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All medium-density housing must meet the requirements of the New Zealand Building Code for structural performance.

Which structural system is most appropriate for a medium-density housing development strongly depends on the type of building. When considering the requirements for seismic and wind design, the developer has a wide variety of options available.

Structural design

In most cases, more than one structural option is suitable for multi-unit developments, and the designer should carefully consider which structural system is most appropriate.

Typical MDH construction options are:

  • 1–2-storey attached MDH – lightweight timber or steel framing, structural steel beams, precast concrete or concrete masonry
  • 2–4-storey attached MDH – lightweight timber or steel framing, structural steel, proprietary panellised systems, precast concrete, concrete masonry or CLT timber
  • apartments – in situ concrete, precast concrete, structural steel, CLT timber or concrete masonry.

When designing low-rise MDH up to 3 storeys, key decisions include:

  • choice of structural system
  • means to resist lateral loads from wind and seismic events particularly on the more open (glazed) front and back walls where they will be subject to loads form inter-tenancy walls
  • span requirements for mid-floor and roof members in large, open spaces.

For taller MDH of 4–6 storeys, key design decisions include:

  • choice of structural system
  • design performance
  • level of ductility
  • adoption of base isolation
  • incorporation of damage-reducing technologies such as rocking steel frames, special friction joints and PRESSS systems
  • design for inter-storey drift and its impact on the detailing of cladding, ceilings and other non-structural components
  • restraints of non-structural components such as ceilings and plant
  • location of heavy plant and equipment such as water tanks
  • location of vertical shafts.

Complexity

As the complexity of a building increases, it can have a significant effect on other aspects of the building.

The added complexity can increase costs, hinder access, undermine seismic performance, potentially compromise weathertightness as detailing is trickier and generally increase the maintenance burden.

Aspects of medium-density housing design that increase complexity include:

  • curved, sloped, stepped or angled walls
  • decorative or structural components that restrict access
  • steep roof pitches
  • multiple building junctions, elements, materials, components and finishes
  • the desire to be bespoke – building components that are non-standard or not readily available
  • large overhangs or projections of roofs or floors, particularly at higher levels
  • multiple functions within the building envelope, such as ground-floor retail and commercial tenants
  • traditional materials used in non-traditional ways
  • increased reliance on mechanical services and plant
  • lack of maintenance, as a result of complexity.

Generally, these design aspects result in a larger number of junctions between building elements and increased likelihood that those junctions will not perform as expected and in a way that is difficult to predict.

Seismic considerations

All of New Zealand has a degree of seismic risk, which must be allowed for during the design of MDH structures. Typically, structural design of large buildings, such as those used in many MDH developments, uses a deformation-focused force-based design, which places emphasis on the expected plastic deformations of critical structural elements.

Options to reduce possible damage to buildings in an earthquake event include:

  • base isolation – several types and options are applicable to MDH
  • PRESSS or PRESSLAM – these systems enable the structure to self-centre and return to vertical after an earthquake
  • minimising inter-storey drift by designing a more rigid structure.

See the BRANZ Seismic Resilience website for more information on these seismic design approaches.

Wind

Wind must be considered when designing MDH in most parts of New Zealand. Design wind speeds are influenced by:

  • adjacent landforms
  • site slope and location of the building on that slope
  • site elevation above sea level
  • location of the building on the site
  • wind region and lee zones
  • proximity of adjacent buildings.

As buildings become taller, designing for wind should also consider:

  • the building form, including the roof to moderate wind impact
  • the increase in wind loads/pressures
  • the effect of downdrafts on adjacent spaces for taller buildings
  • the impact of wind on weathertightness and deflections in structural cladding elements
  • the negative pressures created on walls and roofs, particularly in lee areas around the building and across low-slope and pitched roofs
  • the effect of existing and proposed or permitted adjacent development on the proposed building.