ACI 350.3-20 Code Requirements for Seismic Analysis and Design of Liquid-Containing Concrete Structures (ACI 350.3-20) and Commentary.
R4.1—Earthquake pressures above base of tank wall The general equation for the total base shear normally encountered in the earthquake-design sections of governing building codes, V = C s W, is modifed in Eq. (4.1.1a) through (4.1.1e) by replacing the term W with the four efective weights: the efective weight of the tank wall, εW w , and roof, W r ; the impulsive component of the liquid weight W i and the convective component W c ; and the term C s with C i , C c , or C v as appropriate. Because the impulsive and convective components are not in phase with each other, accepted practice is to combine them using the square-root-sum-of-the-squares method (Eq. (4.1.2)) (NZS 3106; Tavakkoli et al. 2012; Veletsos and Shivakumar 1997). A more detailed discussion of the impulsive and convec- tive components, W i and W c , is contained in Section R9.1. The imposed ground motion is represented by an elastic response spectrum that is either derived from an actual earthquake record for the site or is constructed by analogy to sites with known soil and seismic characteristics. The profle of the response spectrum is defned by S a , which is a function of the period of vibration, and the mapped accelerations S S and S 1 , as described in Section R9.4. Factor I provides a means for the engineer to increase the factor of safety for the categories of structures described in Table 4.1.1a. Engineering judgment may require a factor I greater than tabulated in Table 4.1.1a where it is neces- sary to reduce further the potential level of damage or account for the possibility of an earthquake greater than the design earthquake.
R4.1.1 Earthquake pressures above base of tank wall—A model representation of W i and W c is shown in Fig. R9.1. R4.1.2 Total base shear—Note that the total base shear calculated at the base of the tank wall in accordance with these provisions does not include inertia of the base slab. Inertia of the base slab should be included in the total base shear applied at the bottom of the tank foor slab for tank global stability evaluation. Also note that the total base shear for rectangular tanks is calculated on a unit-width basis when using these provisions. Inertia of the tank side walls (parallel to ground motion) is not included. Inertia of these side walls should be added to the inertia of the perpendicular tank walls and be included in the total base shear for tank global stability evaluation. R4.1.3 Moments at base of tank wall, general equa- tion—IBP overturning moment M o is the hydrodynamic overturning moment on a horizontal plane at the base of the tank wall resulting from the combined efect of the moment due to dynamic fuid pressures acting vertically on the tank bottom, and bending moment on the tank cross section. When considering the moment due to dynamic pressures on the tank foor slab, pressures acting on the top of the tank foor slab may be computed based on M o minus M b . The pressure distribution can be computed as (M o – M b )/(I b /x), as shown in Fig. R4.1.3, where I b is the moment of inertia of the tank bottom footprint.ACI 350.3 pdf download.