## Seismic Design For Vessels [2

Soil profile type (Table 3-9c) Seismic zone factor, Z Numerical coefficient, rw

For zone 4 only:

Seismic source type Distance to fault Near source factor, Nv

Step 2: Determine or calculate seismic coefficients.

Step 3: Determine period of vibration.

Step 4: Calculate the base shear, V.

V is the greater of Vj or V2 but need not exceed V3:

= CylWp RwT

2.5CaIW0

Notation

Z = seismic zone factor (see Figure 3-8)

I = importance factor standard facilities 1.0

hazardous/essential facilities 1.25

Rw = coefficient self-supporting stacks 2.9

vertical vessel on skirt 2.9

spheres and vessels on braced legs 2.2

horizontal vessel on pier 2.9

vertical vessel on unbraced legs 2.2

Cv = coefficient from Table 3-9b Ca = coefficient from Table 3-9a

S =site coefficient (1.0-2.0 based on soil profile) W„ = operating weight of vessel, lb w = uniform weight of vessel or stack, lb/ft Ft = lateral force applied at top of structure, lb Ft = 0.07TV or 0.25V whichever is less, or =0, if T < 0.7 sec H = overall height of vessel, ft D = outside diameter of vessel, ft T = period of vibration, sec (see Figure 3-7) y = deflection, in. N = number of column legs A = cross-sectional area of leg braces, in.2 g = acceleration due to gravity, 386 in./sec2 Im = moment of inertia of pier, legs, stack, etc., in.4 E = modulus of elasticity, psi Sa— Sf = soil profile type from Table 3-9c Na, Nv = near source factor from Table 3-9d Seismic source type = from Table 3-9e

Design Procedure

Step 1: Determine the following.

For all zones:

Weight, WD Importance factor, I

For zone 4 there is the additional requirement that the base shear shall be at least equal to V,4

Step 5: Since the seismic design for pressure vessels is based on allowable stress rather than ultimate strength, the base shear may be reduced by a factor of 1.4.

Step 6: Determine if some percentage of the base shear needs to be applied at the top of the vessel, Ft.

For all other cases Ft = 0.07TV

but need not exceed 0.25V =

For zone 4 there is the additional requirement that the base shear shall be at least equal to V,4

Figure 3-8. Seismic risk map of the United Slates. Reproduced from the Uniform Building Code, 1997 Edition. Copyright 1997, with permission of the publisher, the International Conference of Building Officials.

Figure 3-8. Seismic risk map of the United Slates. Reproduced from the Uniform Building Code, 1997 Edition. Copyright 1997, with permission of the publisher, the International Conference of Building Officials.

Table 3-9a

Seismic Coefficient Ca'

 Soil Profile Seismic Zone Factor, Z Type Z= 0.075 Z= 0.15 Z= 0.2 Z= 0.3 Z= 0.4 SA

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