Important Changes to Part 4 of the NBC 2020: Earthquake Design
Anne Gribbon, Technical Advisor, Codes Canada
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Video transcript
Introduction
Hi, my name is Anne Gribbon and I am a Technical Advisor supporting the Canadian Board for Harmonized Construction Codes—also referred to as the CBHCC.
This committee is the federal-provincial-territorial body responsible for developing and maintaining the National Model Codes in Canada.
I am a member of the Codes Canada team at the National Research Council of Canada.
Codes Canada acts as the secretariat to the CBHCC, providing administrative, technical and policy support, including publishing the National Model Codes.
To learn more about code changes and provisions in the National Model Codes, or about Canada’s national model code development system, please visit CBHCC’s website.
This presentation is part of a video series led by the CBHCC on the key technical changes incorporated in the 2020 editions of the National Model Codes.
This presentation will focus on important changes to earthquake design in Part 4 of the National Building Code of Canada 2020.
Updated seismic hazard values
The seismic hazard values have been updated in the National Building Code of Canada (NBC) 2020. The underlying research for new estimates was provided by Earthquakes Canada. One of the major reasons for revising the hazard values is to incorporate the ongoing improvement in knowledge on seismic hazard.
In general, the hazard has increased for most places in Canada. As a result, the values in the NBC have been updated to mitigate the increase in risk due to higher estimates of hazard.
The values in the NBC 2015 were no longer the best estimates of seismic hazard in Canada. The use of outdated values could increase the risk of injuries, loss of life and property damage from an earthquake in many locations across the country.
Hazard values were updated in the NBC 2020 on the basis of a large amount of new data, new ground motion models (GMMs) and a new approach to site amplification in order to mitigate the increase in risk.
For almost all locations, the revised GMMs are the most significant reason for changes in the seismic hazard.
Site effects
The influence of the ground on the effect of an earthquake on a building is covered on what is called “site effects” in the NBC. Seismic waves get amplified or deamplified when they travel through the soil. The NBC uses the properties of the ground profile, such as the average shear wave velocity, Vs30, and others, to determine these soil amplification effects.
In the NBC 2015, the soils were grouped into Site Classes A through F based on average shear wave velocity, Vs30, and similar properties (N60 and Su). The soil amplification effects were represented by site coefficients provided in Tables. Codes users were required to determine site class, calculate the applicable site coefficient using Tables in NBC and multiply the two to quantify the hazard on a site. This method of grouping soils and site coefficients was approximate, and in some cases, created ambiguity.
In order to address the issue, the soil amplification effects were integrated within the hazard values in the NBC 2020. A separate calculation for site coefficient is no longer required.
Some other changes are:
- The log-log interpolation of hazard values between time periods have been added. This provides a more realistic estimate as compared to linear interpolation.
- An anomaly in the NBC 2015, which in some cases penalized a better soil profile, has also been corrected.
- Site determination was introduced, where site designation X when determined from Vs30 is called Xv and when determined from other properties, such as N60 or Su, is called Xs.
- Hazard values are provided for every value of Vs30 from 140 to 3 000.
- Hazard values are also provided for all values of Site Classes A through E.
- These values are provided using a web tool and do not require calculations for site coefficients.
Web-based tool for seismic hazards
As mentioned, the hazard values are now provided using a web interface:
2020 National Building Code of Canada Seismic Hazard Tool
To find the hazard at a location, users can input the coordinates of the site or drop the pin in Google Maps at the desired location and provide a value of Vs30 or site class. The tool provides hazard values including site effects.
Additional useful features—such as plots of uniform hazard spectra and hazard curves—are available. An option to use Application Program Interface—or API—is provided, which allows downloading of values into software programs for seismic design.
The tool can also be accessed via mobile phone.
For archival purposes, a subset of hazard data is provided for the 679 locations in the NRC Publications Archive or NPARC:
Seismic Hazard Data: NBC 2020, Part 4
These locations are the same as those typically used for NBC Tables.
New seismic categories
Triggers, such as IEFaSa(0.2), were used in the NBC 2015 as conditions under which certain requirements for earthquake design in the NBC were to be applied. In most cases, these triggers were either based on short-period demand Sa(0.2) or on long-period demand. This approach does not take into account all factors that affect the expected magnitude of the inertial seismic force. Designing buildings in accordance with the current approach could lead to inadequate designs. The use of seismic categories instead of triggers addresses the shortcoming of the NBC 2015 approach. In addition, referring to seismic categories will be faster and easier than referring to values of IES(0.2) and IES(1.0).
- Seismic categories are based on both IES(0.2) and IES(1.0). S(0.2) is an indicator of the force demand expected in the first mode of a low-rise building and in higher modes for taller buildings.
- S(1.0) reflects the energy content of ground motions at longer periods and is an indicator of the force demand expected in the first mode for taller buildings.
Vertical stiffness irregularity (VSI)
The definition of vertical stiffness irregularity (VSI), Type 1, in the NBC 2015 was based only on the lateral stiffness of the seismic force resisting system (SFRS).
In some cases, this resulted in false positives. For SFRSs other than concrete and masonry shear walls, inter-storey drift under lateral earthquake design forces reflects the deformation behaviour of the SFRS and is therefore a better parameter than storey stiffness for ascertaining whether a VSI exists in an SFRS.
Changes were made to the definition of VSI in the NBC 2020 to address these issues.
Sloped column designs
Studies have shown that buildings with sloped columns need to be designed for significantly increased earthquake demand because differential horizontal acceleration at the top and bottom of the sloped columns causes the vertical acceleration of the mass supported by the columns. If the effects of this interaction are not accounted for in the design of a building, the use of sloped columns may result in the building not meeting the structural safety and sufficiency objectives of the NBC.
New requirements for analysis and design that account for the influence of sloped columns were provided.
High Importance Category buildings
High Importance Category buildings, which include community centres, elementary, middle and secondary schools, are likely to be used as post-disaster shelters. Such buildings had the same restrictions on structural irregularities as Normal Importance Category buildings. These restrictions were inadequate in the context of the expected performance of such buildings.
In the NBC 2020, new restrictions have been added. Certain structural irregularities that are allowed for Normal Importance Category buildings were prohibited. Also, the NBC now requires the use of SFRSs with at least a minimum level of ductility for such buildings at locations where the seismic hazard is significant.
Note that there is no increase in design base shear or earthquake forces for High Importance Category buildings as a result of these changes.
New seismic force resisting systems (SFRSs)
New SFRSs were added to the NBC as acceptable solutions. These SFRSs will allow Code users to harness the advantages offered by these systems without the additional effort and resources required for their use as an alternative solution. The systems were added in response to requests from industry stakeholders after careful analysis.
Two SFRSs using cross-laminated timber: limited ductility platform-type shear walls and moderately ductile platform-type shear walls.
Two SFRSs using steel: moderately ductile truss moment-resisting frames (MDTMRF) and moderately ductile plate walls (Type MD).
Terminology used for connections
Terminology such as “shallow expansion, chemical, epoxy … anchors” used in NBC 2015 was not consistent with that used in practice and caused confusion among NBC users. The requirements in the NBC 2015 did not explicitly specify that post-installed anchors must be pre-qualified for seismic applications by cyclic load testing.
The misinterpretation of a requirement was preventing the use of power-actuated fasteners and drop-in anchors for all types of tension loads. The intent was to limit their use for cyclic tension loads only.
The gaps in these requirements have been addressed.
Connections of cladding to buildings
Cladding panels are generally considered non-loadbearing components subject to wind or earthquake loads in addition to carrying their own dead load. These components and their connections need to be designed and detailed so that they retain their integrity. If not designed adequately, they can become detached or fall off the structure during the design earthquake.
Absence of explicit requirements in the NBC was creating confusion and hardship for the users and, in some cases, resulting in inadequate design. The requirements have now been explicitly stated.
Photovoltaic arrays (solar panels on roofs)
The lateral displacement of rooftop solar panel arrays in an earthquake may present a hazard to life or a risk of injury as a result of the arrays sliding off the edge of the roof or damaging other rooftop features or equipment in a way that threatens life safety. In order to prevent their lateral displacement, as required by the NBC, such arrays are typically anchored to the structure through penetrations in the roof membrane. This anchoring leads to additional costs and has a significant negative impact on the durability and performance of the roofing membrane.
The change provides a holeless method for resisting lateral earthquake loads on rooftop solar arrays by allowing the use of friction between the supporting base of the array and the roofing membrane, subject to the conditions specified.
In some cases, installers use unacceptable methods of mounting rooftop solar arrays to circumvent anchoring the arrays to the structure. This can lead to potentially unsafe conditions in a seismic event.
Additional performance requirements
In addition to the new requirements for High Importance Category buildings at design ground motion level (2% in 50 year earthquakes), the NBC 2020 introduces additional performance requirements for post-disaster and High Importance Category buildings at lower intensity ground motions, which means less intensity earthquakes that occur more often. In a nutshell, post-disaster and High Importance Category buildings must behave elastically and must meet reduced drift limits at 5% or 10% probability of exceedance in 50 years, respectively.
The performance objective for post-disaster buildings subjected to the design ground motions (DGM) having a probability of exceedance of 2% in 50 years is “functional,” while the performance objective for High Importance Category buildings for the DGM is “immediate occupancy.”
In addition, there are requirements for the connection of elements and components to remain elastic. The NBC 2020 also introduces additional performance requirements for Normal Importance Category buildings with a height above grade more than 30 m in Seismic Category SC4.
In these buildings, only the structural framing elements not considered part of the SFRS must be shown to behave elastically when subjected to ground motions having a 10% probability of exceedance in 50 years.
Conclusion
This concludes the presentation about important changes related to earthquake design in Part 4 of the National Building Code of Canada 2020.
How to get involved
To participate in the code development process, visit the CBHCC’s website to find information about upcoming events and meetings, to submit a code change request, to comment on proposed changes during an open public review or to volunteer to participate on a code development committee.
How to access the Codes
The National Model Codes are published by the National Research Council of Canada.
Visit Codes Canada publications web page on the NRC’s website to purchase a paper copy of the Codes or to access them in free electronic format.
Thank you.