Page 132 - Pressure Vessel Design Manual

112 Pressure Vessel Design Manual

system there is no interaction between the shell and the Method 1, ring analysis. The eccentric load points are trans-
support rings. lated into radial loads in the rings by the gussets. The com-
The analysis for the design of the rings and the stresses posite ring section comprised of the shell and ring is then
induced in the shell employs the same principles as Lug analyzed for the various loads.

WIND DESIGN PER ASCE [ 11

Notation The ASME Code does not give specific procedures for
-
designing vessels for wind. However, Para. UG-22,
Af= projected area, sq ft “Loadings,” does list wind as one of the loadings that must
Cf= force coefficient, shape factor 0.7 to 0.9 be considered. In addition, local, state, or other governmen-
De =vessel effective diameter, from Table 3-4 tal jurisdictions will require some form of analysis to account
f = fundamental natural frequency, I/T, cycles for wind loadings. Client specifications and standards also
per second, Hz frequently require consideration of wind. There are two
F = design wind force, lb main, nationally recognized standards that are most fre-
g = 3.5 for vessels quently used for wind design. They are:
G = gust effect factor, Cat A and B = 0.8, Cat
C and D = 0.85 1. ASCE 7-95 (formerly ANSI A58.1)
Gf= gust response factor for flexible vessels 2. Uniform Building Code (UBC)
h =height of vessel, ft
I =importance factor, see Table 3-1 This section outlines the wind design procedures for both
Iz = the intensity of turbulence at height z of these standards. Wind design is used to determine the
Kz = velocity pressure exposure coefficient from forces and moments at each elevation to check if the calcu-
Table 3-3a, dimensionless lated shell thicknesses are adequate. The overturning
Km =topographic factor, use 1.0 unless vessel is moment at the base is used to determine all of the anchorage
located near or on isolated hills. See ASCE and support details. These details include the number and
for specific requirements size of anchor bolts, thickness of skirt, size of legs, and thick-
M =overturning moment at base, ft-lb ness of base plates.
Ni.NhrNb,Nd = calculation factors As a loading, wind differs from seismic in that it is more or
Q =background response less constant; whereas, seismic is of relatively short duration.
qz=velocity pressure at height z above the In addition, the wind pressure varies with the height of the
ground, PSF vessel. A vessel must be designed for the worst case of wind
= 0.00256 KzKz=V21 or seismic, but need not be designed for both simulta-
R = resonant response factor neously. While typically the worst case for seismic design is
Rn,Rh,Rd = calculation factors with the vessel full (maximum weight), the worst design case
T =period of vibration, sec for wind is with the vessel empty. This will produce the
V= basic wind speed from map, Figure 3-1, maximum uplift due to the minimum restraining weight.
mPh The wind forces are obtained by multiplying the projected
Vref = basic wind speed converted to ft/sec area of each element, within each height zone by the basic
V, = mean hourly wind speed at height z, fv‘sec wind pressure for that height zone and by the shape factor
z = equivalent height of vessel, ft for that element. The total force on the vessel is the sum of
zmin = minimum design height, ft, from the forces on all of the elements. The forces are applied at
Table 3-3 the centroid of the projected area.
= structure, damping coefficient, 1% of Tall towers or columns should be checked for dynamic
critical damping response, If the vessel is above the critical line in Figure
rock or pile foundation: 0.005 3-9, Rdt ratio is above 200 or the h/D ratio is above 15,
compacted soil: 0.01 then dynamic stability (elastic instability) should be investi-
vessel in structure or soft soils: 0.015 gated. See Procedure 4-8, “Vibration of Tall Towers and
cr,b,c,l,E =coefficients, factors, ratios from Table 3-3 Stacks,” for additional information.