Code of Massachusetts Regulations › 780 CMR - STATE BOARD OF BUILDING REGULATIONS AND STANDARDS › Chapter 16 - Structural Design › Section 1613 - EARTHQUAKE LOADS › Subsection 1613.5 - Seismic ground motion values

Current through Register 1525, July 5, 2024
Seismic ground motion values shall be determined in
				  accordance with this section.
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FIGURE 1613.5(1) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR THE CONTERMINOUS UNITED STATES OF 0.2 SEC SPECTRAL RESPONSE
				  ACCELERATION (5% OF CRITICAL DAMPING), SITE CLASS B
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FIGURE 1613.5(1)-continued MAXIMUM CONSIDERED
				  EARTHQUAKE GROUND MOTION FOR THE CONTERMINOUS UNITED STATES OF 0.2 SEC SPECTRAL
				  RESPONSE ACCELERATION (5% OF CRITICAL DAMPING), SITE CLASS B
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FIGURE 1613.5(2) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR THE CONTERMINOUS UNITED STATES OF 1.0 SEC SPECTRAL RESPONSE
				  ACCELERATION (5% OF CRITICAL DAMPING), SITE CLASS B
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FIGURE 1613.5(2)-continued MAXIMUM CONSIDERED
				  EARTHQUAKE GROUND MOTION FOR THE CONTERMINOUS UNITED STATES OF 1.0 SEC SPECTRAL
				  RESPONSE ACCELERATION (5% OF CRITICAL DAMPING), SITE CLASS B
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FIGURE 1613.5(3) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR REGION 1 OF 0.2 SEC SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL
				  DAMPING), SITE CLASS B
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FIGURE 1613.5(3)-continued MAXIMUM CONSIDERED
				  EARTHQUAKE GROUND MOTION FOR REGION 1 OF 0.2 SEC SPECTRAL RESPONSE ACCELERATION
				  (5% OF CRITICAL DAMPING), SITE CLASS B
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FIGURE 1613.5(4) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR REGION 1 OF 1.0 SEC SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL
				  DAMPING), SITE CLASS B
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FIGURE 1613.5(4)-continued MAXIMUM CONSIDERED
				  EARTHQUAKE GROUND MOTION FOR REGION 1 OF 1.0 SEC SPECTRAL RESPONSE ACCELERATION
				  (5% OF CRITICAL DAMPING), SITE CLASS B
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FIGURE 1613.5(5) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR REGION 2 OF 0.2 SEC SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL
				  DAMPING), SITE CLASS B
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FIGURE 1613.5(6) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR REGION 2 OF 1.0 SEC SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL
				  DAMPING), SITE CLASS B
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FIGURE 1613.5(7) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR REGION 3 OF 0.2 SEC SPECTRAL RESPONSE ACCELERATION (5% PERCENT OF
				  CRITICAL DAMPING), SITE CLASS B
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FIGURE 1613.5(8) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR REGION 3 OF 1.0 SEC SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL
				  DAMPING), SITE CLASS B
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FIGURE 1613.5(9) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR REGION 4 OF 0.2 AND 1.0 SEC SPECTRAL RESPONSE ACCELERATION (5% OF
				  CRITICAL DAMPING), SITE CLASS B
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FIGURE 1613.5(10) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR HAWAII OF 0.2 AND 1.0 SEC SPECTRAL RESPONSE ACCELERATION (5% OF
				  CRITICAL DAMPING), SITE CLASS B
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FIGURE 1613.5(11) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR ALASKA OF 0.2 SEC SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL
				  DAMPING), SITE CLASS B
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FIGURE 1613.5(12) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR ALASKA OF 1.0 SEC SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL
				  DAMPING), SITE CLASS B
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FIGURE 1613.5(13) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR PUERTO RICO, CULEBRA, VIEQUES, ST. THOMAS, ST. JOHN AND ST. CROIX OF
				  0.2 AND 1.0 SEC SPECTRAL RESPONSE ACCELERATION (5% OF CRITICAL DAMPING), SITE
				  CLASS B
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FIGURE 1613.5(14) MAXIMUM CONSIDERED EARTHQUAKE GROUND
				  MOTION FOR GUAM AND TUTUILLA OF 0.2 AND 1.0 SEC SPECTRAL RESPONSE ACCELERATION
				  (5% OF CRITICAL DAMPING), SITE CLASS B
1613.5.1
Mapped acceleration
				parameters.
The parameters
S
s
				and
S
1 shall be determined from the 0.2
				and 1-second spectral response accelerations shown on Figures 1613.5(1) through
				1613.5(14). Where
S
1 is less than or
				equal to 0.04 and
S
s is less than or
				equal to 0.15, the structure is permitted to be assigned to
Seismic
				Design Category
A.
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 Class
D shall
				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.
TABLE 1613.5.2 SITE CLASS DEFINITIONS
SITE
CLASS
SOIL PROFILE
NAME
AVERAGE PROPERTIES IN TOP 100
							 feet, SEE SECTION 1613.5.5
Soil shear wave velocity,
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, (ft/s)
Standard penetration resistance,
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Soil undrained shear strength,
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, (psf)
A
Hard rock
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> 5,000
N/A
N/A
B
Rock
2,500 <
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<= 5,000
N/A
N/A
C
Very dense soil and soft rock
1,200 <
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<= 2,500
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>50
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>= 2,000
D
Stiff soil profile
600 <=
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<= 1,200
15 <=
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<= 50
1,000 <=
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<= 2,000
E
Soft soil profile
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< 600
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< 15
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< 1,000
E z
-
Any profile with more than 10
							 feet of soil having the following characteristics:
1.Plasticity
							 index
PI
> 20,
2.Moisture content
w
>= 40%, and
3.Undrained shear strength
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< 500 psf
F
-
Any profile containing soils
							 having one or more of the following characteristics:
1.Soils
							 vulnerable to potential failure or collapse under seismic loading such as
							 liquefiable soils, quick and highly sensitive clays, collapsible weakly
							 cemented soils.
2.Peats and/or highly organic clays
							 (
H
> 10 feet of peat and/or highly organic clay where
H
= thickness of soil)
3.Very high plasticity
							 clays (
H
> 25 feet with plasticity index
PI
> 75)
4.Very thick soft/medium stiff clays
							 (
H
> 120 feet)
For SI: 1 foot = 304.8 mm, 1 square foot = 0.0929
							 m2, 1 pound per square foot = 0.0479 kPa. N/A = Not
							 applicable
1613.5.3
Site coefficients and adjusted
				maximum considered earthquake spectral response acceleration parameters.
The maximum considered earthquake spectral response acceleration for short
				periods,
S
MS, and at 1-second period,
S
M1, adjusted for
site
				class
effects shall be determined by Equations 16-36 and 16-37,
				respectively:
S
MS=
F
a
S
s
(Equation 16-36)
S
M1 =
F
v
S
1
(Equation 16-37)
where:
F
a = Site coefficient
				  defined in Table 1613.5.3(1).
F
v = Site coefficient
				  defined in Table 1613.5.3(2).
S
S = The mapped
				  spectral accelerations for short periods as determined in Section 1613.5.1.
S
1 = The mapped
				  spectral accelerations for a 1-second period as determined in Section 1613.5.1.
TABLE 1613.5.3(1) VALUES OF SITE COEFFICIENT
F
a
										a
SITE
CLASS
MAPPED SPECTRAL RESPONSE
							 ACCELERATION AT SHORT PERIOD
S
s
							 <= 0.25
S
s =
							 0.50
Ss = 0.75
Ss = 1.00
S
s >= 1.25
A
0.8
0.8
0.8
0.8
0.8
B
1.0
1.0
1.0
1.0
1.0
C
1.2
1.2
1.1
1.0
1.0
D
1.6
1.4
1.2
1.1
1.0
E
2.5
1.7
1.2
0.9
0.9
F
Note b
Note b
Note
							 b
Note b
Note b
a. Use straight-line interpolation for intermediate
							 values of mapped spectral response acceleration at short period,
S
s.
b. Values shall be
							 determined in accordance with Section 11.4.7 of ASCE 7.
TABLE 1613.5.3(2) VALUES OF SITE COEFFICIENT
F
V
										a
SITE
CLASS
MAPPED SPECTRAL RESPONSE
							 ACCELERATION AT 1-SECOND PERIOD
S
1
							 <= 0.1
S
1 =
							 0.2
S1 = 0.3
S1 = 0.4
S
1 >= 0.5
A
0.8
0.8
0.8
0.8
0.8
B
1.0
1.0
1.0
1.0
1.0
C
1.7
1.6
1.5
1.4
1.3
D
2.4
2.0
1.8
1.6
1.5
E
3.5
3.2
2.8
2.4
2.4
F
Note b
Note b
Note
							 b
Note b
Note b
a. Use straight-line interpolation for intermediate
							 values of mapped spectral response acceleration at 1-second period,
S
1.
b. Values shall be
							 determined in accordance with Section 11.4.7 of ASCE 7.
1613.5.4
Design spectral response acceleration parameters.
Five-percent damped
				design spectral response acceleration at short periods,
S
DS, and at 1-second period,
S
D1, shall be determined from Equations
				16-38 and 16-39, respectively:
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(Equation 16-38)
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(Equation 16-39)
where:
S
MS = The maximum
				  considered earthquake spectral response accelerations for short period as
				  determined in Section 1613.5.3.
S
M1 = The maximum
				  considered earthquake spectral response accelerations for 1-second period as
				  determined in Section 1613.5.3.
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 below apply 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:
v
si = The shear wave
				  velocity in feet per second (m/s).
d
i = The thickness of
				  any layer between 0 and 100 feet (30 480 mm).
where:
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(Equation 16-40)
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= 100 feet (30 480 mm)
N
i is the Standard
				  Penetration Resistance (ASTM D 1586) not to exceed 100 blows/foot (328 blows/m)
				  as directly measured in the field without corrections. When refusal is met for
				  a rock layer,
N
i shall be taken as 100
				  blows/foot (328 blows/m).
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(Equation 16-41)
where
N
i and
d
i in Equation 16-41 are for
				  cohesionless soil, cohesive soil and rock layers.
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(Equation 16-42)
where:
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Use
d
i and
N
i for cohesionless soil layers only in
				  Equation 16-42.
d
s = The total
				  thickness of cohesionless soil layers in the top 100 feet (30 480 mm).
m
= The number of cohesionless soil layers
				  in the top 100 feet (30 480 mm).
s
ui = The undrained
				  shear strength in psf (kPa), not to exceed 5,000 psf (240 kPa), ASTM D 2166 or
				  D 2850.
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(Equation 16-43)
where:
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d
c = The total
				  thickness of cohesive soil layers in the top 100 feet (30 480 mm).
k
= The number of cohesive soil layers in
				  the top 100 feet (30 480 mm).
PI
= The plasticity index, ASTM D
				  4318.
w
= The moisture content in percent, ASTM D
				  2216.
Where a site does not qualify under the criteria for
Site Class
F and there is a total thickness of soft clay
				  greater than 10 feet (3048 mm) where a soft clay layer is defined by:
				  Su < 500 psf (24 kPa),
w
>= 40
				  percent, and PI > 20, it shall be classified as
Site Class
E.
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 shear wave 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.
TABLE 1613.5.5 SITE
				  CLASSIFICATIONa
SITE CLASS
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or
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E
< 600 ft/s
< 15
< 1,000 psf
D
600 to 1,200 ft/s
15 to
							 50
1,000 to 2,000 psf
C
1,200 to 2,500 ft/s
>
							 50
> 2,000
For SI: 1 foot per second = 304.8 mm per second, 1
							 pound per square foot = 0.0479 kN/m2.
a. If the S
u
method is used and the
							 N
ch
and criteria differ, select the
							 category with the softer soils (for example, use Site Class E instead of
							 D).
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.
3.1. Vs  for the top
				100 feet (30 480 mm) (Vs method).
3.2. N for the top 100 feet (30 480 mm) (
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method).
3.3. N
ch
for cohesionless soil layers (
PI
<
				20) in the top 100 feet (30 480 mm) and average,
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for cohesive soil layers (
PI
> 20) in the
				  top 100 feet (30 480 mm) ( Su method).