Reducing noise transfer

This Issue This is a part of the Noise in buildings feature

By - , Build 149

BRANZ investment combined with Scion expertise has developed a new structural connection system for timber-framed multi-residential units that maintains acoustic performance.

Prototype ‘across plate’ connector installed in a test wall prior to acoustic testing.

DOUBLE-STUD inter-tenancy wall systems provide good acoustic isolation performance between residential units.

An effective double-stud system consists of two frames that are not rigidly connected. This structural disconnection between units means that each unit requires a lot of structural bracing to maintain the gaps when under seismic or wind load.

Both structural and acoustic performance

It can be difficult to design sufficient bracing to meet these structural performance requirements. The amount of bracing required in each individual unit can be reduced by transferring the axial and shear loads between individual unit floor diaphragms. Coupling of the units also helps to eliminate individual unit overturning forces by creating a more squat structure.

There is now a new structural connection system that provides the necessary connection between light-framed timber multi-residential units without significantly compromising acoustic performance.

The project to develop the connectors was part funded by the Building Research Levy and led by the Built Environment team at Scion.

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New design developed

After testing some initial concept connector designs, a final prototype design was developed.

The connector spans the gap across the bottom plates of the double stud wall. It works by minimising the rigidity of the connection from one side of the connector to the other while maximising the load transfer between the frames.

The connector is attached to the floor diaphragm by four 160 mm long self-tapping wood screws that penetrate through the bottom plate and flooring sheet into the edge joists. This provides the required structural connection to the floor diaphragm.

Prototype ‘across plate’ connector installed in a test wall prior to acoustic testing.

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Structural performance tested

Testing at Scion simulated earthquake sequences by adopting the earthquake test procedure loading sequences in the BRANZ Evaluation Method No. 1 (1999) Structural joints – Strength and stiffness evaluation. Results enabled load-displacement curves to be plotted for shear and axial loads applied to the connectors.

Each connector has a shear and axial load transfer of:

• 500 N (average) at displacements of 2 mm – assumed serviceability limit state

• 2,100 N minimum at displacements of 10 mm – assumed ultimate limit state.

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Good acoustic performance

Full-scale acoustic transmission loss wall measurements were performed at Auckland University’s Acoustic Test Laboratories on a common intertenancy double-stud wall system. The wall had two layers of 13 mm fire-rated plasterboard, a lining separation of 205 mm and acoustic infill. The acoustic isolation ratings of the wall were Rw = 62 dB and STC = 63 dB.

The connectors were mounted at 600 mm centres, the minimum expected spacing of connectors for structural load transfer in high-risk seismic zones.

The acoustic test results showed that the connectors reduced the acoustic isolation ratings of the wall by a maximum of 1 dB (Rw = 61 dB, STC = 62 dB). This is an insignificant amount, not noticed by people.

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Connectors will be available

The design of the connectors and installation instructions are in report ER1 Better acoustically performing structural connections available from the BRANZ website, It is possible to fabricate the connectors using this information.

Scion is currently working with industry partners to develop refined versions of these connectors.

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Other project members include Andrea Stocchero and Doug Gaunt from Scion, Warwick Banks from Knowles Consulting, Daniel Scheibmair from Pryda, In Ling Ng from MiTek NZ, Keith Ballagh from Marshall Day Acoustics and George Dodd from the University of Auckland.

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Prototype ‘across plate’ connector installed in a test wall prior to acoustic testing.