but I'm not sure how economical or feasible that solution is.

 

[msquared48]

Pretty well covers the gamut...

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

 

[gobsmacked]

Consider using the existing wall as permanent formwork for a new rc wall. Alternatively. if you can anchor them in the foundation, use Macalloy bars in the centre of the wall to prestress the wall from top to bottom. Add a ring beam at top to provide the anchorages.

 

[SteelPE]

Thanks for the help guys.

I still don’t quite know what I am going to do yet. Is reinforcing an existing un-reinforced masonry wall even allowed by the code (IBC/IEBC 2009)?

My suspicions on the existing building being un-reinforced stem from the buildings age together with some cracking patterns that existed in the building. Considering the building is 60 years old, the existing masonry appears to be in decent condition.

 

[JAE]

The code would not disallow the reinforcing of the masonry.

The question would be, can you reinforce the building in such a way as it would then meet all the provisions of the code.

You would have to ensure that all the load paths through the building would be properly handled by the structural members and the connections.

 

[SteelPE]

JAE,

I agree with your assessment of what would be required.

I am figuring that three things would need to be done.

1) Install new vertical reinforcing to comply with the requirements for intermediate shear walls (specific code requirement).

2) Install a new horizontal bond beam with enough reinforcing to comply with the requirements for intermediate shear walls.

3) Make new attachment of decking to masonry walls.

Seems like a very large/expensive task.

The only way around these requirements is to architecturally figure out a way to not modify the existing structure while meeting the owners requirements (not remove the sections of bearing/shear walls shown in the drawings). That way we are not involved with chapter 34 of the building code.

 

[msquared48]

You could always build a new structural wall to the inside of the existing one and anchor the existing one to it. Lose some area internally, but cheaper...

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

 

[SteelPE]

msquared48,

I was investigating the possibility of doing this (I just wasn't going to bother discussing it here at this time). The architectural drawings show a new stud wall on the inside of the building. I was thinking of making the new stud wall a structural element. I was then thinking of treating the existing masonry as a facade. However, I'm not sure if this would be compliant with the building code.

There is also the problem the problem of resolving the lateral forces. Currently the URM takes these forces out of the building. The new wall would have to be capable of supporting these loads if we decided we could make the system work.

 

[ltlspf]

JAE,
With the following,"cut vertical slots in the wall, opening up the empty cells and install vertical reinforcing bars (with dowels to footings) and fill cells with grout after forming the cells closed." would you cut out a continuous vertical slot from top to bottom of the wall, slide the vert reinforcement in from the side, then add forms to the face of the CMU wall filling the empty cells and the vertical slots with grout? There would be no issues with the discontinuous CMU face walls?
Would this only be if you could not get to the cells from the top? Seems that would be preferred, if possible?

One other question on this topic, what if the existing wall does not meet the minimal requirements for horizontal/ladder reinforcement? What would be the most cost effective way to add that?

Thanks!

 

[mmillerpe]

Could you possibly shotcrete the inside face of the CMU wall at certain locations and use these sections at the LFRS? You would likely need to dowel the shotcrete wall to the exisitng masonry, and would need to develop drag struts to get the loads to these walls as well. Maybe better stiffness compatibility than trying to use a stud wall as a structural element.

 

[msquared48]

SteelPE:

It would be a pain for the contractor, but if you installed anchor bolts in grounted cells on, well, say a 4' grid pattern, it cound work, depending on the wall height, stud size, and the capacity of the existing diaphragm at the top of the wall. Probably would have to use a minimum of 4X_ studs at the AB locations.

If the wall was going to be furred out anyway, then the amount of lost area would be minimized.

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

 

[SteelPE]

msquared48,

I think it would work (almost anything is better than what is there now), but I'm not sure it would be in compliance with the building code . Especially if I approached the URM wall as a facade. I'm not even sure if you could classify it as a facade due to the fact that it would be load bearing. Can't say I have ever seen a load bearing facade (of course I have only been in the business for a little while).

 

[JAE]

ltlspf

Yes - a continuous vertical slot would be preferred in my view.

There would be no issues with the discontinuous CMU face walls?

I don't think so because the new grout would bond it together in
shear and for out-of-plane flexure you'd be better off.

Getting the bars in from the top would be better but usually not possible.

 

[msquared48]

SteelPE:

I have a similar situation where that is exactly the approach I am going to take. I think it can be treated as a masonry fascade...

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

 

[SteelPE]

msquared48,

Looking in the code IEBC 2009 section A106.3.1.

"Unreinforced masonry walls used to carry vertical loads or seismic forces parallel and perpendicular to the wall plane shall be tested as specified in this section. All masonry that does not meet the minimum standards established by this chapter shall be removed and replaced with new materials, or alternatively, shall have its structural functions replaced with new materials and shall be anchored to supporting elements"

So theoretically, the code does allow you to replace the function of the masonry wall with something else as long as the masonry is anchored to the elements (or other supporting elements).

Interesting.......

 

[SteelPE]

Update,

In going through the code again, I believe I can get out of the option of upgrading all of the existing exterior URM walls. IEBC section 807.4.3 basically states that if my structural alterations are less than 30% of the structure (calculated as tributary area to the altered elements) then I only am only required to upgrade the URM walls that are being modified with new openings. If I go over the 30% then I am required to upgrade the entire structure.

I have spoken to the owner about the reinforcing requirements and he does not want to upgrade the entire structure if he does not have to. I made him aware of the consequences, especially with current events, which he is willing to accept.

 

[a2mfk]

I've used JAE's described methods on dozens of existing CMU structures, it works well and you can dictate how many filled cells you want and thus almost treat it like a new wall in your design. Its good to do a good visual review of the entire structure to identify any problem areas, cracks, separations, etc. before you draw up your final plan.

Once involved on a project where we upgraded a large space that consisted of several old warehouse buildings that were all side by side and about 70+ years old and were largely URM using these methods. I am sure it was expensive and labor intensive, but due to the downtown metro location, some historical value of the buildings, value of the property, saving the roof and walls, etc. it was very much worth it. Actually on that job we had a complete as-built done by a separate firm indicating all reinforcing and grouted cells, bond and tie beams, footing sizes, steel joist sizes, connections, etc. before we started our drawings.