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

If the option to incorporate the effects of soil-structure interaction is exercised, the requirements of this section are permitted to be used in the determination of the design earthquake forces and the corresponding displacements of the structure if the model used for structural response analysis does not directly incorporate the effects of foundation flexibility (i.e., the model corresponds to a fixed-based condition with no foundation springs). The provisions in this section shall not be used if a flexible-base foundation is included in the structural response model.
   The provisions for use with the equivalent lateral force procedure are given in Section 19.2, and those for use with the modal analysis procedure are given in Section 19.3.
The following requirements are supplementary to those presented in Section 12.8.
To account for the effects of soil-structure interaction, the base shear (V) determined from Eq. 12.8-1 shall be reduced to

= V - ΔV (19.2-1)

The reduction (ΔV) shall be computed as follows and shall not exceed 0.3V:

(19.2-2)
where
CS

=

the seismic design coefficient computed from Eqs. 12.8-2, 12.8-3, and 12.8-4 using the fundamental natural period of the fixed-base structure (T or Ta) as specified in Section 12.8.2
 

=

the value of CS computed from Eqs. 12.8-2, 12.8-3, and 12.8-4 using the fundamental natural period of the flexibly supported structure () defined in Section 19.2.1.1
 β̃

=

the fraction of critical damping for the structure-foundation system determined in Section 19.2.1.2



=


the effective seismic weight of the structure, which shall be taken as 0.7W, except for structures where the effective seismic weight is concentrated at a single level, it shall be taken as equal to W
The effective period () shall be determined as follows:

               (19.2-3)
where
T = the fundamental period of the structure as determined in Section 12.8.2
= the stiffness of the structure where fixed at the base, defined by the following:
             (19.2-4)
where
 


=


the effective height of the structure, which shall be taken as 0.7 times the structural height (hn), except for structures where the gravity load is effectively concentrated at a single level, the effective height of the structure shall be taken as the height to that level
Ky


=


the lateral stiffness of the foundation defined as the horizontal force at the level of the foundation necessary to produce a unit deflection at that level, the force and the deflection being measured in the direction in which the structure is analyzed
Kθ


=


the rocking stiffness of the foundation defined as the moment necessary to produce a unit average rotation of the foundation, the moment and rotation being measured in the direction in which the structure is analyzed
 g = the acceleration of gravity
      The foundation stiffnesses (Ky and Kθ) shall be computed by established principles of foundation mechanics using soil properties that are compatible with the soil strain levels associated with the design earthquake motion. The average shear modulus (G) for the soils beneath the foundation at large strain levels and the associated shear wave velocity (vs) needed in these computations shall be determined from Table 19.2-1 where
vso

=

the average shear wave velocity for the soils beneath the foundation at small strain levels (10-3 percent or less)
Go = γv2so/g = the average shear modulus for the soils beneath the foundation at small strain levels
  γ = the average unit weight of the soils
      Alternatively, for structures supported on mat foundations that rest at or near the ground surface or are embedded in such a way that the side wall contact with the soil is not considered to remain effective during the design ground motion, the effective period of the structure is permitted to be determined from
       (19.2-5)
where
a = the relative weight density of the structure and the soil defined by
        (19.2-6)

                    Table 19.2-1 Values of vsvso and G/Go
                  Value of vs/vso Value of G/Go
                  SDS/2.5 SDS/2.5
Site Class ≤0.1 0.4 ≥0.8 ≤0.1 0.4 ≥0.8
A
B
C
D
E
F
1.00
1.00
0.97
0.95
0.77
α
1.00
0.97
0.87
0.71
0.22
α
1.00
0.95
0.77
0.32
α
α
1.00
1.00
0.95
0.90
0.60
α
1.00
0.95
0.75
0.50
0.05
α
1.00
0.90
0.60
0.10
α
α
Note: Use straight-line interpolation for intermediate values of SDS/2.5.
αShould be evaluated from site-specific analysis.

             Table 19.2-2 Values of αθ
rm/vsT αθ
<0.05
  0.15
  0.35
0.5
1.0
  0.85
0.7
0.6

ra and rm = characteristic foundation lengths defined by
             (19.2-7)
and
              (19.2-8)
where
Ao = the area of the load-carrying foundation
Io = the static moment of inertia of the load-carrying foundation about a horizontal centroidal axis normal to the direction in which the structure is analyzed
αθ = dynamic foundation stiffness modifier for rocking as determined from Table 19.2-2
vs = shear wave velocity
T = fundamental period as determined in Section 12.8.2
The effective damping factor for the structure-foundation system (β̃) shall be computed as follows:
(19.2-9)
where
βo = the foundation damping factor as specified in Fig. 19.2-1
     For values of between 0.10 and 0.20 the values of βo shall be determined by linear interpolation between the solid lines and the dashed lines of Fig. 19.2-1.
     The quantity r in Fig. 19.2-1 is a characteristic foundation length that shall be determined as follows:
(19.2-10)
FIGURE 19.2-1 Foundation Damping Factor


(19.2-11)
where
          Lo = the overall length of the side of the foundation in the direction being analyzed
ra and rm = characteristic foundation lengths defined in Eqs. 19.2-7 and 19.2-8, respectively
     For intermediate values of , the value of r shall be determined by linear interpolation.      EXCEPTION: For structures supported on point-bearing piles and in all other cases where the foundation soil consists of a soft stratum of reasonably uniform properties underlain by a much stiffer, rock-like deposit with an abrupt increase in stiffness, the factor βo in Eq. 19.2-9 shall be replaced by β'o if < 1 where Ds is the total depth of the stratum. β'o shall be determined as follows:
 (19.2-12)
     The value of β̃ computed from Eq. 19.2-9, both with or without the adjustment represented by Eq. 19.2-12. shall in no case be taken as less than β̃ = 0.05 or greater than β̃ = 0.20.
The distribution over the height of the structure of the reduced total seismic force () shall be considered to be the same as for the structure without interaction.
The modified story shears, overturning moments, and torsional effects about a vertical axis shall be determined as for structures without interaction using the reduced lateral forces.
     The modified deflections (δ̃) shall be determined as follows:

(19.2-13)
where
Mo = the overturning moment at the base using the unmodified seismic forces and not including the reduction permitted in the design of the foundation
hx = the height above the base to the level under consideration
δx = the deflections of the fixed-base structure as determined in Section 12.8.6 using the unmodified seismic forces
     The modified story drifts and P-delta effects shall be evaluated in accordance with the provisions of Sections 12.8.6 and 12.8.7 using the modified story shears and deflections determined in this section.
The following provisions are supplementary to those presented in Section 12.9.
To account for the effects of soil-structure interaction, the base shear corresponding to the fundamental mode of vibration (V1) shall be reduced to

          1 = V1 - ΔV1 (19.3-1)

The reduction (ΔV1) shall be computed in accordance with Eq. 19.2-2 with taken as equal to the effective seismic weight of the fundamental period of vibration, , and Cs computed in accordance with Eq. 12.8-1, except that SDS shall be replaced by design spectral response acceleration of the design response spectra at the fundamental period of the fixed-base structure (T1).
    The period shall be determined from Eq. 19.2-3 or from Eq. 19.2-5 where applicable, taking T = T1, evaluating from Eq. 19.2-4 with = 1, and computing as follows:

(19.3-2)


where
wi = the portion of the total gravity load of the structure at level i
φil = the displacement amplitude at the ith level of the structure when vibrating in its fundamental mode
hi = the height above the base to level i
    The preceding designated values of , , T, and also shall be used to evaluate the factor α from Eq. 19.2-6 and the factor βo from Fig. 19.2-1. No reduction shall be made in the shear components contributed by the higher modes of vibration. The reduced base shear (i) shall in no case be taken less than 0.7V1.
The modified modal seismic forces, story shears, and overturning moments shall be determined as for structures without interaction using the modified base shear (1) instead of V1. The modified modal deflections (δ̃xm) shall be determined as follows:

(19.3-3)
and
        δ̃xm = δxm for m = 2, 3, ... (19.3-4)
where
Mo1

=

the overturning base moment for the fundamental mode of the fixed-base structure using the unmodified modal base shear V1
δxm

=

the modal deflections at level x of the fixed-base structure using the unmodified modal shears, Vm
    The modified modal drift in a story (Δ̃m) shall be computed as the difference of the deflections (δxm) at the top and bottom of the story under consideration.
The design values of the modified shears, moments, deflections, and story drifts shall be determined as for structures without interaction by taking the square root of the sum of the squares (SRSS) of the respective modal contributions. In the design of the foundation, it is permitted to reduce the overturning moment at the foundation-soil interface determined in this manner by 10% as for structures without interaction.
    The effects of torsion about a vertical axis shall be evaluated in accordance with the provisions of Section 12.8.4, and the P-delta effects shall be evaluated in accordance with the provisions of Section 12.8.7 using the story shears and drifts determined in Section 19.3.2.
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