What is MDH?



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Many elements within a medium-density housing development need to be restrained, particularly when the building is subject to seismic loads.

Damage to and repair of vulnerable non-structural components can present a high economic burden after an earthquake.

Common non-structural components in a large MDH development include:

  • above-ceiling services, such as ducts, cable trays and air-handling units
  • roof-mounted plant and equipment
  • suspended ceilings
  • wall cladding panels attached to the main structure
  • internal partitions and walls
  • fire sprinkler pipework

All components and associated supports and restraints should be attached to the supporting structure. This ensures that seismic forces are properly transferred to the structure and greatly increases the likelihood that the building will continue to function immediately after a seismic event.

Installing services and non-structural components, particularly those in the ceiling space and partitions, requires careful planning and cooperation between the relevant trades. Coordination meetings between the trades and the development’s seismic specialists should begin as soon in the design process as possible.

Plant and equipment

The most applicable standard for securing plant and equipment in an MDH development is NZS 4219:2009 Seismic performance of engineering systems in buildings. This standard requires the arrangement and layout of engineering systems within a building to allow for earthquake actions.

When following the NZS 4219:2009 non-specific design pathway, the first step is to determine the earthquake demand on all of the components that make up the engineering systems within the building. The procedure to determine earthquake demand on individual plant or equipment is then to:

  • classify the building importance level and component categories
  • determine the static earthquake load on the plant or equipment
  • obtain the building’s expected seismic displacement from the structural design.

The design should take into account the resilience of plant, equipment and other engineering systems. In particular, it is important to consider:

  • how these systems protect life and property in an emergency such as by providing safe egress
  • the effect of heavy equipment on the structure and other building components – provide sufficient space between components to allow movement without collision
  • the seismic resilience of connections of public utility services – use flexible couplings to accommodate movement of services such as water and sewerage pipes
  • the consequences of leakage from damaged pipes and vessels
  • how seismic bracing will affect, accommodate and integrate with the engineering system’s non-seismic requirements, such as thermal expansion in pipework and wind forces on roof-mounted equipment.

Suspended ceilings

Where suspended ceilings are used in MDH developments, they should be designed and constructed in accordance with NZS 1170.5:2004 Structural design actions – Part 5: Earthquake actions – New Zealand and AS/NZS 2785:2000 Suspended ceilings – Design and installation.

AS/NZ 2785:2000 provides both specific and non-specific pathways to design a suspended ceiling system. The pathway the designer should choose depends on the seismic demand determined by NZS 1170.5:2004. This takes into account the location of the MDH development, component categories and requirements of the ceiling being designed. This information and AS/NZ 2785:2000 are then used to determine whether the suspended ceiling requires a specific design or if a non-specific design will be sufficient.

Non-specific design guidance is usually provided by the suspended ceiling manufacturer. A suspended ceiling may require a specific design because of:

  • ceiling complexity
  • ceiling size
  • floor-to-ceiling height
  • ceiling-to-structure height
  • building components in the ceiling voids
  • working life of the design.

The Association of Wall and Ceilings Industries of New Zealand (AWCI) has developed a seismic grade from 1–4 to describe the seismic requirements of suspended ceilings.

Interior partitions and walls

There are no Acceptable Solutions containing specific references or guidance for non-structural partitioning systems, so specific engineering design (SED) with reference to AS/NZS 1170 may be required. Verification method B1/VM1 to Clause B1 of the NZBC references the AS/NZS 1170 suite of New Zealand Standards. Relevant parts of the standard include:

  • AS/NZS 1170.0:2002 Structural design actions – Part 0: General principles
  • NZS 1170.5:2004 Structural design actions – Part 5: Earthquake actions – New Zealand.

These standards should be referred to when designing the installation of non-structural partitions.

AS/NZS 1170.0:2002 requires buildings to be designed and constructed so that during their design working life, they are able to withstand all actions and environmental influences that may be imposed on them.

Section 3 of AS/NZS 1170.0:2002 requires that buildings must be able to withstand seismic actions so that:

  • the building does not collapse
  • parts of the building do not collapse if they present a hazard to people inside the building
  • the non-structural systems that will enable people to evacuate the building do not collapse.