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Section 1616 Earthquake Loads — Criteria Selection

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1616.1 Structural Design Criteria
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 lateral-and vertical-force-resisting 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 soil-structure 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 SDSD in Equations 16-50, 16-51, 16-52 and 16-53 or Equations 9.5.2.7-1, 9.5.2.7-2, 9.5.2.7.1-1 and 9.5.2.7.1-2 of ASCE 7 is permitted to be taken equal to zero. When the load combinations of Section 1605.3.2 are utilized, a one-third increase in soil allowable stresses is permitted for all load combinations that include W or E.
1616.2 Seismic Use Groups and Occupancy Importance Factors
Each structure shall be assigned a seismic use group and a corresponding occupancy importance factor (IE) as indicated in Table 1604.5.
1616.2.1 Seismic Use Group I
Seismic Use Group I structures are those not assigned to either Seismic Use Group II or III.
1616.2.2 Seismic Use Group II
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.
1616.2.3 Seismic Use Group III
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.
1616.2.4 Multiple Occupancies
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.
1616.3 Determination of Seismic Design Category
All structures shall be assigned to a seismic design category based on their seismic use group and the design spectral response acceleration coefficients, SDS and SD1,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:

  1. The approximate fundamental period of the structure, Ta, 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 Ts determined in accordance with Section 1615.1.4,
  2. Equation 9.5.5.2.1-1 of ASCE 7 is used to determine the seismic response coefficient, Cs, and
  3. The diaphragms are rigid as defined in Section 1602.

TABLE 1616.3(1)
SEISMIC DESIGN CATEGORY BASED ON
SHORT-PERIOD RESPONSE ACCELERATIONS
VALUE OF SDS SEISMIC USE GROUP
I II III
SDS < 0.167g A A A
0.167g ≤ SDS < 0.33g B B B
0.33g ≤ SDS < 0.50g C C C
0.50 ≤ SDS Da Da Da
  1. 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


TABLE 1616.3(2)
SEISMIC DESIGN CATEGORY BASED ON
1-SECOND PERIOD RESPONSE ACCELERATION
VALUE OF SD1 SEISMIC USE GROUP
I II III
SD1 < 0.067g A A A
0.067g ≤ SD1 < 0.133g B B C
0.133g ≤ SD1 < 0.20g C C D
0.20 ≤ SD1 D D D
1616.3.1 Seismic Design Category A, E or F
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.
1616.4 Reserved
1616.5 Building Configuration
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.
1616.5.1 Building Configuration (For Use in the Simplifiedanalysis Procedure of Section 1617.5)
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.
1616.5.1.1 Plan Irregularity
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.

TABLE 1616.5.1.1
PLAN STRUCTURAL IRREGULARITIES
IRREGULARITY TYPE AND DESCRIPTION REFERENCE
SECTION
SEISMIC DESIGN
CATEGORYa
APPLICATION
1a Torsional Irregularity - to be considered when diaphragms are not flexible as determined in section 1602.1.1

Torsional irregularity shall be considered to exist when the maximum story drift, computed including accidental torsion at one end of the structure transverse to an axis is more than 1.2 times average of the story drifts at the two ends of the structure.
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 lateral-force-resisting 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 Out-of-Plane Offsets
Discontinuities in a lateral-force-resistance path, such as out-of-plane 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 lateral-force-resisting elements are not parallel to or symmetric about the major orthogonal axes of the lateral-force-resisting system.
1620.3.2 C and D
  1. Seismic design category is determined in accordance with Section 1616.
1616.5.1.2 Vertical Irregularity
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:

  1. 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.
  2. Irregularities of Types 1a, 1b and 2 of Table 1616.5.1.2 are not required to be considered for one-story buildings in any seismic design category or for two-story buildings in Seismic Design Category B, C or D.

TABLE 1616.5.1.2
VERTICAL STRUCTURAL IRREGULARITIES
IRREGULARITY TYPE AND DESCRIPTION REFERENCE
SECTION
SEISMIC DESIGN
CATEGORYa
APPLICATION
1a Stiffness Irregularity—Soft Story

A soft story is one in which the lateral stiffness is less than 70 percent of that in the story above or less than 80 percent of the average stiffness of the three stories above.
9.5.2.5.1 of ASCE 7 D
1b Stiffness Irregularity—Extreme Soft Story

An extreme soft story is one in which the lateral stiffness is less than 60 percent of that in the story above or less than 70 percent of the average stiffness of the three stories above
9.5.2.5.1 of ASCE 7 D
2 Weight (Mass) Irregularity

Mass irregularity shall be considered to exist where the effective mass of any story is more than 150 percent of the effective mass of an adjacent story. A roof that is lighter than the floor below need not be considered.
9.5.2.5.1 of ASCE 7 D
3 Vertical Geometric Irregularity

Vertical geometric irregularity shall be considered to exist where the horizontal dimension of the lateral-force-resisting system in any story is more than 130 percent of than in an adjacent story.
9.5.2.5.1 of ASCE 7 D, E and F
4 In-plane Discontinuity in Vertical Lateral-Force-Resisting Elements

An in-plane offset of the lateral-force-resisting elements greater than the length of those elements or a reduction in stiffness of the resisting element in the story below.
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

A weak story is one in which the story lateral strength is less than 80 percent of that in the story above.The story strength is the total strength of seismic-resisting elements sharing the story shear for the direction under consideration.
1620.2.3
9.5.2.5.1 of ASCE 7
B, C and D,
D
  1. Seismic design category is determined in accordance with Section 1616.
1616.6 Analysis Procedures
A structural analysis conforming to one of the types permitted in Section 9.5.2.5.1 of ASCE7 or to the simplified procedure in Section 1617.5 shall be made for all structures. The analysis shall form the basis for determining the seismic forces, E and Em, to be applied in the load combinations of Section 1605 and shall form the basis for determining the design drift as required by Section 9.5.2.8 of ASCE 7 or Section 1617.3.

Exception: Design drift need not be evaluated in accordance with Section 1617.3 when the simplified analysis method of Section 1617.5 is used.
1616.6.1 Simplified Analysis
A simplified analysis, in accordance with Section 1617.5, shall be permitted to be used for any structure in Seismic Use Group I, subject to the following limitations, or a more rigorous analysis shall be made:

  1. Buildings of light-framed construction not exceeding three stories in height, excluding basements.
  2. Buildings of any construction other than light-framed construction, not exceeding two stories in height, excluding basements, with flexible diaphragms at every level as defined in Section 1602.

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