B1/AS1: Structural provisions for buildings

B1 STRUCTURE ACCEPTABLE SOLUTION B1/AS1 Buildings and structures 2.3.8 Slab-on-ground in expansive soils 2.3.8.1 In NZS 3604 Section 1.1 Scope: a) for buildings covered by the standard, modify 1.1.2(a) to read: “Buildings founded on good ground or on expansive soils where the requirements of 1.1.5 are met”; and b) add a new clause “Clause 1.1.5 Buildings on expansive soils”: i) “Buildings on expansive soils shall be supported on slab-on-ground foundations complying with 7.5.13 and in addition to 1.1.2 shall be limited as follows: (a) single storey, stand-alone household unit, and (b) maximum length or width of floor of 24.0 m including any attached garage, and (c) simple plan shapes such as rectangular, L, T or boomerang, and (d) concrete slab-on-ground with a minimum thickness of 100 mm and a minimum concrete compressive strength of 20 MPa, and (e) simple roof forms, incorporating hips, valleys, gables or mono pitches, and (f) maximum overall height of 7.0 m to roof apex from lowest cleared ground level, and (g) maximum roof height of 3.0 m, and (h) roof slope between 10° and 35° from the horizontal, and (i) maximum span of roof truss 12.0 m, and (j) external walls maximum of 2.4 m height studs, other than gable end walls and walls to mono-pitched roofs, which shall not exceed 4.0 m.” COMMENT: 1. Floor plans – Where floor plans incorporate re-entrant corners then continuity of the exterior ground beam shall be maintained by continuing it as an internal beam, with the exterior beam details continued for a length of at least 1.0 m into the internal beam. This is only applicable where internal beams are specified in Tables 7.4A and 7.4B. This is aimed to bring the solution in NZS 3604 in line with Clause 5.3.8 of AS 2870. 2. Ground movement – Provision for the additional ground movement effects from trees near to foundations in expansive soils should be considered. Trees remove moisture from the soil for a radius equal to the height of the tree. This causes expansive soils to shrink to varying degrees, and when near houses leads to differential settlement occurring under foundations. Movement of the foundations may lead to cracks in the building and door jamming. Where existing trees (including trees that have been recently removed) are located closer to the foundations than 1.5 times the mature height of a tree, then additional geotechnical advice should be obtained. Planting of new trees should be avoided near foundations of new buildings or neighbouring buildings on sites with expansive soils. 2.3.8.2 In NZS 3604 Clause 7.5, a) add the following paragraph at the end of Clause 7.5.1: i) “Slabs on expansive soils for buildings meeting the requirements of 1.1.5 shall, in addition to meeting the requirements of 7.5.1 to 7.5.12, meet the requirements of 7.5.13. Where there is conflict the requirements of 7.5.13 shall apply.”; and MINISTRY OF BUSINESS, INNOVATION AND EMPLOYMENT 28 JULY 2025 Page 10