Important Changes to Part 4 of the NBC 2020: Structural Design
Zaineb Al-Faesly, Technical Advisor, Codes Canada
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Video transcript
Introduction
Hi, my name is Zaineb Al-Faesly 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 structural design in Part 4 of the National Building Code of Canada 2020.
It is important to note that Part 4 addresses structural design and earthquake design of buildings.
This presentation will focus on structural design, such as design for wind and snow, but not with the earthquake loads.
Outline
In total, 30 changes have been introduced in Part 4 of the NBC 2020 in the area of structural design.
This presentation focuses on the most significant changes in the following areas:
- classification of buildings,
- serviceability limit states,
- design of rooftop parking,
- design of floor slabs,
- snow drifting,
- design of roof-mounted solar panels,
- design of parapets,
- design of attached canopies,
- design of storage garages, and
- update to wind design data.
Importance Categories
This addresses the classification of buildings.
In the context of Part 4, Importance Categories are defined as Low, Normal, High, and Post-disaster buildings, as well as the associated load factors for snow, wind and seismic loads.
Often, Code users tend to interpret the Importance Categories in Part 4 on the basis of the specific examples provided, rather than an understanding of the intent of the categories. This may lead to inappropriate application of the categories.
To address this concern, in the NBC 2020, a generic definition of the Importance Categories is included and the specific examples from Table 4.1.2.1. are moved as explanatory Notes. In addition, the definition of post-disaster buildings in Division A was updated to include additional types of buildings that have been identified by the authorities having jurisdiction (AHJs) and its duplication in Part 4 was deleted to avoid any confusion.
This change will provide AHJs with more flexibility in interpreting and identifying the categories and will simplify the enforcement by allowing the proper loading conditions to be used for serviceability assessment of the structures such as deflection and vibration.
Serviceability
The second change that I would like to present relates to serviceability. As all of you know, the serviceability limit states are associated with poor performance of the structure, such as excessive deflection and cracking. Although not life-threatening, these effects must be avoided.
The loading criteria for serviceability limit states design conditions had always been included in the body of NBC up to the 2005 edition.
With the introduction of the companion load approach in the 2005 edition of the NBC, the serviceability loading criteria was not included in the body of the Code due to time constraints.
The serviceability loading criteria are now reintroduced in the body of the NBC 2020 and harmonized with all design material standards referenced in the NBC.
Rooftop parking
Another change that was introduced in the NBC 2020 relates to the design of exterior areas.
The roof of a building can be used as a parking, which is regularly cleared of snow to allow for the storage and movement of vehicles throughout the winter.
However, when a snowstorm occurs, the roof is not likely to be cleared of snow until after the storm is over. In addition, vehicles may be parked on the roof for the winter with no attempt to clear the roof of snow that collects. The current NBC does not take these scenarios into consideration which could be a safety issue.
In the NBC 2020, the current requirements for the combination of live loads and snow loads for rooftop parking are revised to suit their intended use.
Floor slab systems
In this slide, the focus is on the design of floor slabs.
It is clear that beams, walls and columns are structural members that support tributary area of live load on floor or slab systems.
However, the floor systems themselves are not supporting structural members but rather are the supported structural members. Current practice generally excludes the use of live load reduction factors for the design of floor slab system.
A clarification to exclude the use of floor slab live load reduction factors is now provided in the NBC 2020. This change will result in a consistent and safe interpretation of the use of live load reduction factors for the design of structural members.
Snow drifting
As you can see in this photo, snow drifting depends on many factors, including the height of the step, the size of the lower roof and the canopy, etc.
In the NBC 2015, there was insufficient guidance on determining the snow drift loads on multi-level roofs where the difference in elevation is large between the lower roof surface and the top of the parapet on the upper roof. This can result in an overestimation of the snow drift loads in the roof step.
Additionally, the 2015 Structural Commentary on snow loads only provided guidance regarding canopies and roofs with an area less than 25 m2 and lacked guidance on larger areas. The inappropriate design of canopies and lower roofs could lead to their structural failure.
In the NBC 2020, these gaps are addressed by providing a new formula that deals with snow drifting with all cases of lower roofs, including canopies, and ensuring their safer design.
Solar panels
Another significant change in the NBC 2020 is the introduction of a new design approach for roofs with solar panels both for wind and snow effects.
There has been much demand from industry and regulators for Code provisions on the use of solar collectors on building roofs. A great number of such installations have already been executed and more are being installed due to the focus on alternative energy sources in Canada to support this important initiative.
New wind load provisions for roof-mounted solar panels are introduced in the NBC based on the guidance from the 2015 Structural Commentaries.
In addition, new requirements are included for the determination of design snow loads for roofs with solar panels as the addition of solar panels affects the quantity and distribution of snow on a roof, and, hence, the design snow loads.
These changes will ensure a harmonized method is used for the safer design of solar panels as well as accounting for the additional loads due to the installation of solar panels in the design of the building structure.
Parapets
In the NBC 2015, wind provisions provided insufficient guidance for wind loads on roof parapets. It is important to properly address these requirements to avoid roof failures, which could lead to water leakage that may cause significant damage to the roofing system.
Designers are now provided with proper wind load requirements for parapets on low buildings, including balcony guards near the tops of the buildings, for their appropriate design.
Canopies
Here, you can see a low building with attached canopies that could experience high loads due to wind and should be designed to withstand these loads. The NBC 2015 did not provide requirements for their design.
The gap is now addressed in the NBC 2020, which provides a new harmonized design approach for the safer design of attached canopies in low buildings.
Storage garages
Another item that was clarified in the NBC 2020 is the design of storage garages.
In this figure, you can see a storage garage building, which could be subjected to the effects of de-icing salts, a corrosive element that would impact the durability of the structure as any other parking structure.
The provision in the NBC 2015 could be interpreted as requiring the application of the CSA S413, "Parking structures," for the design of parking structures that are standalone buildings, but not for the design of other types of storage garages. Additionally, it was not clear whether the existing provision applied to associated ramps and pedestrian areas. Furthermore, the provision could be interpreted as not applying to storage garages that are constructed of materials not specifically identified as falling within the scope of the referenced standard.
The inappropriate design and construction of storage garages could lead to structural failure or collapse due to the ingress of precipitation, water and moisture from the exterior or due to the deterioration resulting from aggressive garage environments and the expected service conditions.
The design approach for all storage garages, including associated ramps and pedestrian areas, and the durability of these structures under their expected service conditions, is now clarified in the NBC 2020.
Wind data
Finally, the last item I would like to touch on is the update to climatic design data.
Every code cycle, Table C-2 of Appendix C of the NBC is reviewed and updated with more recent data.
For the 2020 edition of the NBC, the wind data in Table C-2 was updated and the explanatory Note was revised accordingly.
These changes will ensure that the wind data in the NBC are based on the most up-to-date observed data and reflect recent climate trends for a safer design.
Takeaways
In summary, the following highlights for structural design changes in Part 4 are to be retained:
- Clarification of the Importance Categories will provide more flexibility to the AHJs and also simplify enforcement.
- Re-introduction of serviceability loading conditions and harmonization with the design material standards will ensure the proper performance of the building.
- Revision to the design of rooftop parking to suit the intended use will ensure safer design.
- Exclusion of the application of the reduction factor for the design of floor slabs will provide a consistent and safe interpretation of the use of live load reduction factors for the design of structural members.
- Introduction of a new formula that deals with snow drifting in all cases of lower roofs, including canopies, will ensure their safer design.
- Introduction of a new design approach for roof-mounted solar panels will ensure their safer installation on buildings.
- Introduction of wind load requirements for parapets will ensure the use of the appropriate design for the roofing system.
- Introduction of wind load requirements for canopies will ensure their safer design.
- Clarification that all storage garages, including associated ramps and pedestrian areas, are now to be designed in conformance with the performance criteria of the CSA standard on parking structures.
- And, finally, the update of design wind data to include climate trends will ensure a safer design.
Conclusion
This concludes the presentation about important changes to structural 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’s 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.