Out of Plumb CMU Wall 1

[birneys]

I have a project with multiple quality issues caused by the masonry subcontractor. The latest is that several load-bearing CMU walls have been built out of plumb. The walls in question are exterior walls of a hardened room area (elementary school in the midwest) that is designed for 250 mph wind speeds (tornado, FEMA 361) and support a 10" solid precast plank roof. Bearing height is 12'-8". Per FEMA 361, the walls are 8" CMU reinforced w/ #5 verts @ 8" o.c.

The worst case is actually a "wave" shape; it goes one way out of plumb and then back the other way. Overall, the top is only ~1/2" out of plumb from the bottom of the wall. However, the bulge in the wall results in closer to 1" in the last five feet.

Can anyone provide some guidance on how to analyse a curved wall section like this? besides FEA? If it was just leaning one way, I would just account for the eccentricity and reduce my capacity. I'm not sure how to account for the wave shape in my analysis of the wall section.

Note: the precast roof is already in place and the topping has been poured. So, if I tell them it's no good, there will be a huge confrontation. I have to have some solid evidence if I say it's not adequate. Thanks.

 

[mike20793]

How close to capacity are you with the applied eccentricity? Did you apply any eccentricity when the wall was originally designed? I assume this is an exterior wall, so how much are the planks bearing on the CMU? Is it a FEMA rated shelter or just designed to the FEMA specs? That is pretty far out of plumb on a highly sensitive structure and if it is FEMA rated (meaning FEMA is giving funds for the shelter to be built), I would not let it slide. It actually exceeds the tolerances according to the masonry code. The max of out plumb is 1/4" in 10 feet or 3/8" in 20 feet and 1/2" maximum.

Personally, I always use 12 inch block on FEMA shelters regardless of the bearing height.

 

[JStructsteel]

well if you cut the wall at one of the waves, and did your statics, would it still work? I know its not a true way to do it, but it could give you some insight.

If you are comfortable with it, let it go, 1, if it gets hit by a tornado, who can quantify the 250 psf is correct, and if a cow hits it at 250 mph, is that included in the design? A cow hitting a wall with 250 mph speed and a 250 mph windload on it, more than likely is not going to work.

 

[KootK]

Other than general concern for poor workmanship and the violation of specified tolerances, I wouldn't worry about this much. Obviously, the plank needs however much bearing it needs. The wall shouldn't be materially affected however. If a little extra out of tolerance in a not so slender wall is really a deal breaker, it was probably no good to start with. I vote for writing them up for crap work and then letting it go.

If you want to run an analysis, I'd take a one foot strip of wall and analyze it with the slender wall provisions, building in an estimate of the out of plumbness into your analsyis.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mike20793]

I mentioned I wouldn't let it slide on a FEMA rated shelter because they can pull funding from it if it is not up to ICC 500-14 which references the same standards as IBC 2012. It almost happened on one of my FEMA shelters I did earlier this year, but it was caught before the roof was up. If it's just designed to the FEMA specs, then I agree; if you feel good about it, let it go.

 

[tstructural]

And don't forget to send the masonry contractor a bill for your added analysis and increased liability.

 

[KootK]

Great points about the FEMA stuff Mike. Tough to beat that "been there" advice.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[birneys]

Thanks for the advice. It's good to know about the FEMA funding issue; fortunately, we're not getting funding, just designing to the FEMA spec. I did apply an eccentricity for the original design as the precast planks bear 3" on one side of the wall.

The static analysis of the worst case in either direction is within capacity, so I feel better about letting it stay. The plan right now is to write them up for the poor quality and determine a credit to the owner (including my fee).

Mike, quick question: Is there a specific reason you go to 12" block on your shelters? FEMA 361 references that 8" solid grouted block is adequate for missile penetration. Do you go to 12" just as an added safety factor or something else?

 

[mike20793]

I've done about 25 FEMA shelters in the past couple of years and most are double tee roofs with over 60 ft spans so the uplift forces were large; that's one reason. The bearing of double tees is a function of the span, so when we have an interior bearing wall, the extra bearing helps. We also have a hard time getting the jambs to work on wider doors with 8 inch block. I've found that when we design to FEMA specs, architects like to include large, wide roll up doors since they don't necessarily have to meet the missile impact criteria. We found that it's usually better just to make them 12" CMU and save ourselves the trouble down the road. The extra cell width helps with installation of our uplift bars as well; they can be tricky with the camber of the double tees. Overall, it's a combination of constructability and need for design, so we tell architects up front that if it's a FEMA shelter, they need 12 inch CMU.

As a side note, I have been getting quite a few concrete dome FEMA shelters, too; some have diameters of over 160 ft. It's amazing how efficiently a 4 inch roof can perform at such large diameters. It seems that FEMA is really pushing these hard and is offering funding to build these things. Everyone is ramping up for the IBC 2015 when they will make FEMA shelters required on new schools; there is some question to whether local jurisdictions will adopt this provision.

 

[birneys]

Mike, would you be willing to share a detail of a typical CMU wall to precast roof attachment? I've tried several different details over the last year, and the contractor has either responded "we can't build that" (after they win the bid) or I show up at the site and they just didn't do anything. Then I either have to come up with a retro fit detail or delay the project and make them rip it out. I think the main problem is that my area is just starting to build these shelters, and so the contractors are all whining about the extra cost and telling the owners (school districts) that I'm being way too conservative, wasting their money, etc. I would love to see what other engineers are using, so that I could have more evidence to convince my clients that I'm not going overboard. Thanks!

How do they get around the missile impact with the roll up doors? I would think that you would be more susceptible to missile impact and suction with the larger openings.