4" Interlocking Block Wall with Pilasters 2

[swm25]

Mesa, AZ has a guide for Residential Fence Walls and Retaining walls with the attached 4" interlocking block wall W/ Pilasters detail in it. I sometimes get asked to design a wall that is similar to this wall i.e. a little retaining, or a little bit higher, but I am unable to get this wall to pencil out as far as overturning moments from wind forces and the bearing pressures from the overturning wind forces. My designs end up being more robust, and I get questioned and accused of being overly conservative.

Is there a different way of looking at these systems that I am missing? I don't believe that there have been many issues with these systems failing.

 

[KootK]

Some options:

1) Low importance factor?
2) Treat the panel between piers as three sided with some shear going straight to ground?

Does the little pile work in flexure as an unreinforced bit? If so, I'm surprised.

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.

 

[concretemasonry]

The Arizona Masonry Guild standard applies only to the interior walls of a development and not the exterior walls that can have different potential winds and importance.

The material requirement standards (CMUs and grout) are extremely low and are fraction of what is normally possible to find and use. I don't think it is even possible to produce and deliver a CMU with a strength as low as the spec allows.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[swm25]

I saw that and figure that it is related to their use of a 10 mph wind load. But I couldn't get it to pencil out at 10 mph even treating the whole wall and panel concrete footing with the pilaster to resist overturning. Perhaps they just don't need to meet 1.5 overturning F.S.

 

[dhengr]

Swm25:
Those walls are all over the place in Arizona, many communities and cities have a std. detail for them which they seem to pretty well accept. I can only imagine that the std. detail (essentially the detail that you show) is o.k’ed. because it has proven to work over a long period of time, so they look the other way. There are minor variations in the block materials used, exact max. dimensions and spacings of piers, etc., but they are all essential what your detail shows. I find it hard to imagine that an EOR could provide a set of calcs., etc. which really meet the current codes, newest lateral loads, etc. They have pulled out all the stops and used every conceivable favorable adjustment factor and design consideration to start to really justify that system. BUT, they do generally seem to work, and if they fail they are on your property, and are your problem. As mentioned above, your detail is for interior privacy walls of the development. The exterior perimeter walls which may involve the greater/general public safety are always huskier structural walls and foundations. One of the biggest issues is quality of workmanship and attention to the exact details, and this varies greatly. The conc. blk. products used vary quite a bit too, many of them are pretty weak, quick made, poorly compacted blk. units, and poorly cured, crumbly, weak, every negative descriptive possible, to build a weak privacy screen wall. But, they’re cheap and they usually work. You really have to read the notes and fine print to understand all the exceptions and various code liberties taken to even start to justify these walls.

 

[swm25]

thanks dhengr, I agree with you that these walls seem to work, and that they are prescriptive, i.e. history shows they do good enough even though they may be difficult to get the numbers to work, it is a bit frustrating when people want something that is very similar but a little different such as a bit higher, or a bit of retaining, and my design comes out much more robust when I make the numbers work to code, and then the client is surprised/upset by the design. Warning them up front helps, but I was just wondering if I was missing something that would help.

 

[woodman88]

Got to love the government, using an engineer's detail that is out of date and getting away with doing it.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[concretemasonry]

If I recall correctly, there were full scale test on the masonry portion of the wall/pilaster assembly done when the standard was researched and established years ago.

Engineers often get lost in masonry design and especially, the walls with full bed joints and mortarless head joints. Usually, this involve some testing and masonry portion to determine the wall properties. The Arizona Masonry Guild or similar names previously had a big budget for testing, just as the SoCal masonry groups. - Masonry is properly analyzed and designed on a wall performance basis and not by picking out individual items or components.

As far as the more or less "global" over turning, that is an individual site problem depending on the soils and geometry of the specific foundation dimensions.

dhenry - the masonry units may appear to be sub-standard in you opinion, but the standards are really woefully low, considering current manufacturing methods in AZ. I sit as voting method on the ASTM C90 specifications for CMUs and we get a bit frustrated because the standards (diluted by a consensus membership) are ignored by producers because it actually costs more to make, sell and deliver a poor CMU than it does to make a superior unit. The cheapest way to make a stronger block is to use more water than to use more cement as an example.

Personally, I do not like 4" thick CMUs because of the aspect ratio. In the real world, a contractor will charge more for a single exposed 4" wall than a 6" or even an 8" wall.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msquared48]

Well, this looks like a pole footing situation where the CMU spans the wind forces horizontally to the pilasters. The interstitial footing is for placing the CMU and to take the vertical load from the CMU. I would only take about 1/4 of the span to either side of the pilaster as going to the pole footing in vertical bearing. The rest to the strip footing.

They are only using 10 psf wind - a little light here I think. Plus the 200 psf/ft value can be doubled as more deflection can be allowed being a screen wall.

The 4" CMU seems light to me to span 10 feet, but maybe the wind never blows greater than 10 psf in Arizona...

Mike McCann, PE, SE (WA)

 

[swm25]

He does put an assumed minimum bearing pressure, wind pressure and passive pressure on the detail which seem to indicate conditions that it would be designed for.

 

[woodman88]

Using ASCE 7-05 eq 6-28 with
exp B, and an importance factor of I=.87, Kz=0.57, Kzt=1.0, Kd=1.0, V=90, G=0.85, Cf=1.2.
I get a 10.6 psf wind pressure.

As for the lateral design of the bearing, I wonder if he used the table value for class 5 materials (100psf) with the two times factor for structures not adversely affected by an 1/2" motion?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[ruready]

It doesn't pencil out, just like the City standards in San Diego don't pencil out. Our firm designs the majority of retaining walls in Las Vegas and we cannot compete with City and County standards from dozens of municipalities in California and Arizona. One reason why is they treat cmu fence walls just like a wood fence wall, if it blows over (which they do all the time in Arizona) then you just stand it back up. Second reason, why update a design if they don't have failures (City of San Diego - took me a while to get City staff to finally admit this). So, most likely you are not doing anything wrong, they are just allowed to cheat. On one end they make your life miserable during plan check process and on the other they exclude themselves from following the rules they make us adhere to.