Each structure shall be assigned to a seismic design category in accordance with Section 1616.3. Seismic design categories are used in this code to determine permissible structural systems, limitations on height and irregularity, those components of the structure that must be designed for seismic resistance and the types of lateral force analysis that must be performed. Each structure shall be provided with complete lateraland verticalforceresisting systems capable of providing adequate strength, stiffness and energy dissipation capacity to withstand the design earthquake ground motions determined in accordance with Section 1615 within the prescribed deformation limits of Section 1617.3. The design ground motions shall be assumed to occur along any horizontal direction of a structure. A continuous load path, or paths, with adequate strength and stiffness to transfer forces induced by the design earthquake ground motions from the points of application to the final point of resistance shall be provided.
Allowable stress design is permitted to be used to evaluate sliding, overturning and soil bearing at the soilstructure interface regardless of the approach used in the design of the structure, provided load combinations of Section 1605.3 are utilized. When using allowable stress design for proportioning foundations, the value of 0.2 S_{DS}D in Equations 1650, 1651, 1652 and 1653 or Equations 9.5.2.71, 9.5.2.72, 9.5.2.7.11 and 9.5.2.7.12 of ASCE 7 is permitted to be taken equal to zero. When the load combinations of Section 1605.3.2 are utilized, a onethird increase in soil allowable stresses is permitted for all load combinations that include W or E.
Allowable stress design is permitted to be used to evaluate sliding, overturning and soil bearing at the soilstructure interface regardless of the approach used in the design of the structure, provided load combinations of Section 1605.3 are utilized. When using allowable stress design for proportioning foundations, the value of 0.2 S_{DS}D in Equations 1650, 1651, 1652 and 1653 or Equations 9.5.2.71, 9.5.2.72, 9.5.2.7.11 and 9.5.2.7.12 of ASCE 7 is permitted to be taken equal to zero. When the load combinations of Section 1605.3.2 are utilized, a onethird increase in soil allowable stresses is permitted for all load combinations that include W or E.
Each structure shall be assigned a seismic use group and a corresponding occupancy importance factor (IE) as indicated in Table 1604.5.
Seismic Use Group I structures are those not assigned to either Seismic Use Group II or III.
Seismic Use Group II structures are those, the failure of which would result in a substantial public hazard due to occupancy or use as indicated by Table 1604.5, or as designated by the commissioner.
Seismic Use Group III structures are those having essential facilities that are required for post earthquake recovery and those containing substantial quantities of hazardous substances, as indicated in Table 1604.5, or as designated by the commissioner. Where operational access to a Seismic Use Group III structure is required through an adjacent structure, the adjacent structure shall conform to the requirements for Seismic Use Group III structures. Where operational access is less than 10 feet (3048 mm) from an interior lot line or less than 10 feet (3048 mm) from another structure, access protection from potential falling debris shall be provided by the owner of the Seismic Use Group III structure.
Where a structure is occupied for two or more occupancies not included in the same seismic use group, the structure shall be assigned the classification of the highest seismic use group corresponding to the various occupancies. Where structures have two or more portions that are structurally separated in accordance with Section 1620, each portion shall be separately classified. Where a structurally separated portion of a structure provides required access to, required egress from or shares life safety components with another portion having a higher seismic use group, both portions shall be assigned the higher seismic use group.
All structures shall be assigned to a seismic design category based on their seismic use group and the design spectral response acceleration coefficients, S_{DS} and S_{D1},determined in accordance with Section 1615.1.3 or 1615.2.5. Each building and structure shall be assigned to the most severe seismic design category in accordance with Table 16 16.3(1) or 16 16.3(2), irrespective of the fundamental period of vibration of the structure, T. Seismic Design Category B is the minimum design category allowed.
Exception: The seismic design category is permitted to be determined from Table 1616.3(1) alone when all of the following apply:
 The approximate fundamental period of the structure, T_{a}, in each of the two orthogonal directions determined in accordance with Section 9.5.5.3.2 of ASCE 7, is less than 0.8 T_{s} determined in accordance with Section 1615.1.4,
 Equation 9.5.5.2.11 of ASCE 7 is used to determine the seismic response coefficient, C_{s}, and
 The diaphragms are rigid as defined in Section 1602.
VALUE OF S_{DS}  SEISMIC USE GROUP  

I  II  III  
S_{DS} < 0.167g  A  A  A 
0.167g ≤ S_{DS} < 0.33g  B  B  B 
0.33g ≤ S_{DS} < 0.50g  C  C  C 
0.50 ≤ S_{DS}  D_{a}  D_{a}  D_{a} 
 Building structures in Seismic Use Groups I or II and on Site Class E may be designed in Seismic Design Category C if their fundamental period of vibration is not between 1 and 2 seconds or a dynamic structural analysis based on a site specific spectrum is performed
VALUE OF S_{D1}  SEISMIC USE GROUP  

I  II  III  
S_{D1} < 0.067g  A  A  A 
0.067g ≤ S_{D1} < 0.133g  B  B  C 
0.133g ≤ S_{D1} < 0.20g  C  C  D 
0.20 ≤ S_{D1}  D  D  D 
Requirements for Seismic Design Categories A, E and F have been eliminated from the New York City Building Code as such categories do not apply in New York City. References to these categories can be found in ASCE 7.
Buildings shall be classified as regular or irregular based on the criteria in Section 9.5.2.3 of ASCE 7.
Exception: Buildings designed using the simplified analysis procedure in Section 1617.5 shall be classified in accordance with Section 1616.5.1.
Buildings designed using the simplified analysis procedure in Section 1617.5 shall be classified as regular or irregular based on the criteria in this section. Such classification shall be based on the plan and vertical configuration. Buildings shall not exceed the limitations of Section 1616.6.1.
Buildings having one or more of the features listed in Table 1616.5.1.1 shall be designated as having plan structural irregularity and shall comply with the requirements in the sections referenced in that table.
IRREGULARITY TYPE AND DESCRIPTION  REFERENCE SECTION 
SEISMIC DESIGN CATEGORY^{a} APPLICATION 


1a  Torsional Irregularity  to be considered when diaphragms are not flexible as determined in section 1602.1.1 
9.5.5.5.2 of ASCE 7 1620.4.1 9.5.2.1 of ASCE 7 9.5.5.7.1 of ASCE 7 
C and D, D, D, C and D 
1b  Extreme Torsional Irregularity — to be considered when diaphragms are not flexible as determined in Section 1602.1. Extreme torsional irregularity shall be considered to exist when the maximum story drift. 
9.5.5.5.2 of ASCE 7 1620.4.1 9.5.5.5.1 of ASCE 7 9.5.5.7.1 of ASCE 7 
C and D, D, D, C and D 
2  Reentrant Corners Plan, configurations of a structure and its lateralforceresisting system contain reentrant corners where both projections of the structure beyond a reentrant corner are greater than 15 percent of the plan dimension of the structure in the given direction. 
1620.4.1  D 
3  Diaphragm Discontinuity Diaphragms with abrupt discontinuities or variations in stiffness including those having cutout or open areas greater than 50 percent of the gross enclosed diaphragm area or changes in effective diaphragm stiffness of more than 50 percent from one story to the next. 
1620.4.1  D 
4  OutofPlane Offsets Discontinuities in a lateralforceresistance path, such as outofplane offsets of the vertical elements. 
1620.4.1 9.5.2.5.1 of ASCE 7 1620.2.9 
D, D, B, C and D 
5  Nonparallel Systems The vertical lateralforceresisting elements are not parallel to or symmetric about the major orthogonal axes of the lateralforceresisting system. 
1620.3.2  C and D 
 Seismic design category is determined in accordance with Section 1616.
Buildings having one or more of the features listed in Table 1616.5.1.2 shall be designated as having vertical irregularity and shall comply with the requirements in the sections referenced in that table.
Exceptions:
 Structural irregularities of Type 1a, 1b or 2 in Table 1616.5.1.2 do not apply where no story drift ratio under design lateral load is greater than 130 percent of the story drift ratio of the next story above. Torsional effects need not be considered in the calculation of story drifts for the purpose of this determination. The story drift ratio relationship for the top two stories of the building is not required to be evaluated.
 Irregularities of Types 1a, 1b and 2 of Table 1616.5.1.2 are not required to be considered for onestory buildings in any seismic design category or for twostory buildings in Seismic Design Category B, C or D.
IRREGULARITY TYPE AND DESCRIPTION  REFERENCE SECTION 
SEISMIC DESIGN CATEGORYa APPLICATION 


1a  Stiffness Irregularity—Soft Story 
9.5.2.5.1 of ASCE 7  D 
1b  Stiffness Irregularity—Extreme Soft Story 
9.5.2.5.1 of ASCE 7  D 
2  Weight (Mass) Irregularity 
9.5.2.5.1 of ASCE 7  D 
3  Vertical Geometric Irregularity 
9.5.2.5.1 of ASCE 7  D, E and F 
4  Inplane Discontinuity in Vertical LateralForceResisting Elements 
1620.4.1 9.5.2.5.1 of ASCE 7 1620.2.9 
D, D, B, C and D 
5  Discontinuity in Capacity—Weak Story 
1620.2.3 9.5.2.5.1 of ASCE 7 
B, C and D, D 
 Seismic design category is determined in accordance with Section 1616.