Pre LSD Unreinforced Masonry Walls 1

[KarlT]

I am working on a project (in Canada where it's cold and snowing!) in which we are removing the existing brick veneer in a concrete block cavity wall, applying a new air/vapour barrier, new brick ties, insulation and installing a new brick veneer wythe.

The structural wythe is an existing 12" block wall, with a clear span of 22'6" from the bottom of the wall to the underside of some existing 42" deep joists. The bottom of the joists are tied to the block wall with weld plates, and the diagonal bridging is tied to the non-load bearing block walls with plates at the underside of joist elevation.

We are using newer rigid "l-ties", which will be fastened to the existing block with 2 epoxy set inserts, allowing the wall to act partially composite with the brick. The problem is that even accounting for the benefits of partial composite action, the allowable tensile stresses in the existing block are exceeded implying that I now need to reinforce the wall.

The stupid thing is that this wall has been in service since 1977 and has been performing fine since then. The wall was well constructed (based on site observations)and there are no visible signs of any cracking or distress. There are many examples of walls like this one that are all performing well, but they do not meet the newest codes. Is it typically assumed that once you touch the wall, you need to bring it up to codes, or can you say since it has been performing well we can leave it alone?

By the way, I am strongly leaning towards grouting in bars to satisy the code because in the grand scheme of things it's cheap insurance to do what I assume is required.

How does everyone else deal with type of problem?

 

[JAE]

Structural Engineers always get hit with "But..its been doing fine for 27 years and now you say its not strong enough?"

What is not well understood by non-engineers is the concept of safety factor that is REQUIRED by the codes. The wall may have been designed say, for a 25 psf wind and with safety factors its ultimate "fall down" wind would be about 32 psf. But over its 27 year life, it perhaps only received an 18 psf wind. But now you must meet a 30 psf wind (these are all fictitious numbers). It won't fall down, and with the 18 psf, never reached its cracking moment, but it does not meet code.

You, as an engineer, must use your best judgement as to what the actual numbers are - cracking moment, safe moment, ultimate moment of the as-is condition and compare that with the code requirements. Perhaps a 5% overage in the allowable would be acceptable....but that's your call.

Other things to consider - Wall components and cladding forces are based on tributary area....what's the true trib. area of a wall, all interlocked and deflecting as a half barrel vault? Its more than 4' wide...but this is a subjective choice. Also, are there any other overly-conservative assumptions that have gone into the analysis of the wall? Think through everything as many times you can find small things that better reflect reality.

 

[TFL]

WHEN WE REPLACE FACADES WE UPGRADE THE WALL BEHIND IT. WE TYPPICALLY TRY TO DO IT WITH STEEL TUBE WIND COLUMNS. THE TUBE ARE USUALLY EXTERIOR TO THE EXISTING WALL.

AS A SIDE NOTE WE ARE DOING SOME FORENSIC ENGINEERING ON THREE SCHOOLS THAT WERE BUILT IN THE LAST THREE YEARS IN A NY TOWN THAT HAVE GYM AND AUDITORIUM WALLS THAT ARE APPROX 24 FEET TALL THAT ARE 8" UNREINFORCED MASONRY! THE CRAZY ARCHITECT PUT HIS ARCHITECTURAL STAMP ON THE STRUCTURAL AND MECHANICAL DRAWINGS FOR THESE SCHOOLS. NEEDLES TO SAY HE IS BEING INVESTIGATED BY ALL SORTS OF PEOPLE

 

[KarlT]

In my initial analysis, I assumed a simply supported wall spanning 22'-6" from bottom of wall to underside of joists, resulting in a max. moment of wl^2/8.

To try to squeeze out additional capacity, I actually tried to take advantage of contuniuty by running moment distribution. I assumed the wall base was pinned, the connections between the underside of joists and the wall acted as a pin and the top of the wall was pinned.

This actually reduced my wl^2/8 moment at mid-ht between u/s joist and bottom of wall by about 40%, but I got a negative moment at the underside of joists connection of 85% of wl^2/8 of the original simple span...so even that moment still exceeds allowable tensile stresses...

I know what you mean JAE, sometimes you take a big step back and then you can rationalize a different way of making it work. However, in this case i don't think I can do the same here, the wall will need reinforcing.

 

[jheidt2543]

KarlT,

In your first note you say that you are planning to grout the cores and add rebar in the wall. I would be interested in knowing how you plan to do that. Did you look at other alternative methods of stiffening the wall?

 

[KarlT]

jheidt2543

Since the existing brick veneer is being removed, the full height of the block wall will be accessible from the outside. I was going to have the contractor cut continuous vertical slots into the face shell @ 5'-4" o/c and grout in a continuous 20M rebar.

I have also heard of another way of grouting in which the face shells are locally opened up, and rebar is fed into the cores and lapped as required.

Beyond that, I had considered trying to cut down on the clear span of the wall by diagonally bracing back to the underside of the joists but that didn't look too appealing.

 

[JAE]

KarlT - did you include self-weight of the wall in your calcs? That will add axial compression to your wall and help a bit.

 

[KarlT]

JAE
I actually added self weight of the block and the dead load reaction from the roof, with a 0.85 factor on the dead load (since it helps me in this case), but it still wasn't enough to keep those tensile stresses below the limit.

I was talking to a former masonry professor of mine and he was telling me that when the LSD code for masonry came out, some people actually considered putting out an alert that some of these existing walls may be unsafe. However, since these walls were performing satisfactorily, it was decided that the issue would be left alone. He also mentioned that there is a provision in the National Building Code of Canada that says if a building has been performing satisfactorily since it was built, then it may be possible to leave it as-is.

Ultimately, it comes down to a judgement call. By the letter of the law, if we are doing other work to the building anyway, then it's my responsibility to ensure that the new wall system meets modern codes. If I ignore the fact that the old wall doesn't meet today's codes and something bad happens, then guess who gets the blame.....it's certainly not the people who said "what are you worried about, the wall's been performing fine!"

You're right JAE, sometimes our decisions are not popular, but at least I can sleep at night!

 

[dutchie]

even under section 16 of CAN/CSAS304-94, that wall would need to be 13.5" wide, so it's already outside of empirical rules.

Depending on the extent of the renovations to the building, you may not have to improve the wall by code. In Ontario, refer to Section 11 of the OBC (i think the same goes for the NBC). If you are not changing the occupant load or hazard index to the building, or adding additonal loads to the wall, then the wall is ok to stay the way it is. If you are affecting any of these things, NBC will require you to update the wall to current day codes.

The nice thing is that using engineered masonry design, you can get quite a bit of strength out of a 12" wall. A repair we once did to a wall involved breaking off a face shell at the top of the wall and feeding in 15M rebar. We then grouted the core with the benefit of some holes throughout the height to confirm the grout got where it was supposed to go. It was pretty painless in the end.

There is program out there distributed by the Ontario Block Producers Association which does engineered masonry design in accordance with CSA S304-94. It is pretty easy to use and is very handy when working out reinforced masonry walls (as you well know, the hand calc method is quite time consuming).

I think if you try 15M at 600 o.c. you will find you have plenty of capacity. It seems to me that you may not have adequate min steel area with the size and spacing you are proposing according to code.

Sorry to run on, hopefully this is of some help

good luck
dutchie

 

[KarlT]

Thanks Dutchie for the helpful info!

However, i was thinking I would keep the bar spacing more spread out, to minimize the additional dead load on the grade beams and piles below. Once you start grouting a 12" block wall, you can add quite a lot of weight that wasn't included in the original design of the support structure below.

PS-Here in Winnipeg this morning it's -40C, and with the wind chill it feels like -52C!