8" CMU wall with multiple 6" CMU courses around the buildng

[bldgdesign]

Architect is showing a 6" CMU course at multiple locations in an 8" CMU wall so that it creates an architectural band (reveal) around the buiding perimeter.

Does anyone know where I can find out how to address this in ACI 530? Or any other document or article that can help me determine if this is actually allowed and how to adjust the wall reinforcing accordingly.

thanks.

 

[SteelPE]

Usually when I run into this I check the design of the wall for the forces at the reduced section (moments etc.). If everything checks out then fine.... if not then I don't usually let them do this.

 

[ToadJones]

I'd be most worried about the bond between the two courses since the faces will not line up.

 

[dhengr]

The problem is that the face shells of the conc. blk. are the primary compressive strength elements in that wall system and usually the only mudded shells. And, on one or the other face they do not line up to transmit their compressive loading. Thus you have a face shell bearing on a few cross shells, assuming that the cross shells even have mortar on them, and that doesn’t usually happen unless you call for it on the drawings and in the specs. Obviously, the reinforcing effectiveness and the wall thickness is significantly reduced at these bands. You would have to call for a full bond beam above and below these bands to start to make structural sense out of this. Architectural sense is a whole different matter, and not easily rationalized at times. What the Arch wants to achieve can be pretty much accomplished by using a full width conc. blk. of a different color or texture, or with break-off ribs or some such, without compromising the strength of the walls.

 

[Ron]

What he is proposing is a waterproofing nightmare as well. Pain in the a$$ all the way around.

 

[msquared48]

Why doesn't he just mix 8" regular CMU block with 8" split face? Same effect for the reveal, and it is more sound structurally.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[bldgdesign]

I don't like it either. I can't find anything that says you can't do it, unless the tolerance portion of the code applies, 3/4" max offset. However, that is meant to be for construction tolerance from the documents.

If anyone uses Enercalc, I did notice in the masonry wall module, you can input a reveal distance and height, and it shows the picture with the block inset and the reinforcing lapped.

 

[msquared48]

Oh! I know the answer! IT'S TOO SIMPLE!!!!!

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[concretemasonry]

There is also the concept of "corbeling" that allows for a prescribed maximum offset IF there is full face shell bedding.

How high is the wall? Very possibly it could be a 6" structural wall with courses of 8" block (bond beams or filled top units) used as projections beyond the face of the 6" block. 6" block can carry large loads and the 8" can be minor elements if the architect is knowledgeable. - There I go again about attacking the U.S. tradition of using low (assumed minimal) strength 8" units with no accepatance standards.

Dick

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

 

[DST148]

@bldgdesign: I presume it is a nonloadbearing wall. Ref ACI 530-05: For non-loadbearing corbels, the maximum projection of one unit shall not exceed (1) one-half the nominal unit height (2) 0ne-third the nominal thickness of the unit. So 8" wall bearing on top of 6" nominal thickness would be acceptable. The code requires units to be solid. Like dhengr suggested, the coursing below and above 6" thick block could be bond beams or fully grouted, including the 6" thick band. The vertical reinforcing could be in the center of the 6" thick wall. On a conservative side, the capacity may be assigned based on 6" thick wall.

 

[concretemasonry]

Assuming that the capacity is based on the 6" thickness is fine IF it is a properly engineered CM wall. they have been known to be used on very tall load-bearing structures. All that changes is the specified strength and recognition of the actual wall height.

Dick

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

 

[bldgdesign]

This is an exterior load bearing / shear wall. Wall height is 12'-8" to the roof and has a 5'-0" parapet. Wind speed is 90 mph, but insured by FM Global. Calculations show that the wall works. Just trying to determine if it is allowed as described (8" cmu with 6" reveal course); not 6" cmu with 8" corbelling.
Only thing mentioned in ACI 530 for reinforced masonry is to desgin by one of the design chapters. Reinforced masonry doesn't appear to be limited in the amount of corbelling per course the way unreinforced is. Just needs to be stable and meet design requirements.