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# AWWA D103 TANK SEISMIC DESIGN FACTORS

## AWWA D103 TANK SEISMIC DESIGN FACTORS

(OP)
Hi All

Reference tank design for seismic to AWWA D103 standard:

I'm a bit puzzled trying to understand the wording included in the above standard under Section 14: Seismic Design.

It says:

"The design design seismic forces have been reduced by a factor of 1.4 and shall be used with the allowable strength design method".

This reduction is applied in the formula for calculating the design acceleration, A, which is used to determine the lateral forces for overturning moments and base shear. In that formula (Eq. 14-16) which is generally written as
A = S*I/R
where
S is the spectral response acceleration
I is the importance factor
R is the Response modification factor depending on type of structure and extent of anchorage

The "A" is reduced by a factor of 1.4

My question is that in API-650 which has a similar design procedure, the reduction factor of 1.4 is not included.
I believe both AWWA D103 and API-650 both use the allowable strength design method.

Could this be explained by those more familiar with the above codes.

Thanks
Michael

### RE: AWWA D103 TANK SEISMIC DESIGN FACTORS

I don't have the D103 code handy.
The D100 code uses R=3.0 anchored and 2.5 unanchored.
API-650 uses R=4.0 anchored and 3.5 unanchored.
If D103 uses the same R values as D100, part or all of that 1.4 factor may be included in the R values.

### RE: AWWA D103 TANK SEISMIC DESIGN FACTORS

#### Quote (The "A" is reduced by a factor of 1.4 My question is that in API-650 which has a similar design procedure, the reduction factor of 1.4 is not included. I believe both AWWA D103 and API-650 both use the allowable strength design method.)

Both API 650 and AWWA use ASD and 1.4 factor is included in R (Response Modification Factors for ASD Methods ) values for API tanks.

API 650 =
for Self-anchored tanks Ri= 3.5 , Rc =2
for Mechanically-anchored Ri= 4 , Rc= 2

AWWA =

for Self-anchored tanks Ri= 2.5 , Rc =1.5

for Mechanically-anchored Ri= 3.0 , Rc= 1.5

for elevated tanks Ri= 3.0 , Rc= not applicable ( if the fluid–structure interaction procedure is not used.)

### RE: AWWA D103 TANK SEISMIC DESIGN FACTORS

(OP)
I see now that the 1.4 factor is found in the differences in the value of 'R' between the different standards.
It makes sense - thanks

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