AWWA D-100 and Vertical Acceleration 3

[BR0]

Hi All,

I'm working on a steel tank project and was comparing the vertical acceleration between D-100 (2011) and D-100 (2021). The Av went from 0.14*Sds to .48*Sds in that single code cycle for responses limited by buckling. Nearly a 350% increase. I may have missed something, but I checked the equations that use Av and I don't see any type of change that would bring this down any.

Is this what others are seeing as well?

Thanks, Bob

 

[HTURKAK]

I have the copy D-100 (2005) and apparently the same vertical acceleration Av =0.14*Sds. I do not have the copy of D-100 (2021) which proposes the use of Av = .48*Sds .

Can you post the excerpt showing the vertical acc. Av = .48*Sds with relevant wording ?
I looked ASCE 7-16 ( non bldg. str. Section 15.7) . ASCE 7-16 suggests the use of Sav ( vertical seismic parameter ) which will be defined from Section 11.9, and determined with vertical period of vibration and the use of Seismic Coefficients at Table 15.4-2 Seismic Coefficients for Nonbuilding Structures Not Similar to Buildings .

...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[JStephen]

API-650 says 0.47SDS, so pretty much the same.

 

[HTURKAK]

Dear Mr JStephen;
In general i prefer not to respond any post if my nickname is not stated explicitly. This time , i responded because i was thinking the same reasoning should be valid for AWWA D100 (2021) similar with API 650.

I have two points which i think valid for this argument;

1- It is true that API-650 says 0.47SDS but i do not know the situation for AWWA D100 (2021). I have copy and pasted of relevant commentary clause of API 650 (2020)
******
EC.6.1.3 Vertical Seismic Effects
The vertical seismic acceleration parameter, Av is defined by E.2.2 as Av = (2/3) x 0.7 x SDS = 0.47 SDS. The 2/3 factor
represents the typically applied relation between horizontal and vertical design acceleration. The 0.7 (or more precisely, 1.0 divided by 1.4) factor is the ASCE 7 adjustment for allowable stress design.ASCE 7 sets Av = 0.2 SDS. As compared to API, this value does not include the 0.7 allowable stress factor and it does include a load combination factor of 0.3. Av = (2/3) x 0.3 x SDS = 0.2 SDS. The load combination factor is not included in the API definition, because the individual load and stress equations in E.6.2.1, E.6.2.2, and E.6.2.3 include load combination effects.
Increasing Av to 0.47 SDS from the previous 0.14 SDS causes the vertical seismic component of equation E.6.1.4-6 to
become the largest component for determining dynamic hoop stress. This equation incorporates Rw for the impulsive
and convective forces but not for vertical force. Although applying an Rw value greater than 1.0 to vertical seismic
applications is not appropriate for any actions that involve buckling, applying the fully amplified elastic response of the shell hoop tension caused by the breathing response mode is also not correct. Therefore, the vertical component of
the hoop stress equation is conservatively divided by a factor of 2.5. For situations where the Rwi is less than 2.5, the
adjusting factor should be reduced to Rwi.

2- API 650 uses Response Modification Factor (R =2.5 ) for hoop stresses,for anchors ..
EC.6.1.4 Dynamic Liquid Hoop Forces
Calculations of hydrodynamic hoop forces were not included in previous editions of the Annex since it was not usually
a governing condition for the typical petroleum storage tank. However, with larger diameter tanks, products with higher specific gravity, and vertical seismic effects, this additional check for hoop stresses was deemed to be necessary.
Increasing Av to 0.47 SDS from the previous 0.14 SDS causes the vertical seismic component of equation E.6.1.4-6 to
become the largest component for determining dynamic hoop stress. This equation incorporates Rw for the impulsive
and convective forces but not for vertical force. Although applying an Rw value greater than 1.0 to vertical seismic
applications is not appropriate for any actions that involve buckling, applying the fully amplified elastic response of the shell hoop tension caused by the breathing response mode is also not correct. Therefore, the vertical component of
the hoop stress equation is conservatively divided by a factor of 2.5. For situations where the Rwi is less than 2.5, the
adjusting factor should be reduced to Rwi.
****

So , i'm thinking the same reasoning , ( the use of R values ) for AWWA D100 . Otherwise , ( .48*Sds) will be overkill..

My opinion.











He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[BR0]

So , i'm thinking the same reasoning , ( the use of R values ) for AWWA D100 . Otherwise , ( .48*Sds) will be overkill..

API-650 says 0.47SDS, so pretty much the same.

Thanks for the responses. That commentary is interesting and helpful.

Here is the section from AWWA 2021 for vertical acceleration. The commentary in this document doesn't mention anything similar to what API 650 states.

13.5.4.3 Vertical design acceleration. The design of the tank and anchorage shall include load effects from vertical design acceleration, Av, equal to 0.48SDS for responses limited by buckling (Eqs 13-35 and 13-36) and 0.19SDS for responses not limited by buckling (Eqs 13-32, 13-38a, and 13-38b), except as permitted in Sec. 13.5.4.5.​

Do you know the criteria for determining this limit "responses limited by buckling"? Eqs 13-35 and 13-36 are to determine the stresses in self-anchored tanks but don't specifically indicate if they are limited by buckling. Perhaps they are just saying if it is "self-anchored" the response is limited by buckling?

Thanks

 

[JStephen]

For flat bottom tanks, design limited by buckling would be the shell compression checks of 13.5.4.2.1 and 13.5.4.2.2.

 

[HTURKAK]

My understanding is , the longitudinal shell compression stress (σc ) calculated with Av=0.48SDS shall not be higher than Allowable shell compression stress .( I have AWWA D100 2005 and the relevant clause 13.5.4.2.4 Allowable shell stress )

Apparently this is Ground-Supported Flat-Bottom Tank. IMO, in this case , you can follow API 650.

..

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48