Todd,
The storage height is the total height of storage of all commodities. If
you have plastic on the first tier and class 3 above to a total of 12
ft., you have mixed commodity storage to 12 ft., and the standard tells
you how to handle mixed commodities (protect for the worst one). So, you
either protect for 12 ft. of plastic or you segregate the storage. Thats
the literal interpretation of the syandard.
Regardless of what the standard says, the owner's logic is severely
flawed. He's saying that 12 ft. of mixed plastic and class 3 requires no
more protection than 12 ft. of class III. Following his train of
thought, you ought to be able to put hot coffee and cold soda in a
thermos bottle and have the coffee stay hot and the soda stay cold.
If you need hard data, FM did a test with 20 ft. rack storage in 4
tiers. The bottom tier was plastic, and the top 3 were class II. The
resulting fire behaved like a high end class 4 commodity. (The same
thing happened with plastic on the top and class 2 in the bottom 3
tiers, and also with a homogeneous mix of 25% plastic and 75% class 2).
This is the data that led the committee to create the current rules on
mixed commodities, and it would lead to the conclusion that a mixture of
class III and plastic will behave like plastic.
Joe
Todd Williams wrote:
I am working on a project where the owner is going to store a Class 3
commodity in cardboard boxes and a Group A Plastic in cardboard boxes. These
are to be stored on pallets in racks. Based on the available density, the
Class 3 can be stored to 12 feet and the plastic to 5 ft. The owner wants to
store the plastic on the first level and the Class 3 on the second level,
immediately over the plastic. His argument is that it meets both criteria,
but I am suspect that the total fire load of a class 3 on top of plastic
might be greater than the system (currently designed for OH2) can handle.
However, I don't see anything that backs me up. I want him to dedicate
separate racks for each. Any thoughts?
Todd G. Williams, PE
Fire Protection Design/Consulting
Stonington, Connecticut
860.535.2080
www.fpdc.com
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