Table of Contents
| Section | Title | Page |
| Purpose | 2 | |
| Scope | 2 | |
| Related Documents | 2 | |
| Design Basis | 2 | |
| Design Methods | 3 | |
| Horizontal Vessels | 4 | |
| Vertical Vessels | 4 | |
| Allowable Stresses | 5 | |
| Example 1 – Horizontal Vessel (Not Elevated) | 6 | |
| Example 2 – Vertical Vessel on Legs | 7 | |
| Example 3 – Vertical Vessel on Skirt – Calculated Period = 1.50 Seconds | 8 | |
| Example 4 – Vertical Vessel on Skirt – Calculated Period = 0.75 Seconds | 9 | |
| Example 5 – Vertical Vessel on Skirt – Calculated Period = 0.44 Seconds | 10 |
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1. PURPOSE
1.1 This engineering specification provides guidance in the selection and application of the factors and formulas in the 1997 Uniform Building Code™.
2. SCOPE
2.1 This specification applies to all pressure vessels and storage tanks when the design calls for seismic analysis in accordance with the 1997 Uniform Building Code.
- This specification shall be used in conjunction with a Project-Specific Equipment Specification (PES). The PES will state when a seismic analysis is required.
2.3 Flat bottom storage tanks are outside the scope of this document.
- RELATED DOCUMENTS
3.1 American Institute of Steel Construction, Inc. (AISC)
Manual of Steel Construction
3.2 American Society of Mechanical Engineers (ASME)
BPV-VIII ASME Boiler and Pressure Vessel Code, Section VIII –
Division 1, Pressure Vessels
3.3 International Conference of Building Officials (ICBO)
1997 Uniform Building Code™ (UBC)
3.4 International Fire Code Institute (IFCI)
Uniform Fire Code™
4. DESIGN BASIS
4.1 The seismic design of pressure vessels and storage tanks shall be in accordance with the rules and methods described in the 1997 Uniform Building Code (UBC).
4.2 Seismic Zone Factor Z can be read directly from UBC Table 16-I (also see UBC Figure 16.2 for the Seismic Zone Map of the United States). The applicable seismic zone will be stated in the PES.
4.3 Soil Profile Types: SA through SF are given in UBC Table 16-J. The UBC directs that when the soil profile is not known, then soil type SD shall be used. When the PES does not specify a soil type, then SD shall be assumed.
4.4 Seismic Coefficients: Ca and Cv are obtained from UBC Tables 16-Q and 16-R.
4.5 Occupancy Category: Importance Factors I and Ip (UBC Table 16-K)
4.5.1 Essential Facilities are the exception rather than the rule and, when applicable, will be so identified in the PES. When no instructions are given, it shall be assumed that the Essential Facilities Occupancy Category does not apply.
4.5.1.1 The California Building Code (sometimes referred to as Title 24) requires the use of an Importance Factor of 1.50 for Essential Facilities. This exceeds the UBC specified factor of 1.25.
Note: Essential Facilities applications in California shall use an Importance Factor of 1.50.
4.5.2 Hazardous Facilities are defined as those applications involving toxic or explosive chemicals and shall require the use of an Importance Factor of 1.25 as opposed to the Standard Occupancy Importance Factor of 1.00.
4.5.2.1 Argon, nitrogen, or oxygen storage vessels located outside of California shall be classified as Standard Occupancy. However, hydrogen storage shall be always considered hazardous.
- For applications within California, it is common to refer to the Uniform Fire Code to determine whether or not a substance should be classified as hazardous. The Uniform Fire Code classifies all cryogenic liquid storage as hazardous. Therefore, for applications within California, all cryogenic liquid storage shall be treated as hazardous and shall use a Seismic Importance Factor of 1.25.
4.5.3 In all other cases, and unless specifically instructed otherwise in the PES, the Seismic Importance Factor shall be taken as 1.00, corresponding to Standard Occupancy.
4.6 Near Source Factors: Na and Nv (UBC Tables 16-S and 16-T)
4.6.1 Near Source Factors are only applicable to Seismic Zone 4 and are therefore limited to California and surrounding areas.
4.6.2 Values of Near Source Factors are site specific and shall be assumed equal to 1.00 unless specifically instructed otherwise in the PES.
5. DESIGN METHODS
5.1 The UBC recognizes Strength Design and Allowable Stress Design methods of design analysis.
Note: Pressure vessels and storage tanks shall be designed using the Allowable Stress Design analysis method (Reference UBC Section 1612 – Combination of Loads).
5.1.1 UBC Section 1612.3 describes the combination of the various loads to be used for the Allowable Stress Design. UBC Section 1612.3.2, Alternate Basic Load Combinations, shall be used to determine the loads for pressure vessels and storage tanks. Since the live load (L) and snow load (S) are usually zero, and wind load (W) is not assumed to be acting simultaneously with earthquake load (E), the load combination simplifies to the dead load (D) plus earthquake load (E) divided by 1.4 (as a formula: D + E / 1.4).
Note: For the Allowable Stress Design, the calculated earthquake load (E) shall be divided by 1.4.
5.2 UBC Section 1630.1.1 describes the earthquake load (E). For the Allowable Stress Design, the vertical component (Ev) shall be taken as zero.
5.2.1 UBC Section 1634.1.2 states that for Nonbuilding Structures designed using UBC Sections 1634.3, 1634.4, or 1634.5, the Reliability/Redundancy Factor () may be taken as 1.00. Therefore, for shop‑fabricated pressure vessels and tanks, this factor is always 1.00, and the earthquake load as determined from UBC Formula (30-1) is simply the base shear (V).
5.3 UBC Section 1634 specifically addresses the requirements for Nonbuilding Structures. This section shall be used for pressure vessels and storage tanks.
- HORIZONTAL VESSELS
6.1 UBC Section 1634.3 describes a simplified approach to be used for Rigid Structures. Rigid structures are described as those that have a period (T) that is less than 0.06 seconds, or conversely, a natural frequency greater than 16.7 cycles per second.
6.2 The method described in UBC Section 1634.3 shall be used for horizontal vessels because these are rigid by virtue of their low profile.
Note: The responsible engineer is cautioned that although this rule applies to the vast majority of horizontal tanks, unusual configurations such as elevated tanks on piers can result in flexible inverted pendulum design models that may require special consideration.
6.3 UBC Section 1634.3 requires the distribution of the base shear according to the distribution of mass. For a simple vessel, the seismic force is considered to act at the center of gravity. For Seismic Zone 4 applications, Seismic Coefficient Ca from UBC Table 16-Q includes the near source factor.
- VERTICAL VESSELS
7.1 UBC Section 1634.5 describes the method to be used for Other Nonbuilding Structures and is the section to be used for vertical pressure vessels and storage tanks.
7.2 UBC Section 1634.5 requires that the seismic shear force be calculated in accordance with Section 1630.2, Static Force Procedure, with the added provision that the values for base shear (V) shall not be less than the values given by UBC Formulas (34-2) and (34-3). These override UBC Formulas (30-6) and (30-7) contained in Section 1630.2.1.
7.2.1 The total base shear need not exceed the value given by UBC Formula (30-5) contained in Section 1630.2.1.
7.3 Base shear is calculated in accordance with UBC Section 1630.2.1. Formula (30-4) requires the determination of the vessel’s fundamental period (T). This is straightforward for a vertical vessel on a skirt. However, for a vertical vessel on legs, the determination of the fundamental period is more difficult and will vary depending upon the specific geometry. In addition, the analysis itself is subject to a variety of interpretations depending upon the assumptions made. Therefore, for a vessel on legs, UBC Formula (30-5) shall be used because this does not require determination of the period and is a conservative approach.
7.4 Vertical distribution of force shall be in accordance with UBC Section 1630.5, Formula (30-15). This distribution can be summarized by the following table.
| Distribution of Seismic Load: |
| Type of Support | R Value | Point of Seismic Load Application | |
| Legs | 2.2 | Center of gravity of vessel and its contents (or dynamic analysis) | |
| Skirt | 2.9 | 2/3 height of vessel (or dynamic analysis) assuming uniform diameter and wall thickness |
7.5 For Seismic Zone 4 applications, seismic coefficients Ca and Cv include Near Source Factors Na and Nv.
7.6 The UBC requires that a concentrated force (Ft) be placed at the top of the vessel. The determination of Ft is dependent upon the vessel fundamental period (T).
7.6.1 Vessels on legs are usually short with an aspect ratio of less than five and a period (T) of 0.7 seconds or less. UBC Section 1630.5 states that Ft may be considered as zero when the fundamental period (T) is 0.7 seconds or less. The use of Ft is intended for tall, flexible vessels such as distillation columns.
Note: For vessels on legs, Ft shall be assumed to be zero.
7.6.2 In all other cases, Ft shall be determined based upon the period (T) and shall be located at the top of the vessel as required by the UBC [Formula (30-14)].
7.7 Do not treat vertical vessels as rigid structures unless the fundamental period can be clearly shown to meet the definition of rigid structures in UBC Section 1634.3. Rigid structures are described as those with a period (T) that is less than 0.06 seconds. This is very stiff, and any vertical pressure vessel or storage tank is unlikely to meet this requirement.
- ALLOWABLE STRESSES
8.1 Pressure vessel pressure-containing parts shall limit stress levels to those permitted by the controlling pressure vessel code or appropriate National Approval Body. In the United States, the controlling Pressure Vessel Code is the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1.
8.2 Nonpressure structural parts shall limit stress levels to those permitted by recognized structural design codes or applicable regulations. In the United States, the stress levels shall be limited to the values permitted by the AISC Manual of Steel Construction.
8.3 UBC Section 1612.3.2 permits an increase of one-third in the basic allowable stress for hold-down bolts, supports, and other nonpressure, structural components that fall outside of the jurisdiction of the controlling pressure vessel code.
Appendix A
Example 1
| Application | Horizontal Vessel (Not Elevated) |
| Storage | Cryogenic – Liquid Nitrogen |
| UBC Design Basis | · Section 1634 – Nonbuilding Structures
· Treat as a Rigid Structure (T < 0.06 seconds) per Section 1634.3 |
| Applicable Formula | V = 0.7*Ca*I*W (34-1)
|
| Seismic Zone | Figure 16-2 | 1 | 2A | 2B | 3 | 4 | ||
| Seismic Zone Factor, Z | Table 16-I | 0.075 | 0.15 | 0.20 | 0.30 | 0.40 | ||
| Near Source Factor, Na | Table 16-S | N/A | N/A | N/A | N/A | 1.00 | ||
| Importance Factor, I | Table 16-K | 1.00 | 1.00 | 1.00 | 1.00 | 1.25
(Note 2) |
||
| Seismic Coefficient, Ca | Table 16-Q | 0.12 | 0.22 | 0.28 | 0.36 | 0.44 | ||
| V/W = 0.7*Ca*I | 0.0840 | 0.1540 | 0.1960 | 0.2520 | 0.3850 | |||
| and V/(1.4*W) | 0.0600 | 0.1100 | 0.1400 | 0.1800 | 0.2750 | |||
|
Notes:
1. Ca varies depending upon the specific site soil conditions or may be assumed to be the default value based upon soil type SD. In the above example, the default values are assumed. 2. The Importance Factor (I) for nitrogen is normally 1.00. However, in California, cryogenic liquids are treated as hazardous and therefore I = 1.25. The example for Seismic Zone 4 assumes a California location. 3. Near Source Factor Na will vary depending upon specific location. In the above example,
|
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Appendix A (continued)
Example 2
| Application | Vertical Vessel on Legs |
| Storage | Cryogenic – Liquid Nitrogen |
| UBC Design Basis | · Section 1634 – Nonbuilding Structures
· Subsection 1634.5 – Other Nonbuilding Structures |
| Applicable Formula | V = 2.5*Ca*I *W/R (30-5)
But not less than:
V = 0.56*Ca*I*W (34-2) or V = 1.6*Z*Nv*I*W/R (34-3) for Zone 4 only
|
| Seismic Zone | Figure 16-2 | 1 | 2A | 2B | 3 | 4 | ||||||||
| Seismic Zone Factor, Z | Table 16-I | 0.075 | 0.15 | 0.20 | 0.30 | 0.40 | ||||||||
| Near Source Factor, Na | Table 16-S | N/A | N/A | N/A | N/A | 1.00 | ||||||||
| Near Source Factor, Nv | Table 16-T | N/A | N/A | N/A | N/A | 1.20 | ||||||||
| Factor, R | Table 16-P | 2.2 | 2.2 | 2.2 | 2.2 | 2.2 | ||||||||
| Importance Factor, I | Table 16-K | 1.00 | 1.00 | 1.00 | 1.00 | 1.25
(Note 2) |
||||||||
| Seismic Coefficient, Ca | Table 16-Q | 0.12 | 0.22 | 0.28 | 0.36 | 0.44 | ||||||||
| V/W = 2.5*Ca*I/R | (30-5) | 0.1364 | 0.2500 | 0.3182 | 0.4091 | 0.6250 | ||||||||
| V/W = 0.56*Ca*I | (34-2) | 0.0672 | 0.1232 | 0.1568 | 0.2016 | 0.3080 | ||||||||
| V/W = 1.6*Z*Nv*I/R | (34-3) | N/A | N/A | N/A | N/A | 0.4364 | ||||||||
| MAX V/W | 0.1364 | 0.2500 | 0.3182 | 0.4091 | 0.6250 | |||||||||
| and V/(1.4*W) | 0.0974 | 0.1786 | 0.2273 | 0.2922 | 0.4464 | |||||||||
|
Notes:
1. Ca varies depending upon the specific soil conditions or may be assumed to be the default value based upon soil type SD. In the above example, the default values are assumed. 2. The importance Factor (I) for nitrogen is normally 1.00. However, in California, cryogenic liquids are treated as hazardous and therefore I = 1.25. The example for Seismic Zone 4 assumes a California location. 3. Near Source Factors Na and Nv will vary depending upon specific location. In the above example,
|
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Appendix A (continued)
Example 3
| Application | Vertical Vessel on Skirt –
Calculated Period = 1.50 Seconds |
| Storage | Cryogenic – Liquid Nitrogen |
| UBC Design Basis | · Section 1634 – Nonbuilding Structures
· Subsection 1634.5 – Other Nonbuilding Structures |
| Applicable Formula | V = Cv*I*W/(R*T) (30-4)
But not less than:
V = 0.56*Ca*I*W (34-2) or V = 1.6*Z*Nv*I*W/R (34-3) for Zone 4 only
And not more than:
V = 2.5*Ca*I*W/ R (30-5)
|
| Seismic Zone | Figure 16-2 | 1 | 2A | 2B | 3 | 4 |
| Seismic Zone Factor, Z | Table 16-I | 0.075 | 0.15 | 0.20 | 0.30 | 0.40 |
| T – Calculated | Seconds | 1.50 | 1.50 | 1.50 | 1.50 | 1.50 |
| Near Source Factor, Na | Table 16-S | N/A | N/A | N/A | N/A | 1.00 |
| Near Source Factor, Nv | Table 16-T | N/A | N/A | N/A | N/A | 1.20 |
| Seismic Coefficient, Cv | Table 16R | 0.18 | 0.32 | 0.40 | 0.54 | 0.768 |
| Factor, R | Table 16P | 2.9 | 2.9 | 2.9 | 2.9 | 2.9 |
| Importance Factor, I | Table 16-K | 1.00 | 1.00 | 1.00 | 1.00 | 1.25
(Note 2) |
| Seismic Coefficient, Ca | Table 16-Q | 0.12 | 0.22 | 0.28 | 0.36 | 0.44 |
| V/W = Cv*I/(R*T) | (30-4) | 0.0414 | 0.0736 | 0.0920 | 0.1241 | 0.2207 |
| but not less than | ||||||
| V/W = 0.56*Ca*I | (34-2) | 0.0672 | 0.1232 | 0.1568 | 0.2016 | 0.3080 |
| V/W = 1.6*Z*Nv*I/R | (34-3) | N/A | N/A | N/A | N/A | 0.3310 |
| and not more than | ||||||
| V/W = 2.5*Ca*I/R | (30-5) | 0.1034 | 0.1897 | 0.2414 | 0.3103 | 0.4741 |
| therefore V/W | 0.0672 | 0.1232 | 0.1568 | 0.2016 | 0.3310 | |
| and V/(1.4*W) | 0.0480 | 0.0880 | 0.1120 | 0.1440 | 0.2365 | |
Notes:
- Cavaries depending upon the specific soil conditions or may be assumed to be the default value based upon soil type SD.
In the above example, the default values are assumed.
- The importance Factor (I) for nitrogen is normally 1.00. In California, cryogenic liquids are treated as hazardous and therefore I = 1.25. The example for Seismic Zone 4 assumes a California location.
- Near Source Factors Naand Nv will vary depending upon specific location. In the above example,
Na is assumed to be 1.00 and Nv to be 1.20.
Appendix A (continued)
Example 4
| Application | Vertical Vessel on Skirt –
Calculated Period = 0.75 Seconds |
| Storage | Cryogenic – Liquid Nitrogen |
| UBC Design Basis | · Section 1634 – Nonbuilding Structures
· Subsection 1634.5 – Other Nonbuilding Structures |
| Applicable Formula | V = Cv*I*W/(R*T) (30-4)
But not less than:
V = 0.56*Ca*I*W (34-2) or V = 1.6*Z*Nv*I*W/R (34-3) for Zone 4 only
And not more than:
V = 2.5*Ca*I*W/ R (30-5)
|
| Seismic Zone | Figure 16-2 | 1 | 2A | 2B | 3 | 4 |
| Seismic Zone Factor, Z | Table 16-I | 0.075 | 0.15 | 0.20 | 0.30 | 0.40 |
| T – Calculated | Seconds | 0.75 | 0.75 | 0.75 | 0.75 | 0.75 |
| Near Source Factor, Na | Table 16-S | N/A | N/A | N/A | N/A | 1.00 |
| Near Source Factor, Nv | Table 16-T | N/A | N/A | N/A | N/A | 1.20 |
| Seismic Coefficient, Cv | Table 16R | 0.18 | 0.32 | 0.40 | 0.54 | 0.768 |
| Factor, R | Table 16P | 2.9 | 2.9 | 2.9 | 2.9 | 2.9 |
| Importance Factor, I | Table 16-K | 1.00 | 1.00 | 1.00 | 1.00 | 1.25
(Note 2) |
| Seismic Coefficient, Ca | Table 16-Q | 0.12 | 0.22 | 0.28 | 0.36 | 0.44 |
| V/W = Cv*I/(R*T) | (30-4) | 0.0828 | 0.1471 | 0.1839 | 0.2483 | 0.4414 |
| but not less than | ||||||
| V/W = 0.56*Ca*I | (34-2) | 0.0672 | 0.1232 | 0.1568 | 0.2016 | 0.3080 |
| V/W = 1.6*Z*Nv*I/R | (34-3) | N/A | N/A | N/A | N/A | 0.3310 |
| and not more than | ||||||
| V/W = 2.5*Ca*I/R | (30-5) | 0.1034 | 0.1897 | 0.2414 | 0.3103 | 0.4741 |
| therefore V/W | 0.0828 | 0.1471 | 0.1839 | 0.2483 | 0.4414 | |
| and V/(1.4*W) | 0.0591 | 0.1051 | 0.1314 | 0.1773 | 0.3153 |
Notes:
- Cavaries depending upon the specific soil conditions or may be assumed to be the default value based upon soil type SD.
In the above example, the default values are assumed.
- The importance Factor (I) for nitrogen is normally 1.00. In California, cryogenic liquids are treated as hazardous and therefore I = 1.25. The example for Seismic Zone 4 assumes a California location.
- Near Source Factors Naand Nv will vary depending upon specific location. In the above example,
Na is assumed to be 1.00 and Nv to be 1.20.
Appendix A (continued)
Example 5
| Application | Vertical Vessel on Skirt –
Calculated Period = 0.44 Seconds |
| Storage | Cryogenic – Liquid Nitrogen |
| UBC Design Basis | · Section 1634 – Nonbuilding Structures
· Subsection 1634.5 – Other Nonbuilding Structures |
| Applicable Formula | V = Cv*I*W/(R*T) (30-4)
But not less than:
V = 0.56*Ca*I*W (34-2) or V = 1.6*Z*Nv*I*W/R (34-3) for Zone 4 only
And not more than:
V = 2.5*Ca*I*W/ R (30-5)
|
| Seismic Zone | Figure 16-2 | 1 | 2A | 2B | 3 | 4 |
| Seismic Zone Factor, Z | Table 16-I | 0.075 | 0.15 | 0.20 | 0.30 | 0.40 |
| T – Calculated | Seconds | 0.44 | 0.44 | 0.44 | 0.44 | 0.44 |
| Near Source Factor, Na | Table 16-S | N/A | N/A | N/A | N/A | 1.00 |
| Near Source Factor, Nv | Table 16-T | N/A | N/A | N/A | N/A | 1.20 |
| Seismic Coefficient, Cv | Table 16R | 0.18 | 0.32 | 0.40 | 0.54 | 0.768 |
| Factor, R | Table 16P | 2.9 | 2.9 | 2.9 | 2.9 | 2.9 |
| Importance Factor, I | Table 16-K | 1.00 | 1.00 | 1.00 | 1.00 | 1.25
(Note 2) |
| Seismic Coefficient, Ca | Table 16-Q | 0.12 | 0.22 | 0.28 | 0.36 | 0.44 |
| V/W = Cv*I/(R*T) | (30-4) | 0.1411 | 0.2508 | 0.3135 | 0.4232 | 0.7524 |
| But not less than | ||||||
| V/W = 0.56*Ca*I | (34-2) | 0.0672 | 0.1232 | 0.1568 | 0.2016 | 0.3080 |
| V/W = 1.6*Z*Nv*I/R | (34-3) | N/A | N/A | N/A | N/A | 0.3310 |
| And not more than | ||||||
| V/W = 2.5*Ca*I/R | (30-5) | 0.1034 | 0.1897 | 0.2414 | 0.3103 | 0.4741 |
| therefore V/W = | 0.1034 | 0.1897 | 0.2414 | 0.3103 | 0.4741 | |
| and V/(1.4*W) | 0.0739 | 0.1355 | 0.1724 | 0.2217 | 0.3387 |
Notes:
- Cavaries depending upon the specific soil conditions or may be assumed to be the default value based upon soil type SD.
In the above example, the default values are assumed.
- The importance Factor (I) for nitrogen is normally 1.00. In California, cryogenic liquids are treated as hazardous and therefore I = 1.25. The example for Seismic Zone 4 assumes a California location.
- Near Source Factors Naand Nv will vary depending upon specific location. In the above example, Na is assumed to be 1.00 and Nv to be 1.20.