Reinforcing Existing unreinforced CMU 2

[mrpid]

I have been asked to evaluate an existing structure which was originally built as unreinforced CMU. One story, 10,000 sf (non seismic). I am familiar with CMU design and can evaluate the capacity accordingly; however, I am exploring different strategies to reinforce the walls if needed (it may be acceptable as is because it is an existing structure and modifications are cosmetic, so I basically need to determine if it can resists the loads it was designed for and whether or not they intend to add any elements that would increase the loads or adversly affect the structure).

Thanks in advance for any opinions!

 

[JohnWylie]

Fiber Reinforced Polymer (FRP)is one way to go...I did my masters thesis with reinforcing unreinforced masonry wall with FRP...very comparable to steel with respect to strength and ductility...my thesis focused on out-of-plane bending, but FRP is great in in-plane bending...

There are a variety of installation methods for FRP...you can apply FRP externally with sheets or strips...or you can apply FRP internally as rods (circular or square), of course if you have access to the top of the wall...or what I did for my thesis was a method called "near surface mounted", which entails cutting grooves into the surface of the wall and inserting rods or strips with epoxy...

If you would like some literature, I would be happy to send you some papers I wrote in grad school about it...

 

[jike]

Many options come to mind....

Inserting reinforcing rods in vertical cores and grouting
add wind girts
add wind columns or pilasters
etc.

The selection of the appropriate method has alot to do with the function of the building, current occupancy and possible disruption.

 

[dm3415]

JohnWylie,
I've always been curious, would you apply the FRP, or carbon strips if that's a solution, to both sides of the wall for the tensile strength in both directions out-of-plane? I heard of an engineer only using one side, which doesn't make sense if the EQ load goes in the opposite direction, where there's no tensile capacity. Also, when you say in plane bending, are you referring to the shear, or the chord forces at the ends of a shearwall?
Thanks

 

[BAretired]

If you apply FRP to one or both faces of the wall, you change the appearance of the wall which will likely be a consideration for the architect.

Unreinforced masonry walls were commonplace in the 1950's but nowadays, most engineers specify nominal vertical bars at about four foot centers even when stresses are low. Probably most current codes require reinforcement, but there must be large numbers of buildings still in existence today whose walls are not reinforced.

If the walls met the code of the day and have behaved satisfactorily, the owner is not obliged to bring them to the current standard, at least not by my code, NBC.



BA

 

[dm3415]

Ender the IEBC, the cousin to the IBC, you may be required to retrofit the CMU walls with as little as a 10% change in the structure as a trigger.

 

[dm3415]

Sorry, Under, not ender

 

[JohnWylie]

dm3415

It's funny that you ask that question because that was the very first question I had to answer during my defense. During my defense, a few of my professors and myself got into a debate over this exact question....It comes down to what application that is applied...with regards to the near-surface mounted (NSM) FRP (my thesis), it doesn't matter which side of the wall. Now, this is assuming one (1) wythe (which is what my thesis focused on). I believe that with multiply wythes, it would be better to reinforce both sides of the wall for the reverse loading. The NSM method is a relatively new method and to my knowledge, EQ testing hasn't been done with multiple wythes.

In fact, I don't think I have come across any papers/articles for any FRP application under EQ loading. I have seen blast loading though.

To answer your other question, I did mean shear when mentioning in-plane bending...if fact, that was the original intent of the FRP....

BAretired

That was the problem with other methods of FRP application. The NSM FRP method actually hides the FRP within the masonry/CMU...therefore "improving" the aesthetics...

If you guys have any more questions, feel free to ask!!

 

[JAE]

I don't see how applying an FRP to the inside surface of a wall helps in the flexural resistance for outward bending of the wall. Your "d" on the FRP would be zero.

 

[hokie66]

JW,

Agree with JAE. I think you will have to repeat your defence here. Even if not right on the surface, the d would not be enough.

 

[dm3415]

JohnWylie,
Are you saying the FRP is applied inside the cells? If so then I guess you do get a little d (about 1.375 inches or so)for O.O.P. loading, I'm sure everyone here would be interested in knowing more on this, it sounds fascinating.

 

[BAretired]

Here is a file which may be of interest.

BA

 

[hokie66]

For out of plane vertical bending, the FRP looks a lot more costly than retrofitting reinforcing steel.

You saw slots in the face shells, not full height but long enough to lap bars, clean out the cores, insert and tie the bars, space off the faces, form and grout. Labor intensive but not complicated.

 

[JohnWylie]

I think some of the questions might be best explained with the two papers I have attached in addition to a link for my thesis.

I should have been a little more specific about the reverse loading issue. I can really only speak for my research with NSM FRP strips. The paper that BAretired attached is a good article and I believe I actually cited it in my thesis.

Anyway, my unreinforced masonry walls (URM) were reinforced with FRP strips in both the horizonally and vertical directions (pretty much a grid). The idea was to resist not only out-of-plane loading, but in-plane loading as well (as in real life situations). In essence, the FRP strengthened the wall significantly, in addition to an increase in ductility. Now, with that being said, I see all of your points into how the reinforcement on a reverse loading wouldn't do anything...well, I believe that since the walls were strengthening to a point that regardless of were the FRP was located and the direction of load, it was going to resist. Again, this is only for one (1) wythe and NSM FRP strips.

I think its hard to answer questions sometimes via forums, so I hope with the attachments and explanation, it helped...

 

[JohnWylie]

Another journal attachment...

 

[JohnWylie]

Thesis attachment...

 

[asixth]

where did you get the FRPRCS conference proceedings?

 

[JohnWylie]

asixth

I guess I don't understand your question completely?....I wrote the FRPRCS paper that is attached in grad school if that is what your asking. My advisor went to the conference (he is from Australia) and presented the paper...

 

[asixth]

JohnWylie

I have been looking at FRP strengthening lately and in particular have been looking at papers written about FRP design examples in the field. I thought the FRPRCS proceedings would be a good place to find some papers, and seeing that the 2009 conference was in Australia, I though the papers would have been released but this hasn't been the case (normally proceedings from conferences are posted on

http://www.informit.com.au.

My old university has the proceedings from the 2007 conference in Greece but I guess the 2009 proceedings hasn't been released yet.

 

[structuresguy]

I have to agree with the others above, unless you laminate both faces with FRP reinforcing, or insert the bars into the cells near the middle of the wall, out of plane bending will not be resisted adequately in both directions, at least not for any significant hurricane wind pressures. While the wall may not "fail" completely, certainly excessive flexural cracking would occur on an unreinforced tension face.

That said, we get into situations quite often here in Florida where we are renovating an unreinforced masonry wall building. We find that sawing in steel rebar is the most cost effective solution 99% of the time. Any kind of FRP repair is very costly for materials and experienced labor is both scarce (at least in our area in Florida) and more costly than typical block masons.

Amazingly, many of these buildings have survived several significant hurricanes with little to no damage. We have a local hospital that hired us to perform a hurricane resistance evaluation. THe hospital was built in phases over the last 50 years. The construction types of the buildings are unreinforced double wythe brick, unreinforced concrete block, metal stud and EIFS, and the newest is metal stud with stucco over rigid sheathing (not EIFS). Our evaluation showed that the double wythe brick (50's vintage) was actually stronger than both the unreinforced block (70's) and the 80's vintage metal stud walls. Only the new metal stud walls we designed a couple years ago were stronger than the brick. Also, in this case, the unreinforced block would not meet the current code requirements for hurricane winds, while all of the others would.