BUILDING CODE COUNCIL
[Filed May 6, 2022, 4:14 p.m., effective May 6, 2022, 4:14 p.m.]
Effective Date of Rule: Immediately upon filing.
Purpose: Amending chapter 51-50 WAC of the International Building Code, Chapter 35 Reference Standards and Section 1613, specifically addressing amendments to ASCE 7.
This emergency rule is aligning with WSR 22-05-096, an emergency rule related to elevator pit fire sprinklers.
Summary of Proposed Changes: Section 1613: This amendment provides a simplified method to develop seismic design parameters for seismic design of buildings. The current method in ASCE 7-16 for developing seismic design response spectra is very complex, and it requires additional ground motion hazard analyses for many more building sites than required in previous versions of the code.
Ground motion hazard analyses are an advanced approach to develop the earthquake ground motions and response spectra needed for seismic design. They require additional geophysical testing of the soil and advanced computer modeling. The process of obtaining a ground motion hazard analysis requires [a] geotechnical engineer with significant seismic expertise, and greatly increases the cost and time needed to complete a project.
The requirement for ground motion hazard analyses for more types of sites in Washington state also makes the job of municipal review agencies more difficult. Most jurisdictions do not have the expertise to review these analyses. Thus, they will need to contract with third-party reviewers or accept the analyses with little to no review. The first option is costly and time-consuming; the second option is dangerous and a critical life/safety issue because ground motion hazard analyses require a geotechnical engineer with significant seismic expertise to perform them correctly.
This proposal provides an alternative to the ground motion hazard analysis requirements in ASCE 7-16 by permitting an optional multi-period response spectra (MPRS) approach as described in ASCE 7-22. The primary inputs to this simplified method are the latitude/longitude of the site and the average shear wave velocity of the site, which can be obtained through standard geotechnical testing. The engineer would then obtain the equivalent of ground motion hazard analysis results from a United States Geological Survey website developed as part of the National Seismic Hazard Mapping project and adopted in ASCE 7-22. This simplified approach reduces the complexities, and it will result in more consistent, understandable estimation of ground motions for building design. This simplified process also results in ground motion parameters for seismic design that achieve the same level of risk and earthquake return periods that are assumed in ASCE 7-16.
This alternative would be allowed for all soil site classes except Site Class F (e.g. liquefiable sites), meaning it could be used for most sites in the state of Washington. In addition, the MPRS may also be used to develop the code minimum spectrum when ground motion hazard analysis is required. The resulting MPRS would continue to be used within the framework of the current code, ASCE 7-16.
The use of the ASCE 7-22 MPRS as an option in lieu of the ground motion hazard analysis requirements of ASCE 7-16 will simplify the estimation of seismic forces for building design and streamline the design and review process of buildings throughout Washington.
Chapter 35: The purpose of this amendment is to adopt the supplements to 2016 edition of ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7-16), developed by the ASCE 7 Standard Committee to address important issues in between cycles of development. Some of the noted deficiencies in the ASCE 7-16 standard affect high seismic hazard locations such as Washington state and could potentially result in unconservative structural design.
The ASCE 7-16 standard now has three published supplements: Supplement No. 1 was published on December 11, 2018; Supplement No. 2 was published on October 19, 2021; and Supplement No. 3 was published on November 3, 2021. Supplement No. 1 was adopted into the 2021 International Building Code, but Supplement No. 2 and No. 3 were not included as they have just been recently published. Please refer to the attached copies of the documents for specific and detailed information of the changes, including the commentaries from ASCE 7 Standard Committee that explain the technical background of the problems addressed by the document. In general, these documents are developed to correct errors and deficiencies, and clarify the intent in the originally published 2016 standard.
Citation of Rules Affected by this Order: Amending 2.
Statutory Authority for Adoption: RCW 19.27.031
Under RCW 34.05.350
the agency for good cause finds that immediate adoption, amendment, or repeal of a rule is necessary for the preservation of the public health, safety, or general welfare, and that observing the time requirements of notice and opportunity to comment upon adoption of a permanent rule would be contrary to the public interest.
Reasons for this Finding: The amendment in Section 1613 provides a simplified method to develop seismic design parameters for seismic design of buildings. The current method in ASCE 7-16 for developing seismic design response spectra is very complex, and it requires additional ground motion hazard analyses for many more building sites than required in previous versions of the code.
The purpose of the amendment in Chapter 35 is to adopt the Supplements to 2016 edition of ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7-16), developed by the ASCE 7 Standard Committee to address important issues in between cycles of development. Some of the noted deficiencies in the ASCE 7-16 standard affect high seismic hazard locations such as Washington state and could potentially result in unconservative structural design.
Number of Sections Adopted in Order to Comply with Federal Statute: New 0, Amended 0, Repealed 0; Federal Rules or Standards: New 0, Amended 0, Repealed 0; or Recently Enacted State Statutes: New 0, Amended 0, Repealed 0.
Number of Sections Adopted at the Request of a Nongovernmental Entity: New 0, Amended 2, Repealed 0.
Number of Sections Adopted on the Agency's own Initiative: New 0, Amended 0, Repealed 0.
Number of Sections Adopted in Order to Clarify, Streamline, or Reform Agency Procedures: New 0, Amended 0, Repealed 0.
Number of Sections Adopted using Negotiated Rule Making: New 0, Amended 0, Repealed 0; Pilot Rule Making: New 0, Amended 0, Repealed 0; or Other Alternative Rule Making: New 0, Amended 0, Repealed 0.
Date Adopted: April 22, 2022.
AMENDATORY SECTION(Amending WSR 20-21-021, filed 10/9/20, effective 11/9/20)
WAC 51-50-1613Section 1613—Earthquake loads.
1613.4 Amendments to ASCE 7. The provisions of Section 1613.4 shall be permitted as an amendment to the relevant provisions of ASCE 7. The text of ASCE 7 shall be amended as indicated in Sections 1613.4.1 through ((1613.4.2))1613.4.6.
1613.4.1 ASCE 7 Section 22.214.171.124. Amend ASCE 7 Section 126.96.36.199 as follows:
188.8.131.52 Increased structural height limit for steel eccentrically braced frames, steel special concentrically braced frames, steel buckling-restrained braced frames, steel special plate shear walls, and special reinforced concrete shear walls. The limits on height, hn, in Table 12.2-1 are permitted to be increased from 160 ft (50 m) to 240 ft (75 m) for structures assigned to Seismic Design Categories D or E and from 100 ft (30 m) to 160 ft (50 m) for structures assigned to Seismic Design Category F, provided that the seismic force-resisting systems are limited to steel eccentrically braced frames, steel special concentrically braced frames, steel buckling-restrained braced frames, steel special plate shear walls, or special reinforced concrete cast-in-place shear walls and all of the following requirements are met:
1. The structure shall not have an extreme torsional irregularity as defined in Table 12.3-1 (horizontal structural irregularity Type 1b).
2. The steel eccentrically braced frames, steel special concentrically braced frames, steel buckling-restrained braced frames, steel special plate shear walls or special reinforced concrete shear walls in any one plane shall resist no more than 60 percent of the total seismic forces in each direction, neglecting accidental torsional effects.
3. Where floor and roof diaphragms transfer forces from the vertical seismic force-resisting elements above the diaphragm to other vertical force-resisting elements below the diaphragm, these in-plane transfer forces shall be amplified by the overstrength factor, Ωo for the design of the diaphragm flexure, shear, and collectors.
4. The earthquake force demands in foundation mat slabs, grade beams, and pile caps supporting braced frames and/or walls arranged to form a shear-resisting core shall be amplified by 2 for shear and 1.5 for flexure. The redundancy factor, ρ, applies and shall be the same as that used for the structure in accordance with Section 12.3.4.
5. The earthquake shear force demands in special reinforced concrete shear walls shall be amplified by the over-strength factor, Ωo.
1613.4.2 ASCE 7 Section 12.6. Amend ASCE 7 Section 12.6 and Table 12.6-1 to read as follows:
12.6 ANALYSIS PROCEDURE SELECTION
12.6.1 Analysis procedure. The structural analysis required by Chapter 12 shall consist of one of the types permitted in Table 12.6-1, based on the structure's seismic design category, structural system, dynamic properties, and regularity, or with the approval of the authority having jurisdiction, an alternative generally accepted procedure is permitted to be used. The analysis procedure selected shall be completed in accordance with the requirements of the corresponding section referenced in Table 12.6-1.
Permitted Analytical Procedures
Seismic Design Category
Equivalent Lateral Force Procedure, Section 12.8a
Modal Response Spectrum Analysis, Section 12.9.1, or Linear Response History Analysis, Section 12.9.2
Nonlinear Response History Procedures, Chapter 16a
D, E, F
Risk Category I or II buildings not exceeding two stories above the base
Structures of light frame construction
Structures with no structural irregularities and not exceeding 160 ft in structural height
Structures exceeding 160 ft in structural height with no structural irregularities and with T <3.5Ts
Structures not exceeding 160 ft in structural height and having only horizontal irregularities of Type 2, 3, 4, or 5 in Table 12.3-1 or vertical irregularities of Type 4, 5a, or 5b in Table 12.3-2
All other structures ≤ 240 ft in height
All structures ˃ 240 ft in height
P: Permitted; NP: Not Permitted; Ts= SD1/SDS.
1613.4.3 ASCE 7 Section 11.2. Amend ASCE 7 Section 11.2 to include the following definition:
USGS SEISMIC DESIGN GEODATABASE: A U.S. Geological Survey (USGS) database of geocoded values of seismic design parameters and geocoded sets of multiperiod 5%-damped risk-targeted maximum considered earthquake (MCER) response spectra. The parameters obtained from this database may only be used where referenced by Section 184.108.40.206.
User Note: The USGS Seismic Design Geodatabase is intended to be accessed through a USGS Seismic Design web service that allows the user to specify the site location, by latitude and longitude, and the site class to obtain the seismic design data. The USGS web service spatially interpolates between the gridded data of the USGS geodatabase. Both the USGS geodatabase and the USGS web service can be accessed at https://doi.org/10.5066/F7NK3C76. The USGS Seismic Design Geodatabase is available at the ASCE 7 Hazard Tool https://asce7hazardtool.online/ or an approved equivalent.
1613.4.4 ASCE 7 Section 11.4.8. Amend ASCE 7 Section 11.4.8 to include the following section:
220.127.116.11 Multiperiod design response spectrum. As an alternative to the ground motion hazard analysis requirements of Section 11.4.8, and suitable for all structures other than those designated Site Class F (unless exempted in accordance with Section 20.3.1), a multiperiod design response spectrum may be developed as follows:
1. For exclusive use with the USGS Seismic Design Geodatabase in accordance with this section, the site class shall be determined per Section 20.6.
2. Where a multiperiod design response spectrum is developed in accordance with this section, the parameters SM, SM1, SD, SD1, and TL as obtained by the USGS Seismic Design Geodatabase shall be used for all applications of these parameters in this standard.
3. The SS and S1 parameters obtained by the USGS Seismic Design Geodatabase are only permitted to be used in development of the multiperiod design response spectrum and are not permitted to be used in other applications in this standard. The mapped parameters SS and S1 as determined by Section 11.4.2 and peak ground acceleration parameter PGAM as determined by Section 11.8.3 shall be used for all other applications in this standard.
4. At discrete values of period, T, equal to 0.0s, 0.01s, 0.02s, 0.03s, 0.05s, 0.075s, 0.1s, 0.15s, 0.2s, 0.25s, 0.3s, 0.4s, 0.5s, 0.75s, 1.0s, 1.5s, 2.0s, 3.0s, 4.0s, 5.0s, 7.5s, and 10.0s, the 5%-damped design spectral response acceleration parameter, Sa, shall be taken as 2/3 of the multiperiod 5%-damped MCER response spectrum from the USGS Seismic Design Geodatabase for the applicable site class.
5. At each response period, T, less than 10.0s and not equal to one of the discrete values of period, T, listed in Item 4 above, Sa, shall be determined by linear interpolation between values of Sa, of Item 4 above.
6. At each response period, T, greater than 10.0s, Sa shall be taken as the value of Sa at the period of 10.0s, factored by 10/T, where the value of T is less than or equal to that of the long-period transition period, TL, and shall be taken as the value of Sa at the period of 10.0s factored by 10TL/T2, where the value of T is greater than that of the long-period transition period, TL.
7. Where an MCER response spectrum is required, it shall be determined by multiplying the multiperiod design response spectrum by 1.5.
8. For use with the equivalent lateral force procedure, the spectral acceleration Sa at T shall be permitted to replace SD1/T in Equation (12.8-3) and SD1TL/T2 in Equation (12.8-4).
1613.4.5 ASCE 7 Section 20.6. Amend ASCE 7 Chapter 20 to include the following section:
Section 20.6 Site classification procedure for use with Section 18.104.22.168. For exclusive use in determining the multiperiod design response spectrum and associated spectral parameters in accordance with Section 22.214.171.124, the site class shall be determined in accordance with this section. For all other applications in this standard the site class shall be determined per Section 20.1.
20.6.1 Site classification. The site soil shall be classified in accordance with Table 20.6-1 and Section 20.6.2 based on the average shear wave velocity parameter, , which is derived from the measured shear wave velocity profile from the ground surface to a depth of 100 ft (30 m). Where shear wave velocity is not measured, appropriate generalized correlations between shear wave velocity and standard penetration test (SPT) blow counts, cone penetration test (CPT) tip resistance, shear strength, or other geotechnical parameters shall be used to obtain an estimated shear wave velocity profile, as described in Section 20.6.3. Where site-specific data (measured shear wave velocities or other geotechnical data that can be used to estimate shear wave velocity) are available only to a maximum depth less than 100 ft (30 m), shall be estimated as described in Section 20.6.3.
Where the soil properties are not known in sufficient detail to determine the site class, the most critical site conditions of Site Class C, Site Class CD and Site Class D, as defined in Section 20.6.2, shall be used unless the authority having jurisdiction or geotechnical data determine that Site Class DE, E or F soils are present at the site. Site Classes A and B shall not be assigned to a site if there is more than 10 ft (3.1 m) of soil between the rock surface and the bottom of the spread footing or mat foundation.
20.6.2 Site class definitions. Site class types shall be assigned in accordance with the definitions provided in Table 20.6.2-1 and this section.
126.96.36.199 Soft clay Site Class E. Where a site does not qualify under the criteria for Site Class F per Section 20.3.1 and there is a total thickness of soft clay greater than 10 ft (3 m), where a soft clay layer is defined by su<500psf (su<25 kPa), w ≥ 40%, and PI ˃ 20, it shall be classified as Site Class E. This classification is made regardless of , as computed in Section 20.4.
188.8.131.52 Site Classes C, CD, D, DE and E. The assignment of Site Class C, CD, D, DE and E soils shall be made based on the average shear wave velocity, which is derived from the site shear wave velocity profile from the ground surface to a depth of 100 ft (30 m), as described in Section 20.4.
184.108.40.206 Site Classes B and BC (medium hard and soft rock). Site Class B can only be assigned to a site on the basis of shear wave velocity measured on site. If shear wave velocity data are not available and the site condition is estimated by a geotechnical engineer, engineering geologist, or seismologist as Site Class B or BC on the basis of site geology, consisting of competent rock with moderate fracturing and weathering, the site shall be classified as Site Class BC. Softer and more highly fractured and weathered rock shall either be measured on site for shear wave velocity or classified as Site Class C.
220.127.116.11 Site Class A (hard rock). The hard rock, Site Class A, category shall be supported by shear wave velocity measurement, 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 ft (30 m), surficial shear wave velocity measurements to maximum depths less than 100 ft are permitted to be extrapolated to assess .
Table 20.6.2-1 Site Classification
Calculated Using Measured or Estimated Shear Wave Velocity Profile (ft/s)
A. Hard Rock
B. Medium Hard Rock
˃ 3,000 to 5,000
BC. Soft Rock
˃ 2,100 to 3,000
C. Very Dense Sand or Hard Clay
˃ 1,450 to 2,100
CD. Dense Sand or Very Stiff Clay
˃ 1,000 to 1,450
D. Medium Dense Sand or Stiff Clay
˃ 700 to 1,000
DE. Loose Sand or Medium Stiff Clay
˃ 500 to 700
E. Very Loose Sand or Soft Clay
20.6.3 Estimation of shear wave velocity profiles. Where measured shear wave velocity data are not available, shear wave velocity shall be estimated as a function of depth using correlations with suitable geotechnical parameters, including standard penetration test (SPT) blow counts, shear strength, overburden pressure, void ratio, or cone penetration test (CPT) tip resistance, measured at the site.
Site class based on estimated values of shall be derived using , /1.3, and 1.3 when correlation models are used to derive shear wave velocities. Where correlations derived for specific local regions can be demonstrated to have greater accuracy, factors less than 1.3 can be used if approved by the authority having jurisdiction. If the different average velocities result in different site classes per Table 20.6.2-1, the most critical of the site classes for ground motion analysis at each period shall be used. Where the available data used to establish the shear wave velocity profile extends to depths less than 100 ft (30 m) but more than 50 ft (15 m), and the site geology is such that soft layers are unlikely to be encountered between 50 and 100 ft, the shear wave velocity of the last layer in the profile shall be extended to 100 ft for the calculation of in Equation (20.4-1). Where the data does not extend to depths of 50 ft (15 m), default site classes, as described in Section 20.6.1, shall be used unless another site class can be justified on the basis of the site geology.
1613.4.6 ASCE 7 Section 21.3.1. Amend ASCE 7 Section 21.3 to include the following section:
Section 21.3.1 Alternate minimum design spectral response accelerations. As an alternate approach to Section 21.3, the lower limit of Sa is permitted to be determined according to this section. The design spectral response acceleration at any period shall not be taken less than 80% of the multiperiod design response spectrum as determined by Section 18.104.22.168.
For sites classified as Site Class F requiring site-specific analysis in accordance with Section 11.4.8, the design spectral response acceleration at any period shall not be less than 80% of Sa determined for Site Class E.
Where a different site class can be justified using the site-specific classification procedures in accordance with Section 22.214.171.124, a lower limit of 80% of Sa for the justified site class shall be permitted to be used.
AMENDATORY SECTION(Amending WSR 20-21-021, filed 10/9/20, effective 11/9/20)
WAC 51-50-3500Chapter 35—Referenced standards.
Add the reference standards as follows:
Standard reference number
Referenced in code section number
Standard for Performance-Rated Cross-Laminated Timber (revised 2018)
Minimum Design Loads and Associated Criteria for Buildings and Other Structures with Supplement No.1, Supplement No. 2, and Supplement No.3
Standard for Fixed Guideway Transit and Passenger Rail Systems
Standard for the Installation of Sprinkler Systems (except 126.96.36.199(5))
403.3.3, 7188.8.131.52, 903.3.1.1, 903.2, 903.3.8.2, 903.8.5, 904.13, 905.3.4, 907.6.4, 1019.3