B1/AS1: Structural provisions for buildings

B1 STRUCTURE ACCEPTABLE SOLUTION B1/AS1 Buildings and structures COMMENT: 1. Design constraints a. The characteristic surface movements and the corresponding expansivity classifications have been calculated based on design for ultimate limit state (ULS) conditions for a 1 in 1000 year “extreme” drought event, and the serviceability limit state (SLS) conditions for a 1 in 500 year drought event. b. Maximum soil movements are calculated to be based on a 500 year return period for SLS, and a 1000 year return period for ULS*. (*NB: This differed from the recommendations contained within BRANZ Study Report 120A (BSR120A) which used a 300 year return period for the design level drought conditions) c. Climate parameters adopted from BSR120A of Δu = 1.2 pF, Hs = 1.5 m, and a crack depth of 0.5 Hs d. The I (shrink swell index) ranges attributed to the expansivity classifications as defined ss in Paragraph 2.3.8.2 b) have been calculated using the parameters presented in BSR120A and Equation 2.3.1 of AS 2870. e. Sites subject to parameters that differ from those mentioned above, in particular sites where the crack depth is less than 0.75 m, such as cut natural ground or clay backfill, require specific engineering assessment to confirm their appropriate site classification. f. The effects of nearby trees (whether existing, recently removed, or future planting) are not considered in these solutions. It is recommended that specific geotechnical engineering advice is obtained where a tree is within a lateral distance of 1.5 times its mature height of the foundations. 2. Maintenance of foundations in expansive soils Normal maintenance is that work generally recognised as necessary to achieve the expected performance over time of the foundation located on expansive soils. Unless otherwise specified by the designer and noted on the drawings, basic normal maintenance tasks should ensure that: a. the drainage and wetting of the site is controlled so that extremes of wetting and drying of the soils are prevented; and b. the position and operation of gardens adjacent to the dwelling are controlled, and the planting of trees near to foundations is suitably restricted; and c. any leaks which develop in plumbing, storm water or sanitary sewage systems are repaired promptly. MINISTRY OF BUSINESS, INNOVATION AND EMPLOYMENT 28 JULY 2025 Page 12