BC Building Code 2024
Wind and Seismic Bracing

Requirements for Wind and Earthquake Bracing

The requirements for bracing to resist lateral loads due to wind and earthquake for Part 9 buildings can be found in the 2024 BC Building Code, Division B, Section 9.23 ‘Wood Frame Construction’. BCBC 9.23.13. ‘Bracing to Resist Lateral Loads Due to Wind and Earthquake’ contains the majority of the information referring to other parts of BCBC 9.23. as needed. The Notes to Part 9 also include very helpful information and useful diagrams in understanding the requirements.

Determination of Site Class for Seismic Design

The Site Class is based on the average soil condition present within the top 30 m (100 ft.) of the ground surface. The site is given a designation between A and E, with A being the most stable and E being the least stable. A Geotechnical Engineer determines the Site Class for a property by measuring the shear wave velocity, by soil drilling, or other acceptable methods. Where the Geotechnical Engineer has significant knowledge of the site area he may be permitted to estimate the Site Class for a property.

Determining the Smax Value

The Smax value for the various locations in the Province can be either found in Table C-3 of Appendix C of the 2024 BC Building Code or, if not listed in the Tables, by using the 2020 NBC Seismic Hazard Tool. (See below) Table C-3 Lists the Smax value for the various Site Class designations, and if the Site Class is unknown uses the Site Class E designation as the value.

The designer will need to determine whether the additional cost of meeting the bracing requirements justifies hiring a Geotechnical Engineer to determine a potentially lower Site Class designation.

Determining the 1 in 50 Hourly Wind Pressure Value (HWP)

The Hourly Wind Pressure can also be found in Table C-2 of Appendix C of the 2024 BC Building Code. The value used for determining bracing requirements is the 1 in 50 value shown in the far right column of the table. Values are measured in Kilopascals (kPa), 1 kPa to psf is equal to 20.88543 psf. If you cannot find your location you will need to contact the Authority Having Jurisdiction (AHJ) to get the load information.

Determining Roof Snow Loads

As with the hourly wind pressure the specified roof snow load can also be found in Table C-2 of Appendix C of the 2024 BC Building Code. To determine the roof snow load you will need to use the following formula in conjunction with the values listed in Table C-2. If you cannot find your location you will need to contact the Authority Having Jurisdiction (AHJ) to get the load information.

S=CbSs+Sr

S=Specified Snow Load.
Cb=0.45 where the entire width of the roof is under 4.3 meters/0.55 where the entire width of the roof is over 4.3 meters.
Ss=Ground Snow Load
Sr=Rain Load

Minimum Load: In no case shall the specified snow load be les than 1 kPa. (21 psf)

Roofs used as a balcony or platform: Balconies shall be designed to carry the specified roof load or 1.9 kPa whichever is greater. (40 psf)

(Example: In Penticton for a roof with a width over 4.3 m the roof snow load would be 1 kPa [S=(0.55x1.3) + 0.1=0.815 kPa] because the calculated load is 0.812 kPa it would be the minimum 1 kPa.)

2020 National Building Code of Canada Seismic Hazard Tool

The procedure to determine the seismic design parameter, Smax, for a site is as follows:

1. Go to the 2020 National Building Code of Canada Seismic Hazard Tool

2. Select “Site Class (Xs)” as the site designation.

3. Select the appropriate Site Class (A, B, C, D or E).

4. Enter the latitude and longitude of the site’s location manually, or determine the latitude and longitude from an address or current location.

5. Click “Obtain Seismic Hazard Values.” The following values are needed to determine Smax: Sa(0.2,XS) and Sa(0.5,XS).

6. Determine S(0.2,XS) in accordance with Sentence 4.1.8.4.(6): S(0.2,XS) = Sa(0.2,XS) or Sa(0.5,XS), whichever is greater

7. Determine S(0.5,XS) in accordance with Sentence 4.1.8.4.(6): S(0.5,XS) = Sa(0.5,XS)

8. Determine Smax: Smax = (2/3)S(0.2,XS) or S(0.5,XS), whichever is greater

9. Round Smax to three significant digits. In cases where the Site Class is determined in accordance with Sentence 4.1.8.4.(3), Smax is calculated for the determined Site Class.

In cases where the Site Class is unknown, Smax must be calculated for each of Site Classes A, B, C, D and E, by repeating Steps 1 to 9, and the highest value of Smax among all the Site Classes used for design.

Determining Weight of Construction

BCBC 2024 9.23.13.2.(3) and the Notes to Part 9 A-9.23.13.2.(3) contains a detailed description of Normal-weight and Heavy-weight construction. Essentially a typical wood frame structure will normally qualify as being of Normal-weight. It is important to note that the weight is take as the average weight of an assembly in a building, this is referred to in the code as the Area-weighted Average. For example a floor assembly may have an area where the weight is in excess of 0.5 kPa (but below 1.5 kPa), but when averaged with the remaining floor area could be considered of Normal-weight if the Area-weighted Average was 0.5 kPa or less.

Normal-weight construction, the average dead weight per storey shall not exceed
i) 0.5 kPa for floors and 0.5 kPa for partitions and interior walls,
ii) 0.5 kPa for the roof, and
iii) 0.4 kPa for exterior walls

Heavyweight construction, the average dead weight per storey shall not exceed
i) 1.5 kPa for floors and 0.5 kPa for partitions and interior walls,
ii) 1.0 kPa for the roof, or
iii) 1.2 kPa for exterior walls,

In a building clad with masonry veneer, the average dead weight of the masonry veneer shall not exceed 1.9 kPa, and in a building clad with stone veneer, the average dead weight of the stone veneer shall not exceed 3.2 kPa.(See BCBC 2024 Note A-9.23.13.2.(3).)

Determining Rough Terrain

BCBC 2024 Notes to Part 9 A-9.23.13.7.(3) and (4) explains that Rough terrain is a suburban, urban or wooded terrain extending upwind from the building uninterrupted for at least 1 km. The final decision on the terrain type in an individual location rests with the AHJ.

Determining Wall Height

The unsupported wall height for brace wall panels is measured from the bottom of the bottom plate to the top of the top plate, the maximum height is 3.1 m (10’-2”). Other portions of the wall within the braced wall band can be taller than 3.1 m as permitted in other parts of the code, but the braced wall panels themselves must not be taller than 3.1 m. For example a wall framed under a scissor truss may have sections taller than 3.1 m.

Determining Roof Height

The roof eave-to-ridge height is determined by measuring the distance from the bottom of the eave to the top of the ridge. For use of the simplified approach for braced wall panel length (BCBC 9.23.13.11.) the maximum eave-to-ridge height allowed is 3.0 m. In no case can the eave-to-ridge height exceed 6.0 m for the purposes of BCBC 9.23.13.