In the previous edition of Straight Up we covered the emergence of particularly Mid-Rise and Medium Density Housing (MDH) construction in NZ. In this article we’ll further explore the implications this likely has on Building Control and other construction related professions.

One of the considerations already touched on in the April ‘19 Straight Up article was weathertightness. And indeed further work may be needed to better understand how cladding systems used for traditional residential construction can be applied to mid-rise and MDH developments. For example the overall settlement/shrinkage of taller buildings, particularly prevalent in taller timber buildings, must be designed for in the exterior cladding and associated junction details. Similarly with cladding performance during seismic events where movement in taller buildings differs substantially to the that of residential dwellings. This is an area BRANZ has identified as being of high importance and is researching to identify potential weak spots and factors that may limit the performance of different claddings designed for residential construction but used in medium density housing or midrise buildings.

There are other relatively obvious impacts; clearly dwellings with multiple tenancies will require different noise and fire considerations to ensure tenancies remain ‘separated’. Consideration of acoustic performance in broad terms requires two feasible sound transmission paths to be consider in mid-rise and MDH buildings:

• Airborne sound such as voices, music, or traffic noise for example is evaluated using the sound transmission class (STC) system.

• Impact sound transferred through the structure or its elements such as footsteps, moving furniture or ‘knocking’ plumbing is measured using the impact insulation class (IIC).

The STC rating ranges from poor sound control with little privacy at STC<30, to STC 40–50 where raised voices can be heard in adjacent tenancies, and STC >50 providing reasonable acoustic privacy. Occupants in mid-rise and MDH developments do expect a reasonable level of privacy and the ability to enjoy their own private space without undue influence of other tenancies within the same building. And while the NZ Building Code clause G6 sets minimum sound insulation at STC ≥55 for inter-tenancy walls and floors and the IIC ≥55 for inter-tenancy floors, ultimately what we consider an ‘acceptable acoustic influence’ can vary between individuals. The likely occupancy group of a mid-rise and MDH development should hence be taken into consideration also, rather than simply achieving minimum required levels.

Fire considerations essentially follow a very similar approach to the acoustic design in controlling gas/smoke and heat transfer. Current Acceptable Solutions provide prescriptive guidance which can cover some MDH developments under C/AS1 Houses, small multi-unit dwellings and outbuildings, where each dwelling has an independent egress route and no more than one dwelling unit above another (eg. townhouses/terraced housing), or C/AS2 Buildings with sleeping (noninstitutional, multi-unit dwellings) for dwellings that have shared escape routes or are more than two dwelling units high. Where buildings are designed for mixed-use then obviously other Acceptable Solutions will also need to be considered; for example C/AS5 Buildings for business, commercial or low-level storage or C/AS7 Vehicle parking. BRANZ provides further detail in their Guide to the Acceptable Solutions: Protection from Fire which can be downloaded from the BRANZ website.

An alternative to the Acceptable Solutions is the Verification Method C/VM2 and engineering analysis to show compliance with fire safety requirements of the NZ Building Code. Smaller MDH developments should be feasible within the bounds of C/ VM2, but its limitations will likely see larger MDH or mid-rise construction requiring more sophisticated fire protection analysis and systems. Aspects usually included as specified systems on building compliance schedules and under annual building warrant of fitness regimes. This is as a result of higher occupancy density and increased Fire and Emergency complexities – such as time required to set up fire-fighting equipment, adequate unobstructed access to fight a fire, and required infrastructure such as hydrants or standpipes being available. All of which clearly are very different in comparison to residential dwellings.

Further compliance impacts that might be less obvious could be overall stability (bracing) of the building for example. While the same structural principles apply to bracing design of stand-alone residential dwellings and mid-rise and MDH buildings, common residential solutions often no longer provide the required resistance or aren’t cost effective. Bracing in residential dwellings typically relies on sheathing (eg. plywood, plasterboard, fibre cement) fixed to framing with the framing tied together using connectors (steel straps, screws, hold-down brackets etc). In taller light timber/stick frame MDH or mid-rise construction the bracing on each floor level can still be achieved using sheathing, however transferring those loads through the framing down to the foundation/ground level using the ‘piecemeal’ connector approach becomes highly inefficient and load capacities of connectors is often exceeded on the lower levels as loads from each of the above floors accumulate.

Overseas mid-rise buildings using light timber frame utilise a continuous Anchor Tie-down Systems (ATS) consisting of steel threaded rods that run the full height of the building and are tied into each floor level using steel bearing plates. The ATS also deals with shrinkage of timber buildings (refer weathertightness paragraph and April ‘19 Straight Up article) using take-up devices installed at each floor level that self adjust over the buildings life to ensure a tight fit between bearing plates, washers, and nuts to transfer each floor level’s loads into the ATS steel rods.

Similar to how bracing systems in residential construction are anchored into foundations/sub-floors, ATS also terminates into the foundation with anchors to dissipate the loads into the supporting ground (though ATS loads are much greater than loads in residential construction). Further detail on ATS installation and components can be seen in this YouTube video. Heavier duty connectors and ATS components required for timber mid-rise and MDH developments have been introduced to the NZ market over the past years as mid-rise and MDH construction gained traction.

These are just a handful of the numerous subtle yet substantial differences between traditional residential and MDH/mid-rise construction. And with the impact of these clearly not restricted to just building control, MBIE is looking at how it as the regulator of building and construction in NZ can aid in encouraging greater uptake of MDH and mid-rise without detrimental effect on safeguarding human life. You’ll be able to read more about the MBIE initiatives and proposed regulatory changes in the next Straight Up.

Daniel Scheibmair Specification Engineer Simpson Strong-Tie