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CMU Wall question

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Title: CMU Wall question

How high can I go with a 12" unreinforced cmu wall before I should be concerned about it's integrity?

I am designing a municiple structure in which the facility ( a small wastewater treatment plant accessory building with a portion sitting on an reinforced concrete tank, and the other walls extending to grade) has three walls that need to be 24 feet high above grade but thaey are only 19 feet wide which suggests that horizontal bending is more critical than vertical bending.

I calculated the moment to be 902.5 ft-lbs (19' span and 20psf wind, wl^2/8), and the section modulus of the cmu is listed to be 160 in^3.  It looks like the allowable flexural tension is 50 psi (ACI 530) and my bending stress is (902.5/160)= 67.7psi which exceeds the 50psi allowed.

Is this reasonable? It doesn't seem like a 24' high cmu wall spanning 19' should be too much of a concern.

Thanks in advance for your response.

Tom Bodkin, E.I.T.