12" and 8" Block

[msa123]

I'm tackling construction of my new home. Will be subbing out a lot of the work, and even though I'm trying to use very qualified subs, I want to be as knowledgeable as possible to check their work. This site is a great resource!

Because I'm building in a 100-year floodplain (wife's family land, never floods, but still need to meet FEMA req'ts just in case), I will be blocking up (enclosing) an entire lower level for parking, storage, game room, etc. I will then stick frame a 1-1/2 story on top of lower level. I won't be doing any excavating (lower level will be on existing grade), and plan on "berming" (backfilling) against front of house so that I can bring circular driveway up to front door on main level.

Where I will fill against wall (up to ~6' off grade) I plan on using 12" block (9 courses) with vertical rebar 48"o.c., horizontal ladur-type reinforcing every other course, and fill all blocks solid. Upper-most 12" block course will have horizontal rebar and be filled to create a bond beam. Final 4 courses (to reach 8'8" height) will be 8" block. Stepping down from 12s to 8s allows me to create a brick ledge.

FIRST QUESTION: Civil Engineer friend spoke with structural friend who said I should use #6 rebar (3/4") and place within 1" of inside wall. #6 sure seems like major overkill, especially since fill will not support any structures, except the driveway approximately 15' away. Any advice?

Side and back walls that will be exposed (not backfilled against) will be 8" block with vertical rebar every 48"o.c., horizontal ladur-type every other course, and only cells with rebar filled solid. Also will reinforce around any openings and fill.

SECOND QUESTION: Structural friend of civil friend again said to use #6 rebar in the 8" exposed wall. Major overkill? Steel ain't cheap these days!

THIRD QUESTION: I had planned to use 12" block for first course around entire house. This would give me a brick ledge for the exposed elevations when I stepped the second (thru 13th) course down to 8". Make sense? Other option, I guess, would be to use 8" block on first course also and use footer as brick ledge.

My only other problem is visualizing how the 12" block and 8" block will match up, especially at corners. I've intentionally laid out house so that all dimensions add to 8" intervals, but at corners where I'm stepping back from 12" to 8" to create the brick ledge, won't that make the block cells vertically off by 4"?
Any advice would be helpful.

I apologize for the length of this post. I just wanted to provide enough background information to anyone interested in offering advice.

thanks!

[Rich]

I'm not much on the masonry (except the simplest walls) so I'll let Shanley answer this one.

[Shanley]

Sorry about the late reply been working out of town...I hesitate to to offer my opinion regarding the size and spacing of rebar as I'm not a Structural engineer...That said my answers to your questions in no discernable order:..The cost difference between using #6's instead of # 5's should not be prohibitive and erring on the side of caution is my preferred method, particularly with your house...When building a 12 inch block corner you use an 8" block corner to maintain running bond and you drop a cement brick sailor (brick laid standing with wide side exposed) in the 4" exposed void on the inside to maintain continuity...Are you laying a brick veneer on this shelf?...If so, you should consider a deeper shelf to allow an air space between the block back-up and brick veneer or you will surely have an unsighty effloresence problem which will considerably reduce the life expectancy of your brick...The 1" requirement has me a bit puzzled, it is my understanding that the wall will perform much better if the reinforcing is centered...The rebar and grout are needed for shear not compressive strength, in other words the block themselves perform very well when exposed to down loads but not very well when exposed to lateral forces (ie. wind, earth et al) Hope this helps

[msa123]

Thanks for the information, Shanley.

I was planning on using the ledge when stepping down from the 12" to the 8" block for brick. I understand the need for the air gap, but what are my options for creating a deeper shelf? I plan on having 9 courses of 12" block which will be backfilled against, then five courses of 8" with the brick veneer.

Thanks

[mjpliv]

Placing the rebar within an inch of the inside block face puts it where the block in under the highest lateral tension loads created by the backfill. That is the engineer's rational.

[Shanley]

You could use 6' block above the 12's creating a couple inches for an air space

[msa123]

Shanley,
Only the front wall will be backfilled against, the sides and back will be bricked all the way up from the footer. So I'd have to go with 6" block all the way around to match up the brick, right? Either that or step the 8" block wall in 2" everywhere else? Are 13 or 14 courses of 6" OK for supporting the house everywhere else?

[Shanley]

I'd be more comfortable with 8" cmu stepping in say an inch and a half...make sure you use flashing, drainage material, weeps and minimize the mortar dropping in the cavity

[msa123]

Shanley,
I've seen construction details showing the brick veneer actually overhanging the foundation by an inch or more. Makes sense to me to do it that way, letting my brick veneer hang out over the 12" block and inch or so, still bearing on 3" of the block, providing a 1" air space. See sketch attached.

Do you see any problems with that?

[Shanley]

No. just be careful with that first course it will want to roll on you