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AWWA D100-05 - 1/3 Stress Increase

AWWA D100-05 - 1/3 Stress Increase

AWWA D100-05 - 1/3 Stress Increase

Section 3.3.3 of the AWWA D100-05 Standard states:
"Members subject to stresses produced by wind or seismic loads may be proportioned for unit stresses one-third greater than those specified in Sec. 3.2 and Sec 3.4..."

When looking at the combined stress Eq. 3-3 (fa/Fb + fb/Fb ≤ 1) for a pedesphere water tower, it's my understanding that the actual axial unit stress, fa, is produced by dead, live, and snow (gravity) loads only, where the actual bending unit stress, fb, is produced by wind or seismic (lateral) loads only.

It's not clear to me if the 1/3 stress increase for wind or seismic can be applied to both Fa and Fb, or just Fb in the combined stress equation. I've seen other calculations use the increase for both Fa and Fb, but I feel the statement from Sec. 3.3.3 above would imply that the increase should only be applied to the part of the equation affected by the wind or seismic load (Fb only).

Any insight into the matter would be much appreciated. Thanks.

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