Plain-English answers to the questions I get asked most

In plain English: the building has to stand up, and keep standing up.

Every building in New Zealand must meet the Building Code. Clause B1 (Structure) says a building must carry the loads it will see over its life, including its own weight, people and contents, wind, snow, earthquakes and soil pressure, with a low probability of failure or damage. Clause B2 (Durability) says the materials must keep doing their job for a set period: 50 years for structure and anything hard to replace, 15 years for parts that are moderately difficult to replace, and 5 years for parts that are easy to replace.

Engineers show compliance through Verification Methods such as B1/VM1 (which points to the loadings and material standards), Acceptable Solutions such as NZS 3604 for timber-framed houses, or an Alternative Solution.

In plain English: it’s not in the rulebook, so I’ll prove it works another way.

If a design sits outside the Acceptable Solutions and Verification Methods, because it uses a new product, an overseas code or an unusual structure, it can still be consented as an Alternative Solution. The designer shows, with calculation, testing, published research or comparable overseas standards, that the design meets the performance requirements of the Building Code. Council then assesses the evidence.

Parts of mass timber design currently go down this path, because New Zealand standards haven’t fully caught up with the products. That’s normal, and it’s one reason an experienced timber engineer saves time at consent.

In plain English: some small jobs are exempt, but they still have to be built properly.

Schedule 1 of the Building Act 2004 lists building work that doesn’t need a building consent. Examples include like-for-like repairs and maintenance, some small single-storey detached buildings (up to 30 m² under set conditions), decks and platforms up to 1.5 m above ground, fences up to 2.5 m, and retaining walls up to 1.5 m that don’t support a surcharge such as a driveway or building. There are also exemptions for certain work designed or reviewed by a Chartered Professional Engineer.

Two catches: exempt work must still comply with the Building Code, and the conditions are detailed. Check with your council, or ask me, before you start.

In plain English: the rulebooks engineers design to, and why we sometimes borrow overseas ones.

In New Zealand, loads come from the AS/NZS 1170 series (1170.0 general, 1170.1 dead and live loads, 1170.2 wind, 1170.3 snow, and NZS 1170.5 earthquake). Materials have their own standards: NZS 3101 concrete, NZS 3404 steel, NZS AS 1720.1 and NZS 3603 timber, NZS 3604 light timber-framed buildings, and NZS 4229/4230 masonry.

Australian standards (AS 4100 steel, AS 3600 concrete) and the European Eurocodes (EC0 to EC9) are often used where New Zealand standards are silent, such as some timber connection design (Eurocode 5), usually as part of an Alternative Solution. Having worked in the UK, I’m comfortable in both systems.

In plain English: buildings where a mistake would hurt a lot of people, so council wants a second set of eyes.

Auckland Council treats a structure as high-risk when the consequences of failure are serious: typically buildings over three storeys, Importance Level 3 and 4 buildings, and structures with complex features such as transfer structures, irregular layouts or unusual materials. For these, council expects the PS1 to come from an engineer on its High-Risk register and requires an independent PS2 peer review before consent.

If your engineer isn’t on the register, I can review and sign off their design (PS1) or provide the independent PS2.

In plain English: how bad would it be if this building failed? The worse the answer, the stronger we make it.

AS/NZS 1170.0 gives every building an Importance Level, which sets how big an earthquake or storm it must be designed for.

• IL1: minor structures where failure is unlikely to hurt anyone, such as farm sheds and fences.
• IL2: normal buildings: houses, offices, shops, most apartments and warehouses.
• IL3: buildings that hold crowds, high-value contents, or many vulnerable people: schools and early childhood centres above set capacities, tertiary buildings, theatres and large assembly spaces, aged care and healthcare with resident patients, airport terminals, prisons, and utilities not needed after a disaster.
• IL4: post-disaster facilities that must keep working after a major earthquake: hospitals with emergency or surgery units, fire, police and ambulance stations, civil defence centres, and the utilities that serve them.
• IL5: exceptional structures such as major dams.

In practice, an IL3 building is designed for a roughly 1-in-1000-year earthquake and an IL4 building for about 1-in-2500, compared with 1-in-500 for an ordinary IL2 building.

In plain English: steel frames resist earthquakes by bending, bracing or deliberately crumpling in the right places.

• Portal frames: columns and rafters rigidly connected, the standard for warehouses, sheds and industrial buildings.
• Moment-resisting frames (MRF): beams and columns with rigid joints that resist earthquake and wind by bending. Open and flexible in plan, but relatively flexible under load.
• Concentrically braced frames (CBF): diagonal braces that meet at the joints. Stiff and economical, but the braces can buckle in a large earthquake.
• Eccentrically braced frames (EBF): braces offset so a short “link” beam yields first, absorbing earthquake energy like a fuse. A favourite in New Zealand’s high seismic zones; T35 Mountain Oak in Queenstown is one.
• Buckling-restrained braces (BRB): a steel core inside a casing that stops it buckling, so it works equally in tension and compression.
• Composite floors: steel beams with a concrete slab on profiled steel decking, acting together.

Ductile means a structure can deform a long way past its elastic limit without collapsing. That’s what keeps people safe in a large earthquake, and it’s designed in deliberately.

In plain English: a concrete strip under the walls. Simple, cheap, and fine on good ground.

A continuous reinforced concrete strip under load-bearing walls, usually 300 to 600 mm wide and sitting on firm natural ground. The standard foundation for houses and light buildings on good ground. Not suitable where the ground is soft, filled or liquefiable, or where loads are concentrated.

In plain English: a slab with polystyrene blocks inside, so the concrete forms a grid of beams.

A slab cast on the ground around polystyrene pods, so the concrete forms a grid of stiff ribs with a thin slab on top. It sits on the ground rather than in it, so there’s little excavation, and the ribbed slab is stiff enough to tolerate some ground movement. Common for houses, especially on expansive clay or moderately poor ground.

In plain English: one big thick slab that spreads the whole building’s weight.

A thick reinforced concrete slab under the whole building, spreading the load over a large area and tying the structure together. Used where the ground is soft or liquefiable and some settlement is expected: the raft moves as one and limits differential settlement. Larger rafts are often thickened under columns and walls.

In plain English: when the good ground is deep, we go down to it.

Piles carry the building’s loads down through weak soil to stronger strata, either by bearing on them (end bearing) or by friction along the pile shaft. The type depends on the ground, the loads, the site access, and how much noise and vibration the neighbours will tolerate.

In plain English: a giant steel screw wound into the ground.

A steel shaft with one or more helical plates, screwed into the ground with a torque motor. Quick to install, with little noise, vibration or spoil, and the installation torque gives a direct check on capacity. Good in tension as well as compression. Common for homes, decks, lightweight buildings and restricted sites.

In plain English: drill a hole, drop in a reinforcing cage, fill it with concrete.

A hole is bored to the design depth, a reinforcing cage is lowered in, and the hole is filled with concrete. Can be large diameter and very high capacity, and can be socketed into rock, as at Highbury Building B. In unstable or wet ground the hole needs a casing or drilling fluid to stop it collapsing.

In plain English: a bored pile where the concrete goes in as the drill comes out, so the hole never stands open.

A hollow-stem auger is drilled to depth, then concrete is pumped through the stem as the auger is withdrawn, and the reinforcing cage is pushed into the wet concrete. Fast, with low noise and vibration, and well suited to unstable or wet ground because the hole is never left open. The trade-off is that you can’t inspect the hole, so quality control relies on rig instrumentation.

In plain English: a screw-shaped auger pushes the soil aside rather than bringing it up.

A displacement screw pile: a specially shaped auger is screwed into the ground, pushing the soil sideways rather than removing it, and concrete is cast as the auger is withdrawn. There’s almost no spoil to cart away, the surrounding soil is compacted, and friction capacity is high. Useful on contaminated sites and where spoil disposal is expensive.

In plain English: not really a pile. Columns of compacted gravel that make the ground itself stronger.

Holes are formed and filled with gravel that is rammed in layers to form dense columns. Rather than carrying the building as a pile would, they stiffen and densify the ground, reducing settlement and improving resistance to liquefaction. Often paired with a shallow raft on soft or liquefiable sites.

In plain English: a steel tube or H-section hammered into the ground until it won’t go any further.

Steel tubes or H-sections driven by a hydraulic hammer to a set depth or until they reach a target resistance. High capacity, easy to splice to any length, and the driving record confirms capacity. The downsides are noise and vibration, which rules them out on some urban sites.

In plain English: treated timber poles hammered in. The traditional New Zealand house foundation.

Treated timber poles driven into the ground, widely used for houses, decks, light commercial buildings and retaining walls. Economical and quick, with moderate capacity and depth. The timber must be treated to a high hazard class, and durability depends on keeping it protected at and above ground level.

In plain English: a timber pile wearing a concrete collar where the ground meets the air.

A driven timber pile whose top section is encased in concrete. The timber is most at risk of decay in the zone where moisture comes and goes near the surface, so the concrete “sock” protects it there and gives a solid connection to the foundation beam or floor. A practical way to get the economy of timber piles with better durability and a cleaner structural connection.