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

Every seismically isolated structure and every portion thereof shall be designed and constructed in accordance with the requirements of this section and the applicable requirements of this standard.
The analysis of seismically isolated structures, including the substructure, isolators, and superstructure, shall consider variations in seismic isolator material properties over the projected life of the structure, including changes due to aging, contamination, environmental exposure, loading rate, scragging, and temperature.
      DISPLACEMENT:
      Design Displacement: The design earthquake lateral displacement, excluding additional displacement due to actual and accidental torsion, required for design of the isolation system.
      Total Design Displacement: The design earthquake lateral displacement, including additional displacement due to actual and accidental torsion, required for design of the isolation system or an element thereof.
      Total Maximum Displacement: The maximum considered earthquake lateral displacement, including additional displacement due to actual and accidental torsion, required for verification of the stability of the isolation system or elements thereof, design of structure separations, and vertical load testing of isolator unit prototypes.
      DISPLACEMENT RESTRAINT SYSTEM: A collection of structural elements that limits lateral displacement of seismically isolated structures due to the maximum considered earthquake.
      EFFECTIVE DAMPING: The value of equivalent viscous damping corresponding to energy dissipated during cyclic response of the isolation system.
      EFFECTIVE STIFFNESS: The value of the lateral force in the isolation system, or an element thereof, divided by the corresponding lateral displacement.
      ISOLATION INTERFACE: The boundary between the upper portion of the structure, which is isolated, and the lower portion of the structure, which moves rigidly with the ground.
      ISOLATION SYSTEM: The collection of structural elements that includes all individual isolator units, all structural elements that transfer force between elements of the isolation system, and all connections to other structural elements. The isolation system also includes the wind-restraint system, energy-dissipation devices, and/or the displacement restraint system if such systems and devices are used to meet the design requirements of this chapter.
      ISOLATOR UNIT: A horizontally flexible and vertically stiff structural element of the isolation system that permits large lateral deformations under design seismic load. An isolator unit is permitted to be used either as part of, or in addition to, the weight-supporting system of the structure.
      MAXIMUM DISPLACEMENT: The maximum considered earthquake lateral displacement, excluding additional displacement due to actual and accidental torsion.
      SCRAGGING: Cyclic loading or working of rubber products, including elastomeric isolators, to effect a reduction in stiffness properties, a portion of which will be recovered over time.
      WIND-RESTRAINT SYSTEM: The collection of structural elements that provides restraint of the seismically isolated structure for wind loads. The wind-restraint system is permitted to be either an integral part of isolator units or a separate device.
        BD = numerical coefficient as set forth in Table 17.5-1 for effective damping equal to βD
        BM = numerical coefficient as set forth in Table 17.5-1 for effective damping equal to βM
          b = shortest plan dimension of the structure, in ft (mm) measured perpendicular to d
        DD

=

design displacement, in in. (mm), at the center of rigidity of the isolation system in the direction under consideration, as prescribed by Eq. 17.5-1
       D'D

=

design displacement, in in. (mm), at the center of rigidity of the isolation system in the direction under rigidity of the isolation system in the direction under consideration, as prescribed by Eq. 17.6-1
        DM = maximum displacement, in in. (mm), at the center of rigidity of the isolation system in the direction under consideration, as prescribed by Eq. 17.5- 3
       D'M = maximum displacement, in in. (mm), at the center of rigidity of the isolation system in the direction under consideration, as prescribed by Eq. 17.6- 2
       DTD = total design displacement, in in. (mm), of an element of the isolation system including both translational displacement at the center of rigidity and the component of torsional displacement in the direction under consideration, as prescribed by Eq. 17.5-5
       DTM = total maximum displacement, in in. (mm), of an element of the isolation system, including both translational displacement at the center of rigidity and the component of torsional displacement in the direction under consideration, as prescribed by Eq. 17.5-6
          d = longest plan dimension of the structure, in ft (mm)
     Eloop = energy dissipated in kips-in. (kN-mm), in an isolator unit during a full cycle of reversible load over a test displacement range from Δ+ to Δ-, as measured by the area enclosed by the loop of the force-deflection curve
          e = actual eccentricity, in ft (mm), measured in plan between the center of mass of the structure above the isolation interface and the center of rigidity of the isolation system, plus accidental eccentricity, in ft. (mm), taken as 5% of the maximum building dimension perpendicular to the direction of force under consideration
         F- = minimum negative force in an isolator unit during a single cycle of prototype testing at a displacement amplitude of Δ-
         F+ = maximum positive force in kips (kN) in an isolator unit during a single cycle of prototype testing at a displacement amplitude of Δ+
         Fx = total force distributed over the height of the structure above the isolation interface as prescribed by Eq. 17.5-9
     kDmax = maximum effective stiffness, in kips/in. (kN/mm), of the isolation system at the design displacement in the horizontal direction under consideration, as prescribed by Eq. 17.8-3
     kDmin = minimum effective stiffness, in kips/in. (kN/mm), of the isolation system at the design displacement in the horizontal direction under consideration, as prescribed by Eq. 17. 8-4
     kMmax = maximum effective stiffness, in kips/in. (kN/mm), of the isolation system at the maximum displacement in the horizontal direction under consideration,as prescribed by Eq. 17.8-5
     kMmin = minimum effective stiffness, in kips/in. (kN/mm), of the isolation system at the maximum displacement in the horizontal direction under consideration, as prescribed by Eq. 17.8-6
        keff = effective stiffness of an isolator unit, as prescribed by Eq. 17.8-1
          L = effect of live load in Chapter 17
         TD = effective period, in s, of the seismically isolated structure at the design displacement in the direction under consideration, as prescribed by Eq.17.5- 2
         TM = effective period, in s, of the seismically isolated structure at the maximum displacement in the direction under consideration, as prescribed by Eq. 17.5- 4
         Vb = total lateral seismic design force or shear on elements of the isolation system or elements below isolation system, as prescribed by Eq. 17.5-7
         Vs = total lateral seismic design force or shear on elements above the isolation system, as prescribed by Eq. 17.5-8
          y = distance, in ft (mm), between the center of rigidity of the isolation system rigidity and the element of interest measured perpendicular to the direction of seismic loading under consideration
        βD = effective damping of the isolation system at the design displacement, as prescribed by Eq.17.8-7
        βM = effective damping of the isolation system at the maximum displacement, as prescribed by Eq.17.8-8
        βeff = effective damping of the isolation system, as prescribed by Eq. 17.8-2
         Δ+ = maximum positive displacement of an isolator unit during each cycle of prototype testing
         Δ- = minimum negative displacement of an isolator unit during each cycle of prototype testing
      ΣED = total energy dissipated, in kips-in. (kN-mm), in the isolation system during a full cycle of response at the design displacement, DD
      ΣEM = total energy dissipated, in kips-in. (kN-mm), in the isolation system during a full cycle of response at the maximum displacement, DM
= sum, for all isolator units, of the maximum absolute value of force, in kips (kN), at a positive displacement equal to DD
= sum, for all isolator units, of the minimum absolute value of force, in kips (kN), at a positive displacement equal to DD
= sum, for all isolator units, of the maximum absolute value of force, in kips (kN), at a negative displacement equal to DD
= sum, for all isolator units, of the minimum absolute value of force, in kips (kN), at a negative displacement equal to DD
= sum, for all isolator units, of the maximum absolute value of force, in kips (kN), at a positive displacement equal to DM
= sum, for all isolator units, of the minimum absolute value of force, in kips (kN), at a positive displacement equal to DM
= sum, for all isolator units, of the maximum absolute value of force, in kips (kN), at a negative displacement equal to DM
= sum, for all isolator units, of the minimum absolute value of force, in kips (kN), at a negative displacement equal to DM
All portions of the structure, including the structure above the isolation system, shall be assigned a risk category in accordance with Table 1.5-1. The importance factor, Ie, shall be taken as 1.0 for a seismically isolated structure, regardless of its risk category assignment.
The MCER spectral response acceleration parameters SMS and SM1 shall be determined in accordance with Section 11.4.3.
Each structure shall be designated as having a structural irregularity based on the structural configuration above the isolation system.
In addition to the requirements for vertical and lateral loads induced by wind and earthquake, the isolation system shall provide for other environmental conditions, including aging effects, creep, fatigue, operating temperature, and exposure to moisture or damaging substances.
Isolated structures shall resist design wind loads at all levels above the isolation interface. At the isolation interface, a wind-restraint system shall be provided to limit lateral displacement in the isolation system to a value equal to that required between floors of the structure above the isolation interface in accordance with Section 17.5.6.
Fire resistance for the isolation system shall meet that required for the columns, walls, or other such gravity-bearing elements in the same region of the structure.
The isolation system shall be configured to produce a restoring force such that the lateral force at the total design displacement is at least 0.025W greater than the lateral force at 50% of the total design displacement.
The isolation system shall not be configured to include a displacement restraint that limits lateral displacement due to the maximum considered earthquake to less than the total maximum displacement unless the seismically isolated structure is designed in accordance with the following criteria where more stringent than the requirements of Section 17.2:
  1. Maximum considered earthquake response is calculated in accordance with the dynamic analysis requirements of Section 17.6, explicitly considering the nonlinear characteristics of the isolation system and the structure above the isolation system.
  2. The ultimate capacity of the isolation system and structural elements below the isolation system shall exceed the strength and displacement demands of the maximum considered earthquake.
  3. The structure above the isolation system is checked for stability and ductility demand of the maximum considered earthquake.
  4. The displacement restraint does not become effective at a displacement less than 0.75 times the total design displacement unless analysis demonstrates that earlier engagement does not result in unsatisfactory performance.
Each element of the isolation system shall be designed to be stable under the design vertical load where subjected to a horizontal displacement equal to the total maximum displacement. The design vertical load shall be computed using load combination 5 of Section 2.3.2 for the maximum vertical load and load combination 7 of Section 12.4.2.3 for the minimum vertical load where SDS in these equations is replaced by SMS. The vertical loads that result from application of horizontal seismic forces, QE, shall be based on peak response due to the maximum considered earthquake.
The factor of safety against global structural overturning at the isolation interface shall not be less than 1.0 for required load combinations. All gravity and seismic loading conditions shall be investigated. Seismic forces for overturning calculations shall be based on the maximum considered earthquake, and W shall be used for the vertical restoring force.
    Local uplift of individual elements shall not be allowed unless the resulting deflections do not cause overstress or instability of the isolator units or other structure elements.
  1. Access for inspection and replacement of all components of the isolation system shall be provided.
  2. A registered design professional shall complete a final series of inspections or observations of structure separation areas and components that cross the isolation interface prior to the issuance of the certificate of occupancy for the seismically isolated structure. Such inspections and observations shall indicate that the conditions allow free and unhindered displacement of the structure to maximum design levels and that all components that cross the isolation interface as installed are able to accommodate the stipulated displacements.
  3. Seismically isolated structures shall have a monitoring, inspection, and maintenance program for the isolation system established by the registered design professional responsible for the design of the isolation system.
  4. Remodeling, repair, or retrofitting at the isolation system interface, including that of components that cross the isolation interface, shall be performed under the direction of a registered design professional.
A quality control testing program for isolator units shall be established by the registered design professional responsible for the structural design.
A horizontal diaphragm or other structural elements shall provide continuity above the isolation interface and shall have adequate strength and ductility to transmit forces (due to nonuniform ground motion) from one part of the structure to another.
Minimum separations between the isolated structure and surrounding retaining walls or other fixed obstructions shall not be less than the total maximum displacement.
Nonbuilding structures shall be designed and constructed in accordance with the requirements of Chapter 15 using design displacements and forces calculated in accordance with Sections 17.5 or 17.6.
Parts or portions of an isolated structure, permanent nonstructural components and the attachments to them, and the attachments for permanent equipment supported by a structure shall be designed to resist seismic forces and displacements as prescribed by this section and the applicable requirements of Chapter 13.
Elements of seismically isolated structures and nonstructural components, or portions thereof, that are at or above the isolation interface shall be designed to resist a total lateral seismic force equal to the maximum dynamic response of the element or component under consideration.
   EXCEPTION: Elements of seismically isolated structures and nonstructural components or portions designed to resist seismic forces and displacements as prescribed in Chapter 12 or 13 as appropriate.
Elements of seismically isolated structures and nonstructural components, or portions thereof, that cross the isolation interface shall be designed to withstand the total maximum displacement.
Elements of seismically isolated structures and nonstructural components, or portions thereof, that are below the isolation interface shall be designed and constructed in accordance with the requirements of Section 12.1 and Chapter 13.
The site-specific ground motion procedures set forth in Chapter 21 are permitted to be used to determine ground motions for any structure. For structures on Site Class F sites, site response analysis shall be performed in accordance with Section 21.1. For seismically isolated structures on sites with S1 greater than or equal to 0.6, a ground motion hazard analysis shall be performed in accordance with Section 21.2. Structures that do not require or use site-specific ground motion procedures shall be analyzed using the design spectrum for the design earthquake developed in accordance with Section 11.4.5.
   A spectrum shall be constructed for the MCER ground motion. The spectrum for MCER ground motions shall not be taken as less than 1.5 times the spectrum for the design earthquake ground motions.
Where response-history procedures are used, ground motions shall consist of pairs of appropriate horizontal ground motion acceleration components developed per Section 16.1.3.2 except that 0.2T and 1.5T shall be replaced by 0.5TD and 1.25TM, respectively, where TD and TM are defined in Section 17.5.3.
Seismically isolated structures except those defined in Section 17.4.1 shall be designed using the dynamic procedures of Section 17.6.
The equivalent lateral force procedure of Section 17.5 is permitted to be used for design of a seismically isolated structure provided that
  1. The structure is located at a site with S1 less than 0.60g.
  2. The structure is located on a Site Class A, B, C, or D.
  3. The structure above the isolation interface is less than or equal to four stories or 65 ft (19.8 m) in structural height, hn.
  4. The effective period of the isolated structure at the maximum displacement, TM, is less than or equal to 3.0 s.
  5. The effective period of the isolated structure at the design displacement, TD, is greater than three times the elastic, fixed-base period of the structure above the isolation system as determined by Eq. 12.8-7 or 12.8-8.
  6. The structure above the isolation system is of regular configuration.
  7. The isolation system meets all of the following criteria:
    1. The effective stiffness of the isolation system at the design displacement is greater than one-third of the effective stiffness at 20% of the design displacement.
    2. The isolation system is capable of producing a restoring force as specified in Section 17.2.4.4.
    3. The isolation system does not limit maximum considered earthquake displacement to less than the total maximum displacement.
The dynamic procedures of Section 17.6 are permitted to be used as specified in this section.
Response-spectrum analysis shall not be used for design of a seismically isolated structure unless
  1. The structure is located on a Site Class A, B, C, or D.
  2. The isolation system meets the criteria of Item 7 of Section 17.4.1.
The response-history procedure is permitted for design of any seismically isolated structure and shall be used for design of all seismically isolated structures not meeting the criteria of Section 17.4.2.1.
Where the equivalent lateral force procedure is used to design seismically isolated structures, the requirements of this section shall apply.
Minimum lateral earthquake design displacements and forces on seismically isolated structures shall be based on the deformation characteristics of the isolation system. The deformation characteristics of the isolation system shall explicitly include the effects of the wind-restraint system if such a system is used to meet the design requirements of this standard. The deformation characteristics of the isolation system shall be based on properly substantiated tests performed in accordance with Section 17.8.
The isolation system shall be designed and constructed to withstand minimum lateral earthquake displacements, DD, that act in the direction of each of the main horizontal axes of the structure using Eq. 17.5-1:

(17.5-1)
where

   g = acceleration due to gravity. The units for g are in./s2 (mm/s2) if the units of the design displacement, DD, are in. (mm)
SD1 = design 5% damped spectral acceleration parameter at 1-s period in units of g-s, as determined in Section 11.4.4
TD = effective period of the seismically isolated structure in seconds, at the design displacement in the direction under consideration, as prescribed by Eq. 17.5-2
BD = numerical coefficient related to the effective damping of the isolation system at the design displacement, βD, as set forth in Table 17.5-1
The effective period of the isolated structure at design displacement, TD, shall be determined using the deformational characteristics of the isolation system and Eq. 17.5-2:
(17 .5-2)
where
   W = effective seismic weight of the structure above the isolation interface as defined in Section 12.7.2
kDmin = minimum effective stiffness in kips/in. (kN/rnm) of the isolation system at the design displacement in the horizontal direction under consideration, as prescribed by Eq. 17.8-4
   g = acceleration due to gravity
The maximum displacement of the isolation system, DM, in the most critical direction of horizontal response shall be calculated using Eq. 17.5-3:
(17.5-3)
where
   g = acceleration of gravity
SM1 = maximum considered earthquake 5% damped spectral acceleration parameter at 1-s period, in units of g-s, as determined in Section 11.4.3
TM = effective period, in seconds, of the seismically isolated structure at the maximum displacement in the direction under consideration, as prescribed by Eq. 17.5-4
BM = numerical coefficient related to the effective damping of the isolation system at the maximum displacement, βM, as set forth in Table 17.5-1

Table 17.5-1 Damping Coefficient, BD or BM

Effective Damping, βD or βM (percentage of critical)a,b BD or BM Factor
≤2
5
10
20
30
40
≥50
0.8
1.0
1.2
1.5
1.7
1.9
2.0
aThe damping coefficient shall be based on the effective damping of the isolation system determined in accordance with the requirements of Section 17.8.5.2.
bThe damping coefficient shall be based on linear interpolation for effective damping values other than those given.
The effective period of the isolated structure at maximum displacement, TM, shall be determined using the deformational characteristics of the isolation system and Eq. 17.5-4:
(17.5-4)
where
    W = effective seismic weight of the structure above the isolation interface as defined in Section 12.7.2 (kip or kN)
kMmin = minimum effective stiffness, in kips/in. (kN/mm), of the isolation system at the maximum displacement in the horizontal direction under consideration, as prescribed by Eq. 17.8-6
    g = the acceleration of gravity

The total design displacement, DTD, and the total maximum displacement, DTM, of elements of the isolation system shall include additional displacement due to actual and accidental torsion calculated from the spatial distribution of the lateral stiffness of the isolation system and the most disadvantageous location of eccentric mass.
    The total design displacement, DTD, and the total maximum displacement, DTM, of elements of an isolation system with uniform spatial distribution of lateral stiffness shall not be taken as less than that prescribed by Eqs. 17.5-5 and 17.5-6:
(17.5-5)
 (17.5-6)
where
DD = design displacement at the center of rigidity of the isolation system in the direction under consideration as prescribed by Eq. 17.5-1
DM = maximum displacement at the center of rigidity of the isolation system in the direction under consideration as prescribed by Eq. 17.5-3
  y = the distance between the centers of rigidity of the isolation system and the element of interest measured perpendicular to the direction of seismic loading under consideration
  e = the actual eccentricity measured in plan between the center of mass of the structure above the isolation interface and the center of rigidity of the isolation system, plus accidental eccentricity, in ft (mm), taken as 5% of the longest plan dimension of the structure perpendicular to the direction of force under consideration
  b = the shortest plan dimension of the structure measured perpendicular to d
  d = the longest plan dimension of the structure

   EXCEPTION: The total design displacement, DTD, and the total maximum displacement, DTM, are permitted to be taken as less than the value prescribed by Eqs. 17.5-5 and 17.5-6, respectively, but not less than 1.1 times DD and DM, respectively, provided the isolation system is shown by calculation to be configured to resist torsion accordingly.
The isolation system, the foundation, and all structural elements below the isolation system shall be designed and constructed to withstand a minimum lateral seismic force, Vb, using all of the appropriate requirements for a nonisolated structure and as prescribed by Eq. 17.5-7:
(17.5-7)
where
kDmax = maximum effective stiffness, in kips/in. (kN/mm), of the isolation system at the design displacement in the horizontal direction under consideration as prescribed by Eq. 17.8-3
DD = design displacement, in in. (mm), at the center of rigidity of the isolation system in the direction under consideration, as prescribed by Eq. 17.5-1

    Vb shall not be taken as less than the maximum force in the isolation system at any displacement up to and including the design displacement.
The structure above the isolation system shall be designed and constructed to withstand a minimum shear force, Vs, using all of the appropriate requirements for a nonisolated structure and as prescribed by Eq. 17.5-8:
(17.5-8)
where
kDmax = maximum effective stiffness, in kips/in. (kN/mm), of the isolation system at the design displacement in the horizontal direction under consideration
DD = design displacement, in in. (mm), at the center of rigidity of the isolation system in the direction under consideration, as prescribed by Eq. 17.5-1
RI = numerical coefficient related to the type of seismic forceresisting system above the isolation system

The RI factor shall be based on the type of seismic force-resisting system used for the structure above the isolation system and shall be three-eighths of the value of R given in Table 12.2-1, Table 15.4-1, or Table 15.4-2, as appropriate, with a maximum value not greater than 2.0 and a minimum value not less than 1.0.
The value of Vs shall not be taken as less than the following:
  1. The lateral seismic force required by Section 12.8 for a fixed-base structure of the same effective seismic weight, W, and a period equal to the isolated period, TD.
  2. The base shear corresponding to the factored design wind load.
  3. The lateral seismic force required to fully activate the isolation system (e.g., the yield level of a softening system, the ultimate capacity of a sacrificial wind-restraint system, or the break-away friction level of a sliding system) multiplied by 1.5.
The shear force Vs shall be distributed over the height of the structure above the isolation interface using Eq. 17.5-9:
(17.5-9)
where
Fx = portion of Vs that is assigned to level x
Vx = total lateral seismic design force or shear on elements above the isolation system as prescribed by Eq. 17.5-8
wx = portion of W that is located at or assigned to level i, n, or x, respectively
hx = height above the base of level i, n, or x, respectively

   At each level designated as x, the force, Fx, shall be applied over the area of the structure in accordance with the mass distribution at the level.
The maximum story drift of the structure above the isolation system shall not exceed 0.015hsx. The drift shall be calculated by Eq. 12.8-15 with Cd for the isolated structure equal to RI, as defined in Section 17.5.4.2.
Where dynamic analysis is used to design seismically isolated structures, the requirements of this section shall apply.
The mathematical models of the isolated structure including the isolation system, the seismic force-resisting system, and other structural elements shall conform to Section 12.7.3 and to the requirements of Sections 17.6.2.1 and 17.6.2.2.
The isolation system shall be modeled using deformational characteristics developed and verified by test in accordance with the requirements of Section 17.5.2. The isolation system shall be modeled with sufficient detail to
  1. Account for the spatial distribution of isolator units;
  2. Calculate translation, in both horizontal directions, and torsion of the structure above the isolation interface considering the most disadvantageous location of eccentric mass;
  3. Assess overturning/uplift forces on individual isolator units; and
  4. Account for the effects of vertical load, bilateral load, and/ or the rate of loading if the force-deflection properties of the isolation system depend on one or more of these attributes.
   The total design displacement and total maximum displacement across the isolation system shall be calculated using a model of the isolated structure that incorporates the force-deflection characteristics of nonlinear elements of the isolation system and the seismic force-resisting system.
The maximum displacement of each floor and design forces and displacements in elements of the seismic force-resisting system are permitted to be calculated using a linear elastic model of the isolated structure provided that both of the following conditions are met:
  1. Stiffness properties assumed for the nonlinear components of the isolation system are based on the maximum effective stiffness of the isolation system; and
  2. All elements of the seismic force-resisting system of the structure above the isolation system remain elastic for the design earthquake.
   Seismic force-resisting systems with elastic elements include, but are not limited to, irregular structural systems designed for a lateral force not less than 100% of Vs and regular structural systems designed for a lateral force not less than 80% of Vs, where Vs is determined in accordance with Section 17.5.4.2.
Response-spectrum and response-history procedures shall be performed in accordance with Section 12.9 and Chapter 16 and the requirements of this section.
The design earthquake ground motions shall be used to calculate the total design displacement of the isolation system and the lateral forces and displacements in the isolated structure. The maximum considered earthquake shall be used to calculate the total maximum displacement of the isolation system.
Response-spectrum analysis shall be performed using a modal damping value for the fundamental mode in the direction of interest not greater than the effective damping of the isolation system or 30% of critical, whichever is less. Modal damping values for higher modes shall be selected consistent with those that would be appropriate for response-spectrum analysis of the structure above the isolation system assuming a fixed base.
   Response-spectrum analysis used to determine the total design displacement and the total maximum displacement shall include simultaneous excitation of the model by 100% of the ground motion in the critical direction and 30% of the ground motion in the perpendicular, horizontal direction. The maximum displacement of the isolation system shall be calculated as the vectorial sum of the two orthogonal displacements.
   The design shear at any story shall not be less than the story shear resulting from application of the story forces calculated using Eq. 17.5-9 and a value of Vs equal to the base shear obtained from the response-spectrum analysis in the direction of interest.
Where a response-history procedure is performed, a suite of not fewer than three pairs of appropriate ground motions shall be used in the analysis; the ground motion pairs shall be selected and scaled in accordance with Section 17.3.2.
   Each pair of ground motion components shall be applied simultaneously to the model considering the most disadvantageous location of eccentric mass. The maximum displacement of the isolation system shall be calculated from the vectorial sum of the two orthogonal displacements at each time step.
   The parameters of interest shall be calculated for each ground motion used for the response-history analysis. If seven or more pairs of ground motions are used for the response-history analysis, the average value of the response parameter of interest is permitted to be used for design. If fewer than seven pairs of ground motions are used for analysis, the maximum value of the response parameter of interest shall be used for design.
The isolation system, foundation, and all structural elements below the isolation system shall be designed using all of the appropriate requirements for a nonisolated structure and the forces obtained from the dynamic analysis without reduction, but the design lateral force shall not be taken as less than 90% of Vb determined in accordance as prescribed by Eq. 17.5-7.
   The total design displacement of the isolation system shall not be taken as less than 90% of DTD as specified by Section 17.5.3.5. The total maximum displacement of the isolation system shall not be taken as less than 80% of DTM as prescribed by Section 17.5.3.5.
   The limits on displacements specified by this section shall be evaluated using values of DTD and DTM determined in accordance with Section 17.5.3.5 except that D'D is permitted to be used in lieu of DD, and D'M is permitted to be used in lieu of DM as prescribed in Eqs. 17.6-1 and 17.6-2:
  (17.6-1)
(17.6-2)
where
DD = design displacement, in in. (mm), at the center of rigidity of the isolation system in the direction under consideration, as prescribed by Eq. 17.5-1
DM = maximum displacement in in. (mm), at the center of rigidity of the isolation system in the direction under consideration, as prescribed by Eq. 17.5-3
  T = elastic, fixed-base period of the structure above the isolation system as determined by Section 12.8.2
TD = effective period of seismically isolated structure in s, at the design displacement in the direction under consideration, as prescribed by Eq. 17.5-2
TM = effective period, in s, of the seismically isolated structure, at the maximum displacement in the direction under consideration, as prescribed by Eq. 17.5-4

Subject to the procedure-specific limits of this section, structural elements above the isolation system shall be designed using the appropriate requirements for a nonisolated structure and the forces obtained from the dynamic analysis reduced by a factor of RI, as determined in accordance with Section 17.5.4.2. The design lateral shear force on the structure above the isolation system, if regular in configuration, shall not be taken as less than 80% of Vs, or less than the limits specified by Section 17.5.4.3.
   EXCEPTION: The lateral shear force on the structure above the isolation system, if regular in configuration, is permitted to be taken as less than 80%, but shall not be less than 60% of Vs, where the response-history procedure is used for analysis of the seismically isolated structure.
   The design lateral shear force on the structure above the isolation system, if irregular in configuration, shall not be taken as less than Vs or less than the limits specified by Section 17.5.4.3.
   EXCEPTION: The design lateral shear force on the structure above the isolation system, if irregular in configuration, is permitted to be taken as less than 100%, but shall not be less than 80% of Vs, where the response-history procedure is used for analysis of the seismically isolated structure.
Where the factored lateral shear force on structural elements, determined using either response-spectrum or response-history procedure, is less than the minimum values prescribed by Sections 17.6.4.1 and 17.6.4.2, all response parameters, including member forces and moments, shall be adjusted upward proportionally.
Maximum story drift corresponding to the design lateral force including displacement due to vertical deformation of the isolation system shall not exceed the following limits:
  1. The maximum story drift of the structure above the isolation system calculated by response-spectrum analysis shall not exceed 0.015hsx
  2. The maximum story drift of the structure above the isolation system calculated by response-history analysis based on the force-deflection characteristics of nonlinear elements of the seismic force-resisting system shall not exceed 0.020hsx.
   Drift shall be calculated using Eq. 12.8-15 with the Cd of the isolated structure equal to RI, as defined in Section 17.5.4.2.
   The secondary effects of the maximum considered earthquake lateral displacement of the structure above the isolation system combined with gravity forces shall be investigated if the story drift ratio exceeds 0.010/RI, .
A design review of the isolation system and related test programs shall be performed by an independent engineering team, including persons licensed in the appropriate disciplines and experienced in seismic analysis methods and the theory and application of seismic isolation. Isolation system design review shall include, but not be limited to, the following:
  1. Review of site-specific seismic criteria including the development of site-specific spectra and ground motion histories and all other design criteria developed specifically for the project.
  2. Review of the preliminary design including the determination of the total design displacement, the total maximum displacement, and the lateral force level.
  3. Overview and observation of prototype testing (Section 17.8).
  4. Review of the final design of the entire structural system and all supporting analyses.
  5. Review of the isolation system quality control testing program (Section 17.2.4.9).
The deformation characteristics and damping values of the isolation system used in the design and analysis of seismically isolated structures shall be based on tests of a selected sample of the components prior to construction as described in this section.
   The isolation system components to be tested shall include the wind-restraint system if such a system is used in the design.
   The tests specified in this section are for establishing and validating the design properties of the isolation system and shall not be considered as satisfying the manufacturing quality control tests of Section 17.2.4.9.
Prototype tests shall be performed separately on two full-size specimens (or sets of specimens, as appropriate) of each predominant type and size of isolator unit of the isolation system. The test specimens shall include the wind-restraint system and individual isolator units if such systems are used in the design. Specimens tested shall not be used for construction unless accepted by the registered design professional responsible for the design of the structure and approved by the authority having jurisdiction.
For each cycle of each test, the force-deflection and hysteretic behavior of the test specimen shall be recorded.
The following sequence of tests shall be performed for the prescribed number of cycles at a vertical load equal to the average dead load plus one-half the effects due to live load on all isolator units of a common type and size:
  1. Twenty fully reversed cycles of loading at a lateral force corresponding to the wind design force.
  2. Three fully reversed cycles of loading at each of the following increments of the total design displacement–0.25DD, 0.5DD, 1.0DD, and 1.0DM where DD and DM are as determined in Sections 17.5.3.1 and 17.5.3.3, respectively, or Section 17.6 as appropriate.
  3. Three fully reversed cycles of loading at the total maximum displacement, 1.0DTM.
  4. 30SD1/SDSBD, but not less than 10, fully reversed cycles of loading at 1.0 times the total design displacement, 1.0DTD.
   If an isolator unit is also a vertical-load-carrying element, then item 2 of the sequence of cyclic tests specified in the preceding text shall be performed for two additional vertical load cases specified in Section 17.2.4.6. The load increment due to earthquake overturning, QE, shall be equal to or greater than the peak earthquake vertical force response corresponding to the test displacement being evaluated. In these tests, the combined vertical load shall be taken as the typical or average downward force on all isolator units of a common type and size.
If the force-deflection properties of the isolator units depend on the rate of loading, each set of tests specified in Section 17.8.2.2 shall be performed dynamically at a frequency equal to the inverse of the effective period, TD.
   If reduced-scale prototype specimens are used to quantify rate-dependent properties of isolators, the reduced-scale prototype specimens shall be of the same type and material and be manufactured with the same processes and quality as full-scale prototypes and shall be tested at a frequency that represents fullscale prototype loading rates.
   The force-deflection properties of an isolator unit shall be considered to be dependent on the rate of loading if the measured property (effective stiffness or effective damping) at the design displacement when tested at any frequency in the range of 0.1 to 2.0 times the inverse of TD is different from the property when tested at a frequency equal to the inverse of TD by more than 15%.
If the force-deflection properties of the isolator units are dependent on bilateral load, the tests specified in Sections 17.8.2.2 and 17.8.2.3 shall be augmented to include bilateral load at the following increments of the total design displacement, DTD: 0.25 and 1.0, 0.5 and 1.0, 0.75 and 1.0, and 1.0 and 1.0
   If reduced-scale prototype specimens are used to quantify bilateral-load-dependent properties, the reduced-scale specimens shall be of the same type and material and manufactured with the same processes and quality as full-scale prototypes.
   The force-deflection properties of an isolator unit shall be considered to be dependent on bilateral load if the effective stiffness where subjected to bilateral loading is different from the effective stiffness where subjected to unilateral loading, by more than 15%.
Isolator units that carry vertical load shall be statically tested for maximum and minimum downward vertical load at the total maximum displacement. In these tests, the combined vertical loads shall be taken as specified in Section 17.2.4.6 on any one isolator of a common type and size. The dead load, D, and live load, L, are specified in Section 12.4. The seismic load E is given by Eqs. 12.4-1 and 12.4-2 where SDS in these equations is replaced by SMS and the vertical loads that result from application of horizontal seismic forces, QE, shall be based on the peak response due to the maximum considered earthquake.
If a sacrificial wind-restraint system is to be utilized, its ultimate capacity shall be established by test.
Prototype tests are not required if an isolator unit is of similar size and of the same type and material as a prototype isolator unit that has been previously tested using the specified sequence of tests.
The force-deflection characteristics of the isolation system shall be based on the cyclic load tests of prototype isolator specified in Section 17.8.2.
    As required, the effective stiffness of an isolator unit, keff, shall be calculated for each cycle of loading as prescribed by Eq. 17.8-1:

       
         (17.8-1)
where F+ and F- are the positive and negative forces, at △+ and △-, respectively.
    As required, the effective damping, βeff, of an isolator unit shall be calculated for each cycle of loading by Eq. 17.8-2:

(17.8-2)
where the energy dissipated per cycle of loading, Eloop, and the effective stiffness, keff, shall be based on peak test displacements of △+ and △-.
The performance of the test specimens shall be deemed adequate if the following conditions are satisfied:
  1. The force-deflection plots for all tests specified in Section 17.8.2 have a positive incremental force-resisting capacity.
  2. For each increment of test displacement specified in item 2 of Section 17.8.2.2 and for each vertical load case specified in Section 17.8.2.2,
    1. For each test specimen, the difference between the effective stiffness at each of the three cycles of test and the average value of effective stiffness is no greater than 15%.
    2. For each cycle of test, the difference between effective stiffness of the two test specimens of a common type and size of the isolator unit and the average effective stiffness is no greater than 15%.
  3. For each specimen there is no greater than a 20% change in the initial effective stiffness over the cycles of test specified in item 4 of Section 17.8.2.2.
  4. For each specimen there is no greater than a 20% decrease in the initial effective damping over the cycles of test specified in item 4 of Section 17.8.2.2.
  5. All specimens of vertical-load-carrying elements of the isolation system remain stable where tested in accordance with Section 17.8.2.5.
At the design displacement, the maximum and minimum effective stiffness of the isolated system, kDmax and kDmin, shall be based on the cyclic tests of item 2 of Section 17.8.2.2 and calculated using Eqs. 17.8-3 and 17.8-4:

(17.8-3)

  (17.8-4)
    At the maximum displacement, the maximum and minimum effective stiffness of the isolation system, kMmax and kMmin, shall be based on the cyclic tests of item 3 of Section 17.8.2.2 and calculated using Eqs. 17.8-5 and 17.8-6:

  (17.8-5)
        (17.8-6)
    The maximum effective stiffness of the isolation system, kDmax ( or kMmax), shall be based on forces from the cycle of prototype testing at a test displacement equal to DD (or DM) that produces the largest value of effective stiffness. Minimum effective stiffness of the isolation system, kDmin ( or kMmin), shall be based on forces from the cycle of prototype testing at a test displacement equal to DD (or DM) that produces the smallest value of effective stiffness.
    For isolator units that are found by the tests of Sections 17.8.2.2, 17.8.2.3, and 17.8.2.4 to have force-deflection characteristics that vary with vertical load, rate of loading, or bilateral load, respectively, the values of kDmax and kMmax shall be increased and the values of kDmin and kMmin shall be decreased, as necessary, to bound the effects of measured variation in effective stiffness.
At the design displacement, the effective damping of the isolation system, βD, shall be based on the cyclic tests of item 2 of Section 17.8.2.2 and calculated using Eq. 17.8-7:
(17.8-7)
In Eq. 17.8-7, the total energy dissipated per cycle of design displacement response, ΣED, shall be taken as the sum of the energy dissipated per cycle in all isolator units measured at a test displacement equal to DD and shall be based on forces and deflections from the cycle of prototype testing at test displacement DD that produces the smallest values of effective damping.
   At the maximum displacement, the effective damping of the isolation system, βM, shall be based on the cyclic tests of item 2 of Section 17.8.2.2 and calculated using Eq. 17.8-8:
(17.8-8)
In Eq. 17.8-8, the total energy dissipated per cycle of design displacement response, ΣEM, shall be taken as the sum of the energy dissipated per cycle in all isolator units measured at a test displacement equal to DM and shall be based on forces and deflections from the cycle of prototype testing at test displacement DM that produces the smallest value of effective damping.
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