Proposed Change 2048

Climate change effects not yet addressed in the NBC

In the 2020 and previous editions of the National Building Code of Canada (NBC), it was assumed that climatic data statistics used in structural design did not change over time (or were stationary). Accordingly, the climatic design data in the NBC have been updated for each Code cycle using past weather observations collected and analyzed by Environment and Climate Change Canada (ECCC) under the assumption that past statistics will continue to be applicable to the future. In the face of extensive evidence that the climate is changing across Canada, this practice raises real safety concerns regarding the design of the main structural systems and envelope of buildings to withstand climatic loads such as those due to snow and wind.

In addition, in the current edition of the NBC, wind data are based mainly on synoptic wind observations and do not account for the different existing climatic influences in Canada, where some regions are more prone to local convective thunderstorms. This phenomenon is expected to be exacerbated in the future with climate warming.

PCF 1979 proposes to update the climatic data in Table C-2, Climatic Design Data for Selected Locations in Canada.

Further to PCF 1979, this proposed change  develops an approach for the design of Part 9 buildings using the new climatic data, which reflects expected changes to the climate in future.

Uniform hazard approach in load calculations yields non-uniform probability of failure

An additional problem identified is that the current methodology for the structural design of buildings in the NBC uses what has been termed a “uniform hazard” approach. In this approach, reference design wind and snow loads at various locations across Canada are specified at an annual probability of exceedance of 1/50, corresponding to a 50-year return period in a stationary climate. The minimum safety criterion adopted in the NBC corresponds to a probability of failure (i.e., probability that the effects of loads are higher than the resistance of a structural member in a building) of 0.001 during the 50-year assumed service life of a building in the Normal Importance Category. To provide an acceptable probability of failure, the “specified design loads” or so-called “service loads” taken at the 50-year return period have been multiplied by load factors (i.e., 1.5 for snow and 1.4 for wind) to obtain the “ultimate loads” applied in design calculations. These load factors are influenced by the target safety criteria and the variability of the load, and historically have been taken as constant across all regions of Canada. However, reliability studies [1] have shown that this approach leads to a non-uniform probability of failure across the country, due to the different behaviour of wind and snow events in Canada’s various regions and the localized variation in these loads. The probability of failure can differ by as much as a factor of 10, depending on where the project is located, and this variability could be further exacerbated by climate change.

Adjusting loads to account for non-stationarity in a future changing climate

Results obtained using RCP8.5, corresponding to 2.5°C global warming, were adopted to establish future climatic loads for structural design. Comparisons were made based on a building constructed in 2025 having a service life of 50 years, and the differences between the four RCP scenarios included in the ECCC study [2] remained within a narrow range. The decision to adopt the most extreme RCP8.5 scenario reflects a prudent approach, as there is uncertainty in the 50-year estimate. Recent studies [1] revealed the statistical significance of the non-stationarity of extreme wind speeds and ground snow loads based on the future projections provided by ECCC [2] for many regions across Canada.

Therefore, a non-stationary extreme-value analysis approach, known as the “Minimax Method,” is proposed to design for the worst-case year of the building’s service life. This approach ensures that the annual probability of failure remains acceptably low during the entire service life. For instance, future projections for wind loads mostly predict increases in reference pressure, making the last year of service life the worst case; conversely, for ground snow loads, future projections mostly show decreases, making the first year of service life the worst case.

Using ECCC’s climate projections [2], regional climate change factors have been developed that can be applied to the wind and snow reference values in different regions across the country. These factors were used to determine the reference values tabulated in NBC Table C-2, Climatic Design Data for Selected Locations in Canada.

Introducing the uniform risk approach in ultimate limit state (ULS) design

To address the variability of the probability of failure across the country, a so-called “uniform risk” approach is proposed in which “ultimate loads” are specified directly for each location with load factors of 1.0. This approach is similar to the approach used for earthquake design in the NBC and other approaches adopted internationally. Notably, ASCE/SEI 7, “Minimum Design Loads for Buildings and Other Structures,” has used uniform risk for wind loads since the 2010 edition [3] (and more recently for snow loads since the 2022 edition), and the Australian Building Code [4] many years before.

Climate change factors were calculated by comparing the future design levels, determined using the Minmax approach, with design levels based on the conventional stationary approach. For reference design wind pressures, most areas in Canada have a climate change factor of 1.05, while locations in Ontario, the Atlantic provinces, and west of 120°W in British Columbia have a climate change factor of 1.1. For ground snow loads, the northern territories have a climate change factor of 1.05, while most regions have a climate change factor of 1.0 (see [5] and PCF 1979).

In deriving climate change factors for the “uniform risk” approach, target annual probabilities of exceedance of 1/500 for wind and 1/1 000 for snow were selected. With the proposed uniform risk approach, the targeted probabilities are applied with a load factor of 1.0, as opposed to the present method of using a 1/50 target and increasing it by a load factor across the country. The 1/500 and 1/1 000 annual probabilities of exceedance were selected to maintain the average risk of failure across the country. As a result, the same target probability of failure of approximately 1/1 000 is set, but with more precise estimates from location to location.

Calibration studies and benefits of the uniform risk approach

References

(1) RWDI Report No. 1702484, 2020, Development of Climate Change Provisions for Structural Design of Buildings and Implementation Plan in the National Building Code.

(2) Cannon, A.J., Jeong, D.I., Zhang, X., and F. W. Zwiers. (2020). Climate-Resilient Buildings and Core Public Infrastructure: An Assessment of the Impact of Climate Change on Climatic Design Data in Canada. Government of Canada, Ottawa, ON. 106 p. (https://climate-scenarios.canada.ca/?page=buildings-report-overview).

(3) ASCE 7-22, Minimum Design Loads for Buildings and Other Structures.

(4) Australian/New Zealand Standard (AS/NZS) 1170.2:2002, Structural design actions. Part 2: Wind actions.

(5) Li S.H., Irwin P., Lounis Z., Attar A., Dale J., Gibbons M., Beaulieu, S. (2022). Effects of Nonstationarity of Extreme Wind Speeds and Ground Snow Loads in a Future Canadian Changing Climate. Nat Hazard Rev. 23(4):04022022.

Refer to the supporting document titled "Cost impact of climatic load changes on Part 9: Adopting Part 4 proposed new return periods in PCF 2048" for the full cost analysis. A summary is reproduced here.

Cost impact on Part 9 buildings of updated hourly wind pressure with longer return periods

As a result of the new 1-in-500 annual probability hourly wind pressure data and the introduction of a "reference hourly wind pressure" in this proposed change to replace 1-in-50 annual probability hourly wind pressure, the value decreased for six locations, remained the same for eight locations, and increased for the remaining 666 locations out of the 680 locations in Table C-2 included in PCF 1979.

For the structural sufficiency of glass (NBC Sentence 9.6.1.3.(2)), a 128.5 m², two-storey detached house was used as the building archetype, which contained five differently sized windows with areas of glass between 0.57 m² and 1.43 m². In 620 of the 680 locations in Table C-2, the 1-in-50 annual probability hourly wind pressure remained below the maximum limits provided in NBC Tables 9.6.1.3.-A, 9.6.1.3.-B and 9.6.1.3.-C before and after the change, resulting in no impact. In three of the 60 locations with potential impact — Cowley, AB; Cape Race, NL; and Resolution Island, NU — the reference hourly wind pressure before and after the proposed change exceeded the maximum value of 1.0 kPa provided in the prescriptive table in the NBC, which would require consultation with the window manufacturer for glass thickness. For the remaining 57 locations, windows increased in cost between $126.98 and $353.51.

For the nailing of framing — roof trusses, rafters and joists to wall framing (NBC Sentence 9.23.3.4.(3)), a 120 m² bungalow was used as the building archetype. Due to the proposed change, two new locations — Channel-Port aux Basques and St. John's, NL — had reference hourly wind pressure that was equal to or exceeded 0.8 kPa, where roof trusses, rafters or joists would be required to be tied to wall framing with connectors that can resist 3 kN of roof uplift. In these locations, the number of required galvanized-steel connectors was calculated to be approximately 72, resulting in a cost increase of $437.04.

For fasteners for sheathing (NBC Article 9.23.3.5.), a 128.5 m², two-storey detached house was used as the building archetype. In 671 of the 680 locations in Table C-2, the 1-in-50 annual probability hourly wind pressure and reference hourly wind pressure remained below 0.8 kPa, resulting in no impact. Seven of the nine remaining locations had a 1-in-50 annual probability hourly wind pressure and reference hourly wind pressure greater than 0.8 kPa, resulting in no impact. The same remaining two locations noted above exceeded 0.8 kPa as a result of the proposed change, having the following impact:

• For roof sheathing, the two locations would require larger size fasteners and fasteners spaced at 50 mm within 1 m of the roof edge. The cost increase using common wire nails is estimated to be $468.68 for each location.
• For wall sheathing, the two locations would require braced wall panels with wood-based wall sheathing, resulting in a cost increase of $1,125.30 for each location.   

For the anchorage of building frames (NBC Sentence 9.23.6.1.(3)), the same two locations noted above had a 1-in-50 annual probability hourly wind pressure that exceeded 0.8 kPa, resulting in an increase in the number of anchor bolts by 15 for a total cost increase of $94.20.

For required roof sheathing (NBC Sentence 9.23.16.1.(1)), a 128.5 m² two-storey detached bungalow was used. Similar to the above results, the same two locations would be impacted by the proposed change and be required to meet NBC Subsection 9.23.16. The cost increase from sheathing deemed too thin for truss spacing in NBC Sentence 9.23.16.7.(2) to compliant plywood sheathing is approximately $168.82.

For lumber roof sheathing (NBC Article 9.23.16.5.), the roof area of a 128.5 m², two-storey detached house was used as the building archetype. Similar to the above results, the same two locations would be impacted by the proposed change and be required to install lumber roof sheathing diagonally instead of horizontally in accordance with NBC Sentence 9.23.16.5. This results in a cost increase of approximately $311.67 for each location.

For the attachment of cladding to flat ICF wall units (NBC Sentence 9.27.5.4.(2), a 128.5 m², two-storey detached house was used. In 619 of the 680 locations in Table C-2, the 1-in-50 annual probability hourly wind pressure and reference wind pressure was equal to or less than 0.6 kPa before and after the change, resulting in no impact. In 34 of the remaining 61 locations, the 1-in-50 annual probability hourly wind pressure and reference hourly wind pressure were greater than 0.6 kPa before and after the change, so the impact is assumed to be minimal and would account for additional fasteners. The greatest impact is assumed to occur at locations where the 1-in-50 annual probability hourly wind pressure is equal to or less than 0.60 kPa and the reference hourly wind pressure is more than 0.6 kPa after the change, which occurred in the remaining 27 locations. This resulted in approximate cost increases of $2,009.15 in these locations, representing the different material costs of fasteners into concrete, additional labour, and reduced daily output to attach the furring through the flat wall ICF units into the solid concrete backup wall.

Cost impact on Part 9 buildings of updated snow loads with longer return periods

As a result of the new 1-in-1 000 annual probability snow load data and modification to the calculation of specified snow loads in this proposed change, out of the 680 locations in Table C-2 of PCF 1979, the specified snow load remained the same in 41 locations (neutral), increased in 154 locations (adverse), and decreased in the remaining 485 locations (beneficial).

For platforms subject to snow and occupancy loads (NBC Sentence 9.4.2.3.(1)), a 3.5 m × 4 m archetype exterior platform was assessed. Before and after the change, 483 out of the 680 locations had specified snow loads less than 1.9 kPa, resulting in no impact. Of the 197 remaining locations, 115 locations had specified snow loads that remained between the same range before and after the change, resulting in no impact. Using the archetype, NBC Span Tables, and costs from RSMeans, 16 of the 82 remaining locations experienced a cost increase ranging from $47.77 to $291.81, and 37 locations experienced a cost decrease ranging from $47.77 to $126.43. In 24 locations, the same joist and built-up beam size was sufficient before and after the change, resulting in no impact.

For the performance of windows, doors and skylights (NBC Sentence 9.7.3.1.(2)), the magnitude of the cost impact could not be determined without industry knowledge about the structural design of skylights, including the capacity of the skylight frames and glazing, for the 154 locations where snow loads increase.

For columns (NBC Subclause 9.17.1.1.(1)(b)(ii)), a 2.44 m × 4 m exterior platform raised 3 m from the ground by three columns was assessed. In 657 of the 680 locations, the sum of the specified snow load and the occupancy load remained below 4.8 kPa before and after the change, resulting in no impact. In 19 of the 23 remaining locations, the same column size was applicable before and after the change, resulting in no impact. In three of the remaining locations, there was a cost decrease of $290.86. One location experienced a cost increase of $290.86.

For ridge support (NBC Sentence 9.23.14.8.(5) and Table 9.23.14.8.), a 120 m² bungalow was used as the building archetype. In 481 of the 680 locations, the specified snow load remained within the same range, resulting in no impact. Of the 199 remaining locations, 58 were not impacted because the same number of nails were sufficient before and after the change. In the remaining 141 locations, 112 experienced a decrease in the required number of nails (maximum three nails less), and 29 experienced an increase in the number of nails (maximum three nails more) resulting in additional material costs of $5.54.

For ICF lintels (NBC Sentence 9.20.17.4.(3) and Tables 9.20.17.4.-A, 9.20.17.4.-B and 9.20.17.4.-C), a 120 m² bungalow was used assuming 150 mm thick ICF walls. ICF lintel sizes before and after the change where the ground snow load exceeded 3.33 kPa were analyzed. In six locations, the ground snow load before and after the change remained below or equal to 1.5 kPa, resulting in no impact. In 105 locations the ICF lintel size was sufficient to support the snow load before and after the change, resulting in no impact. In 62 locations, the ground snow load exceeded both the values in the NBC Span Tables and those provided by an ICF manufacturer, which would likely require a structural engineer to design using Part 4 and additional material and labour costs. For the remaining 507 locations, there was an increased cost for ICF lintels of between $6.71 and $88.46.

For spans for joists, rafters and beams (NBC Sentence 9.23.4.2.(1)), a 120 m² bungalow was used. In 589 of the 680 locations, the specified snow load before and after the change remained in the same range, resulting in no impact. The impact on the remaining 91 locations was the following:

• For roof joists (NBC Tables 9.23.4.2.-D and 9.23.4.2.-E), 37 of the 91 locations did not experience an impact because the same roof joist size was sufficient before and after the change. The cost impact was not determined for five locations where the specified snow load exceeded 4.0 kPa and the NBC Span Tables could not be used to determine the size of roof joists required either before or after the change. Of the remaining locations, 10 experienced a cost increase of approximately $1,850.00, while 39 locations experienced a cost decrease of the same amount.
• For roof rafters (NBC Tables 9.23.4.2.-F and 9.23.4.2.-G), 26 of the 91 locations did not experience an impact because the size of the roof rafters was sufficient before and after the change. The cost impact was not determined for five locations where the specified snow load exceeded 4.0 kPa and the NBC Span Tables could not be used to determine the size of roof rafters required either before or after the change. Of the remaining locations, 16 experienced a cost increase of between $255.30 and $1,342.89, while 44 locations experienced a cost decrease of between $255.30 and $1,342.89.
• For built-up ridge beams and lintels supporting the roof (NBC Table 9.23.4.2.-L), 12 of the 91 locations did not experience an impact because the size of the built-up ridge beam was sufficient before and after the change. The cost impact was not determined for nine locations where the specified snow load exceeded 3.0 kPa and the NBC Span Tables could not be used to determine the size of roof rafters required either before or after the change. Of the remaining locations, 17 experienced a cost increase of between $140.24 and $262.66, while 53 locations experienced a cost decrease of the same amount.
• For lintels for various wood species (NBC Tables 9.23.12.3.-A, 9.23.12.3.-B, 9.23.12.3.-C and 9.23.12.3.-D), the cost impact was not determined for nine locations where the specified snow load exceeded 3.0 kPa and the NBC Span Tables could not be used to determine the size of the lintel required either before or after the change. Of the remaining locations, 18 experienced a cost increase of between $32.13 and $84.47, while 64 locations experienced a cost decrease of the same amount.

Designers, architects, building regulators and building owners.