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Importance Factor for Elevated Water Tank

Importance Factor for Elevated Water Tank

(OP)
I'm looking to get some opinions on elevated water tower importance factor. We do assessments for old elevated water tanks across the Canadian prairies. Typically they're decommissioned and only a historical structure (i.e. no water storage, and completely useless structures). For one tank in particular the owner wants to keep using it so it's still part of a water distribution system. Therefore, I would consider this a post-disaster structure. NBCC would recommend importance factors in excess of 1 for snow, wind, and seismic analysis (1.25, 1.25, and 1.50 respectively). However, we typically use AWWA D100 as the governing standard which does not mention importance factor. Question is this, would you consider post-disaster importance factors in excess of 1.0 for this tank? Keep in mind the allowable stresses in AWWA D100 are low (in my opinion) so the importance factor may be implicit in the allowable stresses given this is a carbon steel water tank calibrated standard and perhaps they're all "post-disaster" important.

Also, is anyone aware of NBCC importance factors from 1940 onward?

Thanks in advance.

RE: Importance Factor for Elevated Water Tank

AWWA D100 does use importance factors for wind and seismic. For wind, I think only in the last edition or two, in seismic, for a good many years.
However, note that old D100 tanks may not meet the seismic requirements of the current standard- seismic loads have generally increased.

RE: Importance Factor for Elevated Water Tank

(OP)
Thanks Stephen. Can you point me in the direction of the importance factors? Or rather, what specific section are the importance factors specified?

RE: Importance Factor for Elevated Water Tank

AWWA D100-11, Section 3.1.4 gives I=1.15 for wind and Table 24 in Section 13.2 gives I-E = 1.00, 1.25, or 1.50 for seismic.

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