Specific bracing design

By - , Build 147

Changes are needed after houses with a mixture of NZS 3604:2011 wall bracing and specifically designed bracing performed poorly during the Canterbury earthquakes. BRANZ’s new design procedure for seismic bracing elements in new houses will help.

MOST RESIDENTIAL BUILDINGS in New Zealand are light timber-framed buildings constructed according to NZS 3604:2011 Timber-framed buildings. This standard is also cited as an Acceptable Solution to New Zealand Building Code clause B1 Structure.

NZS 3604 gives bracing demand

In NZS 3604:2011, the seismic demand is determined from a predefined table, based on the soil classification, seismic hazard zone, house foundation type and building envelope weight.

Designers only need to match the bracing demand to the capacity provided. NZS 3604:2011 specifies using the P21 test developed by BRANZ to evaluate the seismic bracing capacity of proprietary light timber-framed wall elements.

However, NZS 3604:2011 has limitations on application, and many new timber-framed houses require specifically designed bracing elements, especially those with open-plan design.

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Mixed bracing caused issues in Canterbury

So how did timber-framed houses perform during the Canterbury earthquakes in 2010 and 2011?

Earthquake-damaged timber-framed houses of all ages did meet the current New Zealand Building Code objective of ‘safeguarding people from injury caused by structural failure’. The damage, however, was sometimes significant.

Interestingly, the magnitude of seismic damage varied significantly. It was exacerbated by mixing sheathed timber-framed wall bracing elements (such as gypsum plasterboard) with specifically designed bracing systems (such as steel portal frames or specifically designed plywood walls). Less damage was found on timber-framed houses braced totally by sheet materials.

Stiffness incompatibilities between different types of bracing is thought to be the most likely cause of this variability.

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Elements need to be compatible

Excluding the effects of liquefaction, earthquake damage to timber-framed houses was due to differential lateral deformations between either different levels or different sections within a building.

Differential deformations between two levels of a building depend on the stiffness of the bracing elements. This is often quantified using interstorey drift which is the ratio of the lateral deflection between two levels and the storey height.

Differential deformations between different sections of the buildings occur as a result of deformation incompatibility of the bracing elements over these areas.

Therefore, the stiffness performance of specifically designed bracing elements should be satisfactory for storey drift and also be compatible with the conventional NZS 3604:2011 plasterboard-lined timber-framed wall bracing elements in the same storey.

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Seismic bracing performance investigated

BRANZ examined the expected performance of conventional light timber-framed houses with the minimum seismic bracing required by NZS 3604:2011. This research used typical P21 rating test results of plasterboard-lined light timber-framed walls.

Having this information means specifically designed bracing systems can be chosen to have compatible deformation performance with the NZS 3604:2011 wall bracing systems.

The research did not allow for the adverse effect of an irregular bracing arrangement.

The displacement-based approach was followed and assumed a damping level (an indicator of energy-dissipating capacity) of 20%. The damping level was based on several P21 test results on plasterboard-sheathed light timber-framed walls.

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Findings of flexibility and stiffness

The project had several key findings:

● Theory suggests light timber-framed houses with the minimum seismic bracing required by NZS 3604:2011 are likely to be flexible. They can deflect beyond the Building Code-specified deflection limit of 2.5% storey drift in an ultimate limit state earthquake event.

● However, there is often unquantified stiffening potential in simple and traditional light timber-framed houses that can significantly boost their stiffness. Examples include coupling actions due to lintel beams over doors and windows, panels beneath windows and more available wall length than specified for bracing. However, these types of stiffening effects cannot be guaranteed.

● Where specific bracing elements (such as steel portals) are required in a mainly light timber-framed house, they often have no significant stiffening potential. The performance criterion for designing specific bracing elements needs to include damage control.

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Walls weaker after 1% storey drift

The stiffness performance criterion for specifically designed seismic bracing elements was based on the observed stiffness behaviour of timber-framed wall bracing elements during P21 tests.

Typical P21 test results show that conventional timber-framed bracing walls of reasonable lengths undergo strength degradation after a storey drift of about 1%. The walls are also significantly softer after racking to 1% storey drift.

Consequently, the stiffness performance requirements for specifically engineered bracing were established to be 1% storey drift under the ultimate limit state.

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Seismic design action at ULS defined

The research showed that to satisfy the deflection compatibility, the seismic design action at ultimate limit state of specifically designed bracing should be twice the seismic bracing demands required in NZS 3604:2011.

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New seismic design guidance

BRANZ has developed a step-by-step seismic design procedure for specifically designed seismic bracing elements in houses with mainly NZS 3604:2011 bracing elements.

It can be used where there is a mix of specifically designed bracing and conventional NZS 3604:2011 sheathed timber-framed wall bracing.

This will be available shortly in a BRANZ study report – Design guidance of specifically designed bracing systems in light timber framed residential buildings – from www.branz.co.nz.

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Note

EQC funded this BRANZ project to improve our housing stock and increase community resilience to earthquakes.

Download the PDF

Articles are correct at the time of publication but may have since become outdated.

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