650 608Rev4Pub
650 608Rev4Pub
650 608Rev4Pub
(uplift)
A computation of ASCE 7-02 wind pressure on domes is shown for 3 tanks below, using a dome radius
equal to the tank diameter (D), a typical radius for API 650 tanks (see section 5.10.6 for steel domes and
G.6.2 for aluminum domes), resulting in a dome-height-to-tank-diameter ratio of 0.13:
Dome Roof Wind Pressure Coefficients Cp for 3 Tank Sizes
30 x 40h
80 x 48h
150 x 48h
5,000 bbl
43,000 bbl
151,000 bbl
h/D
1.3
0.6
0.32
Pt. A (windward edge)
-1.6
-1.4
-1.0
Pt. B (center)
-1.0
-1.0
-0.8
Pt. C (leeward edge)
-0.5
-0.5
-0.3
average coefficient
-1.02
-0.97
-0.72
average uplift (psf)
38.1
36.6
28.8
In each case, the uplift on the windward side is about 3 times the uplift on the leeward side, producing a
net horizontal force in a direction opposite to the wind direction. Therefore, the horizontal effect of the
wind counteracts overturning and can be conservatively neglected.
A 30 psf roof uplift pressure was selected as a reasonable average for all roofs based on the above, and
matches that used for steel roofs in 650.
650-608 Design Loads for Tank Roofs 4/8/08
Snow Load
The snow load currently given in API 650 5.2.1(g) is solely a balanced (uniform) load. ASCE 7-02
provides, in addition to balanced snow loads, unbalanced snow loads (ASCE 7 Figure 7-3). The ASCE 702 unbalanced snow load on dome roofs varies from 0.5 times the flat roof snow load pf at the roofs
crown to about 2 times the flat roof snow load at the 30o slope point (see the figure below). In plan, this
load is distributed over a 90o sector and tapers to zero over the 22.5o sectors to either side of the 90o
degree sector.
The ASCE unbalanced distribution can be used to compute an average pressure in the loaded 90 o sector of
1.58 times the flat roof snow load, when the area loaded is accounted for. (Arcs further from the roof
center have more area). This ballot proposes for simplicity to use 1.5 times the flat roof snow load for the
unbalanced load, and apply this over a 90o + 2(22.5 o) = 135 o sector (135/360 = 3/8 of the roofs area).
2.0 pf Cs /Ce
0.5pf
30o slope
Dome
Elevation
This ballot proposes that roof general buckling and tension ring checks for steel and aluminum domes be
based on the unbalanced snow load since its intensity is greater than the balanced snow load, and the
unbalanced load acts over a sufficiently large portion of the roof to cause general buckling and tension
ring failure.
ASCE 7 does not require unbalanced loads for dome roofs with a slope from the eave to the crown of 10 o
or less. If a cone roof is considered to be similar to a dome roof, then a cone roof with a slope of on 12
(3.57o) has a slope less than 10o, for which the only unbalanced load ASCE 7 requires is a partial loading
650-608 Design Loads for Tank Roofs 4/8/08
Slope or Radius r
3.6 o ( on 12); may be
greater
9.5 o < < 37o
Self-Supporting Cone
5.10.5
variable from about 3 to
Roofs
less than 2*
Self-Supporting Dome
5.10.6
0.8D < r < 1.2D
4*
and Umbrella Roofs
Self-Supporting
Appendix G
0.7D < r < 1.2D
1.65 on general buckling;
Aluminum Dome Roofs
1.95 on member buckling
*Safety factors for self-supporting cone and self-supporting dome roofs are given in Jawad and Farr,
Structural Analysis and Design of Process Equipment.
Fixed roofs will be more accurately designed as a result of this ballot by including unbalanced snow loads
in their design. The table above shows that steel domes and self-supporting cone roofs have higher safety
factors than the other roofs. This is partially due to the fact that API 650 places no tolerances on out-ofroundness for steel domes and self-supporting cones, so they can have geometric imperfections that
reduce their buckling strength. However, since unbalanced loads will now be considered in their design,
it is reasonable to reduce the steel dome safety factor for unbalanced loads to 3.5 as proposed in this
ballot. The US unit equation for steel dome thickness given in 5.10.6.1 is modified as follows:
Current equation: t =
rr
200
T
+ C.A. > 3/16 in.
45
rr
r
T
+ C.A. and t = r
200 45
230
where T = balanced load and U = unbalanced load
U
+ C.A. > 3/16 in.
45
Designers may use the balloted equations given in 5.10.5 and 5.10.6 or they may perform more precise
analyses if they wish. An example for a self-supporting steel dome using the current 650 approach and
this ballots approach is given below:
Given:
Balanced snow load = 20 psf
Unbalanced snow load = 30 psf
Tank diameter D = 80 ft
Dome radius rr = tank diameter D
External pressure = 1 w.c. = 5.2 lb/ft2
C.A. = 0
Assume a thickness of 0.375 in., which weighs 15.3 lb/ft2
Current method:
T = balanced snow + 0.4(external pressure) + dead load = 20 psf + 0.4(5.2 psf) + 15.3 psf = 37.4 psf
650-608 Design Loads for Tank Roofs 4/8/08
t=
rr
200
T
80
+ C.A. =
45
200
37.4
+ 0 = 0.365 in.
45
Ballot method:
U = unbalanced snow + 0.4(external pressure) + dead load = 30 psf + 0.4(5.2 psf) + 15.3 psf = 47.4 psf
r
80
47.4
U
t= r
+ C.A. =
+ 0 = 0.357 in.
230
45
230 45
T = balanced snow + 0.4(external pressure) + dead load = 20 psf + 0.4(5.2 psf) + 15.3 psf = 37.4 psf
r
T
80
37.4
t= r
+ C.A. =
+ 0 = 0.365 in.
200 45
200
45
For this example, the current method and the ballot method give the same result.
Aluminum Dome Seismic Load
The aluminum dome seismic load (G.4.2.3) needs to be updated since Appendix E has changed. This
ballot corrects the dome seismic load to correspond with the new Appendix E seismic design
requirements.
Aluminum Dome General Buckling
The aluminum dome general buckling equation is made dimensionless in this ballot to be consistent with
the metric guidelines passed by the committee. The allowable buckling pressure is required to be
compared to the revised loads in G.4.2.1 and G.4.2.2.
Aluminum Dome Tension Ring
The aluminum dome tension ring area equation given in G.4.1.4 has a typo. The US unit version should
be
11D 2
180
An =
tan sin(
) Ft
Written in dimensionless form, making the applied load a variable, and using sin(180 o/) , this
equation is
D 2 ( LL DL)
An =
, which is used in this ballot.
8 Ft tan
BALLOT:
Add underlined words
1.5 Formulas
Where units are not defined in formulas in this standard, use consistent units (e.g., in., in 2, in.3, lbf/in2).
5.2.1 Loads Loads are defined as follows:
(a) Dead Load (DL): The weight of the tank or tank component, including any corrosion
650-608 Design Loads for Tank Roofs 4/8/08
4. Fastest mile wind speed times 1.2 is approximately equal to 3 sec gust wind speed.
5.10.4.10 Center columns shall be designed for both the balanced snow load and unbalanced snow load.
Intermediate columns need only be designed for the balanced snow load.
5.10.5.1 Self-Supporting Cone Roofs
For SI units, change:
D
T
Minimum thickness =
> 5 mm
4.8 sin 2.2
Maximum thickness = 12.5 mm, exclusive of corrosion allowance
where
D = nominal diameter of the tank shell (m)
T = greater of load combinations (e)(1)and (e)(2) of Appendix R (kPa)
= angle of cone elements to the horizontal (deg)
to:
Minimum thickness = greatest of
D
4.8 sin
T
D
+ CA,
2.2
5.5 sin
U
+ CA, and 5 mm
2.2
where
D = nominal diameter of the tank (m)
T = greater of Appendix R load combinations (e)(1)and (e)(2) with balanced snow load Sb (kPa)
U = greater of Appendix R load combinations (e)(1)and (e)(2) with unbalanced snow load Su (kPa)
= angle of cone elements to the horizontal
CA = corrosion allowance
For US units, change:
D
T
> 3/16 in.
400 sin 45
Maximum thickness = in., exclusive of corrosion allowance
Minimum thickness =
where
D = nominal diameter of the tank (ft)
T = greater of load combinations (e)(1)and (e)(2) of Appendix R (lbf/ft 2)
= angle of cone elements to the horizontal (deg)
to:
Minimum thickness = greatest of
D
400 sin
T
D
+ CA,
45
460 sin
U
+ CA, and 3/16 in.
45
where
D = nominal diameter of the tank shell (ft)
T = greater of Appendix R load combinations (e)(1)and (e)(2) with balanced snow load Sb (lbf/ft2)
U = greater of Appendix R load combinations (e)(1)and (e)(2) with unbalanced snow load Su (lbf/ft2)
= angle of cone elements to the horizontal
CA = corrosion allowance
5.10.6.1 Self-Supporting Dome and Umbrella Roofs
650-608 Design Loads for Tank Roofs 4/8/08
Minimum thickness =
where
D = nominal diameter of the tank shell (m)
T = greater of load combinations (e)(1)and (e)(2) of Appendix R (kPa)
r = roof radius (m)
to:
Minimum thickness = greatest of
rr
2.4
rr
T
+ C.A.,
2.2
2.7
U
+ C.A., and 5 mm
2.2
where
D = nominal diameter of the tank shell (m)
T = greater of Appendix R load combinations (e)(1)and (e)(2) with balanced snow load Sb (kPa)
U = greater of Appendix R load combinations (e)(1)and (e)(2) with unbalanced snow load Su (kPa)
r = roof radius (m)
For US units, change:
rr
T
+ C.A. > 3/16 in.
200 45
Maximum thickness = in., exclusive of corrosion allowance
Minimum thickness =
where
D = nominal diameter of the tank shell (ft)
T = greater of load combinations (e)(1)and (e)(2) of Appendix R (lbf/ft 2)
r = roof radius (ft)
to:
Minimum thickness = greatest of
rr
200
r
T
+ C.A., r
45
230
U
+ C.A., 3/16 in.
45
where
D = nominal diameter of the tank shell (ft)
T = greater of Appendix R load combinations (e)(1)and (e)(2) with balanced snow load Sb (lbf/ft2)
U = greater of Appendix R load combinations (e)(1)and (e)(2) with unbalanced snow load Su (lbf/ft2)
r = roof radius (ft)
APPENDIX R LOAD COMBINATIONS
R.1 For the purposes of this standard, loads are combined in the following manner. Design rules account
for these load combinations, including the absence of any load other than DL in the combinations:
(a) Fluid and Internal Pressure:
DL + F + Pi
(b) Hydrostatic Test:
DL + (Ht + Pt)
650-608 Design Loads for Tank Roofs 4/8/08
Replace with:
In SI units:
Wa =
108.1 10 6
I x Ag
LR ( SF )
where
2258 10 6 I x Ag
LR 2 ( SF )
where
Wa = allowable total downward load (lbf/ft2)
Ix = moment of inertia of frame members against
650-608 Design Loads for Tank Roofs 4/8/08
10
In US units:
D2 p
An =
8 Ft tan
11D 2
180
An =
tan (
) Ft
where
An = net area of tension beam (in.2)
D = nominal tank diameter (ft)
= number of dome supports
= the central angle of the dome or roof slope at
the tank shell
Ft = allowable stress of the tension ring (lbf/in2)
In cases where the total dead load plus live load is
greater than 1.34 kPa (28 lbf/ft2), the above formula
shall be multiplied by W/1.34 (or 28), where W =
the total dead load plus live load for the dome.
Note: this formula does not include factors for
bending stresses due to loads from the panel
attached to the beam. These stresses must also be
considered in the tension ring design, as per G.3.1.
G.4.2 DESIGN LOADS
Dome roofs shall be designed for the loads in 5.2.1,
G.4.2, and G.4.3; and for the load combinations (a),
(b), (c), (e), and (f) of Appendix Y.
G.4.2.1 Unbalanced Load
The design shall consider one-half of the uniform
downward load required applied to one-half of the
dome with only the dead load on the other half.
G.4.2.2 Wind Load
G.4.2.2.1 For dome structural design, the
minimum wind load shall be the load resulting
from a design wind speed of 190 km/h (120 mph)
650-608 Design Loads for Tank Roofs 4/8/08
where
An = net area of tension ring
D = nominal tank diameter
p = maximum pressure given in R.1(e)
= the central angle of the dome or roof slope at
the tank shell
Ft = least allowable stress for components of the
tension ring
Note: this formula does not include bending
stresses due to loads from the panel attached to the
beam. These stresses must also be considered in
the tension ring design per G.3.1.
In SI units: (V/190)2
In SI units: (V/190)2
Where
V = wind speed (3 sec gust) in km/h (mph).
Where
V = wind speed (3 sec gust) in km/h (mph).
12
Appendix L
Change 11. Thickness * _______ In. Snow Load* to
Minimum Roof Live Load _______ psf
Balanced Snow Load _________ psf
Unbalanced Snow Load _______ psf
13