CMU Vertical Reinforcement Design

[Kowycz]

I am currently designing a small CMU building with exterior load bearing walls that surpass the IBC's lateral support requirements (2109.4.1) The code states that load bearing walls (non solid units / not fully grouted) not exceed a distance of 18 times the thickness.

8" CMU Block X 18 = 144" [or 12'-0"]

The question I have is I would like to achieve greater heights by grouting vertical reinforcement in the block cores however I am not sure what the design procedure is.

I was browsing through the Masonry Designers' Guide however they seemed to just briefly explain the concept rather than the procedure.

I also have a copy of the Concrete Masonry Design Tables (by the National Concrete Masonry Association). They seem to provide a fairly decent example. This example references the national masonry standard, Chapter 7 - Reinforced Masonry; which I don't have a copy of.

ALL ADVICE IS GREATLY APPRECIATED!

Thank you,

Matthew Peters

 

[concretemasonry]

Take a look at ACI 530. It is the basis for the IBC and a good portion of the IRC. It also describes the design methods for empirical, working stress and strength design.

How high is the wall? If it had piers, pilasters or columns, what is the spacing?

Are you locked into an 8" wall? In many cases, a 12" wall can be more economical if it fits architecturally. You could even go to a 14" or 16" thick wall, that usually a rarity.

I have seen many buildings designed using the concept of a horizontal bond beam to cut the vertical span dimensions.

It is not necessary for the wall to be fully grouted since partial reinforcing/grouting with hollow CMUs is used very commonly.

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

 

[JedClampett]

You need to get out of the Empirical Method (Section 2109 and ACI 530 Chapter 5) and get into the number crunching, load path following, stress block checking sections of the code such as IBC Section 2107 or ACI Chapter 3.
Unless the building is over 20'-0" tall to the roof level, 8 inch blocks should not be a problem. And if you use the Strength Design Sections, you can exceed that.
It's a lot of work, even for a small building. You need to check wind loads, seismic, the roof diaphragm, etc.. If there's someone experienced in your office, check with them. It's not hard, but going in blind is pretty scary.

 

[JLNJ]

The taller you get the more critical the areas between windows and next to openings get. If you have punched windows and small piers bewteen them it gets a little dicey at 20' tall.

 

[concretemasonry]

Once you have structural steel reinforcement, you are out of the empirical design option. Steel for continuity and details is OK. It is a conservative and proven method and some experienced engineers use it for certain types of structures whenever possible.

Dick

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

 

[henri2]

When I used to design ages ago, I used MIA's Reinforced Masonry Engineering Handbook. The handbook had solved problems for numerous situations. It seemed to work for most situations I came across. It cut to the chase.

The latest edition is based on ACI 530/530.1-05.

http://www.masonryinstitute.org/book.php?catID=5&pID=146

I am not sure if it has both ASD/WSD and SD approaches.

 

[Kowycz]

I wanted to extend my thanks first of all to everyone who has contributed and delivered insight.

There are a few things I would like to mention, so that everyone may see my perspective on the situation.

I was suppose an important question I must pose is economics related. I noticed concretemasonry mentioned it would be more economical to just use a larger size block. I am unfamiliar with which is more costly (reinforcing 8" CMU or simply bumping up in size to 12" CMU). I had already completed the majority of the design/draft work based on 8" CMU for several buildings and a change in overall ceiling height occurred. I felt it would be less costly (in the office) to simply reinforce the 8" CMU block to achieve the desired height in these cases.

Also the total laterally unbraced wall height in the most extreme cases will be approximately 15'-0" to finished grade.

I am a recent civil graduate who will sit for the FE exam fall of next year. There is one resident structural engineer in the office I am working in and I was simply interested in getting more then one other perspective on the situation.

Thanks again to everyone,

Matt

 

[Kowycz]

I just wanted to bump this thread for further insight on the economical aspect of the situation.

 

[JLNJ]

For 15', 8" CMU with reinforcing is the way to go. Minimal reinforcement will be required.

When you go to 12" block on a union job you often need to add a man to the crew to help with the lifts of the heavier block.