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 1531, September 27, 2024
Seismic ground motion values shall be determined in accordance with this section.
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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 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.
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, Click to view image , (ft/s) |
Standard penetration resistance, Click to view image |
Soil undrained shear strength, Click to view image , (psf) |
||
| A | Hard rock |
Click to view image > 5,000 |
N/A | N/A |
| B | Rock | 2,500 < Click to view image <= 5,000 |
N/A | N/A |
| C | Very dense soil and soft rock | 1,200 < Click to view image <= 2,500 |
Click to view image >50 |
Click to view image >= 2,000 |
| D | Stiff soil profile | 600 <= Click to view image <= 1,200 |
15 <= Click to view image <= 50 |
1,000 <= Click to view image <= 2,000 |
| E | Soft soil profile |
Click to view image < 600 |
Click to view image < 15 |
Click to view image < 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 Click to view image < 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, SMS, and at 1-second period, SM1, adjusted for site class effects shall be determined by Equations 16-36 and 16-37, respectively:
| SMS= FaSs | (Equation 16-36) |
| SM1 = Fv S1 | (Equation 16-37) |
where:
Fa = Site coefficient defined in Table 1613.5.3(1).
Fv = Site coefficient defined in Table 1613.5.3(2).
SS = The mapped spectral accelerations for short periods as determined in Section 1613.5.1.
S1 = 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 Fa a
| SITE CLASS | MAPPED SPECTRAL RESPONSE ACCELERATION AT SHORT PERIOD | ||||
| Ss <= 0.25 | Ss = 0.50 | Ss = 0.75 | Ss = 1.00 | Ss >= 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,
Ss. b. Values shall be determined in accordance with Section 11.4.7 of ASCE 7. |
TABLE 1613.5.3(2) VALUES OF SITE COEFFICIENT FV a
| SITE CLASS | MAPPED SPECTRAL RESPONSE ACCELERATION AT 1-SECOND PERIOD | ||||
| S1 <= 0.1 | S1 = 0.2 | S1 = 0.3 | S1 = 0.4 | S1 >= 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,
S1. 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, SDS, and at 1-second period, SD1, shall be determined from Equations 16-38 and 16-39, respectively:
| Click to view image | (Equation 16-38) |
| Click to view image | (Equation 16-39) |
where:
SMS = The maximum considered earthquake spectral response accelerations for short period as determined in Section 1613.5.3.
SM1 = 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:
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).
where:
| Click to view image | (Equation 16-40) |
|
Click to view
image = 100 feet (30 480 mm) |
Ni 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, Ni shall be taken as 100 blows/foot (328 blows/m).
| Click to view image | (Equation 16-41) |
where Ni and di in Equation 16-41 are for cohesionless soil, cohesive soil and rock layers.
| Click to view image | (Equation 16-42) |
where:
Use di and Ni for cohesionless soil layers only in Equation 16-42.
ds = 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).
sui = The undrained shear strength in psf (kPa), not to exceed 5,000 psf (240 kPa), ASTM D 2166 or D 2850.
| Click to view image | (Equation 16-43) |
where:
dc = 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 | Click to view image |
Click to view
image or Click to view image | Click to view image |
| 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 Su method is used and the Nch and criteria differ, select the category with the softer soils (for example, use Site Class E instead of D). |
Click to view image method).
for cohesive soil layers (PI > 20) in the top 100 feet (30 480 mm) ( Su method).
1613.5.6 Determination of seismic design category. Structures classified as Occupancy Category I, II or III that are 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 E. Structures classified as Occupancy Category IV that are 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.
TABLE 1613.5.6(1) SEISMIC DESIGN CATEGORY BASED ON SHORT-PERIOD RESPONSE ACCELERATIONS
| VALUE OF SDS | OCCUPANCY CATEGORY | ||
| I or II | III | IV | |
| SDS < 0.167g | A | A | A |
| 0.167g <= SDS < 0.33g | B | B | C |
| 0.33g <= SDS < 0.50g | C | C | D |
| 0.50g <= SDS | D | D | D |
TABLE 1613.5.6(2) SEISMIC DESIGN CATEGORY BASED ON 1-SECOND PERIOD RESPONSE ACCELERATION
| VALUE OF SD1 | OCCUPANCY CATEGORY | ||
| I or II | III | IV | |
| SD1 < 0.067g | A | A | A |
| 0.067g <= SD1 < 0.133g | B | B | C |
| 0.133g <= SD1 < 0.20g | C | C | D |
| 0.20g <= SD1 | D | D | D |
