Equivalent CMU design? 3

[CivilNJ]

Locale: Northeast US
Contruction: Residential 2 Story Addition - Basement Area (approx: 500 SF)
Wall is 8'H. and is on a 12x24 footing.

I have a question regarding a substitution for CMU wall design. My Architect designed a 12" CMU wall with top and bottom 2 courses filled solid. The Contractor changed this to 10" CMU with inline piers (about every 6') for the lateral load on the wall. The Architect accepted this design change and this raised a couple of questions for me.
1. Is the cost truly equivalent or negligible?
2. Do the girder pockets need to be filled solid (piers)? There is only one, but it will bear part of the load for both stories. In other words, is it acceptable to place the girder in the 10" CMU without additional support?

p.s. I already stopped work and met to discuss future design changes.

Thanks,
Jim

 

[structbear]

OK this would be my answer to your questions.

1. I believe that the pilasters would cost more. You have to locate them and cut block etc. I would think that it would be cheaper just to pay more for the block than to build it. But if the contractor thinks he can do it, power to him, he is the one who has to build it.

2. I have been told that if the bar spacing is less than 24" o.c.. Grout it solid. The cost to block the grout flow is more than just grouting it solid. As for any thing bearing. grout and reinforce directly below it. Depending on the load you may even have to put a pilaster below it.

 

[CivilNJ]

First, thank you for the quick response.
Second, let me clarify further.
The inline piers (pilasters) are simply concrete filled blocks in the wall itself. Therefore, there are no special cuts in the block. (it does not protrude as a normal pilaster would) It is just a pair of cores with rebar from bottom to top of wall to prevent the lateral load from collapsing the wall.

I tend to agree with part 2 and was pretty much looking for confirmation.

 

[UcfSE]

What do you mean by cost to block the grout flow? The cost to keep it from flowing into everycell instead of only those with rebar?

If that's the case I strongly disagree. The "block" is just a roll of screen in between down pours. You do have to stop and screen the course below the lintel but I doubt very seriously that it is comparable to the extra concrete cost and labor and pump rental. We're talking a few minutes versus a lot of minutes and extra material. I'm thinking I misunderstood, it makes no sense to pour it solid "to save money or time".

I agree that the wall should have been just as easy to leave it at 12". Even though the pilasters are the same depth you still have to have reinforcement for the cmu between the pilasters in order to transfer the load to the pilaster. You could have vertical reinforcement spanning to a bond beam at the top that transfers load to the pilaster depending on exactly your situation. I think my point is just having pilasters isn't merely that simple, there is more detailing required to make it work. If that's all done correctly and you save money, that's great.

 

[SperlingPE]

In my neck of the woods, the cost savings realized from switching to 10" block is purely a labor savings. The local masonry unions require two people to handle 12" cmu and they price accordingly (I do not know if two people actually handle the block on site).

The difference in the two walls (10" cmu v. 12" cmu) is more reinforcing required for the 10" wall and slightly less expensive block for the 10" wall. The same number of block will be placed regardless of the thickness. Assume that there is no extra labor required to place 12" cmu. The cost savings on the block is probably negated by the use of more reinforcing and more grout. Keep in mind that prices vary for different regions. If you are really concerned about the cost, ask the contractor to submit his cost breakdown numbers for both walls and ask an independant third party to review. Or ask the architect to verify the numbers. My first concern would be the design of the pilasters and the horizontal reinforcing required to transfer load to the pilasters. If this design was not checked for the project conditions (soil conditions), then the cost issue is moot.

Contractor's two motivator's to change design are to save money or to make construction easier.

 

[bjb]

Is this a basement wall? In my opinion the design of this wall should be verified by a qualified structural engineer, I would not leave it up to architects or contractors. I have seen many basement walls fail, sometimes by bulging and horizontal cracking in the span, sometimes by an anchorage failure between the wall and the floor system.

A 6 foot spacing for rebar sounds to far apart to me. Also, if you consider these reinforced cells to be hidden pilasters, then something has to provide a horizontal reaction at the top of the pilaster. How is this being handled, especially when the wall is parallel to the floor joists?

Girders should absolutely bear on solid masonry.

In my opinion this masonry wall should be designed to resist the horizontal loads by spanning vertically, by providing rebar of the correct size and spacing. This way, the horizontal load is transfered to the floor system as a uniform load in pounds per foor at the top of wall. Pilasters will concentrate this load, making it harder to transfer.

Is there a high water table? What are the soils like? If your soils are clays, you can have problems with the magnitude of the horizontal soil load. I recently designed a repair for a house less than 5 years old that had a concrete basement wall crack in the middle, and also had a failure of the connection between the wall and the floor joists.

In my area, cast-in-place concrete basement walls are the norm. Another issue I have with masonry basement walls is that water can get into the hollow cells and cause problems. The damproffing that contractors spray on the wall won't act as waterproofing. If you are in a dry area this is not a major concern for you.

To sum up my wordy response, it is best to have a qualified structural engineer review the design of your wall. A failure of your wall will be big bucks, and it is not uncommon for residential basement walls to have problems. I would ask for proof in the form of calculations stamped by a preofessional engineer that the wall, its anchorage, and the girder bearing are structurally adequate. Don't just take the contractors of the architects word for it.

 

[LPPE]

I believe the masonry code does not allow rebar spacing greater than 48 inches on center.

I'd hire a structural engineer to take a look at the CMU basement wall. I've seen many CMU basement walls that were improperly built and cause many many problems later down the line.