Partially Buried Reinforced Concrete Tanks

[EDub24]

I'm currently designing a partially buried reinforced fixed-base concrete tank. When determining the seismic loads I am using ACI 350.3-06 with the modifications per §15.7.7.3 of ASCE 7-16. That section of ASCE 7 requires you to replace the impulsive response modification factor of ACI 350.3, Ri, with the response modification factor, R, given in Table 15.4-2 of ASCE 7. In Table 4.1.1(b) of ACI 350.3 for a fixed base tank you are allowed to increase the response modification factor, Ri, from 2.0 for an at-grade tank to 3.0 for a buried tank with the ability to use linear interpolation if the tank is partially buried. It seems like ASCE 7 doesn't provide this option and only gives values for at-grade tanks. I'm interested in seeing what other people have done in this situation. Do you increase the response modification factor if the tank is partially buried or just stick with the factor given in ASCE 7?

 

[strucbells]

Following as I have a similar structure I'm just starting to design.

The commentary in ASCE 7-16 C15.7.7.3 seems to suggest that ACI 350.3-06 is non-conservative compared to the seismic provisions of the since updated versions of ASCE 7.

My approach will likely to be to just use the ASCE 7 lower R value, which won't be too punitive in my case as the structure is only about 40% buried. I have heard rumor that ACI 350 is supposed to finally get updated this year, hopefully they will provide more clarification in that update on this topic.

 

[r13]

I agree with strucbells approach, when jammed in between code conflicts/discrepancies, take the more conservative route, as nobody knows better and take the responsibility as you do when bad thing happens. But for huge difference, we might need to stop and look around the historical performance of designs based on the same rule with same coefficient, then determine the route to take with confidence.

 

[EDub24]

Yeah I've been using the lower R to date as well but wondered what other people did. Last I checked in the October ACI committee meeting they were still going over the public response comments for the upcoming ACI 350.3. The public response period was in the summer of 2018 so the updated version has taken a long time in coming out.I reviewed that draft version and they show roughly the same thing as before. The impulsive response modification factor for almost all tank types did increase by 0.25. The only one that didn't change was for unanchored, contained or uncontained tanks at grade which stayed at 1.5. I think ACI is pretty clear but if you need to use the modifications given in ASCE 7 then they are the ones that should clarify. I might e-mail ASCE just to get their opinion.

 

[HTURKAK]

My opinion is that, ACI 350.3-06 shall be used with combining the modifications required in Section 15.7.7.3 .

ASCE 7-16 C15.7 TANKS AND VESSELS explains the issue and one of the paragraph ' ACI has published ACI 350.3-06 (2006), Seismic Design of Liquid-Containing Concrete Structures. This document, which addresses all types of concrete tanks (prestressed and nonprestressed, circular, and rectilinear), has provisions that are unfortunately not consistent with the seismic criteria of ASCE/SEI 7. However, the document, when combined with the modifications required in Section 15.7.7.3, serves as both a practical “how-to”....'

The major differences are not only the R values . Others combination of the impulsive and
convective component in ASCE 7-16 V =Vi + Vc (linear comb) while in ACI 350.3-06 V = sqrt(Vi**2+ Vc**2) (SRSS)

Moreover, dynamic earth and groundwater pressures on the buried portion of the tank shall also be considered.

 

[strucbells]

HTURKAK-

FYI the code does specify the linear combination (Vi + Vc) but also states just below this in 15.7.6.1 note C that: "Impulsive and convective seismic forces for tanks are permitted to be combined using the square root sum of the squares (SRSS) method in lieu of the direct sum method shown in Section 15.7.6 and its related subsections."

It also states in 15.7.2 that "Impulsive and convective components shall be combined by the direct sum or the square root of the sum of the squares (SRSS) method where the modal periods are separated. If significant modal coupling may occur, the complete quadratic combination (CQC) method shall be used."

 

[EDub24]

My opinion is that, ACI 350.3-06 shall be used with combining the modifications required in Section 15.7.7.3 .

Yes, I do include the other modifications. My only question was in reference to the impulsive factor for an at-grade tank vs buried.