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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.

RE: AWWA D100-05 - 1/3 Stress Increase

Axial stress can be produced by wind - overturning of a water tank can cause tension-compression axial forces in tower columns.

RE: AWWA D100-05 - 1/3 Stress Increase

I wouldn't necessarily classify the stresses produced by the wind on a pedesphere as purely axial stresses, but rather stresses (tension/compression) that act in the axial direction as a result of bending. The lateral loads produce the fb stresses, and the gravity loads produce the fa stresses. Both are acting in the same direction, but are produced differently.

fa = P/A
fb = M/S = My/I

If this were a leg-supported water tank, I would agree that the axial forces in the tower legs are produced by both lateral and gravity loads. But I think a pedesphere water tower stresses should be broken down into gravity (fa) and lateral (fb) components because of its shell configuration.

RE: AWWA D100-05 - 1/3 Stress Increase

We haven't used the 1/3 stress increase in the work we do for some years as current building codes have done away with it.

For the towers - based on past practice - the 1/3 increase was applied to all allowable stresses within the load combination and never taken as separate applications to specific types of stress.

You still would have the full (non-adjusted) load combinations for gravity that would be checked.

RE: AWWA D100-05 - 1/3 Stress Increase

I've copied the responses from Franklin Kurtz (AWWA Standards Engineer) and Stephen Meier (AWWA Standards Committee member) below.
Mr. Meier's response is consistent with using the 1/3 stress increase for the entire equation.

Mr. Kraus,

See response below from Steve Meier of Tank Industry Consultants. Mr. Meier is chair of our AWWA Standards Committee on Steel Tanks, the body responsible for AWWA D100.

Bear in mind that Mr. Meier’s response is his own opinion and does not represent an official AWWA interpretation of the standard, since it is AWWA policy not to interpret standards. Mr. Meier is, however, a leading authority and expert on steel water storage tanks and a long-time chair of the committee responsible for D100.

If I can assist further, please let me know.


Franklin S. Kurtz, P.E.
AWWA Standards Engineer

As currently written, this eqn is applied such that the sum is less than 1.33 for combined stress due to wind and seismic which is the intent, not separate value for the load components as is used in load factor approaches. It was intended that the lower factor of safety be applied for the combined stress, not different FS for different loads in combination as is used for load factor design. This was and is consistent with historic ASD design methods. The inquirer should also remember that vertical seismic forces can be included in some of these load cases.

That being said, we are working on load cases to address issues similar to that described in the inquiry so the future answer to this question may well be different. For now, the above is the correct application.


Stephen W. Meier SE,PE

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