Part 9 NBCC Foundation Wall Requirements 2

[Woody1515]

Hello everyone,

I have some questions regarding Part 9 of the National Building Code of Canada, specifically regarding foundation walls. Part 9 of the Code generally deals with residential and smaller commercial buildings. It is described as a prescriptive code. In this section, it states normal concrete foundation walls (no reinforcement) may be used provided the fill depth does not exceed a certain height for a certain wall thickness. In my opinion, I can’t see how a concrete foundation wall can work without any reinforcement added. But if an engineer specs a wall with no reinforcement and it meets the requirements of the building code I mentioned above, is that considered to be acceptable? If the wall were to fail but the engineer can prove it meets the Part 9 building code requirements, would they be liable for the damages caused by the wall failing?

Thanks in advance! I have been asking quite a few question on this forum lately and really appreciate the thoughtful responses.

 

[jayrod12]

I don't want to get into the semantics of whether the engineer would be liable or not. However, if it meets the building code in force then I would say no they would not be liable.

That being said, many jurisdictions have their own amendments to the NBCC, locally where I practice they mandate a nominal amount of reinforicng be installed in a wall, depending on the situation it is somewhere around 10m @ 900 o/c verts and 3 - 20m horizontals. It's ridiculous how little, but guess it's better than nothing.

 

[r13]

I don't use plain concrete in the foundations, for its performance is difficult to predict, and the repair is difficult and costly. So I always provide minimum reinforcement. But this practice may not accepted by others, include the owner. Lucky that I don't deal with residential works, thus haven't received objections so far.

 

[PEinc]

How can an unreinforced concrete foundation wall be any worse than a rubble stone foundation wall with questionable, if any, mortar?

http://www.PeirceEngineering.com

 

[r13]

Not worse, until problems occur. The joint can be repaired with less effort than cracks on concrete.

 

[PEinc]

I disagree.

http://www.PeirceEngineering.com

 

[r13]

But this practice may not accepted by others,..

 

[Woody1515]

I really struggle with this. All my engineering knowledge tells me there must be reinforcement in the wall, but if the Code says there doesn’t need to be, why put it in? It would most likely be cheaper and quicker to construct without it. Even if you specific the code minimums (roughly 0.002Ag), that’s equivalent to approximately 15M bars every 16” or so. The difference between no reinforcement and that is a shocking amount. I guess you can split the difference between the two options?

 

[Woody1515]

How does one justify the amount of reinforcement specified if it falls short of the minimum shrinkage requirements, but more than having no reinforcement (ie, more than what the building code specifies but less than what is required from CSA A23.3)?

 

[r13]

Since no reinforcement is required by calculation, you don't need to justify anything from the perspective of code, it is a judgement call.
Provide T & S steel at interior face, if this is basement wall.

 

[oldestguy]

Ya can't argue with the long term foundation made from concrete without reinforcement, for the typical residence. In my experience (my work and hundreds of others) darn few have failed. Then it usually some factor such as frost action and lots of water preset. I think the saving grace of these jobs is the fact the foundation walls carry a load of the house placing the concrete in vertical compression and resulting in minimal tension. Another probably is the fact no compaction is given to the backfill. Take the typical foundations in many areas consisting of concrete blocks and most work OK.

 

[jayrod12]

Unreinforced walls have failed regularly where I practice. Rubblestone do not due to the general thickness and height. Rarely are they full basements with rubble walls, and when they are they are on the order of 16" thick.

I've had to specify repair to many a concrete wall that was poured 8" thick with minimal to no vertical reinforcing and so I would never suggest doing as such.

 

[PEinc]

I am not recommending thin, unreinforced, concrete walls. If the unreinforced concrete is thick enough, I don't see why it should not be at least as strong as a rubble stone foundation wall. Also, concrete block walls are thin and have little to no reinforcement inside. How little reinforcing can you have in a concrete wall before you consider it unreinforced? I don't think it is proper to say that no foundations or foundation walls should be made of unreinforced concrete.

http://www.PeirceEngineering.com

 

[jayrod12]

I would agree with that. Thick concrete walls can perform adequately.

No one locally ever does masonry foundation walls, around here masons are the highest, or close to, paid people on a job site. You can have a couple of lackeys build formwork and slap some reinforcing together to save on concrete costs in lieu of masonry.

 

[Woody1515]

For a 8” thick, 9’ tall basement wall I am calculation the following minimums for reinforcement (from CSA A23.3):

Horizontal - 0.002Ag = 6-15M bars
Vertical - for flexure, 15M bars @ 16” O/C
- for shrinkage, 0.0015Ag = 15M bars
@ 24” O/C

So unless I’ve miscalculated, the difference between no reinforcement and just the minimums is staggering. In the Canadian Code, the minimum flexural steel can be neglected if the moment of resistance is 1.3333 times greater than the factored moment. However, 15M bars both ways at roughly 24” O/C would still govern. Hard to believe how they allow no reinforcement in walls... so when people are spec’ing minimum steel, that still requires 15M bars @ 24” O/C?

 

[r13]

How about 10M @12" O/C.? Smaller bars at closer spacing is better for crack control.

 

[Woody1515]

Yes, that would be better for crack control. My main point is the contractor could argue no reinforcement is needed as per the code, when I’m finding quite a bit is still needed. I know that reinforcement isn’t super expensive and at the end of the day the engineer has the stamp, but I feel like this causes a lot of unnescessary problems.

 

[r13]

I wouldn't worry about contractor's complain, as he/she is paid to do the work accordingly - reinforced or not, not paid to comment or argue.

You shall check if your concrete design code permit "structural plain concrete" applications. If the wall is permitted to be designed as thus, and it satisfies all code provisions, then you shouldn't need to provide any reinforcement just for the sake of "good practice", for which you feel difficult/uneasy to justify. Otherwise, you need to provide As_req, or the lesser of As_min and 1.33*As_req, when As_req < As_min.

 

[GeoEnvGuy]

I don't suppose the OP has read the section on Reinforcement for Flat insulating Concrete Form foundation Walls which is different than a precast solid concrete wall which craned in and doweled to the footing and laterally supported at the top.