ei-004 _rev. 16_wind, earthquake and snow loading_sin anexo c.pdf

ENGINEERING STANDARD

EI - 004

WIND, EARTHQUAKE AND SNOW LOADING

Please discard any previous issue of this Standard.

Revision Number Log Rev. Date By Pages App. Remarks

0 1.07.96 PCA HPJ ISSUED FOR APPROVAL 1 16.08.96 PCA HPJ APPROVED FOR DESIGN

2 7.01.97 PCA 1,5 HPJ REVISION TABLE 1, PAGE 5

3 30.01.97 PCA 3,4,5,8,11,12,13,16 HPJ REVISED PAGES 3, 4, 5, 8, 11, 12, 13 AND 16

4 9.05.97 PCA 3,6,7,8,14 HCS REVISED PAGES 3, 6, 7, 8, AND 14

5 7.7.97 PCA 6,8,14 HCS REVISED PAGES 6, 8 AND 14

6 20.8.97 PCA 3 HCS REVISED PAGE 3

7 15.9.97 PCA 3,8,13,19 HCS REVISED PAGES 3, 8, 13 AND 19



10 7.10.98 PCA All HCS GENERAL REVISION

11 7.5.01 S y S All HCS GENERAL REVISION

12 30.1.04 S y S 10 HLM REVISED PAGE 10

13 17.3.05 PCA ALL HLM GENERAL REVISION

14 26.4.05 S y S 1 TO 131 OF APPENDIX B HLM APPENDIX B REPLACED

15 12.7.05 S y S 1, 35 TO 50 HLM REVISED PAGES 1, 35 TO 50

16 21.10.06 S y S i, ii, 1, 4 TO 6, 8 TO 11, 13 TO 17, 19 TO 27, 32, 34 TO

42 AND 49

HLM REVISED PAGES i, ii, 1, 4 TO 6, 8 TO 11, 13 TO 17, 19 TO 27, 32, 34 TO 42 AND 49

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Rev. 16 i

1.0 GENERAL REQUIREMENTS ........................................................................................1 1.1 Scope ....................................................................................................................................3 1.2 General ................................................................................................................................3 2.0 REFERENCES...................................................................................................................4 2.1 Enap Refineras Engineering Standards..........................................................................4 2.2 U.S.A Specifications and Codes ........................................................................................4 2.3 Chilean Codes .....................................................................................................................4 3.0 COMBINATIONS OF LOADS ........................................................................................5 3.1 General ................................................................................................................................5 3.2 Symbols and Notation ........................................................................................................5 3.3 Combining Nominal Loads using Allowable Stress Design............................................6

3.3.1 Basic Combinations....................................................................................................6 3.4 Combining Factored Loads Using Strength Design........................................................7

3.4.1 Applicability................................................................................................................7 3.4.2 Basic Combinations....................................................................................................7

4.0 WIND LOADS....................................................................................................................8 4.1 General ................................................................................................................................8

4.1.1 Scope............................................................................................................................8 4.1.2 Allowed Procedures....................................................................................................8 4.1.3 Minimum Design Wind Loading ..............................................................................8

4.1.3.1 Main Wind Force Resisting System.....................................................................8 4.1.3.2 Components and Cladding ...................................................................................8

4.2 Basis of Loads .....................................................................................................................9 4.3 Determination of wind forces ..........................................................................................10

4.3.1 Buildings....................................................................................................................10 4.3.2 Open type structures................................................................................................11 4.3.3 Structures subjected to induced wind vibration....................................................11

4.3.3.1 Vortex Shedding.................................................................................................11 4.3.3.2 Prevention of Excessive Vibration.....................................................................11

5.0 SEISMIC LOADS ............................................................................................................13 5.1 General Provisions ...........................................................................................................13 5.2 Seismic Design Criteria....................................................................................................14 5.3 Allowable Deflection and Drift........................................................................................16 5.4 Seismic Analysis Procedures ...........................................................................................18

5.4.1 General ......................................................................................................................18 5.4.1.1 Introduction ........................................................................................................18 5.4.1.2 Direction of the seismic solicitation...................................................................19 5.4.1.3 Combination of effects of horizontal components of the earthquake ................20 5.4.1.4 Mathematical modeling......................................................................................20

5.4.2 Static Method of Analysis ........................................................................................22 5.4.3 Dynamic Method of Analysis ..................................................................................25

6.0 DESIGN SNOW LOADING ...........................................................................................33 7.0 GROUND-SUPPORTED FLAT-BOTTOM STEEL TANKS.....................................34 7.1 Tanks Classification .........................................................................................................34

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Rev. 16 ii

7.2 Seismic Design ..................................................................................................................34 7.2.1 Scope..........................................................................................................................34 7.2.2 Generalities ...............................................................................................................34 7.2.3 Design Spectral Response Accelerations ................................................................34 7.2.4 Base Shear.................................................................................................................35 7.2.5 Overturning Moment...............................................................................................36 7.2.6 Anchorage Ratio for Self-anchored Tanks ............................................................37 7.2.7 Sliding Resistance.....................................................................................................38 7.2.8 Anchorage .................................................................................................................39

7.2.8.1 Anchor Bolts ......................................................................................................39 7.2.8.2 Anchor Chairs ....................................................................................................39 7.2.8.3 Shear Device ......................................................................................................40

7.2.9 Freeboard..................................................................................................................40 7.2.10 Construction requirements......................................................................................41

8.0 SEISMIC DESIGN OF PRESSURE VESSELS............................................................42 8.1 Scope ..................................................................................................................................42 8.2 General requirements ......................................................................................................42

8.2.1 Risk classification .....................................................................................................42 8.2.2 Combination loads....................................................................................................43 8.2.3 Seismic loads .............................................................................................................43 8.2.4 Mathematical model of the pressure vessel............................................................44 8.2.5 Seismic analysis ........................................................................................................45

8.3 Seismic design of vertical pressure vessels .....................................................................46 8.4 Seismic design of horizontal cylindrical pressure vessels .............................................46 8.5 Secondary elements attached to pressure vessel............................................................46 8.6 Seismic design of anchorage system ...............................................................................47 8.7 Other considerations ........................................................................................................50

Table 5.1 Importance Factor....................................................................................................28 Table 5.2 Plan Structural Irregularities .................................................................................29 Table 5.3 Vertical Structural Irregularities ...........................................................................30 Table 5.4 Allowable Story Drift ...............................................................................................30 Table 5.5 Response Modification Coefficients and Deformation Coefficient......................31 Table 5.6 Horizontal Force Factors.........................................................................................32 Table 5.7 Modal Design Damping Ratios................................................................................32 Table 5.8 Hazard Factor...........................................................................................................32 Table 7.1 Anchorage Ratio.......................................................................................................38 Table 8.1 Risk Grade ................................................................................................................42

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Rev. 16 1

1.0 GENERAL REQUIREMENTS

This standard provides the minimum design basis for wind, earthquake and snow loading to be imposed on building, structures, vessels and equipments.

Chilean standards NCh 2369 Earthquake-resistant design of industrial structures and

facilities and NCh 433 Earthquake resistant design of buildings included in the appendices B and C form part of these requirements.

The requirements indicated in this standard shall be applied as a whole. The load combinations of Chapter 3.0 are consistent with the loading conditions indicated in Chapters 4.0 to 8.0.

For wind loads, Chapter 4.0 replaces the Chilean standard NCh 432 Calculation of the

action of wind on structures. For seismic loads, Chapter 5.0 prevails over NCh 2369. However the special NCh 2369

requirements of chapters 8, 9, 10, and 11 for steel structures, reinforced concrete structures, foundations and specific structures respectively are not modified by this standard and have to be fulfilled, except 11.7 and 11.8 for tanks where Chapters 7.0 and 8.0 of this standard prevail over NCh 2369.

The loading conditions indicated in Chapters 4.0 to 8.0, are only applicable to the location of Enap Refineras plant at Talcahuano.

Talcahuano is located near Concepcin, an earthquake prone zone, characterized by large

subduction earthquakes of Richter magnitude Ms = 8.5 with epicenters off-shore at approximately 40[km] from Talcahuano and a focal depth of 40[km]. The seismic zone is 3 according to NCh. 2369.

The studies of seismic risk have already been done and their recommendations are fully

included in these bases, according to NCh 2369 Section 5.8. Soil classification at Enap Refineras plant is Soil Type III. All structures, buildings, equipment and pieces of equipment shall have sufficient strength

to withstand seismic actions in accordance with these specifications. In accordance with section 4.6 of NCh 2369, design of structures shall be done by

engineers authorized to practice in Chile, with at least five years of proven experience in Seismic Design. Exception is made with equipment designed by foreign suppliers.

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Rev. 16 2

Seismic Design of all structures, equipment and their anchorages shall be reviewed and approved by engineers responsible to Enap Refineras, with 10 years or more of experience in Seismic Design, authorized to practice in Chile as indicated in section 4.6 of NCh 2369.

Contractor shall submit for review their calculation sheets and structural drawings, at own

expenses, to a local Seismic Reviewer previously approved by Enap Refineras. It is the Contractors responsibility to ensure that the seismic review considers not only

the building and/or equipment, but also the foundations in the context of an integral system. Parameters and methods for seismic analysis shall be established by Enap Refineras, or

approved by the Seismic Reviewers, upon proposals by structural designers and equipment suppliers. An example of acceptable form for submitting these parameters are shown in Appendix A.

Suppliers must submit the following information for approval:

Structural diagrams, drawings and/or sketches, signed by the designers, detailing the structures or structural components of equipment under analysis.

Identification of material used. Detail of the load cases and load combinations considered in the analysis. Details of the structures mathematical models used, indicating sections, releases,

orientation of members, etc. Identification of the computer programs, and description of the same, if it is requested by

the reviewers. Verification of maximum seismic deformations and drifts. Complete computation sheets, signed by the designers, for all structures, structural

components of equipment, connections and anchorages. If computer programs are used, complete input and result sheets.

Calculation report with a clear explanation of the analysis performed, accompanying the former documents. Exception is made with Class C3 minor structures and equipment, in which it is sufficient

to submit drawings with dimensions, weights, centers of gravity, material properties and anchoring devices, also signed by designers.

The review and approval does not relieve the suppliers responsibility for the supply

fulfillment of all structural and seismic specifications.

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Rev. 16 3

1.1 Scope 1.1.1 These general design criteria shall apply to equipment, piping, structures and buildings to

be designed by the Supplier for Enap Refineras plant at Talcahuano. 1.1.2 All particular seismic design requirements are included in Section 7.0 Ground-Supported

Flat-Bottom Steel Tank and Section 8.0 Seismic Design of Pressure Vessels. 1.1.3 All structural requirements and loading conditions that are not explicit in above

mentioned criteria shall be defined by the Supplier and submitted to the Project Manager for approval.

It shall be mandatory for the Supplier to indicate anyone deviation and not explicit requirement or condition in order to obtain the approval from the Project Manager for all of them.

1.1.4 The approval by the Project Manager shall be granted without detriment of correspondent

responsibility of the Supplier. 1.1.5 Design criteria prepared by the Supplier cannot modify any requirement of this design

standard. If it happens, the modification or substitution will be void and considered as non-existent.

1.1.6 Design and drawings that not fulfilling this design standard must be redone, even if the

Project Manager had mistakenly approved them. 1.1.7 Any complement, modification or substitution to this design standard should be admitted

and agreed before the signature of the contract. 1.2 General 1.2.1 Drawing sizes, titles, notes and numbers shall conform to Enap Refineras Engineering

Standard EI-006. 1.2.2 Drawings, documents and computations prepared by foreign Engineers and/or

Manufactures working for the Supplier shall be prepared in Spanish or English language. Work done in Chile shall be in Spanish language.

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Rev. 16 4

2.0 REFERENCES 2.1 Enap Refineras Engineering Standards 2.1.1 EI-001 Structural Steel Design and Fabrication. 2.1.2 EI-002 Foundations and Elevated Structures. 2.1.3 EI-003 Design Loading for Equipment, Structures, Buildings and Foundations. 2.1.4 EI-005 Anchor Bolts. 2.1.5 EI-006 Dibujos en AutoCad. 2.1.6 EI-007 Underground Piping and Surface Drainage. 2.1.7 EI-010 Fabricacin, Montaje y Controles de Estructuras de Acero. 2.2 U.S.A Specifications and Codes 2.2.1 ASCE/SEI 7-05, Minimum Design Loads for Buildings and Other Structures. 2.2.2 UBC 1997, Uniform Building Code. 2.2.3 AISC, Manual of Steel Construction, Allowable Stress Design, Ninth Edition, 1989. 2.2.4 AISC, Manual of Steel Construction, Load and Resistance Factor Design. Volume I.

Structural Members, Specifications & Codes, Second Edition, 1998. 2.2.5 AISC, Seismic Provisions for Structural Steel Buildings, April 1997. 2.2.6 AISC, Seismic Provisions for Structural Steel Buildings. Supplement No. 2

November, 10, 2000. 2.2.7 ACI 318-99, Building Code Requirements for Structural Concrete. 2.2.8 American Petroleum Institute (API): Welded Steel Tanks for Oil Storage. API Standard

650. Tenth Edition, November 1998. Addendum 1, January 2000. Addendum 2, November 2001. Addendum 3, September 2003. Addendum 4, December 2005.

2.3 Chilean Codes 2.3.1 NCh 2369 Of. 2003, Earthquake-Resistant Design of Industrial Structures and

Facilities. 2.3.2 NCh 433 Of. 1996, Earthquake Resistant Design of Buildings.

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Rev. 16 5

3.0 COMBINATIONS OF LOADS

This Section prevails over Section 4.5 of NCh 2369. 3.1 General

Loading combinations shall be according to the provisions of Enap Refineras engineering standard indicated in 2.1.3 and in this chapter. 3.2 Symbols and Notation

The following nominal loads shall be considered: D = Dead load as defined in Enap Refineras Engineering Standard EI-003 paragraph III.A; Eh = Horizontal earthquake load given in Chapter 5.0; Ev = Vertical earthquake load given in Chapter 5.0; F = Load due to fluids with well-defined pressures and maximum heights; H = Load due to lateral earth pressure, ground water pressure, or pressure of bulk materials; L = Live load as defined in Enap Refineras Engineering Standard EI-003 paragraph III.B; La = Accidental live loads, originated by events that occur only occasionally during the normal

use of installations. They include: Extreme impacts and explosions. Short-circuit loads. Loads due to over filling of tanks and bins.

Lo = Operation live loads as defined in Enap Refineras Engineering Standard EI-003 paragraph III.C;

Lr = Roof live load as defined in Enap Refineras Engineering Standard EI-003 paragraph III.B;

T = Self-straining force (Provision shall be made for anticipated self-straining forces arising from differential settlements of foundations and from restrained dimensional changes due to temperature, moisture, shrinkage, creep, and similar effects). Thermal forces are defined in Enap Refineras Engineering Standard EI-003 paragraph III.D;

W = Wind load given in Chapter 4.0; M = Erection loads; K1 = Live load reduction coefficient defined in paragraph 5.2.13; K2 = Damping correction coefficient; 1.1 for steel structures and equipment and 1.4 for

concrete structures; IR = Hazard Factor from Table 5.8 for piperacks. Equal 1.0 for all other cases.

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Rev. 16 6

3.3 Combining Nominal Loads using Allowable Stress Design 3.3.1 Basic Combinations

Loads listed herein shall be considered to act in the following combinations, whichever produces the most unfavorable effect in the building, foundation, or structural member being considered. Effects of one or more loads not acting shall be considered. D + Lo (1) D + L + F + H + T + Lr + Lo + La (2) D + W + L + Lr + Lo (3) D + K1L +Lr + Lo + La + IR(Eh + Ev) (4) D + W + H (5) D + H + La + IR(Eh + Ev) (6) D + M (7) Exception: Loads Lo and La are combined with earthquake in combination (4) and (6) only if

the following conditions are met: La action is originated by the earthquake; in such a case, it must be included

with its proper sign. It is normal that, at the beginning of the earthquake, Lo is acting and it is not

stopped or interrupted because of it.

If the earthquake originates an effect by which Lo and La actions are interrupted at the beginning of the seismic movements, such actions might not be considered. Load combinations for Chapter 7.0 are indicated in Appendix R of API 650 standard.

The most unfavorable effects from both wind and earthquake loads shall be considered,

where appropriate, but they need not be assumed to act simultaneously.

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Rev. 16 7

3.4 Combining Factored Loads Using Strength Design 3.4.1 Applicability

The load combinations and load factors given in Section 3.4.2 shall be used only in those cases in which they are specifically authorized by the applicable material design standard. 3.4.2 Basic Combinations

Structures, components, and foundations shall be designed so that their design strength equals or exceeds the effects of the factored loads in the following combinations: 1.4(D + F) (1) 1.2(D + F + T) + 1.6(L + H + Lo) + 0.5Lr (2) 1.2D + 1.6Lr + 0.5L + Lo (3) 1.2D + 1.6Lr + 0.8W (4) 1.2D + 1.6W + 0.5(L + Lo) + 0.5Lr (5) 1.2D + K1L + Lo + La + IRK2(Eh + Ev) (6) 0.9D + 1.6W + 1.6H (7) 0.9D + 1.6H + La + IR(K2Eh + 0.3Ev) (8) 1.4(D + F) + 1.6M (9) Exception: The load factor on H shall be set equal to zero in combination (7) and (8) if the

structural action due to H counteracts that due to W, Eh or Ev. Where lateral earth pressure provides resistance to structural actions from other forces, it shall not be included in H, but shall be included in the design resistance.

Loads Lo and La are combined with earthquake in combination (6) and (8) only if the conditions indicated in section 3.3.1 are met

Each relevant strength limit state shall be investigated. Effects of one or more loads not

acting shall be investigated. The most unfavorable effects from both wind and earthquake loads shall be investigated, where appropriate, but they need not be considered to act simultaneously.

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Rev. 16 8

4.0 WIND LOADS 4.1 General

This Chapter replaces the Chilean standard NCh 432. 4.1.1 Scope

Building and other structures, including the main wind force resisting system and all components and cladding thereof, shall be designed and constructed to resist wind loads as specified in ASCE/SEI 7-05 and in this Engineering Standard. 4.1.2 Allowed Procedures

The design wind loads for buildings and other structures, including the main wind force resisting system, components and cladding elements thereof, shall be determined using one of the following procedures specified in the ASCE/SEI 7-05:

Method 1: Simplified Procedure as specified in ASCE/SEI 7-05 Section 6.4 for buildings or structures meeting the requirements specified therein.

Method 2: Analytical Procedure as specified in ASCE/SEI 7-05 Section 6.5 for buildings or structures meeting the requirements specified therein.

Method 3: Wind Tunnel Procedure as specified in ASCE/SEI 7-05 Section 6.6. 4.1.3 Minimum Design Wind Loading

The design wind load, determined by any one of the procedures specified in Section 4.1.2, shall be not less than specified in this section. 4.1.3.1 Main Wind Force Resisting System

The wind load to be used in the design of the main wind force resisting system for an enclosed or partially enclosed building or other structures shall not be less than 84[kgf/m2] (0.83[kN/m2]) multiplied by the area of the building or structures projected onto a vertical plane normal to the assumed wind direction.

The design wind force for open buildings and other structures shall be not less than 84[kgf/m2] (0.83[kN/m2]) multiplied by the area Af. The area Af is defined in ASCE/SEI 7-05 Section 6.3. 4.1.3.2 Components and Cladding

The design wind pressure for components and cladding of buildings shall be not less than a net pressure of 84[kgf/m2] (0.83[kN/m2]) acting in either direction normal to the surface.

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Rev. 16 9

4.2 Basis of Loads 4.2.1 Design wind loads shall be determined by one of the procedures specified in Section

4.1.2. The following are the design parameters required to calculate such forces: 4.2.1.1 The exposure category shall be C, according to ASCE/SEI 7-05 Section 6.5.6.3. 4.2.1.2 All structures shall be classified as Category IV (Table 1-1 of ASCE/SEI 7-05), and

the importance factor shall be 1.15. 4.2.1.3 The basic wind speed at 10[m] above ground level shall be 140[km/hr] (87[mph]). 4.2.1.4 Inside the Enap Refineras plant the topographic factor Kzt shall be 1.0. 4.2.1.5 The following values can be used for the critical damping ratio (). This is valid only

for the determination of the gust effect factor for flexible or dynamically sensitive structures.

Structural System Steel frames with welded connections, with or without bracings. Shell of steel welded; stacks, bin, tanks, vessels, tower, piping, etc.

0.003

Steel frames with field-bolted connections, with or without bracings. 0.005 Structures of reinforced concrete and masonry. 0.008

4.2.2 The gust effect factor Gf for rigid structures defined in ASCE/SEI 7-05 Section 6.2 shall

be considered equal to 0.85. 4.2.3 For chimneys, stack, flare stacks and other aerodynamically excitable structures the value

of Gf shall be calculated according to Section 6.5.8.2 of ASCE/SEI 7-05. The minimum value for Gf shall be 0.85 (Flexible or dynamically sensitive structures are defined in Section 6.2 of ASCE/SEI 7-05).

4.2.4 No allowance for the direct shielding effects of other structures or terrain features shall be

permitted in this standard, as stated in Section 6.5.2.1 of ASCE/SEI 7-05. However, any increase in pressure or suction on structures as a result of such obstructions shall be considered in design.

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Rev. 16 10

4.2.5 Generally, the structure shall be designed based on the wind load applied along the two principal axes of the structure. Other wind directions may require to be considered depending on the shape of the structure, such as a tall polygonal structure.

4.2.6 All structures and foundations subjected to wind load shall be designed to resist the

pressures and suctions over the vertical surfaces and roof, including appurtenances. 4.2.7 Any direction shall be considered as prevailing direction of wind if real ones are not

known. It will be supposed that wind works horizontally. 4.2.8 Maximum horizontal deflection for rigid structures defined in ASCE/SEI 7-05 Section

6.2, in the static wind design will be less than 5[mm] each meter of height, deflection/height = 1/200.

4.2.9 For vertical vessels, chimney, stacks, flare stacks, high tower, the allowable deflection at

the top of vertical equipment shall not exceed H/150, where H is the total equipment height; the calculation of the deflection shall be carried out for equipment in corroded condition.

4.2.10 For flexible or dynamically sensitive structures defined in ASCE/SEI 7-05 Section 6.2,

this figure has to be reduced to 2.5[mm] each meter height, deflection/height = 1/400. 4.2.11 For chimney, high towers, stacks, where ratio height/diameter or height/width were higher

than 10 it shall be necessary to carry out a vibration check for the vortex shedding effect by a recognized procedure, or the procedure indicated in Section 4.3.3.

4.2.12 For Section 4.2.11, a recognized procedure is the one according to the Engineering

Sciences Data Unit (ESDU, London). By this method are checked both the critical velocity of the wind (velocity exciting the phenomenon) and the value of the fatigue stresses induced in any section of the structure. The following values are to be assumed for the logarithmic decrement:

Empty column (without internals): 0.030 Installed column (with trays and structures): 0.033 Column during operation: 0.035

4.3 Determination of wind forces 4.3.1 Buildings

Wind load on enclosed buildings and their components and cladding shall be calculated in accordance with Section 4.1.2.

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Rev. 16 11

4.3.2 Open type structures

The design wind loading for open type structures shall be based on the exposed area of the framing, including the supported equipment and piping, which may be conservatively estimated as a percentage of the gross projected area, and the forces coefficients, Cf, for trussed tower as shown on Figure 6-23 of the ASCE/SEI 7-05. 4.3.3 Structures subjected to induced wind vibration

Structures such as stacks, above grade pipelines and other excitable structures must have a fundamental period of vibration different from, and preferably less than the period of the wind pulsation force (vortex shedding forces). 4.3.3.1 Vortex Shedding

The mean hourly design speed, in meters per second, at the equivalent height of the structure is denoted by VD and is determined from equation 6-14 of ASCE/SEI 7-05.

The critical wind speed, in meters per second, is given by:

STdVC =

Where:

=T The fundamental natural period of a stack, second; =d Mean diameter of upper one-third of stack, meters; =S Strouhal number usually used as 0.2 is for single stack and may vary due to Reynolds

numbers and multiple stacks.

The fundamental period of the structure must be calculated by an acceptable method.

If VC > 1.3VD, then no vortex shedding consideration is required. For other cases see Section 4.3.3.2. 4.3.3.2 Prevention of Excessive Vibration

Many methods have been used in the prevention of excessive vibrations in stack designs. It is not the purpose of this standard to determine the exact method to be used in the design of stacks, but rather to indicate some methods that have been successfully used.

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Rev. 16 12

The resonant vibration of a steel stack can be prevented or diminished by including in the design one or more features that are described below under three general categories: aerodynamic methods, damping methods, and stiffening methods.

Aerodynamic Methods:

These are methods that disrupt the formation of vortices on the sides of the stack. They are preferred over other methods because, unlike the others, they remove the source of vibration, i.e., the periodic vortex shedding forces (for example: Helical Strakes or Shrouds).

Damping Methods:

These methods consist of attachments and auxiliary structures that absorb dynamic energy from the moving stack.

Stiffness Methods:

These methods can be applied to the design to modify the vibration characteristic of the stack and thus reduce the probability that resonant vibration will occur.

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Rev. 16 13

5.0 SEISMIC LOADS 5.1 General Provisions 5.1.1 Industrial structures, equipment and their supporting structures shall be designed in

accordance with Chilean standard NCh 2369 Earthquake-Resistant Design of Industrial Structures and Facilities, complemented or modified by this Standard.

In the following paragraphs, the sections that modify the standard NCh 2369 are explicitly identified; those sections without explicit references to NCh 2369 are complementary to it. Office, residential, assembly and similar buildings, may be designed in accordance with Chilean standard NCh 433 Earthquake Resistant Design of Buildings.

5.1.2 Seismic design considers serviceability requirements besides safe earthquake behavior of

buildings, structures, vessels and equipments.

Serviceability requirements refer to the operation of the buildings, structures, vessels and equipment after the earthquake with minor interruptions. This condition means only partial levels of ductility. By economic reasons a shutdown longer than one week is not acceptable (Category C1 according to Table 5.1).

5.1.3 The term structure refers to open frame structures with or without rigid floor

diaphragms at various level; e.g. equipment supporting structure, steel structure, tower, tank, stack, piperack, piping support, stair, ladder, etc.

5.1.4 The term building refers to enclosed structures with walls and interior partitions

supported by rigid floor diaphragms at various levels. 5.1.5 The term rigid component refers to component, including its attachments, having a

fundamental period less than or equal to 0.06[s]. 5.1.6 The term flexible component refers to component, including its attachments, having a

fundamental period greater than 0.06[s]. 5.1.7 Structures designed to resist earthquake forces should be able of absorbing large

quantities of energy beyond the elastic range before ultimate failure. Such structures should have a consistent stress level or margin of reserve strength throughout. Attention must be given to those structural elements being specially designed for ductility and against sudden brittle or buckling type of failure. The structures shall have enough ductility so as to justify the response modification coefficient R, detailed hereinafter. To comply with this, concrete structures must satisfy the requirement of Chapter 21 of ACI

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Rev. 16 14

318-99, as detailed in Chapter 9 of NCh 2369, and the other requirements set forth therein. Steel structures must satisfy the requirements of Chapter 8 of NCh 2369, or the Seismic Provisions for Structural Steel Buildings, 1999 of AISC.

With the same basic purpose of providing enough ductility and reserve strength, special consideration must be given of meeting the requirements referred to joints, anchorage, bracing, detailing and structural features of NCh 2369, Chapters 8 to 11 and Annex A and B.

5.1.8 An importance factor, I, shall be assigned to each structure or equipment in accordance

with Table 5.1. 5.1.9 In an adequate seismic design, as it is herein recommended, yielding occur in very

specific elements, designed under that scope. However for elements and connections specifically reinforced, their yielding should not produce important effects and deformations on structures and equipments, since the economical criteria to minimize the shutdown will prevail over safety criteria only.

5.1.10 The seismic forces resulting from a large earthquake are in general larger than design

forces and frequently larger than the forces that produce yielding. In this regard these seismic recommendations are stricter than recommendations given by codes for seismic design of normal use in which economical reasons are not explicitly included.

5.1.11 Therefore, the particular requirements of this Engineering Standard shall prevail over all

here mentioned Codes or Standards. 5.2 Seismic Design Criteria 5.2.1 Steel and Concrete Buildings, and other structures and their components shall be designed

to meet the seismic requirements set forth in the following paragraphs.

5.2.2 The design seismic forces, and their distribution over the height of the structure, shall be established in accordance with the procedures indicated in Sections 5.4.2 and 5.4.3. The corresponding internal forces in structural members shall be determined using a linearly elastic model.

5.2.3 Individual members shall be provided with adequate strength to resist the shear, axial

forces, and moments determined in accordance with these provisions. 5.2.4 A continuous load path, or paths, with adequate strength and stiffness shall be provided to

transfer all forces from the point of application to the final point of resistance.

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Rev. 16 15

5.2.5 The foundation shall be designed to resist the forces developed and shall accommodate the movements imparted to the structure by the design ground motions.

5.2.6 The structure shall be single in order to obtain a structural behavior that is easy to understand and to model. All the forces shall be transferred to the foundations in a direct way.

5.2.7 The seismic analysis shall be made using the static or the dynamic analysis. Generally a

static analysis will be sufficient, provided that buildings, structures, vessels and equipment have their masses and stiffnesses regularly distributed in plan and in height. Regular distribution is to be assumed when a deviation of 20 percent of uniform distribution is not exceeded.

5.2.8 Static analysis should be used in buildings, structures, vessels and equipment susceptible

of being reduced to one-degree-of-freedom system. 5.2.9 Dynamic analysis shall be used in buildings, structures, vessels and equipment including

but not limited to the following: 5.2.9.1 Equipment with height larger than 20[m]. 5.2.9.2 Buildings and structures irregular in plan configuration: Structures having one or

more of the irregularity typed listed in Table 5.2. 5.2.9.3 Buildings and structures irregular in vertical configuration: Structures having one or

more of the irregularity typed listed in Table 5.3. 5.2.9.4 Buildings and structures supporting heavy hanging equipment. 5.2.9.5 Concrete or masonry lined stacks or tall vessels of a height to horizontal dimension

ratio of five or more. 5.2.9.6 When specifically indicated by Enap Refineras, for each project, in which the basic

assumptions of the static method do not apply. 5.2.10 When a building or structure has been analyzed both by the static and the dynamic

method, the latter shall prevail. 5.2.11 Whichever method of seismic analysis is chosen, live load upon buildings, structures,

vessels and equipment, are to be reduced in accordance with their probability of occurrence under seismic condition.

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The following coefficients shall be applied in order to determine the seismic live loads acting upon buildings, structures, vessels and equipment: Locations K1 Storage warehouses, file rooms: 1.00 Fuel, water, or any material usually stored: 1.00 Operating floors and others: 0.50 Maintenance areas Access, platforms and walkways: 0.25 Roofs: 0.00

5.2.12 Components mounted on vibration isolation systems shall have a bumper restraint or

snubber in each horizontal direction, and vertical restraints shall be provided, constructed of ductile materials. A viscoelastic pad or similar material of appropriate thickness shall be used between the bumper and equipment item to limit the impact load.

5.3 Allowable Deflection and Drift 5.3.1 For the design of structures the maximum allowable deflection for structural elements

shall be as defined in this Section:

Structural Element Allowable Deflection (of span)

Beams in general due to dead loads plus live loads: 1/300 Beams in pipe racks due to dead loads plus live loads: 1/150 Trusses due to dead loads plus live loads: 1/700 Crane girders, vertical, due to dead loads plus live loads and impact:

1/1000

Crane girders, horizontal due to impact: 1/500 Purlins, roof sheets and wind columns due to dead loads and live loads:

1/200

Siding and girt due to wind loads: 1/200 Floor systems supporting large open areas free of partitions or other sources of damping, where vibration due to pedestrian traffic might be objectionable, shall be designed with due regard for such vibration. Mechanical equipment that can produce objectionable vibrations in any portion of an inhabited structure shall be isolated to minimize the transmission of such vibrations to the structure.

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5.3.2 Seismic Deformations

This Section modifies Chapter 6 of NCh 2369 5.3.2.1 Horizontal seismic deformations of the structural system must be compatible with the

flexibility and strength of piping, ducts, walls, partitions and other non-structural elements attached to the structure as well as with the capacity of deformation of the ducts expansion joints. However, shall not exceed the allowable story drift as obtained from Section 5.3.2.2 at any story.

5.3.2.2 The design story drift () shall be computed as the difference of the deflections at the

top and bottom of the story under consideration. For structures having plan irregularity, the design story drift, , shall be computed as the largest difference of the deflections along any of the edges of the structure at the top and bottom of the story under consideration.

The deflections of level x at any point, x, shall be determined in accordance with the following equation:

IC xed

x=

Where:

=dC Deflection amplification factor from Table 5.5; =xe Deflections determined by an elastic analysis;

=I Importance factor from Table 5.1. The elastic analysis of the seismic force-resisting system shall be made using the prescribed seismic design forces in Section 5.4.2 or 5.4.3. For the purpose of this drift analysis only, the limitation of the minimum seismic base shear specified in Section 5.4.2.3 or 5.4.3.9 is not applicable for computing displacements. For determining compliance with the story drift limitation of Table 5.4, the deflections of level x at any point (x) shall be calculated as required in this section. Flexible diaphragms, that is when the maximum lateral deformation of the diaphragm is more than two times the average story drift of the associated story, shall not be accepted. In case that situation happens, enough horizontal bracings shall be added to fulfill this requirement.

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5.3.2.3 Separation between adjacent buildings must be compatible with seismic deformations according to Section 5.3.2.4.

5.3.2.4 Sufficient distance shall separate adjacent buildings or structures to avoid contact,

when deflected by seismic action or seismic forces.

Separation shall be no less than the larger of the following dimensions:

( ) ( )2xjdj2xidi CCS += h004.0S =

[ ]mm30S = Where:

xi and =xj Computed horizontal seismic deflection of each structure determined in accordance with the procedure indicated in Section 5.3.2.2;

diC and =djC Deflection amplification factor of each structure from Table 5.5; =h Height of the structure at the considered level.

5.3.2.5 An approach to the real seismic deflections of the structure may be obtained from the

horizontal deflections gotten through a mathematical model representative of the structure, when subjected to a response spectrum modal analysis, using the spectrum of acceleration defined in Section 5.4.3.9, multiplied by the factor Cd defined in Table 5.5.

5.4 Seismic Analysis Procedures 5.4.1 General 5.4.1.1 Introduction

This section defines two basic analytical procedures for the seismic design. Both of them are based in the allowable stress design method.

Static Method of Analysis. The Section 5.4.2 of this standard modifies Section 5.3 of NCh 2369.

Dynamic Method of Analysis. The Section 5.4.3 of this standard modifies Section 5.4 of NCh 2369.

The strength design method is permitted with the use of load combination defined in Section 3.4.

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Limitations on the use of static method of analysis are indicated in Sections 5.2.7, 5.2.8 and 5.2.9. The response modification coefficient, R, and the deformation coefficient, Cd, for mixed structural systems for the direction under analysis (including the equipment) shall not exceed the lowest value indicated in the Table 5.5. The modal design damping ratio, , for mixed structural system (including the equipment) shall not exceed the lowest value indicated in the Table 5.7. If an equipment or component is supported above the base by another structure and the weight of the equipment is not more than 25 percent of the seismic weight, P, as defined in section 5.4.2.1, the design seismic forces for the supported equipment shall be determined in accordance with the requirement of section 5.4.2.4. If the weight of supported equipment is more than 25 percent of the seismic weight, P, as defined in section 5.4.2.1, the design seismic forces shall be determined based on an analysis of the combined system (comprising the equipment and supporting structure). For supported equipments that have rigid component dynamic characteristics, the R response modification coefficient for the supporting structural system shall be used for the combined system. For supported equipments that have flexible component dynamic characteristics, the R response modification coefficient for the combined system shall not be greater than 3. The equipment, and its supports and attachments, shall be designed for the forces determined from the analysis of the combined system.

5.4.1.2 Direction of the seismic solicitation The design ground motion can occur along any direction of a structure. The analyses shall be performed, as a minimum, for each one of two orthogonal or approximately orthogonal horizontal directions. The effect of the vertical seismic accelerations should be considered in the following cases:

a. Suspension rods of hanging equipment and their supporting beams. b. Structures and elements of prestressed concrete (pretensioned and post-

tensioned). c. Foundations, anchorage elements and supports of structures and equipment. d. Any structure or element in which the vertical seismic action affects its

design, for example, long span or cantilever structures, shell and shell-like structures, long span vaults or any structures with unusual geometry or mass distribution.

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5.4.1.3 Combination of effects of horizontal components of the earthquake

For the design of the structural earthquake resistant elements, in general, it is not necessary to combine the effects of the two horizontal components of the seismic action. The analyses shall be performed for each one of the two horizontal directions considered separately and independently. Make exception to this rule piperacks and structures that present notorious torsional irregularities or have special or intermediate moment frame in both directions. In such cases, the elements should be designed for 100 percent of the prescribed horizontal forces in one direction, plus 30 percent of the prescribed forces in the perpendicular direction, and the same shall be done in the perpendicular direction. The combination requiring the greater element strength shall be used for the design.

5.4.1.4 Mathematical modeling

(a) Basic assumptions In general, a building or structure should be modeled, analyzed, and verified as a three-dimensional assembly of elements and components. Two-dimensional modeling, analysis, and verification of buildings or structures with stiff or rigid diaphragms are acceptable if torsional effects are either sufficiently small to be ignored or indirectly captured. For irregular structures or structures without independent orthogonal system, a three-dimensional model incorporating a minimum of three degrees of freedom consisting of translation in two orthogonal plan directions and torsional rotation about the vertical axis shall be included at each level of the structure. Where the diaphragms are not rigid compared with the vertical elements of the lateral-force-resisting system, the model should include representation of the diaphragms flexibility and additional dynamic degrees of freedom, as required to account for the participation of the diaphragm in the dynamic response of the structure. In the case of structures having flexible diaphragms, special modeling and computing considerations must be employed. In such case, it shall be verified that the diaphragm fulfills the requirements indicated in the Section 5.3.2.2. The equipment shall be included in the model according to the recommendations of Section 5.4.1.1.

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(b) Horizontal eccentricity The effect of horizontal eccentricity must be considered for building or structures with diaphragms capable of resisting the forces generated by torsion. The total torsional eccentricity at a given floor level shall be set equal to the sum of the following two torsional eccentricities:

The natural eccentricity; that is, the eccentricity between the center of mass of all floors above and including the given floor, and the center of rigidity of the vertical seismic elements in the story below the given floor. This requirement is automatically fulfilled in a three-dimensional model.

The accidental eccentricity; that is, an accidental eccentricity produced by horizontal offset of the center of mass of all floors above and including the given floor, obtained by the following procedures: For both methods (static and dynamic):

HZb1.0e kky = For the seism applied in the X direction.

HZb1.0e kkx = For the seism applied in the Y direction.

Where: =e Design accidental eccentricity; =kxb Dimension in the x direction of the k floor level; =kyb Dimension in the y direction of the k floor level; =kZ Height of floor level k, over the base level; =H Total height of the structure over the base level.

For dynamic method of analysis: A minimum of 5 percent of the horizontal dimension at the given floor level measured perpendicular to the direction of the applied load. The accidental eccentricity must be simulated displacing the center of masses horizontally in 5 percent in the mathematical model. On applying these relations, the same sign must be considered on each level. In general, two cases are necessary to be considered for each directions of analysis.

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5.4.2 Static Method of Analysis

This Section modifies Section 5.3 of NCh 2369

5.4.2.1 The total shear load or base shear shall be computed in the direction of each main axis of buildings, structures, vessels and equipment with the following formula:

PI)T(CV =

Where:

=V Seismic base shear; ( )=TC Seismic coefficient modified by the structure response, i.e., depending on the

fundamental period; =T Fundamental period (Natural period with greater equivalent translational mass

in the direction of analysis); =I Importance factor from Table 5.1;

LKLDP 1o ++= Seismic weight; =1K Live reduction factor for building, structures, vessels and equipment, defined

in Section 5.2.11; =D Vertical dead load (as per Engineering Standard EI-003); =oL Vertical other loads when applicable (as per Engineering Standard EI-003);

=L Unreduced vertical live load (as per Engineering Standard EI-003). 5.4.2.2 Seismic coefficient modified by the structural response: [ ] [ ]

( ) [ ] [ ][ ] [ ][ ] [ ]

[ ]

>

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= Modal design damping ratio from Table 5.7. 5.4.2.3 The minimum value for the seismic base shear (V) shall be:

PI12.002.0PR

I45.0V4.0

min

= Or the value established in NCh 2369 paragraph 5.8.1.3, whichever be greater.

5.4.2.4 Horizontal force on elements of structures, non-structural components and minor

equipment supported by structures, shall be calculated according to the following formula (This article modifies Chapter 7 of NCh 2369):

ppp WCF =

Alternatively, Fp may be calculated using the following formula:

pr

x*pp Wh

h31CF

+=

Except that: Fp shall not be less than pW5.0 Fp need not be more than pW0.2 Where:

=pC Horizontal force factors from Table 5.6; =pW Weight of an element or component; =*pC Horizontal force factor from Table 5.6; =xh Height of element or component attachment level over to the base level. Shall

not be taken less than 0; =rh Total height of the structure over the base level.

5.4.2.5 For the cases indicated in sections 5.4.1.2.a and 5.4.1.2.b a vertical seismic

coefficient equal to 0.50 shall be considered to compute the vertical seismic force acting simultaneously with horizontal forces. For the cases indicated in sections 5.4.1.2.c and 5.4.1.2.d a vertical seismic coefficient equal to 0.33 shall be considered to compute the vertical seismic force acting simultaneously with horizontal forces.

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5.4.2.6 If earthquake forces due to seismic torsion at any level on any element exceed 50% of the earthquake forces without eccentricity, the structure shall be analyzed by the dynamic method or shall be modified to avoid this condition.

5.4.2.7 Vertical distribution of seismic forces. The lateral seismic force (Fx) induced at any

level shall be determined from the following formula:

VWh

WhF n

1kkk

xxx

=

=

Where:

=V Total base shear load at the base of the structure; =xF Lateral seismic force at Level x;

xW and =kW Portion of the total gravity load of the structure (W) located or assigned to Level x or k;

xh and =kh Height from the base to Level x or k; =n Number of level; = Exponent related to the structure period as follows: for structures

having a period of 0.5[s] or less, = 1; for structures having a period of 2.5[s] or more, = 2; and for structures having a period between 0.5[s] and 2.5[s], shall be 2 or shall be determined by linear interpolation between 1 and 2: = 1 + (T-0.5)/2

5.4.2.8 When stack, stack-like, cylindrical tanks and vessels are allowed to be designed by

static analysis, seismic overturning moments shall be calculated as follow: 5.4.2.8.1 In structures with a ratio of height/diameter less than five, the moment diagram

will be calculated from shear forces as obtained in 5.4.2.7. 5.4.2.8.2 In structures with a ratio height/diameter bigger or equal five, the moment diagram

will be obtained from dynamic method of analysis.

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5.4.3 Dynamic Method of Analysis

This Section modifies Section 5.4 of NCh 2369 5.4.3.1 A standard calculation procedure based on two or three-dimensional model will be

applied. The finite elements models used for dynamic analysis should include a sufficient number of dynamic degrees of freedom permitting selection of the most significant modes of vibration of the structure.

5.4.3.2 According to Section 5.2.9 a dynamic seismic analysis shall be carried out as required

in this document. As an additional option, Enap Refineras shall require this analysis when considered necessary due to the importance or characteristic of the structure. Method to be used shall be response spectrum.

5.4.3.3 For doubly symmetric structures, vibrations must be considered uncoupled in both

horizontal directions. Only accidental torsion shall be included in the calculations, evaluating it statically.

5.4.3.4 For structures with only one axis of symmetry, vibration modes in the direction of

this axis can be considered as uncoupled. 5.4.3.5 In general, a three dimensional analysis shall be performed through a computer

program, considering a minimum of three degrees of freedom at each mass level: two horizontal displacements and a rotation around a vertical axis through the center of gravity. In the case of structures having flexible diaphragms special modeling and computing considerations must be employed.

5.4.3.6 Modal shapes and frequencies have to be analyzed. If they are found to be coupled,

without a clear or predominant direction of vibration for each one of them, then a three-dimensional analysis with two directional acceleration input as indicated in section 5.4.1.3, shall be performed instead of two independent analyses.

5.4.3.7 The effect of the vertical seismic component for the cases indicated in sections

5.4.1.2.a, 5.4.1.2.b, 5.4.1.2.c and 5.4.1.2.d shall be considered. Analyses shall be carried out considering horizontal and vertical seismic excitation acting simultaneously. For the vertical component analysis, the design response spectrum defined in section 5.4.3.9 with R=3 and =0.03 shall be used.

5.4.3.8 In special cases, like continuous structures, discrete analysis can be used by dividing

them in segments, but not limited to that case, rotatory inertias around horizontal axis may be important if discrete masses are located quite far from the axis.

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A convenient number of degrees of freedom in connection with rotational inertias have to be added in dynamic analysis for those special cases.

5.4.3.9 Design response spectrum: [ ] [ ]

( ) [ ] [ ][ ] [ ][ ] [ ]

[ ]

>

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5.4.3.12 The analysis shall include a sufficient number of modes to obtain a combined modal mass participation of at least 90% of the actual mass, but never less than the five first modes.

5.4.3.13 The modal damping considered in CQC criterion for the computation of cross

correlation coefficient will be assumed equal for all modes and with the value indicated in paragraph 5.4.1.1.

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Table 5.1 Importance Factor Category I

C1 Structure or vital equipments for the normal operation of the plant. This category will be used by default except in those cases in that Enap Refineras indicates the opposite. Maximum risk grade pressure vessels. Medium risk grade pressure vessels that store products with moderate or high fire or explosion hazard or products biologically or environmentally dangerous.

1.00

C2 Structure or equipment that can have smaller damages than quick repair and which do not cause important stoppage of the plant or damages to structures of the previous category. Medium risk grade pressure vessels that store products with low fire or explosion hazard. Minimum risk grade pressure vessels that store products with high fire or explosion hazard or products biologically or environmentally dangerous.

0.80

C3 Provisional structure whose seismic fails it does not endanger structures of the previous categories. Minimum risk grade pressure vessels that store products biologically or environmentally benign or products with low fire or explosion hazard.

0.67

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Table 5.2 Plan Structural Irregularities Item Irregularity Type and Description

1 Non-approximately double symmetrical geometric configuration. 2 Potential large torsional moments due to significant eccentricity between the seismic

resistant system and the tributary mass at any level. 3 Torsional irregularity to be considered when diaphragms are not flexible. Torsional

irregularity shall be considered to exist when the maximum story drift computed, including accidental torsion at one end of the structure, transverse to an axis is bigger than 1.2 times the average of the story drifts of the two ends of the structure along the axis being considered.

4 Reentrants corners: Plan configurations of a structure and its lateral force resisting system contain reentrants corners, where both projections of the structure beyond a reentrants corner are greater than 15 percent of the plan dimension of the structure in the given direction.

5 Diaphragm discontinuity: Diaphragms with abrupt discontinuities or variations in stiffness, including those having cutout or open areas greater than 50 percent of the gross enclosed diaphragm area, or changes in effective diaphragm stiffness of more than 50 percent from one story to the next.

6 Out of plane offsets: Discontinuities in a lateral force path, such as out of plane offset of the vertical elements.

7 Nonparallel systems: The vertical lateral force-resisting elements are not parallel to or symmetric about the major orthogonal axes of the lateral force resisting system.

8 Heavy-duty equipment, which is not uniformly distributed at each floor level. 9 Theoretical centers of mass at each level, which are non-approximately found in the

same vertical axes.

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Table 5.3 Vertical Structural Irregularities Item Irregularity Type and Description

1 Mass-stiffness ratio between different stories or load levels that varies significantly. 2 Weight (mass) irregularity. Mass irregularity shall be considered to exist where the

effective mass of any story is more than 150 percent of the effective mass of an adjacent story. A roof that is lighter than the floor below need not be considered.

3 Stiffness irregularity or soft story. A soft story is one in which the lateral stiffness is less than 70 percent of that in the story above or less than 80 percent of the average stiffness of the three stories above.

4 Important and obvious differences in stiffness of different resistance lines. 5 Non-approximately symmetrical geometric configuration about the vertical axes or

horizontal offset with significant dimensions. 6 Vertical geometric irregularity. It shall be considered to exist where the horizontal

dimension of the lateral force resisting system in any story is more than 130 percent of that in an adjacent story.

7 In-plane discontinuity in vertical lateral-force-resisting element shall be considered to exist where an in-plane offset of the lateral force-resisting elements is greater than the length of those elements or there exists a reduction in stiffness of the resisting elements in the story below.

8 Discontinuity in lateral strength or weak story. A weak story is one in which the story lateral strength is less than 80 percent of that in the story above. The story strength is the total strength of all seismic-resisting elements sharing the story shear for the direction under consideration.

Table 5.4 Allowable Story Drift

Structure Masonry walls and partitions rigidly attached to structure 0.0030hsx All other structures 0.0075hsx hsx is the story height measured from ground to level x.

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Table 5.5 Response Modification Coefficients and Deformation Coefficient Coefficient Type of Structural System

R Cd Reinforced Concrete Shear Walls 5.0 4.5 Reinforced Concrete Ductile Moment Resisting Frames 5.0 4.5 Masonry Walls (Concrete Blocks or Clay Bricks) 3.5 3.0 Intermediate Steel Moment Frames 5.0 4.0 Ordinary Steel Concentrically Braced Frames 5.0 4.0 Steel Storage Racks 4.0 3.5 Elevated Tanks, Vessels, Bins or Hoppers: On braced legs On unbraced legs Irregular braced legs single pedestal or skirt supported Welded steel Concrete

3.0 3.0 2.0 2.0 2.0

2.5 2.5 2.0 2.0 2.0

Horizontal, saddle supported welded steel vessels 2.5 2.5 Tanks or vessels supported on structural towers similar to building 3.0 2.0 Flat bottom, ground supported tanks, or vessels 3.0 2.5 Reinforced or prestressed concrete: Tanks with reinforced nonsliding base Tanks with anchored flexible base

2.0 3.0

2.0 2.0

Cast-in-place concrete silos, stacks and chimneys having walls continuous to the foundation

3.0

3.0

Steel Tanks, Elevated Tanks, Chimneys and Towers 2.5 2.5 Cooling towers (Concrete, steel or wood frame) 3.5 3.0 Inverted pendulum-type structures (not elevated tank) 2.0 2.0 Heavy Equipment at Ground Level such as Power Transformers, Pumps, Compressors, etc.

2.5 3.0

Hydrocracker Reactors 3.0 3.5 Hydrocracker Towers 2.5 3.0 Cylindrical Heaters 3.0 3.5 Towers Founded on Fixed Base at Ground Level 2.5 3.0 Reactor Founded on Fixed Base at Ground Level 3.0 3.5 Heater Non-cylindrical Type 4.0 3.5 Pipe Rack Supported on Steel Structure 5.0 4.0 Pipe Rack Supported on Reinforced Concrete Structure 4.5 4.0 Other self-supporting structures, tanks or vessels not covered above 1.25 2.5 Large Diameter Pipes 3.5 4.0

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Table 5.6 Horizontal Force Factors Element *

pC Cp

Seismic restraint and guides of all equipment 0.250 1.0 Non-structural components, exterior and interior ornamentations and appendages.

0.425 1.7

Walls and partitions, perpendicular to own plane 0.125 0.5 Cantilever walls and parapets 0.425 1.7 Connections of prefabricated wall panels 0.500 2.0 Other components: Rigid components Flexible components

0.350 0.500

1.4 2.0

Minor tanks and vessels (includes contents), including support system 0.250 1.0 Any flexible equipment laterally braced or anchored to the structural frame at a point below their center of mass

0.425 1.7

Anchorage of expansion anchor bolts, chemical anchor bolts, cast-in-place anchors bolts, anchorage constructed of non-ductile materials or by use of adhesive

0.500 2.0

Table 5.7 Modal Design Damping Ratios

Structural System Steel frames with welded connections, with or without bracings. Shell of steel welded, stacks, bin, tanks, vessels, tower, piping, etc.

0.02

Steel frames with field-bolted connections, with or without bracings. 0.03 Structures of reinforced concrete and masonry. 0.05

Table 5.8 Hazard Factor

Category Contents IR A Content is biologically or environmentally benign, low fire or

low explosion hazard. 1.0

B Content is moderate fire or explosion hazard 1.1 C Content is biologically or environmentally dangerous, high fire

hazard or high explosion hazard 1.2

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6.0 DESIGN SNOW LOADING

No snow loading shall be considered for the design of the structures and building.

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7.0 GROUND-SUPPORTED FLAT-BOTTOM STEEL TANKS

This Section prevails over Section 11.8 of NCh 2369. 7.1 Tanks Classification

In accordance with the characteristic of stored liquid a category from Table 5.8 shall be assigned to each tank. The categories are associated with a hazard factor relative to the risk of the content.

7.2 Seismic Design 7.2.1 Scope

This section provides minimum requirements for the seismic design of welded ground-supported cylindrical liquid storage steel tanks without internal pressure.

7.2.2 Generalities

The design and construction of storage steel tanks shall conform to the requirements of the standard indicated in 2.2.8. The seismic design shall be done as established in the Appendix E of API 650 standard except as indicated in sections 7.2.3 to 7.2.10. Some of these exceptions are corrections to misprint API 650 equations.

7.2.3 Design Spectral Response Accelerations

The design response spectrum for ground supported flat bottom steel tanks is defined by the following parameters:

Ri I36.0A = Self-anchored Tank; Ri I32.0A = Mechanically-anchored Tank;

R40.1c

Rc I04.0T

I94.0A =

iv A32A =

Where:

=iA Impulsive design response spectrum acceleration coefficient, %g (Damping ratio equal to 2 percent);

=cA Convective design response spectrum acceleration coefficient, %g (Damping ratio equal to 0.5 percent);

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=vA Vertical earthquake acceleration coefficient, %g; =RI Hazard factor from Table 5.8; =cT Natural period of the convective (sloshing) mode of behavior of the liquid,

seconds;

D

DH68.3tanh

578.08.1Tc

=

=D Nominal tank diameter, m; =H Maximum design product level, m;

EDt

HCT

u

ii

=

=iT Natural period of vibration for impulsive mode, seconds. The design methods in this standard are independent of impulsive period of the tank. Expression for

iT given in this standard is applicable only to those circular tanks in which wall is rigidly attached to base slab;

+

=2i

DH067.0

DH3.046.0

DH

1C

=ut Equivalent uniform thickness of tank shell, m; = Mass density of fluid, kg/m3; =E Elastic Modulus of tank material, N/m2,

7.2.4 Base Shear

The base shear due to seismic forces applied to the bottom of the tank floor shall be determined in accordance with the following formula:

( )[ ] [ ]2cc2ifrsi WAWWWWAV ++++= Where:

=V Total design base shear, N; =ci A,A Seismic accelerations coefficients determined in accordance to Section 7.2.3;

=sW Total weight of the tank shell and appurtenances, N; =rW Total weight of fixed tank roof including framing, knuckles, and any

permanent attachments, as specified by the purchaser, N; =fW Weight of tank floor, N;

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=iW Effective impulsive weight of the liquid, N;

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H

DH67.3sinh

DH67.3

0.1D

H67.3cosh0.1Xc

=

=isX Height from the bottom of the tank shell to the center of action of the lateral seismic force related to the impulsive liquid force for the slab moment, m;

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=rsw Roof load acting on the shell, N/m; =vA Vertical earthquake acceleration coefficient for self-anchored tanks;

eeyaa GDH201GHFt99w = =aw Resisting force of tank contents per unit of shell circumference that may be

used to resist the shell overturning moment, N/m; =at Thickness of the bottom plate under the shell extending at least the distance, L,

from the inside of the shell, less corrosion allowance, mm; =L Required minimum width of the bottom annulus measured from the inside of

the shell (Defined in Section E.6.2.1.1.1 of the standard indicates in 2.2.8), m; =yF Minimum specified yield strength of bottom annulus, MPa; =eG Effective specific gravity including vertical seismic effects; ( )ve A4.00.1GG = =G Specific gravity of the product.

Table 7.1 Anchorage Ratio

Anchorage Ratio Criteria J < 0.785 No uplift under the design seismic overturning moment. The tank is

self-anchored. 0.785 < J < 1.54 Tank is uplifting, but the tank is stable for the design load providing the

shell compression requirements are satisfied. Tank is self-anchored. J > 1.54 Tank is not stable and shall be mechanically-anchored for the design

load.

7.2.7 Sliding Resistance For self-anchored flat bottom steel tanks, the overall horizontal seismic shear force shall be resisted by friction between the tank bottom and the foundation or subgrade. Self-anchored storage tanks shall be designed such that sliding will not occur where the tank is full of stored product. The maximum calculated seismic base shear, V, does not exceed

sV : ( ) ( )vpfrss A4.00.1WWWW32.0V +++=

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7.2.8 Anchorage

Experience has shown that properly designed anchored tanks retain greater reserve strength with respect to unanchored tank. Considering this property, tanks shall be anchored with the exception of the tanks that fulfill the following conditions (both):

The anchorage ratio, J, defined in Section 7.2.6, is less than 1.54. The sliding resistance, sV , defined in Section 7.2.7, is greater than the base shear

defined in Section 7.2.4.

7.2.8.1 Anchor Bolts 7.2.8.1.1 The anchor bolt shall be of the removable type (Type PC according the standard

indicated in 2.1.4). Therefore, the anchor bolts should take only tensions. 7.2.8.1.2 The seismic base shear force shall be resisted by the sliding resistance force. When

the seismic base shear exceed the sliding resistance force it should be add stoppers or shear keys that prevents the sliding of the tank and should be designed to resist the total base shear due to seismic forces.

7.2.8.1.3 The anchor bolts must be designed in accordance with the requirements of the

standard indicated in 2.2.8, Appendix E.

7.2.8.1.4 The thread length of the anchor bolt under the nut must be longer than four nominal bolt diameters and no less than 75[mm].

7.2.8.1.5 The embedment length of the anchor bolt shall be equal or greater than the length

recommended in the standard indicated in 2.1.4. 7.2.8.2 Anchor Chairs 7.2.8.2.1 The anchor bolts shall have an anchor chair to permit yielding and the inspection

of the anchors after an earthquake. 7.2.8.2.2 The anchor chair should have a continuous upper ring along of all the perimeter of

the tank. 7.2.8.2.3 The base plate should be dimensioned for the bending moments coming from the

contact pressure between the bottom plate of the tank and the foundation. The minimum dimensions should be according to the standard indicates in 2.2.8, sections 3.4 and 3.5.

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7.2.8.2.4 The upper ring, the vertical stiffeners close to the anchor bolts and the tank wall in the area of the upper ring of the anchor chair should be able to supports the anchor attachment design load, PA, defined in the standard indicates in 2.2.8, section E.6.2.1.2.

7.2.8.2.5 The anchor bolt projection, measured from the top of the tank floor plate provided

under the shell, shall be at least eight nominal diameters of the bolt and no less than 300[mm].

7.2.8.2.6 The uncorroded thickness of the vertical chair stiffeners must be equal or greater

than 3/8 times the thickness of the plate on the top of the chair and no less than 10[mm]. Any specified corrosion allowance must be added.

7.2.8.2.7 The uncorroded thickness of the upper ring plate of the chair must be equal or

greater than 16[mm]. Any specified corrosion allowance must be added. 7.2.8.3 Shear Device 7.2.8.3.1 Stoppers or shear keys are device that prevents the sliding of the tank and should

be designed to resist the total base shear defined in the section 7.2.4. 7.2.8.3.2 The stoppers should be designed to resist a seismic force equal to a total base shear

defined in the section 7.2.4, divided by the number of stoppers and multiplied by three. This force should be used for the verification of welding to anchored plate and anchor stud. The stoppers thickness should be at least fifty percent larger than thickness of tank floor plate provided under the shell.

7.2.8.3.3 The shear keys should be located centered with respect to the vertical tank shell. 7.2.9 Freeboard

In order to prevent the overflow of the tank contents or damages whether to the roof or to the top of the shell, a freeboard for the sloshing wave shall be provided. The freeboard is defined as the distance from the maximum liquid level of the tank to the top level of the shell. The minimum freeboard must be calculated by the following formula:

( )D25.010.02.1

IRs +=

Where:

=s Minimum freeboard, m; =RI Hazard factor from Table 5.8; =D Nominal tank diameter, m,

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The minimum freeboard is required unless the roof and tank shell are designed to contain the sloshing liquid.

7.2.10 Construction requirements 7.2.10.1 Annular bottom plates shall be adopted for the design of the self-anchored tank. That

design must be in accordance with sections 3.5 and E.6.2.1.1.2 of the standard indicated in 2.2.8. Exception is made to the requirement that the annular bottom plates shall have a minimum radial width that provides at least 600[mm] between the inside of the shell and any lap-welded joint in the remainder of the bottom and at least a 50[mm] projection outside the shell.

7.2.10.2 The slope of the bottom of the tank must be in accordance with Section B.3.3 of the

standard indicated in 2.2.8. 7.2.10.3 The connections of all piping attached to the tank must be flexible in order to prevent

the damage due to potential uplift or sliding of the tank during earthquake.

Unless otherwise calculated, the following displacement shall be assumed for the design of all side-wall connections: a. Vertical displacement of 50[mm] for mechanically-anchored tanks. b. Horizontal displacement for 50[mm] for mechanically-anchored tanks. c. Vertical displacement of 300[mm] for self-anchored tanks. d. Horizontal displacement of 200[mm] for self-anchored tanks.

7.2.10.4 The support columns of the roof and the column-base shall be designed considering

the force due to sloshing wave. The column-base shall be connected to the tank to prevent lateral movement of the column bottom and the connection shall permit a vertical displacement over the bottom of the tank equal or greater than 300[mm].

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8.0 SEISMIC DESIGN OF PRESSURE VESSELS

This Section prevails over Section 11.7 of NCh 2369. 8.1 Scope 8.1.1 This standard establishes the minimum requirements for the seismic design of vertical or

horizontal pressure vessels and of their respective support structures, connections and anchorage systems.

8.1.2 This standard is applicable to pressure vessels whose support system is located directly on

the ground and to pressure vessels supported on one or more levels of the structure. 8.1.3 In the case of pressure vessels supported on one or more levels of a structure, the seismic

load must be calculated as indicated in Section 5.4.1.1. 8.1.4 This Chapter complements the recommendations indicated in the Chapter 5.0 of this

standard, provided that they do not contradict the recommendations and limitations established in this Chapter.

8.2 General requirements 8.2.1 Risk classification

8.2.1.1 For seismic design, all pressure vessels shall be classified according to the risk

classification indicated in Table 8.1. The risk grade shall be select according to the bad conditions indicated in the Table 8.1.

Table 8.1 Risk Grade

Risk Grade

Exposed people

Direct economic loss

Indirect economic loss

Pollution

Minimum Few (20) The vessel and numerous equipment and installation

Catastrophic Recovery greater than 3 years

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8.2.1.2 When risk classification selection is unclear, the maximum risk grade must be used. 8.2.1.3 Importance Factor

The Importance Factor shall be established according to risk classification and the characteristics of the content. The values of this factor correspond to those indicated in Table 5.1.

8.2.2 Combination loads 8.2.2.1 For the purpose of the pressure vessel seismic design, the seismic loads shall be

combined with the other loads according to the ASME code. The resulting stress shall be less than the allowable stress indicated in the ASME code.

8.2.2.2 For the purpose of the support structure and anchorage system seismic design, the

seismic loads shall be combined with the other loads according to the combinations indicated in Section 3.3 and 3.4. The live loads shall consider service and operation loads, including thermal effects, internal pressure, eventual vibration and other effects caused by operation.

8.2.2.3 Wind and seismic loads should not be considered to act simultaneously. 8.2.3 Seismic loads 8.2.3.1 Seismic loads can be specified in one of the following ways:

a. Using horizontal and vertical forces associated with the weights of the different parts that the pressure vessel has been divided into, for its analysis.

b. Using response spectrum for the horizontal and vertical ground acceleration.

8.2.3.2 The pressure vessel shall be analyzed for seismic loads acting in two horizontal directions, perpendicular to each other and acting separately.

8.2.3.3 The analysis in one horizontal direction is only accepted in vertical pressure vessel

with axial symmetry. 8.2.3.4 Vertical seismic loads shall be considered in the following cases:

c. When calculating the supporting structure of the pressure vessel. d. When calculating the anchorage system. e. When the pressure vessel has eccentric weights or there are projecting elements

attached to the pressure vessel.

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f. When calculating the pedestals and foundations. 8.2.3.5 The seismic vertical or horizontal loads shall be assumed applied in the most

unfavorable direction for the pressure vessels design and for the support systems design.

8.2.3.6 The magnitude of vertical seismic loads shall be equal to 2/3 the magnitude of

horizontal seismic loads. 8.2.4 Mathematical model of the pressure vessel 8.2.4.1 The mathematical model used to analyze the pressure vessel shall adequately

represent its weight and stiffness distribution and the stiffness of supporting structure. 8.2.4.2 When partial analyses of the pressure vessel are carried out, the mathematical model

used shall adequately represent the transfer of loads from the points of application to the attachment and the interaction between the parts.

8.2.4.3 The minimum number of lumped masses and degrees of freedom to be used in the

analytical model of the pressure vessel shall be such to reproduce the real mass distribution and the shape of the vibration modes.

8.2.4.4 The concentrated mass model shall include all the pressure vessel weights, the

weights of the platforms and piping attached to the pressure vessel, the weight of the contents and the other accessories that contribute to the weight of the structure. In pressure vessels that contain liquids, whose volume varies according to the operating conditions of the system, the Designer shall consider the most probable weight of the liquid during its useful life, in according with Section 2.1.3.

8.2.4.5 When the stiffness of the piping attached to the pressure vessel is high, piping shall

be included in the mathematical model. 8.2.4.6 Due to physical characteristics of the fired heaters, reactors and combined feed

exchanger and also due to interconnecting pipes between these equipments, Enap Refineras requires THE CONTRACTOR to carry out a three-dimensional seismic dynamic analysis including all equipment, piping, foundations and supporting structures.

8.2.4.7 The results of this analysis, such as the fundamental period, forces and displacements

shall be considered for the design of equipment, interconnecting pipe, expansion joints, springs, bumpers, stoppers, etc.

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8.2.4.8 Enap Refineras will require the calculation procedure, backup forms and the results of this analysis before placing the purchase order of the involved equipments.

8.2.4.9 In pressure vessel that contains liquid, all the mass of the liquid shall be assumed

rigidly attached to the pressure vessel walls. 8.2.5 Seismic analysis

8.2.5.1 For the seismic analysis two procedures can be used:

a. Static analysis or equivalent forces analysis. b. Dynamic-spectrum modal analysis.

8.2.5.2 Analysis of the pressure vessel shall incorporate the horizontal and vertical

components of seismic loads acting simultaneously for the cases indicated in Section 8.2.3.4.

8.2.5.3 The static analysis shall be used in the following cases:

c. Rigid pressure vessels supported on ground. A pressure vessel that has a fundamental period less than 0.06[s], including their anchorage system, is considered rigid.

d. Flexible pressures vessel whose height is less than or equal to 15[m] when their grade of risk is minimum or medium.

e. Pressure vessels susceptible to being reduced to one-degree-of-freedom system.

For the others cases a dynamic spectrum modal analysis shall be used. 8.2.5.4 The seismic force that acts on rigid pressure vessels is calculated by multiplying their

weight by the seismic coefficient. The seismic coefficient shall be determined according to Section 5.4.2.2. This force is assumed to be applied at the center of gravity of the total weight of the vessel and content.

8.2.5.5 In order to apply the static method of analysis, the recommendations of Section 5.4.2,

shall be followed. 8.2.5.6 In order to apply the dynamic spectrum modal analysis method, the recommendations

of Section 5.4.3

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