AGENDA ITEM: 650-608 Design Loads for Tank Roofs

DATE: January 28, 2008

CONTACT: Randy Kissell, TGB Partnership

PH: 919-644-8250 FX: 919-644-8252, email:

PURPOSE:

1)  To allow reduced live loads for tank roofs where allowed by load standards

2)  To include unbalanced snow loads on tank roofs

SOURCE: Committee members’ request

REVISION: 3

IMPACT:

1)  Reduce cost where reduced live loads are allowed by load standards;

2)  Reduce the likelihood of failures when roofs are subjected to drifting snow loads.

RATIONALE:

General

This version of the ballot addresses comments received in the ballot conducted before the Fall 2007 Refining meeting.

Committee members requested an investigation into more precise loads for tank roofs. The first ballot addressed only aluminum dome roofs, but committee members requested that any changes to roof loads apply to all tank roofs. This ballot, therefore, uniformly addresses snow loads, wind loads, and minimum roof live loads for all tank roofs addressed by API 650.

Loads on tanks were recently revised in agenda item 650-472, which provided more precise loads than previously required by API 650. For example, API 650 previously specified a 25 psf roof live load for all tanks in all locations. 650-472 (published in 650’s 10th edition, addendum 4) changed this, providing rules for determining the uniform snow load for the tank based on its location. Changes were also made to more accurately determine other loads such as minimum roof live load and wind load, and address load combinations.

The 650-472 load changes were based on ASCE 7, Minimum Design Loads for Buildings and Other Structures, but the 650-472 snow load and minimum roof live load were simplified from the ASCE 7 approach. Now that the committee is requesting more precise loads, this ballot proposes to revise 650’s snow load and minimum roof live load requirements to more closely match ASCE 7.

This will have the benefit of reducing costs in low snow load regions where ASCE 7 provides for lower minimum roof live loads, and making tank design more consistent with accepted practice for other structures. Another advantage will be to reduce roof failures due to drifting snow loads by addressing unbalanced snow loads for the first time in API 650.

Minimum Roof Live Load

ASCE 7 section 4.9.1 prescribes minimum roof live loads as a function of the tributary area of a structural member and the rise-to-span ratio of dome roofs. The largest ASCE 7 minimum roof live load value of 20 psf is conservatively prescribed by API 650 5.2.1(e). ASCE 7 allows this value to be reduced to as little as 12 psf for structural components with large tributary areas in domes with large rise-to-span ratios. This ballot proposes to allow lower minimum roof live loads where allowed by ASCE 7, but no less than 15 psf. (AWWA D100 specifies 15 psf as a minimum.) This reduces the design load in regions where the ground snow load is small.

Wind Loads

This ballot would not change wind loads except that they would be applied to all roofs, including aluminum dome roofs, which currently have different wind loads based on outdated ASCE 7 arched roof wind loads. The discussion below provides the rationale for the roof wind load currently in API 650.

ASCE 7-02 (Minimum Design Loads for Buildings and Other Structures) provides wind pressures for dome roofs in Figure 6-7. Dome pressures are a function of the tank-height-to-diameter ratio, distance from the windward edge, and roof profile. These were used in agenda item 650-472 to provide the wind pressure for doubly curved surfaces, and this ballot proposes to apply these to aluminum domes, since the ASCE 7 pressure is more accurate than the current API 650 G.4.2.2.1 provisions. The ASCE 7 dome wind pressure approach is briefly reviewed below.

For typical profiles permitted by API 650 (for steel domes, see 5.10.6.1 and for aluminum domes, see G.6.2) on an 80’ diameter, 48’ tall tank, ASCE 7 Figure 6-7 gives an approximate average Cp = -0.97, so the design wind pressure is

p = qh (GCp - GCpi)

p = 36.4 (-0.97(0.85) – 0.18) = 36.4 (-0.94) = 36.6 psf (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’f x 40’h / 80’f x 48’h / 150’f x 48’h
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.

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 7-02 unbalanced snow load on dome roofs varies from 0.5 times the flat roof snow load pf at the roof’s 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 90o 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 roof’s area).

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 10o or less. If a cone roof is considered to be similar to a dome roof, then a cone roof with a slope of ¾ on 12 (3.57o) has a slope less than 10o, for which the only unbalanced load ASCE 7 requires is a partial loading (the balanced load acting over only part of the roof).

Roof Design

There are four types of fixed roofs addressed by API 650:

Roof Type / API 650 Reference / Slope θ or Radius r / Safety Factor on Buckling
Supported Cone Roofs / 5.10.4 / 3.6 o (¾ on 12); may be greater / 1.67 on column buckling
Self-Supporting Cone Roofs / 5.10.5 / 9.5 o θ 37o / variable from about 3 to less than 2*
Self-Supporting Dome and Umbrella Roofs / 5.10.6 / 0.8D r 1.2D / 4*
Self-Supporting Aluminum Dome Roofs / Appendix G / 0.7D r 1.2D / 1.65 on general buckling; 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-of-roundness 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 = + C.A. 3/16 in.

Ballot equations: Use the greater of t = + C.A. and t = + C.A. 3/16 in.

where T = balanced load and U = unbalanced load

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 is using 5.10.6 using the current 650 approach and this ballot’s 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

t = + C.A. = + 0 = 0.365 in.

Ballot method:

U = unbalanced snow + 0.4(external pressure) + dead load = 30 psf + 0.4(5.2 psf) + 15.3 psf = 47.4 psf

t = + C.A. = + 0 = 0.357 in.

T = balanced snow + 0.4(external pressure) + dead load = 20 psf + 0.4(5.2 psf) + 15.3 psf = 37.4 psf

t = + C.A. = + 0 = 0.365 in.

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

An =

Written in dimensionless form, making the applied load a variable, and using η sin(180o/η) ≈ π, this equation is

An = , which is used in this ballot.

BALLOT:

Add unlined words

5.2.1 Loads Loads are defined as follows:

(a) Dead Load (DL): The weight of the tank or tank component, including any corrosion allowance unless otherwise noted.

(b) Design External Pressure (Pe): Shall not be less than 0.25 kPa (1 in. of water). This standard does not contain provisions for external pressures greater than 0.25 kPa. Design requirements for vacuum exceeding this value and design requirements to resist flotation and external fluid pressure shall be a matter of agreement between the Purchaser and the Manufacturer (see Appendix V).

(c) Design Internal Pressure (Pi): Shall not exceed 18 kPa (2.5 lbf/in2).

(d) Hydrostatic Test (Ht ): The load due to filling the tank with water to the design liquid level.

(e) Minimum Roof Live Load (Lr): 1.0 kPa (20 lb/ft2) on the horizontal projected area of the roof. The minimum roof live load may alternatively be determined in accordance with ASCE 7, but shall not be less than 0.72 kPa (15 psf). The minimum roof live load shall be reported to the purchaser.

(f) Seismic (E): Seismic loads determined in accordance with sections E.1 through E.6 (see Data Sheet, Line 8).

(g) Snow (S): The ground snow load shall be determined from ASCE 7 Figure 7-1 or Table 7-1 unless the ground snow load that equals or exceeds the value based on a 2% annual probability of being exceeded (50 yr mean recurrence interval) is specified by the purchaser.

1) The balanced design snow load (Sb) shall be 0.84 times the ground snow load. Alternately, the balanced design snow load shall be determined from the ground snow load in accordance with ASCE 7.

2) The unbalanced design snow load (Su) for cone roofs with a slope of 10o or less shall be equal to the balanced snow load. The unbalanced design snow load (Su) for all other roofs shall be 1.5 times the balanced design snow load. Unbalanced design snow load shall be applied over a 135o sector of the roof plan with no snow on the remaining 225o sector. Alternately, the unbalanced snow load shall be determined from the ground snow load in accordance with ASCE 7.

The balanced and unbalanced design snow loads shall be reported to the purchaser.

(h) Stored Liquid (F): The load due to filling the tank to the design liquid level (see 5.6.3.2) with liquid with the design specific gravity specified by the purchaser.

(i) Test Pressure (Pt): As required by F.4.4 or F.7.6.

(j) Wind (W): The design wind speed (V) shall be 190 km/hr (120 mph), the 3 sec gust design wind speed determined from ASCE 7 Figure 6-1, or the 3 sec gust design wind speed specified by the purchaser (this specified wind speed shall be for a 3 sec gust based on a 2% annual probability of being exceeded (50 yr mean recurrence interval)). The design wind pressure shall be 0.86 kPa [V/190]2, [(18 lbf/ft2)(V/120)2] on vertical projected areas of cylindrical surfaces and 1.44 kPa(V/190)2, [(30 lbf/ft2)(V/120)2] uplift (2) on horizontal projected areas of conical or doubly curved surfaces, where V is the 3 sec gust wind speed. The 3 sec gust wind speed used shall be reported to the purchaser.

1. These design wind pressures are in accordance with ASCE 7 for wind exposure category C. As an alternative, pressures may be determined in accordance with ASCE 7 (exposure category and importance factor provided by purchaser) or a national standard for the specific conditions for the tank being designed.