Cover [PDF]

Standards [PDF]

Foreword [PDF]

Acknowledgements [PDF]

Dedication [PDF]

Contents [PDF]

Chapter 1 General

Chapter 2 Combinations of Loads

Chapter 3 Dead Loads, Soil Loads, and Hydrostatic Pressure

Chapter 4 Live Loads

Chapter 5 Flood Loads

Chapter 6 Reserved for Future Provisions

Chapter 7 Snow Loads

Chapter 8 Rain Loads

Chapter 9 Reserved for Future Provisions

Chapter 10 Ice Loads - Atmospheric Icing

Chapter 11 Seismic Design Criteria

Chapter 12 Seismic Design Requirements for Building Structures

Chapter 13 Seismic Design Requirements for Nonstructural Components

Chapter 14 Material Specific Seismic Design and Detailing Requirements

Chapter 15 Seismic Design Requirements for Nonbuilding Structures

Chapter 16 Seismic Response History Procedures

Chapter 17 Seismic Design Requirements for Seismically Isolated Structures

Chapter 18 Seismic Design Requirements for Structures with Damping Systems

Chapter 19 Soil-Structure Interaction for Seismic Design

Chapter 20 Site Classification Procedure for Seismic Design

Chapter 21 Site-Specific Ground Motion Procedures for Seismic Design

Chapter 22 Seismic Ground Motion Long-Period Transition and Risk Coefficient Maps

Chapter 23 Seismic Design Reference Documents

Chapter 24

Chapter 25

Chapter 26 Wind Loads: General Requirements

Chapter 27 Wind Loads on Buildings‒MWFRS (Directional Procedure)

Chapter 28 Wind Loads on Buildings‒MWFRS (Envelope Procedure)

Chapter 29 Wind Loads on Other Structures and Building Appurtenances‒MWFRS

Chapter 30 Wind Loads ‒ Components and Cladding (C&C)

Chapter 31 Wind Tunnel Procedure

Appendix 11A Quality Assurance Provisions

Appendix 11B Existing Building Provisions

Appendix C Serviceability Considerations

Appendix D Buildings Exempted from Torisional Wind Load Cases

Nonbuilding structures include all self-supporting structures that carry gravity loads and that may be required to resist the effects of earthquake, with the exception of building structures specifically excluded in Section 11.1.2 and other nonbuilding structures where specific seismic provisions have yet to be developed, and therefore, are not set forth in Chapter 15. Nonbuilding structures supported by the earth or supported by other structures shall be designed and detailed to resist the minimum lateral forces specified in this chapter. Design shall conform to the applicable requirements of other sections as modified by this section. Foundation design shall comply with the requirements of Sections 12.1.5, 12.13, and Chapter 14.
The design of nonbuilding structures shall provide sufficient stiffness, strength, and ductility consistent with the requirements specified herein for buildings to resist the effects of seismic ground motions as represented by these design forces:
  1. Applicable strength and other design criteria shall be obtained from other portions of the seismic requirements of this standard or its reference documents.
  2. Where applicable strength and other design criteria are not contained in, or referenced by the seismic requirements of this standard, such criteria shall be obtained from reference documents. Where reference documents define acceptance criteria in terms of allowable stresses as opposed to strength, the design seismic forces shall be obtained from this section and used in combination with other loads as specified in Section 2.4 of this standard and used directly with allowable stresses specified in the reference documents. Detailing shall be in accordance with the reference documents.
Structural analysis procedures for nonbuilding structures that are similar to buildings shall be selected in accordance with Section 12.6. Nonbuilding structures that are not similar to buildings shall be designed using either the equivalent lateral force procedure in accordance with Section 12.8, the modal analysis procedure in accordance with Section 12.9, the linear response history analysis procedure in accordance with Section 16.1, the nonlinear response history analysis procedure in accordance with Section 16.2, or the procedure prescribed in the specific reference document.
Reference documents referred to in Chapter 15 are listed in Chapter 23 and have seismic requirements based on the same force and displacement levels used in this standard or have seismic requirements that are specifically modified by Chapter 15.
Where nonbuilding structures identified in Table 15.4-2 are supported by other structures and nonbuilding structures are not part of the primary seismic force-resisting system, one of the following methods shall be used.

For the condition where the weight of the nonbuilding structure is less than 25% of the combined effective seismic weights of the nonbuilding structure and supporting structure, the design seismic forces of the nonbuilding structure shall be determined in accordance with Chapter 13 where the values of Rp and ap shall be determined in accordance to Section 13.1.5. The supporting structure shall be designed in accordance with the requirements of Chapter 12 or Section 15.5 as appropriate with the weight of the nonbuilding structure considered in the determination of the effective seismic weight, W.
For the condition where the weight of the nonbuilding structure is equal to or greater than 25% of the combined effective seismic weights of the nonbuilding structure and supporting structure, an analysis combining the structural characteristics of both the nonbuilding structure and the supporting structures shall be performed to determine the seismic design forces as follows:
  1. Where the fundamental period, T, of the nonbuilding structure is less than 0.06 s, the nonbuilding structure shall be considered a rigid element with appropriate distribution of its effective seismic weight. The supporting structure shall be designed in accordance with the requirements of Chapter 12 or Section 15.5 as appropriate , and the R value of the combined system is permitted to be taken as the R value of the supporting structural system. The nonbuilding structure and attachments shall be designed for the forces using the procedures of Chapter 13 where the value of Rp shall be taken as equal to the R value of the nonbuilding structure as set forth in Table 15.4-2, and ap shall be taken as 1.0.
  2. Where the fundamental period, T, of the nonbuilding structure is 0.06 s or greater, the nonbuilding structure and supporting structure shall be modeled together in a combined model with appropriate stiffness and effective seismic weight distributions. The combined structure shall be designed in accordance with Section 15.5 with the R value of the combined system taken as the lesser R value of the nonbuilding structure or the supporting structure. The nonbuilding structure and attachments shall be designed for the forces determined for the nonbuilding structure in the combined analysis.
Architectural, mechanical, and electrical components supported by nonbuilding structures shall be designed in accordance with Chapter 13 of this standard.
Nonbuilding structures having specific seismic design criteria established in reference documents shall be designed using the standards as amended herein. Where reference documents are not cited herein, nonbuilding structures shall be designed in compliance with Sections 15.5 and 15.6 to resist minimum seismic lateral forces that are not less than the requirements of Section 12.8 with the following additions and exceptions:
  1. The seismic force-resisting system shall be selected as follows:
    1. For non building structures similar to buildings, a system shall be selected from among the types indicated in Table 12.2-1 or Table 15.4-1 subject to the system limitations and limits on structural height, hn, based on the seismic design category indicated in the table. The appropriate values of R, Ω0, and Cd indicated in the selected table shall be used in determining the base shear, element design forces, and design story drift as indicated in this standard. Design and detailing requirements shall comply with the sections referenced in the selected table.
    2. For nonbuilding structures not similar to buildings, a system shall be selected from among the types indicated in Table 15.4-2 subject to the system limitations and limits on structural height, hn, based on seismic design category indicated in the table. The appropriate values of R, Ω0 , and Cd indicated in Table 15.4-2 shall be used in determining the base shear, element design forces, and design story drift as indicated in this standard. Design and detailing requirements shall comply with the sections referenced in Table 15.4-2.
    3. Where neither Table 15.4-1 nor Table 15.4-2 contains an appropriate entry, applicable strength and other design criteria shall be obtained from a reference document that is applicable to the specific type of nonbuilding structure. Design and detailing requirements shall comply with the reference document.
  2. For nonbuilding systems that have an R value provided in Table 15.4-2, the minimum specified value in Eq. 12.8-5 shall be replaced by

    Cs = 0.044SDSIe (15.4-1)

    The value of Cs shall not be taken as less than 0.03. And for nonbuilding structures located where S1 ≥ 0.6g, the minimum specified value in Eq. 12.8-6 shall be replaced by

    Cs = 0.8S1/(R/Ie) (15.4-2)

         EXCEPTION: Tanks and vessels that are designed to AWWA D100, AWWA Dl03, API 650 Appendix E, and API 620 Appendix L as modified by this standard, and stacks and chimneys that are designed to ACI 307 as modified by this standard, shall be subject to the larger of the minimum base shear value defined by the reference document or the value determined by replacing Eq. 12.8-5 with the following:

    Cs = 0.044SDSIe (15.4-3)

    The value of Cs shall not be taken as less than 0.01. And for nonbuilding structures located where S1 ≥ 0.6g, the minimum specified value in Eq. 12.8-6 shall be replaced by

    Cs = 0.5S1/(R/Ie) (15.4-4)

    Minimum base shear requirements need not apply to the convective (sloshing) component of liquid in tanks.
  3. The importance factor, Ie, shall be as set forth in Section 15.4.1.1.
  4. The vertical distribution of the lateral seismic forces in nonbuilding structures covered by this section shall be determined:
    1. Using the requirements of Section 12.8.3, or
    2. Using the procedures of Section 12.9, or
    3. In accordance with the reference document applicable to the specific nonbuilding structure.
  5. For nonbuilding structural systems containing liquids, gases, and granular solids supported at the base as defined in Section 15.7.1, the minimum seismic design force shall not be less than that required by the reference document for the specific system.
  6. Where a reference document provides a basis for the earthquake resistant design of a particular type of nonbuilding structure covered by Chapter 15, such a standard shall not be used unless the following limitations are met:
    1. The seismic ground accelerations and seismic coefficients shall be in conformance with the requirements of Section 11.4.
    2. The values for total lateral force and total base overturning moment used in design shall not be less than 80% of the base shear value and overturning moment, each adjusted for the effects of soil-structure interaction that is obtained using this standard.
  7. The base shear is permitted to be reduced in accordance with Section 19.2.1 to account for the effects of soilstructure interaction. In no case shall the reduced base shear be less than 0.7V.
  8. Unless otherwise noted in Chapter 15, the effects on the nonbuilding structure due to gravity loads and seismic forces shall be combined in accordance with the factored load combinations as presented in Section 2.3.
  9. Where specifically required by Chapter 15, the design seismic force on nonbuilding structures shall be as defined in Section 12.4.3.


The importance factor, Ie, and risk category for nonbuilding structures are based on the relative hazard of the contents and the function. The value of Ie shall be the largest value determined by the following:
  1. Applicable reference document listed in Chapter 23,
  2. The largest value as selected from Table 1.5-2, or
  3. As specified elsewhere in Chapter 15.
Nonbuilding structures that have a fundamental period , T, less than 0.06 s, including their anchorages, shall be designed for the lateral force obtained from the following:

V = 0.30SDSWIe (15.4-5)

where
V = the total design lateral seismic base shear force applied to a nonbuilding structure
SDS = the site design response acceleration as determined from Section 11 .4.4
W = nonbuilding structure operating weight
Ie = the importance factor determined in accordance with Section 15.4.1.1

The force shall be distributed with height in accordance with Section 12.8.3.
Nonbuilding structures that have a fundamental period , T, less than 0.06 s, including their anchorages, shall be designed for the lateral force obtained from the following:

V = 0.30SDSWIe (15.4-5)

where
V = the total design lateral seismic base shear force applied to a nonbuilding structure
SDS = the site design response acceleration as determined from Section 11 .4.4
W = nonbuilding structure operating weight
Ie = the importance factor determined in accordance with Section 15.4.1.1

The force shall be distributed with height in accordance with Section 12.8.3.
The seismic effective weight W for nonbuilding structures shall include the dead load and other loads as defined for structures in Section 12.7.2. For purposes of calculating design seismic forces in nonbuilding structures, W also shall include all normal operating contents for items such as tanks, vessels, bins, hoppers, and the contents of piping. W shall include snow and ice loads where these loads constitute 25% or more of W or where required by the authority having jurisdiction based on local environmental characteristics.
The fundamental period of the nonbuilding structure shall be determined using the structural properties and deformation characteristics of the resisting elements in a properly substantiated analysis as indicated in Section 12.8.2. Alternatively, the fundamental period T is permitted to be computed from the following equation

(15.4-6)

The values of fi represent any lateral force distribution in accordance with the principles of structural mechanics. The elastic deflections, δi, shall be calculated using the applied lateral forces, fi. Equations 12.8-7, 12.8-8, 12.8-9, and 12.8-10 shall not be used for determining the period of a nonbuilding structure.
The drift limitations of Section 12.12.1 need not apply to nonbuilding structures if a rational analysis indicates they can be exceeded without adversely affecting structural stability or attached or interconnected components and elements such as walkways and piping . P-delta effects shall be considered where critical to the function or stability of the structure.
The requirements regarding specific materials in Chapter 14 shall be applicable unless specifically exempted in Chapter 15.
Deflection limits and structure separation shall be determined in accordance with this standard unless specifically amended in Chapter 15.
Where required by a reference document or the authority having jurisdiction, specific types of non building structures shall be designed for site-specific criteria that account for local seismicity and geology, expected recurrence intervals, and magnitudes of events from known seismic hazards (see Section 11.4.7 of this standard). If a longer recurrence interval is defined in the reference document for the nonbuilding structure, such as liquefied natural gas (LNG) tanks (NFPA 59A), the recurrence interval required in the reference document shall be used.
Anchors in concrete used for nonbuilding structure anchorage shall be designed in accordance with Appendix D of ACI 318.
Anchors in masonry used for nonbuilding structure anchorage shall be designed in accordance with TMS402/ ACI 530/ ASCE 6. Anchors shall be designed to be governed by the tensile or shear strength of a ductile steel element.
     EXCEPTION: Anchors shall be permitted to be designed so that the attachment that the anchor is connecting to the structure undergoes ductile yielding at a load level corresponding to anchor forces not greater than their design strength, or the minimum design strength of the anchors shall be at least 2.5 times the factored forces transmitted by the attachment.
Post-installed anchors in concrete shall be prequalified for seismic applications in accordance with ACI 355.2 or other approved qualification procedures. Post-installed anchors in masonry shall be prequalified for seismic applications in accordance with approved qualification procedures.
Nonbuilding structures similar to buildings as defined in Section 11.2 shall be designed in accordance with this standard as modified by this section and the specific reference documents . This general category of nonbuilding structures shall be designed in accordance with the seismic requirements of this standard and the applicable portions of Section 15.4. The combination of load effects, E, shall be determined in accordance with Section 12.4.
In addition to the requirements of Section 15 .5 .1, pipe racks supported at the base of the structure shall be designed to meet the force requirements of Section 12.8 or 12.9. Displacements of the pipe rack and potential for interaction effects (pounding of the piping system) shall be considered using the amplified deflections obtained from the following equation:

δx = Cdδxe
Ie
(15.5-1)

where
Cd = deflection amplification factor in Table 15 .4-1
δxe = deflections determined using the prescribed seismic designforces of this standard
Ie = importance factor determined in accordance with Section 15.4.1.1

     See Section 13.6.3 for the design of piping systems and their attachments. Friction resulting from gravity loads shall not be considered to provide resistance to seismic forces.
Steel storage racks supported at or below grade shall be designed in accordance with ANSI/RMI MH 16.1 and its force and displacement requirements, except as follows.
7.1.2 Base Plate Design
Once the required bearing area has been determined from the allowable bearing stress,
F'p, the minimum thickness of the base plate is determined by rational analysis or by appropriate test using a test load 1.5 times the ASD design load or the factored LRFD load. Design forces that include seismic loads for anchorage of steel storage racks to concrete or masonry shall be determined using load combinations with overstrength provided in Section 12.4.3.2 of ASCE/SEI 7. The over strength factor shall be taken as 2.0.
     Anchorage of steel storage racks to concrete shall be in accordance with the requirements of Section 15.4.9 of ASCE/SEI 7. Upon request, information shall be given to the owner or the owner '.s agent on the location, size, and pressures under the column base plates of each type of upright frame in the installation. When rational analysis is used to determine base plate thickness, and other applicable standards do not apply, the base plate shall be permitted to be designed for the following loading conditions, where applicable:
(balance of section unchanged)
7.1.4 Shims
Shims may be used under the base plate to maintain the plumbness and/or levelness of the storage rack. The shims shall be made of a material that meets or exceeds the design bearing strength (LRFD) or allowable bearing strength (ASD) of the floor. The shim size and location under the base plate shall be equal to or greater than the required base plate size and location.
     Shims stacks shall be interlocked or welded together in a fashion that is capable of transferring all the shear forces at the base.
     Bending in the anchor associated with shims or grout under the base plate shall be taken into account in the design of anchor bolts.
As an alternative to ANSI MH 16.1 as modified above, steel storage racks shall be permitted to be designed in accordance with the requirements of Sections 15.1, 15.2, 15.3, 15.5.1, and 15.5.3.5 through 15.5.3.8 of this standard.
Steel storage racks shall satisfy the force requirements of this section.
     EXCEPTION: Steel storage racks supported at the base are permitted to be designed as structures with an R of 4, provided that the seismic requirements of this standard are met. Higher values of R are permitted to be used where the detailing requirements of reference documents listed in Section 14.1.1 are met. The importance factor, Ie, for storage racks in structures open to the public, such as warehouse retail stores, shall be taken equal to 1.5.
Steel storage racks shall be designed for each of the following conditions of operating weight, W or Wp.
  1. Weight of the rack plus every storage level loaded to 67% of its rated load capacity.
  2. Weight of the rack plus the highest storage level only loaded to 100% of its rated load capacity.
     The design shall consider the actual height of the center of mass of each storage load component.
For all steel storage racks, the vertical distribution of seismic forces shall be as specified in Section 12.8.3 and in accordance with the following:
  1. The base shear, V, of the typical structure shall be the base shear of the steel storage rack where loaded in accordance with Section 15.5.3.6.
  2. The base of the structure shall be the floor supporting the steel storage rack. Each steel storage level of the rack shall be treated as a level of the structure with heights hi and hx measured from the base of the structure.
  3. The factor k is permitted to be taken as 1.0.
Steel storage rack installations shall accommodate the seismic displacement of the storage racks and their contents relative to all adjacent or attached components and elements. The assumed total relative displacement for storage racks shall be not less than 5% of the structural height above the base, hn, unless a smaller value is justified by test data or analysis in accordance with Section 11.1.4.
Electrical power generating facilities are power plants that generate electricity by steam turbines, combustion turbines, diesel generators, or similar turbo machinery.
In addition to the requirements of Section 15.5.1, electrical power generating facilities shall be designed using this standard and the appropriate factors contained in Section 15.4.
In addition to the requirements of Section 15.5.1, structural towers that support tanks and vessels shall be designed to meet the requirements of Section 15.3. In addition, the following special considerations shall be included:
  1. The distribution of the lateral base shear from the tank or vessel onto the supporting structure shall consider the relative stiffness of the tank and resisting structural elements.
  2. The distribution of the vertical reactions from the tank or vessel onto the supporting structure shall consider the relative stiffness of the tank and resisting structural elements. Where the tank or vessel is supported on grillage beams, the calculated vertical reaction due to weight and overturning shall be increased at least 20% to account for nonuniform support. The grillage beam and vessel attachment shall be designed for this increased design value.
  3. Seismic displacements of the tank and vessel shall consider the deformation of the support structure where determining P-delta effects or evaluating required clearances to prevent pounding of the tank on the structure.
Piers and wharves are structures located in waterfront areas that project into a body of water or that parallel the shoreline.
In addition to the requirements of Section 15.5.1, piers and wharves that are accessible to the general public, such as cruise ship terminals and piers with retail or commercial offices or restaurants, shall be designed to comply with this standard. Piers and wharves that are not accessible to the general public are beyond the scope of this section.
     The design shall account for the effects of liquefaction and soil failure collapse mechanisms and consider all applicable marine loading combinations, such as mooring, berthing, wave, and current on piers and wharves as required. Structural detailing shall consider the effects of the marine environment.
Nonbuilding structures that do not have lateral and vertical seismic force-resisting systems that are similar to buildings shall be designed in accordance with this standard as modified by this section and the specific reference documents. Loads and load distributions shall not be less demanding than those determined in this standard. The combination of earthquake load effects, E, shall be determined in accordance with Section 12.4.2.
     EXCEPTION: The redundancy factor, ρ, per Section 12.3.4 shall be taken as 1.
This section applies to all earth-retaining structures assigned to Seismic Design Category D, E, or F. The lateral earth pressures due to earthquake ground motions shall be determined in accordance with Section 11.8.3. The risk category shall be determined by the proximity of the earth-retaining structure to other buildings and structures. If failure of the earth-retaining structure would affect the adjacent building or structure, the risk category shall not be less than that of the adjacent building or structure.
     Earth-retaining walls are permitted to be designed for seismic loads as either yielding or nonyielding walls. Cantilevered reinforced concrete or masonry retaining walls shall be assumed to be yielding walls and shall be designed as simple flexural wall elements.
Stacks and chimneys are permitted to be either lined or unlined and shall be constructed from concrete, steel, or masonry. Steel stacks, concrete stacks, steel chimneys, concrete chimneys, and liners shall be designed to resist seismic lateral forces determined from a substantiated analysis using reference documents. Interaction of the stack or chimney with the liners shall be considered. A minimum separation shall be provided between the liner and chimney equal to Cd times the calculated differential lateral drift.
     Concrete chimneys and stacks shall be designed in accordance with the requirements of ACI 307 except that (1) the design base shear shall be determined based on Section 15.4.1 of this standard, (2) the seismic coefficients shall be based on the values provided in Table 15.4-2, and (3) openings shall be detailed as required below. When modal response spectrum analysis is used for design, the procedures of Section 12.9 shall be permitted to be used.
     For concrete chimneys and stacks assigned to SDC D, E, and F, splices for vertical rebar shall be staggered such that no more than 50% of the bars are spliced at any section and alternate lap splices are staggered by the development length. In addition, where the loss of cross-sectional area is greater than 10%, crosssections in the regions of breachings/openings shall be designed and detailed for vertical force, shear force, and bending moment demands along the vertical direction, determined for the affected cross-section using an overstrength factor of 1.5. The region where the overstrength factor applies shall extend above and below the opening(s) by a distance equal to half of the width of the largest opening in the affected region. Appropriate reinforcement development lengths shall be provided beyond the required region of overstrength. The jamb regions around each opening shall be detailed using the column tie requirements in Section 7.10.5 of ACI 318. Such detailing shall extend for a jamb width of a minimum of two times the wall thickness and for a height of the opening height plus twice the wall thickness above and below the opening, but no less than the development length of the longitudinal bars. Where the existence of a footing or base mat precludes the ability to achieve the extension distance below the opening and within the stack, the jamb reinforcing shall be extended and developed into the footing or base mat. The percentage of longitudinal reinforcement in jamb regions shall meet the requirements of Section 10.9 of ACI 318 for compression members.
Special hydraulic structures are structures that are contained inside liquid-containing structures. These structures are exposed to liquids on both wall surfaces at the same head elevation under normal operating conditions. Special hydraulic structures are subjected to out-of-plane forces only during an earthquake where the structure is subjected to differential hydrodynamic fluid forces. Examples of special hydraulic structures include separation walls, baffle walls, weirs, and other similar structures .
Special hydraulic structures shall be designed for out-of-phase movement of the fluid. Unbalanced forces from the motion of the liquid must be applied simultaneously "in front of" and "behind" these elements.
     Structures subject to hydrodynamic pressures induced by earthquakes shall be designed for rigid body and sloshing liquid forces and their own inertia force. The height of sloshing shall be determined and compared with the freeboard height of the structure. Interior elements, such as baffles or roof supports, also shall be designed for the effects of unbalanced forces and sloshing.
Secondary containment systems, such as impoundment dikes and walls, shall meet the requirements of the applicable standards for tanks and vessels and the authority having jurisdiction.
     Secondary containment systems shall be designed to withstand the effects of the maximum considered earthquake ground motion where empty and two-thirds of the maximum considered earthquake ground motion where full including all hydrodynamic forces as determined in accordance with the procedures of Section 11.4. Where determined by the risk assessment required by Section 1.5.3 or by the authority having jurisdiction that the site may be subject to aftershocks of the same magnitude as the maximum considered motion, secondary containment systems shall be designed to withstand the effects of the maximum considered earthquake ground motion where full including all hydrodynamic forces as determined in accordance with the procedures of Section 11.4.
Sloshing of the liquid within the secondary containment area shall be considered in determining the height of the impound . Where the primary containment has not been designed with a reduction in the structure category (i.e., no reduction in importance factor Ie) as permitted by Section 1.5.3, no freeboard provision is required. Where the primary containment has been designed for a reduced structure category (i.e., importance factor Ie reduced) as permitted by Section 1.5.3, a minimum freeboard, δs, shall be provided where

δs = 0.42DSac (15.6-1)

where Sac is the spectral acceleration of the convective component and is determined according to the procedures of Section 15.7.6.1 using 0.5% damping. For circular impoundment dikes, D shall be taken as the diameter of the impoundment dike. For rectangular impoundment dikes, D shall be taken as the plan dimension of the impoundment dike, L, for the direction under consideration.
Self-supporting and guyed telecommunication towers shall be designed to resist seismic lateral forces determined from a substantiated analysis using reference documents.
This section applies to all tanks, vessels, bins, and silos, and similar containers storing liquids, gases, and granular solids supported at the base (hereafter referred to generically as "tanks and vessels"). Tanks and vessels covered herein include reinforced concrete, prestressed concrete, steel, aluminum, and fiber-reinforced plastic materials. Tanks supported on elevated levels in buildings shall be designed in accordance with Section 15.3.
Tanks and vessels storing liquids, gases, and granular solids shall be designed in accordance with this standard and shall be designed to meet the requirements of the applicable reference documents listed in Chapter 23. Resistance to seismic forces shall be determined from a substantiated analysis based on the applicable reference documents listed in Chapter 23.
  1. Damping for the convective (sloshing) force component shall be taken as 0.5%.
  2. Impulsive and convective components shall be combined by the direct sum or the square root of the sum of the squares (SRSS) method where the modal periods are separated. If significant modal coupling may occur, the complete quadratic combination (CQC) method shall be used.
  3. Vertical earthquake forces shall be considered in accordance with the applicable reference document. If the reference document permits the user the option of including or excluding the vertical earthquake force to comply with this standard, it shall be included. For tanks and vessels not covered by a reference document, the forces due to the vertical acceleration shall be defined as follows:
    1. Hydrodynamic vertical and lateral forces in tank walls: The increase in hydrostatic pressures due to the vertical excitation of the contained liquid shall correspond to an effective increase in unit weight, γL, of the stored liquid equal to 0.2SDS γL.
    2. Hydrodynamic hoop forces in cylindrical tank walls: In a cylindrical tank wall, the hoop force per unit height, Nh, at height y from the base, associated with the vertical excitation of the contained liquid, shall be computed in accordance with Eq. 15.7-1.
      (15.7-1)
      where
      Di = inside tank diameter
      HL = liquid height inside the tank
      y = distance from base of the tank to height being investigated
      γL = unit weight of stored liquid
    3. Vertical inertia forces in cylindrical and rectangular tank walls: Vertical inertia forces associated with the vertical acceleration of the structure itself shall be taken equal to 0.2SDSW.
Structural members that are part of the seismic force-resisting system shall be designed to provide the following:
  1. Connections to seismic force-resisting elements, excluding anchors (bolts or rods) embedded in concrete, shall be designed to develop Ω0 times the calculated connection design force. For anchors (bolts or rods) embedded in concrete, the design of the anchor embedment shall meet the requirements of Section 15.7.5. Additionally, the connection of the anchors to the tank or vessel shall be designed to develop the lesser of the strength of the anchor in tension as determined by the reference document or Ω0 times the calculated anchor design force. The overstrength requirements of Section 12.4.3, and the Ω0 values tabulated in Table 15.4-2, do not apply to the design of walls, including interior walls, of tanks or vessels.
  2. Penetrations, manholes, and openings in shell elements shall be designed to maintain the strength and stability of the shell to carry tensile and compressive membrane shell forces.
  3. Support towers for tanks and vessels with irregular bracing, unbraced panels, asymmetric bracing, or concentrated masses shall be designed using the requirements of Section 12.3.2 for irregular structures. Support towers using chevron or eccentrically braced framing shall comply with the seismic requirements of this standard. Support towers using tension-only bracing shall be designed such that the full cross-section of the tension element can yield during overload conditions.
  4. In support towers for tanks and vessels, compression struts that resist the reaction forces from tension braces shall be designed to resist the lesser of the yield load of the brace, AgFy, or Ω0 times the calculated tension load in the brace.
  5. The vessel stiffness relative to the support system (foundation, support tower, skirt, etc.) shall be considered in determining forces in the vessel, the resisting elements, and the connections.
  6. For concrete liquid-containing structures, system ductility, and energy dissipation under unfactored loads shall not be allowed to be achieved by inelastic deformations to such a degree as to jeopardize the serviceability of the structure. Stiffness degradation and energy dissipation shall be allowed to be obtained either through limited microcracking, or by means of lateral force resistance mechanisms that dissipate energy without damaging the structure.
Design of piping systems connected to tanks and vessels shall consider the potential movement of the connection points during earthquakes and provide sufficient flexibility to avoid release of the product by failure of the piping system. The piping system and supports shall be designed so as not to impart significant mechanical loading on the attachment to the tank or vessel shell. Mechanical devices that add flexibility, such as bellows, expansion joints, and other flexible apparatus, are permitted to be used where they are designed for seismic displacements and defined operating pressure.
   Unless otherwise calculated, the minimum displacements in Table 15.7-1 shall be assumed. For attachment points located above the support or foundation elevation, the displacements in Table 15.7-1 shall be increased to account for drift of the tank or vessel relative to the base of support. The piping system and tank connection shall also be designed to tolerate Cd times the displacements given in Table 15.7-1 without rupture, although permanent deformations and inelastic behavior in the piping supports and tank shell is permitted. For attachment points located above the support or foundation elevation, the displacements in Table 15.7-1 shall be increased to account for drift of the tank or vessel. The values given in Table 15.7-1 do not include the influence of relative movements of the foundation and piping anchorage points due to foundation movements (e.g., settlement, seismic displacements). The effects of the foundation movements shall be included in the piping system design including the determination of the mechanical loading on the tank or vessel, and the total displacement capacity of the mechanical devices intended to add flexibility.
   The anchorage ratio, J, for self-anchored tanks shall comply with the criteria shown in Table 15.7-2 and is defined as
(15.7-2)
where
    (15.7-3)
wr = roof load acting on the shell in pounds per foot (N/m) of
shell circumference. Only permanent roof loads shall be
included. Roof live load shall not be included
wa = maximum weight of the tank contents that may be used
to resist the shell overturning moment in pounds per foot
(N/m) of shell circumference. Usually consists of an
annulus of liquid limited by the bending strength of the
tank bottom or annular plate
Mrw = the overturning moment applied at the bottom of the shell
due to the seismic design loads in foot-pounds (N-m)
(also known as the "ringwall moment")
D = tank diameter in feet (m)
Ws = total weight of tank shell in pounds (N)

Table 15.7-1 Minimum Design Displacements for Piping Attachments
Condition Displacements (in.)
Mechanically Anchored Tanks and Vessels
Upward vertical displacement relative to support or foundation 1 (25.4 mm)
Downward vertical displacement relative to support or foundation 0.5 (12 .7 mm)
Range of horizontal displacement (radial and tangential) relative to support or foundation 0.5 (12.7 mm)
Self-Anchored Tanks or Vessels (at grade)
Upward vertical displacement relative to support or foundation
If designed in accordance with a reference document as modified by this standard
Anchorage ratio less than or equal to 0.785 (indicates no uplift)
Anchorage ratio greater than 0.785 (indicates uplift)


1 (25.4 mm)
4 (101.1 mm)
If designed for seismic loads in accordance with this standard but not covered by a reference document
For tanks and vessels with a diameter less than 40 ft
For tanks and vessels with a diameter equal to or greater than 40 ft

8 (202.2 mm)
12 (0.305 m)
Downward vertical displacement relative to support or foundation
For tanks with a ringwall/mat foundation
For tanks with a berm foundation

0.5 (12.7 mm)
1 (25.4 mm)
Range of horizontal displacement (radial and tangential) relative to support or foundation 2 (50.8 mm)


Table 15.7-2 Anchorage Ratio
J 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. The tank is self-anchored.
J > 1.54 Tank is not stable and shall be mechanically
anchored for the design load.

Tanks and vessels at grade are permitted to be designed without anchorage where they meet the requirements for unanchored tanks in reference documents. Tanks and vessels supported above grade on structural towers or building structures shall be anchored to the supporting structure.
   The following special detailing requirements shall apply to steel tank and vessel anchor bolts in SDC C, D, E, and F. Anchorage shall be in accordance with Section 15.4.9, whereby the anchor embedment into the concrete shall be designed to develop the steel strength of the anchor in tension. The steel strength of the anchor in tension shall be determined in accordance with ACI 318, Appendix D, Eq. D-3. The anchor shall have a minimum gauge length of eight diameters. Post-installed anchors are permitted to be used in accordance with Section 15.4.9.3 provided the anchor embedment into the concrete is designed to develop the steel strength of the anchor in tension. In either case, the load combinations with overstrength of Section 12.4.3 are not to be used to size the anchor bolts for tanks and horizontal and vertical vessels.
Ground-supported, flat bottom tanks storing liquids shall be designed to resist the seismic forces calculated using one of the following procedures:
  1. The base shear and overturning moment calculated as if the tank and the entire contents are a rigid mass system per Section 15.4.2 of this standard.
  2. Tanks or vessels storing liquids in Risk Category IV, or with a diameter greater than 20 ft (6.1 m), shall be designed to consider the hydrodynamic pressures of the liquid in determining the equivalent lateral forces and lateral force distribution per the applicable reference documents listed in Chapter 23 and the requirements of Section 15.7.
  3. The force and displacement requirements of Section 15.4.
The design of tanks storing liquids shall consider the impulsive and convective (sloshing) effects and their consequences on the tank, foundation, and attached elements. The impulsive component corresponds to the high-frequency amplified response to the lateral ground motion of the tank roof, the shell, and the portion of the contents that moves in unison with the shell. The convective component corresponds to the low-frequency amplified response of the contents in the fundamental sloshing mode. Damping for the convective component shall be 0.5% for the sloshing liquid unless otherwise defined by the reference document. The following definitions shall apply:
Di = inside diameter of tank or vessel
HL = design liquid height inside the tank or vessel
L = inside length of a rectangular tank, parallel to the direction of the earthquake force being investigated
Nh = hydrodynamic hoop force per unit height in the wall of a cylindrical tank or vessel
Tc = natural period of the first (convective) mode of sloshing
Ti = fundamental period of the tank structure and impulsive component of the content
Vi = base shear due to impulsive component from weight of tank and contents
Vc = base shear due to the convective component of the effective sloshing mass
y = distance from base of the tank to level being investigated
γL = unit weight of stored liquid

The seismic base shear is the combination of the impulsive and convective components:


                 V = Vi + Vc (15.7-4)
where
(15.7-5)
(15.7-6)
Sai = the spectral acceleration as a multiplier of gravity including the site impulsive components at period Ti and 5% damping

For TiTs


            Sai = SDS (15.7-7)
For Ts < TiTL
(15.7-8)
For Ti > TL
(15.7-9)
NOTES:
  1. Where a reference document is used in which the spectral acceleration for the tank shell, and the impulsive component of the liquid is independent of Ti, then Sai = SDS.
  2. Equations 15.7-8 and 15.7-9 shall not be less than the minimum values required in Section 15.4.1, Item 2, multiplied by R/Ie.
  3. For tanks in Risk Category IV, the value of the importance factor, Ie, used for freeboard determination only shall be taken as 1.0.
  4. For tanks in Risk Categories I, II, and III, the value of TL used for freeboard determination is permitted to be set equal to 4 s. The value of the importance factor, Ie, used for freeboard determination for tanks in Risk Categories I, II, and III shall be the value determined from Table 1.5-1.
  5. Impulsive and convective seismic forces for tanks are permitted to be combined using the square root of the sum of the squares (SRSS) method in lieu of the direct sum method shown in Section 15.7.6 and its related subsections.
Sac = the spectral acceleration of the sloshing liquid (convective component) based on the sloshing period Tc and 0.5 percent damping

For TcTL:
(15.7-10)
For Tc > TL:
(15.7-11)
EXCEPTION: For Tc > 4 s, Sac is permitted be determined by a site-specific study using one or more of the following methods: (1) the procedures found in Chapter 21, provided such procedures, which rely on ground-motion attenuation equations for computing response spectra, cover the natural period band containing Tc; (2) ground-motion simulation methods employing seismological models of fault rupture and wave propagation; and (3) analysis of representative strongmotion accelerogram data with reliable long-period content extending to periods greater than Tc. Site-specific values of Sac shall be based on one standard deviation determinations. However, in no case shall the value of Sac be taken as less than the value determined in accordance with Eq. 15.7-11 using 50% of the mapped value of TL from Chapter 22.

The 80% limit on Sa required by Sections 21.3 and 21.4 shall not apply to the determination of site-specific values of Sac, which satisfy the requirements of this exception. In determining the value of Sac, the value of TL shall not be less than 4 s.

where
(15.7-12)
and where
D = the tank diameter in ft (m), H = liquid height in ft (m), and g = acceleration due to gravity in consistent units
Wi = impulsive weight (impulsive component of liquid, roof and equipment, shell, bottom, and internal elements)
Wc = the portion of the liquid weight sloshing
Unless otherwise required by the appropriate reference document listed in Chapter 23, the method given in ACI 350.3 is permitted to be used to determine the vertical and horizontal distribution of the hydrodynamic and inertial forces on the walls of circular and rectangular tanks.
Sloshing of the stored liquid shall be taken into account in the seismic design of tanks and vessels in accordance with the following requirements:
  1. The height of the sloshing wave, δs, shall be computed using Eq. 15.7-13 as follows:
              δs = 0.42DiIeSac          (15.7-13)

    For cylindrical tanks, Di shall be the inside diameter of the tank; for rectangular tanks, the term Di shall be replaced by the longitudinal plan dimension of the tank, L, for the direction under consideration.
  2. The effects of sloshing shall be accommodated by means of one of the following:
    1. A minimum freeboard in accordance with Table 15.7-3.
      Table 15.7-3 Minimum Required Freeboard
      Value of SDS Risk Category
      I or II III IV
      SDS < 0.167g a a δsc
      0.167gSDS < 0.33g a a δsc
      0.33gSDS < 0.50g a 0.7δsb δsc
      SDS ≥ 0.50g a 0.7δsb δsc


      aNOTE: No minimum freeboard is required.
      cFreeboard equal to the calculated wave height, δs, is required unless one of the following alternatives is provided: (1) Secondary containment is provided to control the product spill. (2) The roof and supporting structure are designed to contain the sloshing liquid.
      bA freeboard equal to 0.7δs is required unless one of the following alternatives is provided: (1) Secondary containment is provided to control the product spill. (2) The roof and supporting structure are designed to contain the sloshing liquid.
    2. A roof and supporting structure designed to contain the sloshing liquid in accordance with the following subpoint.
    3. For open-top tanks or vessels only, an overflow spillway around the tank or vessel perimeter.
  3. If the sloshing is restricted because the freeboard is less than the computed sloshing height, then the roof and supporting structure shall be designed for an equivalent hydro static head equal to the computed sloshing height less the freeboard. In addition, the design of the tank shall use the confined portion of the convective (sloshing) mass as an additional impulsive mass.
Equipment, piping, and walkways or other appurtenances attached to the structure shall be designed to accommodate the displacements imposed by seismic forces. For piping attachments, see Section 15.7.4.
The attachments of internal equipment and accessories that are attached to the primary liquid or pressure retaining shell or bottom or that provide structural support for major elements (e.g., a column supporting the roof rafters) shall be designed for the lateral loads due to the sloshing liquid in addition to the inertial forces by a substantiated analysis method.
The transfer of the total lateral shear force between the tank or vessel and the subgrade shall be considered:
  1. For unanchored flat bottom steel tanks, the overall horizontal seismic shear force is permitted to be resisted by friction between the tank bottom and the foundation or subgrade. Unanchored 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, shall not exceed

         V < W tan 30°           (15.7-14)

    W shall be determined using the effective seismic weight of the tank, roof, and contents after reduction for coincident vertical earthquake. Lower values of the friction factor shall be used if the design of the tank bottom to supporting foundation does not justify the friction value above (e.g., leak detection membrane beneath the bottom with a lower friction factor, smooth bottoms, etc.). Alternatively, the friction factor is permitted to be determined by testing in accordance with Section 11.1.4.
  2. No additional lateral anchorage is required for anchored steel tanks designed in accordance with reference documents.
  3. The lateral shear transfer behavior for special tank configurations (e.g., shovel bottoms, highly crowned tank bottoms, tanks on grillage) can be unique and are beyond the scope of this standard.
Local transfer of the shear from the roof to the wall and the wall of the tank into the base shall be considered. For cylindrical tanks and vessels, the peak local tangential shear per unit length shall be calculated by
(15.7-15)
  1. Tangential shear in flat bottom steel tanks shall be transferred through the welded connection to the steel bottom. This transfer mechanism is deemed acceptable for steel tanks designed in accordance with the reference documents where SDS < 1.0g.
  2. For concrete tanks with a sliding base where the lateral shear is resisted by friction between the tank wall and the base, the friction coefficient value used for design shall not exceed tan 30°.
  3. Fixed-base or hinged-base concrete tanks transfer the horizontal seismic base shear shared by membrane (tangential) shear and radial shear into the foundation. For anchored flexible-base concrete tanks, the majority of the base shear is resisted by membrane (tangential) shear through the anchoring system with only insignificant vertical bending in the wall. The connection between the wall and floor shall be designed to resist the maximum tangential shear.
For steel tanks, the internal pressure from the stored product stiffens thin cylindrical shell structural elements subjected to membrane compression forces. This stiffening effect is permitted to be considered in resisting seismically induced compressive forces if permitted by the reference document or the authority having jurisdiction.
Steel tanks resting on concrete ring walls or slabs shall have a uniformly supported annulus under the shell. Uniform support shall be provided by one of the following methods:
  1. Shimming and grouting the annulus,
  2. Using fiberboard or other suitable padding
  3. Using butt-welded bottom or annular plates resting directly on the foundation, and
  4. Using closely spaced shims (without structural grout) provided that the localized bearing loads are considered in the tank wall and foundation to prevent local crippling and spalling.
Repairs, modifications, or reconstruction (i.e., cut down and re-erect) of a tank or vessel shall conform to industry standard practice and this standard. For welded steel tanks storing liquids, see API 653 and the applicable reference document listed in Chapter 23. Tanks that are relocated shall be reevaluated for the seismic loads for the new site and the requirements of new construction in accordance with the appropriate reference document and this standard.
Welded steel water storage tanks and vessels shall be designed in accordance with the seismic requirements of AWWA D100.
Bolted steel water storage structures shall be designed in accordance with the seismic requirements of AWWA D103 except that the design input forces of AWWA D100 shall be modified in the same manner shown in Section 15.7.7.1 of this standard.
Reinforced and prestressed concrete tanks shall be designed in accordance with the seismic requirements of AWWA D110, AWWA D115, or ACI 350.3 except that the importance factor, Ie, shall be determined according to Section 15.4.1.1; the response modification coefficient, R, shall be taken from Table 15.4-2; and the design input forces for strength design procedures shall be determined using the procedures of ACI 350.3 except
  1. Sac shall be substituted for Cc in ACI 350.3 Section 9.4.2 using Eqs. 15.7-10 for TcTL and 15.7-11. for Tc > TL from Section 15.7.6.1; and
  2. The value of Ct from ACI 350.3 Section 9.4.3 shall be determined using the procedures of Section 15.7.2(c). The values of I, Ri, and b as defined in ACI 350.3 shall be taken as 1.0 in the determination of vertical seismic effects.
Welded steel petrochemical and industrial tanks and vessels storing liquids under an internal pressure of less than or equal to 2.5 psig (17.2 kpa g) shall be designed in accordance with the seismic requirements of API 650. Welded steel petrochemical and industrial tanks and vessels storing liquids under an internal pressure of greater than 2.5 psig (17.2 kpa g) and less than or equal to 15 psig (104.4 kpa g) shall be designed in accordance with the seismic requirements of API 620.
Bolted steel tanks are used for storage of production liquids. API 12B covers the material, design, and erection requirements for vertical, cylindrical, and above-ground bolted tanks in nominal capacities of 100 to 10,000 barrels for production service. Unless required by the authority having jurisdiction, these temporary structures need not be designed for seismic loads. If design for seismic load is required, the loads are permitted to be adjusted for the temporary nature of the anticipated service life.
Reinforced concrete tanks for the storage of petrochemical and industrial liquids shall be designed in accordance with the force requirements of Section 15.7.7.3.
The intergranular behavior of the material shall be considered in determining effective mass and load paths, including the following behaviors:
  1. Increased lateral pressure (and the resulting hoop stress) due to loss of the intergranular friction of the material during seismic shaking;
  2. Increased hoop stresses generated from temperature changes in the shell after the material has been compacted; and
  3. Intergranular friction, which can transfer seismic shear directly to the foundation.
The lateral forces for tanks and vessels storing granular materials at grade shall be determined by the requirements and accelerations for short period structures (i.e., SDS).
The increase in lateral pressure on the tank wall shall be added to the static design lateral pressure but shall not be used in the determination of pressure stability effects on the axial buckling strength of the tank shell.
A portion of a stored granular mass will act with the shell (the effective mass). The effective mass is related to the physical characteristics of the product, the height-to-diameter (H/D) ratio of the tank, and the intensity of the seismic event. The effective mass shall be used to determine the shear and overturning loads resisted by the tank.
The effective density factor (that part of the total stored mass of product that is accelerated by the seismic event) shall be determined in accordance with ACI 313.
For granular storage tanks that have a steel bottom and are supported such that friction at the bottom to foundation interface can resist lateral shear loads, no additional anchorage to prevent sliding is required. For tanks without steel bottoms (i.e., the material rests directly on the foundation), shear anchorage shall be provided to prevent sliding.
If separate anchorage systems are used to prevent overturning and sliding, the relative stiffness of the systems shall be considered in determining the load distribution.
Welded steel granular storage structures shall be designed in accordance with the seismic requirements of this standard. Component allowable stresses and materials shall be per AWWA D100, except the allowable circumferential membrane stresses and material requirements in API 650 shall apply.
Bolted steel granular storage structures shall be designed in accordance with the seismic requirements of this section. Component allowable stresses and materials shall be per AWWA D103.
Reinforced concrete structures for the storage of granular materials shall be designed in accordance with the seismic force requirements of this standard and the requirements of ACI 313.
Prestressed concrete structures for the storage of granular materials shall be designed in accordance with the seismic force requirements of this standard and the requirements of ACI 313.
This section applies to tanks, vessels, bins, and hoppers that are elevated above grade where the supporting tower is an integral part of the structure, or where the primary function of the tower is to support the tank or vessel. Tanks and vessels that are supported within buildings or are incidental to the primary function of the tower are considered mechanical equipment and shall be designed in accordance with Chapter 13.
   Elevated tanks shall be designed for the force and displacement requirements of the applicable reference document or Section 15.4.
The design of the supporting tower or pedestal, anchorage, and foundation for seismic overturning shall assume the material stored is a rigid mass acting at the volumetric center of gravity. The effects of fluid-structure interaction are permitted to be considered in determining the forces, effective period, and mass centroids of the system if the following requirements are met:
  1. The sloshing period, Tc is greater than 3T where T = natural period of the tank with confined liquid (rigid mass) and supporting structure, and
  2. The sloshing mechanism (i.e., the percentage of convective mass and centroid) is determined for the specific configuration of the container by detailed fluid-structure interaction analysis or testing.
    Soil-structure interaction is permitted to be included in determining T providing the requirements of Chapter 19 are met.
The lateral drift of the elevated tank shall be considered as follows:
  1. The design drift, the elastic lateral displacement of the stored mass center of gravity, shall be increased by the factor Cd for evaluating the additional load in the support structure.
  2. The base of the tank shall be assumed to be fixed rotationally and laterally.
  3. Deflections due to bending, axial tension, or compression shall be considered. For pedestal tanks with a height-to-diameter ratio less than 5, shear deformations of the pedestal shall be considered.
  4. The dead load effects of roof-mounted equipment or platforms shall be included in the analysis.
  5. If constructed within the plumbness tolerances specified by the reference document, initial tilt need not be considered in the P-delta analysis.
For post-supported tanks and vessels that are cross-braced:
  1. The bracing shall be installed in such a manner as to provide uniform resistance to the lateral load (e.g., pretensioning or tuning to attain equal sag).
  2. The additional load in the brace due to the eccentricity between the post to tank attachment and the line of action of the bracing shall be included.
  3. Eccentricity of compression strut line of action (elements that resist the tensile pull from the bracing rods in the seismic force-resisting systems) with their attachment points shall be considered.
  4. The connection of the post or leg with the foundation shall be designed to resist both the vertical and lateral resultant from the yield load in the bracing, assuming the direction of the lateral load is oriented to produce the maximum lateral shear at the post-to-foundation interface. Where multiple rods are connected to the same location, the anchorage shall be designed to resist the concurrent tensile loads in the braces.
Shell structures that support substantial loads may exhibit a primary mode of failure from localized or general buckling of the support pedestal or skirt due to seismic loads. Such structures may include single pedestal water towers, skirt-supported process vessels, and similar single member towers. Where the structural assessment concludes that buckling of the support is the governing primary mode of failure, structures specified in this standard to be designed to subsections a and b below and those that are assigned as Risk Category IV shall be designed to resist the seismic forces as follows:
  1. The seismic response coefficient for this evaluation shall be in accordance with Section 12.8.1.1 of this standard with IelR set equal to 1.0. Soil-structure and fluid-structure interactions is permitted to be utilized in determining the structural response. Vertical or orthogonal combinations need not be considered.
  2. The resistance of the structure shall be defined as the critical buckling resistance of the element, that is, a factor of safety set equal to 1.0.
Welded steel elevated water storage structures shall be designed and detailed in accordance with the seismic requirements of AWWA D100 with the structural height limits imposed by Table 15.4-2.
Concrete pedestal (composite) elevated water storage structures shall be designed in accordance with the requirements of ACI 371R except that the design input forces shall be modified as follows:
    In Eq. 4-8a of ACI 371R,
    For Ts < T ≤ 2.5 s, replace the term with
(15.7-24)
    In Eq. 4-8b of ACI 371R, replace the term with
(15.7-25)
In Eq. 4-9 of ACI 371R, replace the term 0.5Ca with

                    0.2SDS         (15.7-26)
The equivalent lateral force procedure is permitted for all concrete pedestal tanks and shall be based on a fixed-base, single degree-of-freedom model. All mass, including the liquid, shall be considered rigid unless the sloshing mechanism (i.e., the percentage of convective mass and centroid) is determined for the specific configuration of the container by detailed fluid-structure interaction analysis or testing. Soil-structure interaction is permitted to be included. A more rigorous analysis is permitted.
The fundamental period of vibration of the structure shall be established using the uncracked structural properties and deformational characteristics of the resisting elements in a properly substantiated analysis. The period used to calculate the seismic response coefficient shall not exceed 2.5 s.
Attachments to the pressure boundary, supports, and seismic force-resisting anchorage systems for boilers and pressure vessels shall be designed to meet the force and displacement requirements of Section 15.3 or 15.4 and the additional requirements of this section. Boilers and pressure vessels categorized as Risk Categories III or IV shall be designed to meet the force and displacement requirements of Section 15.3 or 15.4.
Boilers or pressure vessels designed and constructed in accordance with ASME BPVC shall be deemed to meet the requirements of this section provided that the force and displacement requirements of Section 15.3 or 15.4 are used with appropriate scaling of the force and displacement requirements to the working stress design basis.
Attachments to the pressure boundary for internal and external ancillary components (refractory, cyclones, trays, etc.) shall be designed to resist the seismic forces specified in this standard to safeguard against rupture of the pressure boundary. Alternatively, the element attached is permitted to be designed to fail prior to damaging the pressure boundary provided that the consequences of the failure do not place the pressure boundary in jeopardy. For boilers or vessels containing liquids, the effect of sloshing on the internal equipment shall be considered if the equipment can damage the integrity of the pressure boundary.
Where the mass of the operating vessel or vessels supported is greater than 25 percent of the total mass of the combined structure, the structure and vessel designs shall consider the effects of dynamic coupling between each other. Coupling with adjacent, connected structures such as multiple towers shall be considered if the structures are interconnected with elements that will transfer loads from one structure to the other.
Fluid-structure interaction (sloshing) shall be considered in determining the effective mass of the stored material providing sufficient liquid surface exists for sloshing to occur and the Tc is greater than 3T. Changes to or variations in material density with pressure and temperature shall be considered.
Boilers and pressure vessels designated Risk Category IV, but not designed and constructed in accordance with the requirements of ASME BPVC, shall meet the following requirements:
    The seismic loads in combination with other service loads and appropriate environmental effects shall not exceed the material strength shown in Table 15.7-4.
    Consideration shall be made to mitigate seismic impact loads for boiler or vessel elements constructed of nonductile materials or vessels operated in such a way that material ductility is reduced (e.g., low temperature applications).
Table 15.7-4 Maximum Material Strength
Material Minimum
Ratio FulFy
Max. Material
Strength Vessel
Material
Max. Material
Strength Threaded
Materiala
Ductile (e.g., steel,
aluminum, copper)
1.33b 90%d 70%d
Semiductile
1.2c 70%d 50%d
Nonductile (e.g., cast
iron, ceramics, fiberglass)
NA 25%e 20%e


aThreaded connection to vessel or support system.
bMinimum 20% elongation per the ASTM material specification.
dBased on material minimum specified yield strength.
cMinimum 15% elongation per the ASTM material specification.
eBased on material minimum specified tensile strength.
Attachments to the pressure boundary and support for boilers and pressure vessels shall meet the following requirements:
  1. Attachments and supports transferring seismic loads shall be constructed of ductile materials suitable for the intended application and environmental conditions.
  2. Anchorage shall be in accordance with Section 15.4.9, whereby the anchor embedment into the concrete is designed to develop the steel strength of the anchor in tension. The steel strength of the anchor in tension shall be determined in accordance with ACI 318 Appendix D Eq. D-3. The anchor shall have a minimum gauge length of eight diameters. The load combinations with overstrength of Section 12.4.3 are not to be used to size the anchor bolts for tanks and horizontal and vertical vessels.
  3. Seismic supports and attachments to structures shall be designed and constructed so that the support or attachment remains ductile throughout the range of reversing seismic lateral loads and displacements.
  4. Vessel attachments shall consider the potential effect on the vessel and the support for uneven vertical reactions based on variations in relative stiffness of the support members, dissimilar details, nonuniform shimming, or irregular supports. Uneven distribution of lateral forces shall consider the relative distribution of the resisting elements, the behavior of the connection details, and vessel shear distribution.
    The requirements of Sections 15.4 and 15.7.10.5 shall also be applicable to this section.
Attachments to the pressure or liquid boundary, supports, and seismic force-resisting anchorage systems for liquid and gas spheres shall be designed to meet the force and displacement requirements of Section 15.3 or 15.4 and the additional requirements of this section. Spheres categorized as Risk Category III or IV shall themselves be designed to meet the force and displacement requirements of Section 15.3 or 15.4.
Spheres designed and constructed in accordance with Section VIII of ASME BPVC shall be deemed to meet the requirements of this section providing the force and displacement requirements of Section 15.3 or 15.4 are used with appropriate scaling of the force and displacement requirements to the working stress design basis.
Attachments to the pressure or liquid boundary for internal and external ancillary components (refractory, cyclones, trays, etc.) shall be designed to resist the seismic forces specified in this standard to safeguard against rupture of the pressure boundary. Alternatively, the element attached to the sphere could be designed to fail prior to damaging the pressure or liquid boundary providing the consequences of the failure does not place the pressure boundary in jeopardy. For spheres containing liquids, the effect of sloshing on the internal equipment shall be considered if the equipment can damage the pressure boundary.
Fluid-structure interaction (sloshing) shall be considered in determining the effective mass of the stored material providing sufficient liquid surface exists for sloshing to occur and the Tc is greater than 3T. Changes to or variations in fluid density shall be considered.
For post supported spheres that are cross-braced:
  1. The requirements of Section 15.7.10.4 shall also be applicable to this section.
  2. The stiffening effect (reduction in lateral drift) from pretensioning of the bracing shall be considered in determining the natural period.
  3. The slenderness and local buckling of the posts shall be considered.
  4. Local buckling of the sphere shell at the post attachment shall be considered.
  5. For spheres storing liquids, bracing connections shall be designed and constructed to develop the minimum published yield strength of the brace. For spheres storing gas vapors only, bracing connection shall be designed for Ω0 times the maximum design load in the brace. Lateral bracing connections directly attached to the pressure or liquid boundary are prohibited.
For skirt-supported spheres, the following requirements shall apply:
  1. The requirements of Section 15.7.10.5 shall also apply.
  2. The local buckling of the skirt under compressive membrane forces due to axial load and bending moments shall be considered.
  3. Penetration of the skirt support (manholes, piping, etc.) shall be designed and constructed to maintain the strength of the skirt without penetrations.
Tanks and facilities for the storage of liquefied hydrocarbons and refrigerated liquids shall meet the requirements of this standard. Low-pressure welded steel storage tanks for liquefied hydrocarbon gas (e.g., LPG, butane, etc.) and refrigerated liquids (e.g., ammonia) shall be designed in accordance with the requirements of Section 15.7.8 and API 620.
Horizontal vessels supported on saddles (sometimes referred to as "blimps") shall be designed to meet the force and displacement requirements of Section 15.3 or 15.4.
Changes to or variations in material density shall be considered. The design of the supports, saddles, anchorage, and foundation for seismic overturning shall assume the material stored is a rigid mass acting at the volumetric center of gravity.
Unless a more rigorous analysis is performed,
  1. Horizontal vessels with a length-to-diameter ratio of 6 or more are permitted to be assumed to be a simply supported beam spanning between the saddles for determining the natural period of vibration and global bending moment.
  2. For horizontal vessels with a length-to-diameter ratio of less than 6, the effects of "deep beam shear" shall be considered where determining the fundamental period and stress distribution.
  3. Local bending and buckling of the vessel shell at the saddle supports due to seismic load shall be considered. The stabilizing effects of internal pressure shall not be considered to increase the buckling resistance of the vessel shell.
  4. If the vessel is a combination of liquid and gas storage, the vessel and supports shall be designed both with and without gas pressure acting (assume piping has ruptured and pressure does not exist).
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