ABS and Cast Iron Installation - 2010 CPC

I have a 3-story apartment building that is proposing to install ABS at the 2nd and 3rd floors with cast iron below. I believe this meets the intent of CPC 701.1.2.2 (limited to not more than 2 stories of areas of residential accommodation). It was brought to my attention that when a building has cast iron, it must have cast iron throughout...is this true? Where in the code is this mentioned? Thanks so much!
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I have run into this issue more than once. It's a typical example of the poor way our codes are constructed. If the codes were worded to ensure proper performance (using the professional designer's skills/knowledge/experience) instead of trying to be (or at least sound) entirely prescriptive this might be avoided.

The underlying intent??? 1. A concern with plastic piping that is more than two stories in height that fails over time due to induced stresses... how aggregate vertical loads affect the pipe and connections, resulting in cracking/leaks... expansion/contraction/movement in long pipe runs, how this is handled by supports and connections/collars, and other technical issues. 2. Pressure from plumbers unions and pipe manufacturers to write the code in a way that seemingly removes discretion, forcing installation of 'all iron' systems even when combination systems might perform as well.

If a BO interprets the code to mean 'if more than 2 stories all portions must be iron', then you always have the option of presenting an 'alternative' mixed-materials design. You could explain that no portion of the plastic piping will carry loads more than two stories in height. You could convince the BO that there is no reason the plastic portions should be disallowed. You could search and find printed materials that support your interpretation.

You would probably have a hard time explaining to an owner/client why it's going to take longer to get his plans approved - even when you explain that there will be some dollar savings. You would probably also need to get some preliminary estimates from contractors to show your client the magnitude of savings obtained.

The 'alternative design' process could be costly and time consuming. If you have a cooperative BO it might be a breeze. On the other hand, you could have a BO that will insist on documentation - as is the BO's discretion under the code. The documentation could be simple or complicated. It's a crap shoot.

The best time to talk to a BO about such issues is when the design is still in preliminary stages. Most BO's will appreciate a designer coming in early to vet anything that could be a concern (i.e. out of their ordinary, daily experience). It's simply a part of this complicated business we all have to deal with - no matter how simple we 'think' it should be.


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