Page 147 - Pressure Vessel Design Manual

Design of Vessel Supports 125

Notes 0 Hemy tjessels (single large vessel or multiple large
vessels): The vessel(s) is the principal vibrating ele-
1. Vessels mounted in structures at some elevation other ment. It requires a combined seismic model, which
than grade generally will experience amplified base simulates the mass and stiffness properties of vessel
motion near and above the natural frequencies of the and structure.
support qtructure. 2. For tall slender vessels, the main concern is bending.
For short, squat vessels the main concern is base shear.
0 Light uessels (less than 1% of structure weight): 3. The procedures outlined in this chapter are static-force
a. If vessel frequency > structure frequency, then procedures, which assume that the entire seismic force
vessel is subjected to maximum acceleration of due to ground motion is applied instantaneously. This
the structure. assumption is conservative but greatly simplifies the
h. If vessel frequency < structure frequency, then calculation procedure. In reality earth quakes are
vessel will not be affected by structure. It will time-dependent events and the full force is not realized
respond as if it were mounted at grade. instantaneously. The UBC allows, and in some cases
e iMedium vessels (less than 20% of' structure weight): requires, that a dynamic analysis be performed in lieu
Approximate methods may be used to develop the in- of the static force method. Although much more
structure response spectra. The method used should sophisticated, often the seismic loadings are reduced
account for interaction between vessel and structure significantly.
(energ)l feedback). Consideration should be given to
account for ductility of the vessel.

PROCEDURE 3-4

SEISMIC DESIGN-VESSEL ON UNBRACED LEGS 14-71

f, =axial stress, psi
Notation fh =bending stress, psi
E = modulus of elasticity, psi
A = cross-sectional area, leg, in. 2 g = acceleration due to gravity, 386 in./sec2
I7 = base shear, lb e = eccentricity of legs, in.
\V = operating weight, lb MI, =overturning moment at base, in.-lb
n = number of legs M, = overturning moment at tangent line, in.-lb
C, =vertical seismic factor M =bending moment in leg, in.-lb
C:h =horizontal seismic factor I1 =summation of moments of inertias of all legs per-
y =static deflection, in. pendicular to Fl,, ins4
F, = vertical seismic force, lb Iz = summation of moments of inertia of one leg per-
F1, =horizontal seismic factor, see Procedure 3-3 pendicular to Fl,, in.4
F, = allowable axial stress, psi I = moment of inertia of one leg perpendicular to Fh,
FI, = allowable bending stress, psi in.4
F, =seismic force applied at top of vessel, lb C, =distance from centroid to extreme fiber, in.
F:, = Euler stress divided by safety factor, psi C,, = coefficient, 0.85 for compact members
f, = maximum eccentric load, lb K1 = end connection coefficient, 1.5-2.0
V,, = horizontal load on leg, lb T =period of vibration, sec
F,, = maximum axial load, Ib r =least radius of gyration, in.