Page 154 - Pressure Vessel Design Manual

132 Pressure Vessel Design Manual

PROCEDURE 3-5

SEISMIC DESIGN-VESSEL ON BRACED LEGS [7]

r = least radius of gyration, in.
Notation M =overturning moment, in.-lb
N =number of legs
A = cross-sectional area of brace, in.’ d =center line diameter of leg circle, in.
AI = cross-sectional area of leg, in. 2 C1= chord length between legs, in.
V=base shear, Ib CI, =horizontal seismic factor, see Procedure 3-3
E = modulus of elasticity, psi C, = vertical seismic factor
W =operating weight of vessel, lb K1= end connection coefficient
A1 = change in length of brace, lb 11 = moment of inertia, cross brace, in.4
Fh =horizontal seismic force, Ib SI = slenderness ratio
F, = vertical seismic force, Ib tan 8 = h’C1
F, =lateral force at top of vessel, lb 1 = length of cross brace; = h’/sin 8
Fa = allowable axial stress, psi
Fy = minimum specified yield stress, psi This procedure is used for calculating the distribution of
V, =horizontal load on one leg, lb vertical and horizontal forces due to wind or seismic loadings
f = axial load in brace, lb for vessels, spheres, elevated tanks, and bins supported on
dl =distance between extreme legs, in. cross-braced legs or columns.
n =number of active rods per pane1 = 1 for sway- To design the legs, base plates, cross-bracing, anchor
bracing, 2 for cross-bracing bolts, ring girder, and foundations, it is necessary for the
FL = axial load on leg due to overturning moment, lb designer to determine the actual distribution of forces.
FD = axial load on leg due to dead wt, Ib The horizontal load due to wind or seismic is distributed
F, = combined axial load on leg, lb to the legs through the cross-bracing or sway rods. The legs,
fa =axial stress, psi in turn, transfer the forces to the vessel base, ring girder, or
y = static deflection, in. support structure. The angle between the applied force and
T = maximum period of vibration, sec the cross-bracing determines the magnitude of the imposed
g = acceleration due to gravity, 386 in./sec2 load at that point.