International Building Code Section 1613


International Building Code 1613.1

Scope. Every structure, and portion thereof, including
nonstructural components that are permanently attached to
structures and their supports and attachments, shall be
designed and constructed to resist the effects of earthquake
motions in accordance with ASCE 7, excluding Chapter 14 and
Appendix 11A. The seismic design category for a structure is
permitted to be determined in accordance with Section 1613 or
ASCE 7.
Exceptions:
1. Detached one- and two-family dwellings, assigned to
Seismic Design Category A, B or C, or located where
the mapped short-period spectral response acceleration,
SS, is less than 0.4 g.
2. The seismic-force-resisting system of wood-frame
buildings that conform to the provisions of Section
2308 are not required to be analyzed as specified in
this section.
3. Agricultural storage structures intended only for incidental
human occupancy.
4. Structures that require special consideration of their
response characteristics and environment that are not
addressed by this code or ASCE 7 and for which other
regulations provide seismic criteria, such as vehicular
bridges, electrical transmission towers, hydraulic
structures, buried utility lines and their appurtenances
and nuclear reactors.

International Building Code 1613.2

Definitions. The following words and terms shall, for
the purposes of this section, have the meanings shown herein.
DESIGNEARTHQUAKEGROUNDMOTION. The earthquake
ground motion that buildings and structures are specifically
proportioned to resist in Section 1613.
MAXIMUM CONSIDERED EARTHQUAKE GROUND
MOTION. The most severe earthquake effects considered by
this code.
MECHANICAL SYSTEMS. For the purposes of determining
seismic loads in ASCE 7, mechanical systems shall include
plumbing systems as specified therein.
ORTHOGONAL. To be in two horizontal directions, at 90
degrees (1.57 rad) to each other.
SEISMIC DESIGN CATEGORY. A classification assigned
to a structure based on its occupancy category and the severity
of the design earthquake ground motion at the site.
SEISMIC-FORCE-RESISTING SYSTEM. That part of the
structural system that has been considered in the design to provide
the required resistance to the prescribed seismic forces.
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SITE CLASS. A classification assigned to a site based on the
types of soils present and their engineering properties as
defined in Section 1613.5.2.
SITE COEFFICIENTS. The values of Fa and Fv indicated in
Tables 1613.5.3(1) and 1613.5.3(2), respectively.

International Building Code 1613.3

Existing buildings. Additions, alterations, modification,
or change of occupancy of existing buildings shall be in
accordance with Sections 3403.2.3 and 3406.4.

International Building Code 1613.4

Special inspections. Where required by Section
1705.3, the statement of special inspections shall include the
special inspections required by Section 1705.3.1.

International Building Code 1613.5

Seismic ground motion values. Seismic ground
motion values shall be determined in accordance with this section.

Mapped acceleration parameters. The parameters
Ss and S1 shall be determined from the 0.2 and 1-second
spectral response accelerations shown on Figures 1613.5(1)
through 1613.5(14). Where S1 is less than or equal to 0.04
and Ss is less than or equal to 0.15, the structure is permitted
to be assigned to Seismic Design Category A.

Site class definitions. Based on the site soil properties,
the site shall be classified as either Site Class A, B, C,
D, E or F in accordance with Table 1613.5.2. When the soil
properties are not known in sufficient detail to determine the
site class, Site ClassDshall be used unless the building official
or geotechnical data determines that Site Class E or F
soil is likely to be present at the site.

Site coefficients and adjusted maximum considered
earthquake spectral response acceleration
parameters. The maximum considered earthquake spectral
response acceleration for short periods, SMS, and at 1-second
period, SM1, adjusted for site class effects shall be determined
by Equations 16-37 and 16-38, respectively:

Design spectral response acceleration parameters.
Five-percent damped design spectral response acceleration
at short periods, SDS, and at 1-second period, SD1, shall
be determined from Equations 16-39 and 16-40, respectively.

Site classification for seismic design. Site classification
for Site Class C, D or E shall be determined from
Table 1613.5.5.
The notations presented belowapply to the upper 100 feet
(30 480 mm) of the site profile. Profiles containing distinctly
different soil and/or rock layers shall be subdivided
into those layers designated by a number that ranges from 1
to n at the bottom where there is a total of n distinct layers in
the upper 100 feet (30 480 mm). The symbol i then refers to
any one of the layers between 1 and n.
where:
vsi = The shear wave velocity in feet per second (m/s).
di = The thickness of any layer between 0 and 100 feet (30
480 mm).

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The shear wave velocity for rock, Site Class B, shall be
either measured on site or estimated by a geotechnical engineer
or engineering geologist/seismologist for competent
rock with moderate fracturing and weathering. Softer and
more highly fractured and weathered rock shall either be
measured on site for shearwave velocity or classified as Site
Class C.
The hard rock category, Site Class A, shall be supported
by shear wave velocity measurements either on site or on
profiles of the same rock type in the same formation with an
equal or greater degree of weathering and fracturing. Where
hard rock conditions are known to be continuous to a depth
of 100 feet (30 480 mm), surficial shear wave velocity measurements
are permitted to be extrapolated to assess vs.
The rock categories, Site Classes A and B, shall not be
used if there is more than 10 feet (3048 mm) of soil between
the rock surface and the bottom of the spread footing or mat
foundation.

Steps for classifying a site.
1. Check for the four categories of Site Class F requiring
site-specific evaluation. If the site corresponds
to any of these categories, classify the site as Site
Class F and conduct a site-specific evaluation.
2. Check for the existence of a total thickness of soft
clay ??10 feet (3048 mm) where a soft clay layer is
defined by: su ??500 psf (24 kPa), w =?40 percent
and PI ??20. If these criteria are satisfied, classify
the site as Site Class E.
3. Categorize the site using one of the following three
methods with vs , N, and su and computed in all
cases as specified.
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Determination of seismic design category.
Occupancy Category I, II or III structures located where the
mapped spectral response acceleration parameter at 1-second
period, Sl, is greater than or equal to 0.75 shall be
assigned to Seismic Design Category E. Occupancy Category
IV structures located where the mapped spectral
response acceleration parameter at 1-second period, S1, is
greater than or equal to 0.75 shall be assigned to Seismic
Design Category F. All other structures shall be assigned to
a seismic design category based on their occupancy category
and the design spectral response acceleration coefficients,
SDS and SD1, determined in accordance with Section
1613.5.4 or the site-specific procedures of ASCE 7. Each
building and structure shall be assigned to the more severe
seismic design category in accordance with Table
1613.5.6(1) or 1613.5.6(2), irrespective of the fundamental
period of vibration of the structure, T.

Alternative seismic design category determination.
Where S1 is less than 0.75, the seismic design
category is permitted to be determined from Table
1613.5.6(1) alone when all of the following apply:
1. In each of the two orthogonal directions, the
approximate fundamental period of the structure,
Ta, in each of the two orthogonal directions determined
in accordance with Section 12.8.2.1 of
ASCE 7, is less than 0.8 Ts determined in accordance
with Section 11.4.5 of ASCE 7.
2. In each of the two orthogonal directions, the fundamental
period of the structure used to calculate
the story drift is less than Ts.
3. Equation 12.8-2 of ASCE 7 is used to determine
the seismic response coefficient, Cs.
4. The diaphragms are rigid as defined in Section
12.3.1 in ASCE 7 or for diaphragms that are flexible,
the distance between vertical elements of the
seismic-force-resisting system does not exceed 40
feet (12 192 mm).

Simplified design procedure. Where the
alternate simplified design procedure of ASCE 7 is used,
the seismic design category shall be determined in accordance
with ASCE 7.

International Building Code 1613.5.1

Mapped acceleration parameters. The parameters
Ss and S1 shall be determined from the 0.2 and 1-second
spectral response accelerations shown on Figures 1613.5(1)
through 1613.5(14). Where S1 is less than or equal to 0.04
and Ss is less than or equal to 0.15, the structure is permitted
to be assigned to Seismic Design Category A.

International Building Code 1613.5.2

Site class definitions. Based on the site soil properties,
the site shall be classified as either Site Class A, B, C,
D, E or F in accordance with Table 1613.5.2. When the soil
properties are not known in sufficient detail to determine the
site class, Site ClassDshall be used unless the building official
or geotechnical data determines that Site Class E or F
soil is likely to be present at the site.

International Building Code 1613.5.3

Site coefficients and adjusted maximum considered
earthquake spectral response acceleration
parameters. The maximum considered earthquake spectral
response acceleration for short periods, SMS, and at 1-second
period, SM1, adjusted for site class effects shall be determined
by Equations 16-37 and 16-38, respectively:

International Building Code 1613.5.4

Design spectral response acceleration parameters.
Five-percent damped design spectral response acceleration
at short periods, SDS, and at 1-second period, SD1, shall
be determined from Equations 16-39 and 16-40, respectively.

International Building Code 1613.5.5

Site classification for seismic design. Site classification
for Site Class C, D or E shall be determined from
Table 1613.5.5.
The notations presented belowapply to the upper 100 feet
(30 480 mm) of the site profile. Profiles containing distinctly
different soil and/or rock layers shall be subdivided
into those layers designated by a number that ranges from 1
to n at the bottom where there is a total of n distinct layers in
the upper 100 feet (30 480 mm). The symbol i then refers to
any one of the layers between 1 and n.
where:
vsi = The shear wave velocity in feet per second (m/s).
di = The thickness of any layer between 0 and 100 feet (30
480 mm).

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The shear wave velocity for rock, Site Class B, shall be
either measured on site or estimated by a geotechnical engineer
or engineering geologist/seismologist for competent
rock with moderate fracturing and weathering. Softer and
more highly fractured and weathered rock shall either be
measured on site for shearwave velocity or classified as Site
Class C.
The hard rock category, Site Class A, shall be supported
by shear wave velocity measurements either on site or on
profiles of the same rock type in the same formation with an
equal or greater degree of weathering and fracturing. Where
hard rock conditions are known to be continuous to a depth
of 100 feet (30 480 mm), surficial shear wave velocity measurements
are permitted to be extrapolated to assess vs.
The rock categories, Site Classes A and B, shall not be
used if there is more than 10 feet (3048 mm) of soil between
the rock surface and the bottom of the spread footing or mat
foundation.

Steps for classifying a site.
1. Check for the four categories of Site Class F requiring
site-specific evaluation. If the site corresponds
to any of these categories, classify the site as Site
Class F and conduct a site-specific evaluation.
2. Check for the existence of a total thickness of soft
clay ??10 feet (3048 mm) where a soft clay layer is
defined by: su ??500 psf (24 kPa), w =?40 percent
and PI ??20. If these criteria are satisfied, classify
the site as Site Class E.
3. Categorize the site using one of the following three
methods with vs , N, and su and computed in all
cases as specified.
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International Building Code 1613.5.5.1

Steps for classifying a site.
1. Check for the four categories of Site Class F requiring
site-specific evaluation. If the site corresponds
to any of these categories, classify the site as Site
Class F and conduct a site-specific evaluation.
2. Check for the existence of a total thickness of soft
clay ??10 feet (3048 mm) where a soft clay layer is
defined by: su ??500 psf (24 kPa), w =?40 percent
and PI ??20. If these criteria are satisfied, classify
the site as Site Class E.
3. Categorize the site using one of the following three
methods with vs , N, and su and computed in all
cases as specified.
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International Building Code 1613.5.6

Determination of seismic design category.
Occupancy Category I, II or III structures located where the
mapped spectral response acceleration parameter at 1-second
period, Sl, is greater than or equal to 0.75 shall be
assigned to Seismic Design Category E. Occupancy Category
IV structures located where the mapped spectral
response acceleration parameter at 1-second period, S1, is
greater than or equal to 0.75 shall be assigned to Seismic
Design Category F. All other structures shall be assigned to
a seismic design category based on their occupancy category
and the design spectral response acceleration coefficients,
SDS and SD1, determined in accordance with Section
1613.5.4 or the site-specific procedures of ASCE 7. Each
building and structure shall be assigned to the more severe
seismic design category in accordance with Table
1613.5.6(1) or 1613.5.6(2), irrespective of the fundamental
period of vibration of the structure, T.

Alternative seismic design category determination.
Where S1 is less than 0.75, the seismic design
category is permitted to be determined from Table
1613.5.6(1) alone when all of the following apply:
1. In each of the two orthogonal directions, the
approximate fundamental period of the structure,
Ta, in each of the two orthogonal directions determined
in accordance with Section 12.8.2.1 of
ASCE 7, is less than 0.8 Ts determined in accordance
with Section 11.4.5 of ASCE 7.
2. In each of the two orthogonal directions, the fundamental
period of the structure used to calculate
the story drift is less than Ts.
3. Equation 12.8-2 of ASCE 7 is used to determine
the seismic response coefficient, Cs.
4. The diaphragms are rigid as defined in Section
12.3.1 in ASCE 7 or for diaphragms that are flexible,
the distance between vertical elements of the
seismic-force-resisting system does not exceed 40
feet (12 192 mm).

Simplified design procedure. Where the
alternate simplified design procedure of ASCE 7 is used,
the seismic design category shall be determined in accordance
with ASCE 7.

International Building Code 1613.5.6.1

Alternative seismic design category determination.
Where S1 is less than 0.75, the seismic design
category is permitted to be determined from Table
1613.5.6(1) alone when all of the following apply:
1. In each of the two orthogonal directions, the
approximate fundamental period of the structure,
Ta, in each of the two orthogonal directions determined
in accordance with Section 12.8.2.1 of
ASCE 7, is less than 0.8 Ts determined in accordance
with Section 11.4.5 of ASCE 7.
2. In each of the two orthogonal directions, the fundamental
period of the structure used to calculate
the story drift is less than Ts.
3. Equation 12.8-2 of ASCE 7 is used to determine
the seismic response coefficient, Cs.
4. The diaphragms are rigid as defined in Section
12.3.1 in ASCE 7 or for diaphragms that are flexible,
the distance between vertical elements of the
seismic-force-resisting system does not exceed 40
feet (12 192 mm).

International Building Code 1613.5.6.2

Simplified design procedure. Where the
alternate simplified design procedure of ASCE 7 is used,
the seismic design category shall be determined in accordance
with ASCE 7.

International Building Code 1613.6

Alternatives to ASCE 7. The provisions of Section
1613.6 shall be permitted as alternatives to the relevant provisions
of ASCE 7.

Assumption of flexible diaphragm. Add the following
text at the end of Section 12.3.1.1 of ASCE 7:
Diaphragms constructed of wood structural panels or
untopped steel decking shall also be permitted to be idealized
as flexible, provided all of the following conditions are
met:
1. Toppings of concrete or similar materials are not
placed over wood structural panel diaphragms except
for nonstructural toppings no greater than 11/2 inches
(38 mm) thick.
2. Each line of vertical elements of the lateral-
force-resisting system complies with the allowable
story drift of Table 12.12-1.
3. Vertical elements of the lateral-force-resisting system
are light-framed walls sheathed with wood structural
panels rated for shear resistance or steel sheets.
4. Portions of wood structural panel diaphragms that
cantilever beyond the vertical elements of the lateral-
force-resisting system are designed in accordance
with Section 2305.2.5 of the International
Building Code.
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Additional seismic-force-resisting systems for
seismically isolated structures. Add the following exception
to the end of Section 17.5.4.2 of ASCE 7:
Exception: For isolated structures designed in accordance
with this standard, the Structural System Limitations
and the Building Height Limitations in Table
12.2-1 for ordinary steel concentrically braced frames
(OCBFs) as defined in Chapter 11 and ordinary moment
frames (OMFs) as defined in Chapter 11 are permitted to
be taken as 160 feet (48 768 mm) for structures assigned
to Seismic Design Category D, E or F, provided that the
following conditions are satisfied:
1. The value of RI as defined in Chapter 17 is taken as
1.
2. For OMFs and OCBFs, design is in accordance
with AISC 341.
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International Building Code 1613.6.1

Assumption of flexible diaphragm. Add the following
text at the end of Section 12.3.1.1 of ASCE 7:
Diaphragms constructed of wood structural panels or
untopped steel decking shall also be permitted to be idealized
as flexible, provided all of the following conditions are
met:
1. Toppings of concrete or similar materials are not
placed over wood structural panel diaphragms except
for nonstructural toppings no greater than 11/2 inches
(38 mm) thick.
2. Each line of vertical elements of the lateral-
force-resisting system complies with the allowable
story drift of Table 12.12-1.
3. Vertical elements of the lateral-force-resisting system
are light-framed walls sheathed with wood structural
panels rated for shear resistance or steel sheets.
4. Portions of wood structural panel diaphragms that
cantilever beyond the vertical elements of the lateral-
force-resisting system are designed in accordance
with Section 2305.2.5 of the International
Building Code.
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International Building Code 1613.6.2

Additional seismic-force-resisting systems for
seismically isolated structures. Add the following exception
to the end of Section 17.5.4.2 of ASCE 7:
Exception: For isolated structures designed in accordance
with this standard, the Structural System Limitations
and the Building Height Limitations in Table
12.2-1 for ordinary steel concentrically braced frames
(OCBFs) as defined in Chapter 11 and ordinary moment
frames (OMFs) as defined in Chapter 11 are permitted to
be taken as 160 feet (48 768 mm) for structures assigned
to Seismic Design Category D, E or F, provided that the
following conditions are satisfied:
1. The value of RI as defined in Chapter 17 is taken as
1.
2. For OMFs and OCBFs, design is in accordance
with AISC 341.