CMU wall - footers, bondbeam, rebar, et cetera


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Old 02-25-09, 09:42 AM
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CMU wall - footers, bondbeam, rebar, et cetera

I'm building an 85 foot long CMU (concrete block) privacy wall. This is exterior and not load-bearing. The wall will be 8 feet high and I am using standard 8x8x16 inch blocks.

How deep and wide do the poured footers need to be? This is Louisiana - ground does not freeze. Rule of thumb would be 8 inches deep and 16 inches wide. Seems a bit shallow. We have hard clay soil, undisturbed, well-compacted. Should I go deeper with the footer?

I also calculate I need a column or outcrop every 20 feet along the length of the wall. I need vertical rebar every 4 feet along the length, too. How many bond beams do I need? Is one at the top and one in the middle enough for an 8 foot high non load-bearing wall? Too much?

This is all prescriptive method stuff, but I haven't found one comprehensive source for masonry empirical design. Am I on the right track? Thanks guys for your help.
 
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Old 02-25-09, 10:20 AM
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You will need an engineer stamp on the plans for the footer and will probably not be able to build it 8' tall. If you are in the sticks and not subject to permits and code, you will most likely end up building a very expensive sidewalk without engineering.
 
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Old 02-25-09, 10:45 AM
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Tscarborough, could you elaborate. Why couldn't I build it 8 feet tall? I guess your comment about the sidewalk is your way of saying it will fall over? Rebar anchored in the footing and outcrops every 20 feet are there to prevent that.
 
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Old 02-25-09, 11:13 AM
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CMU wall - footers, bondbeam, rebar, et cetera

Very few places permit an 8' high privacy fence.

Common sense dictates the need for an engineered design, especially with the potential for high winds. This is especially true when you have no idea of how to build it.

Could the fence fall on a neighboring property?

Dick
 
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Old 02-25-09, 05:57 PM
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Well, unified development code in the area permits a maximum of 8 foot tall privacy fences.

Common sense also dictates that engineering has already been done with universal building materials that appear in the international building code.

Why do you assume I have no idea how to build it? Did my original post really give you that impression? I thought this might be a good place to ask for a little help with getting the details right.
 
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Old 02-25-09, 06:42 PM
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CMU wall - footers, bondbeam, rebar, et cetera

Your "unified" development code may permit 8' high fences. Does it say anything about the design and materials?

Where does it appear in the IBC or the IRC, which are the major codes in effect in the U.S.? Just because the materials appear in the code, they have to be used and designed properly. Empirical design is contained in ACI 530 and is recognized for some applications in the IBC and IRC.

I questioned the background or knowledge because of the use/design by the questions asked and the use of some common terms (and some incorrect terms) but some unnecessary construction requirements thought of or proposed and a total void of the very important questions. When you are referring to "hard" clay as beneficial when it comes to a shallow footing, all I can think of is a wall turned into a 8' wide concrete sidewalk. There were many 6' wide wood sidewalks in MS and LA a few years ago.

After spending many months after Katrina investigating the damage and destruction, I am well aware of the real lack of enforcement of any codes and the very poor use of any outside standards. In your area, you apparently have no codes that you can say are in effect. - That is unfortunate since you have nothing to keep you from making mistakes.

Since I am an engineer, I would never give someone advice on the construction of something as potentially dangerous as what you are proposing. If you were talking about a reasonable height and had some real soil information and a code to insure that it was constructed correctly, that would be a different story.

In some countries, they routinely build 20 story loadbearing block buildings out of 6" block, but they are designed and built properly according to U.S. standards with approved details and inspection procedures. They would never allow an 8' block privacy wall with the level of design you are proposing.

What you are proposing will stand for a while, but I would suggest you or you children do not get too close to it in any periods of strong wind or heavy rains.

Dick
 
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Old 02-25-09, 07:49 PM
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chicago mason, thank you.

Dick, I appreciate your experience. That is why I was hoping I would get your input when I came here. I've read your posts before. It's unfortunate that you assume I know nothing, and that you assume I can't even name the code in my area. We use the 2006 IRC, instituted by former Gov. Blanco after the Katrina disasters. The unified development code is a local code which dictates setbacks, maximum building and fence heights, et cetera, but leaves design and materials to the IRC.

Which construction requirements did I ask about that are unnecessary?

You also assume that I think our soil is beneficial when it comes to shallow footing. On the contrary, I thought those rule of thumb footing dimensions seemed too shallow and narrow. That was my concern and major reason for posting here. I would think they need to be deeper and wider. I also think a deep and wide spread footing, with vertical rebar anchored into it and continuing up grouted cores of the CMU wall all the way to the top, would be the most important element of the design to keep it from falling.

I mentioned clay because different soil types have different bearing capacities. Clay is often graded by type, like soft, ordinary, hard, but usually does not have very good bearing capacity. But firm clay is better than soft clay. Next time I guess I'll just say "clay."

Which questions do you think are the most important ones that I left out?

Ok, so you think that an 8 foot high CMU wall is impossible to safely build, no matter what the footing is like, no matter how much rebar, no matter how many pilasters or how close together the pilasters are. I disagree with you, but you have more experience than I do. I would like to engage you in conversation (ok, typed conversation) to learn more, but I'm guessing I've missed my chance.

But since you've assumed so much about me, let me tell you what I assume about you. I assume that you are a professional with a good bit of experience. And I assume that since you take the time to frequently participate in an online forum named "doityourself" that you actually want to help people complete their projects in a safe manner. But by just dismissing people you run the risk of making them angry or upset and disregarding anything you have to say. Not me, because I'll just keep looking for good information. But how many others do you think just say "aw the heck with it - I guess I'll just do it on my own."
 
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Old 02-26-09, 07:15 AM
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Here is the windload map for Louisiana:

BOAL - Building Officials Association of Louisiana - Home

I am familiar with the soils of the southern 2/3 of the state, and the clay soil you have is expansive. It may require drilled piers or spread footings. Your wall, no matter how knowledgeable of building you are, should be engineered.
 
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Old 02-26-09, 06:51 PM
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CMU wall - footers, bondbeam, rebar, et cetera

I try to help if I feel the person will do what is recommended.

Tscarborough clearly said you need to have the wall engineered. I suggested it was prudent to use common sense and have it designed professionally.

I make suggestions if it seems they will be carried out completely. In the case of a wall like you are suggesting, it definitely needs professional engineering and not just using selective sections of a code that is not intended for the application. I do not encourage building without proper planning and controls.

If you are fortunate, you will be able to get a permit and the services of an inspector to make sure it is built correctly.

You may be over-designing the wall, but if you do not have the footings to provide the stability for a high wall or a means to make sure the construction is correct. Without any soil information, it is fruitless to try to design a cantilevered wall. If it was a load bearing wall, it would be much easier and reliable. The wall strength is not the problem. If OSHA ever saw what you are proposing, the job would probably be shut down during construction unless it was braced properly.

I enjoy helping people when I feel that everything is taken into account and that is why I suggest getting a professional to design it and possibly help on the control of construction methods if you do not have a permit and inspector. I know the idea of a professional designing and a permit and inspector may be contrary to the way things have been done in your area (wherever it is) in the past.

I have seen too many problems when some items were not considered and were not caught by someone on the site that could recognize a problem.

Dick
 
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Old 02-26-09, 08:32 PM
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I don't know why you persist in thinking this will be done without permit or inspection. I guess you have one idea about Louisiana and no matter what I write that won't change.

I do have soil information. It's firm clay, has a load-bearing value of 1500 psf. I check the IRC and it says minimum spread footing width of 12 to 16 inches. Gee, that seems really narrow for an 8 foot wall.
 
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Old 02-26-09, 08:41 PM
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It is your deal, good luck with it.
 
 

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