Design guidance of specifically designed bracing systems in light timber-framed residential buildings

Abstract

In the Canterbury earthquakes, residential houses of mainly light timber-framed (LTF) construction achieved the objective of “safeguarding people from injury caused by structural failure”, as required by the New Zealand Building Code. However, the earthquake damage was often significant and beyond homeowners’ expectations. Of special importance was that the mixed use of typical sheathed LTF wall bracing elements with specifically designed bracing systems significantly exacerbated the seismic damage to the LTF residential houses. This was most likely caused by deformation incompatibilities between specifically designed bracing systems and the conventional gypsum plasterboard walls. The objective of this study was to develop design guidance to mitigate, through better designing and detailing, the earthquake damage to LTF residential buildings when the buildings include a mixture of specifically designed bracing elements and conventional gypsum plasterboard wall bracing elements.

Earthquake damage to LTF buildings is a result of significant differential deformations between different levels or between different parts of a building. Differential deformations between two adjacent levels of a building depend on the stiffness of the bracing elements between the two adjacent levels. Differential deformations between different parts of the buildings are due to deformation incompatibility of the bracing elements over these building areas, especially when the floor or ceiling diaphragms are relatively flexible. This is especially the case when LTF buildings have mixed bracing elements, namely specifically designed bracing elements and NZS 3604 LTF wall bracing elements.

In this guidance, the expected seismic performance level of LTF residential buildings constructed to NZS3604 was assessed, based on available seismic rating test results of gypsum plasterboard walls. Subsequently, the seismic performance requirement for specifically designed seismic bracing elements in mainly NZS 3604 buildings was established as 1%, in terms of storey drift at ultimate limit state. Subsequently, a step-by-step design procedure for specifically designed bracing elements is developed and presented.

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Technical Abstract

In the Canterbury earthquakes, residential houses of mainly light timber-framed (LTF) construction achieved the objective of “safeguarding people from injury caused by structural failure”, as required by the New Zealand Building Code. However, the earthquake damage was often significant and beyond homeowners’ expectations. Of special importance was that the mixed use of typical sheathed LTF wall bracing elements with specifically designed bracing systems significantly exacerbated the seismic damage to the LTF residential houses. This was most likely caused by deformation incompatibilities between mixed bracing elements in a mainly LTF residential house. The objective of this study was to develop design guidance to mitigate, through better designing and detailing, the earthquake damage to LTF residential buildings. The scope of the study was limited to the buildings, which include a mixture of specifically designed bracing elements and conventional NZS 3604 sheathed LTF wall bracing elements.

In this guidance, the construction practice and engineering characteristics of mainly LTF residential buildings were examined first. The examination highlighted that potential stiffness incompatibility between conventional LTF bracing walls and specifically designed bracing elements could significantly facilitate earthquake damage to LTF buildings. In order to achieve stiffness compatibility between specific bracing elements and LTF well bracing elements, the expected seismic performance level of LTF residential buildings with minimum NZS 3604 seismic bracing was assessed. The assessment was conducted, based on a displacement-based approach and available P21 test results of sheathed LTF walls. The assessment revealed that the displacement performance achievement of the building system with minimum NZS 3604 seismic bracing is inappropriate for use as the performance criterion for specifically designed seismic bracing elements. The seismic performance requirement for specifically designed seismic bracing elements in mainly NZS 3604 buildings was established as 1%, in terms of storey drift at ultimate limit state. This was based on observed test results of conventional LTF bracing walls and the current seismic loading standard NZS 1170.5. A step-by-step seismic design procedure for specifically designed bracing elements, which was developed according to a displacement-based approach, is presented.