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The importance of bracing elements
All houses are designed to withstand external forces imposed on them, and the two that are most frequently looked at under the Building Code when assessing Structure – B1 of the Code - are Earthquake and Wind loadings.
Both forces react in a different way and it is important to calculate for both, and taking the larger figure into account then to brace the property to sufficiently withstand these loadings.
Imaginary Line Labels are then drawn throughout the house (normally at 5 or 6m intervals), although when using a ceiling diaphragm which stiffens the roof plane, these can be extended out to 12m. Bracing elements measured in BUs (Bracing Units) are then designed at regular intervals along these Line Labels to provide lateral structural bracing for the property.
These elements commonly consist of one or a combination of the following elements:
- Standard plasterboard linings (1 or 2 sided) which are screw fixed off at close centres
- Reinforced plasterboard (such as Braceline) which provides superior bracing to that mentioned above
- Ply or Strandboard type bracing sheets; which is normally fixed to the framing timbers externally
- A combination of plasterboard and ply linings
- Sometimes a steel braced portal is used and engineer designed where standard solutions do not provide sufficient BUs.
When we conduct our Silver and Gold inspections we sometimes identify walls in which these bracing elements have been compromised or no longer exist – affecting the parameters for which the house was originally designed for and potentially making it unsafe.
These elements are typically compromised in one of several ways and often include:
- A complete or partial wall removal (many builders assume incorrectly that a wall can be removed if the roof is supported by pre-nail trusses which carry the roof weight load)
- The installation of, or failure of a shower against the bracing element
- The cutting of a hole larger than a hot-point size inside the bracing element.
Below we see three examples where these important bracing elements have been weakened or destroyed as a result of poor workmanship by a builder, or ignorance by the owner.
A common mistake where the wall has been removed as part of a kitchen renovation to open-plan the area and no consent has been sought. In this case the beam also carried structural roof load weight and was likely undersized.
The tiled shower backed onto the garage lining and had failed earlier so the bracing element was partially replaced but never completed correctly. Subsequent failure has seen the shower fail a second time and caused the plaster board lining to deteriorate and lose most of its bracing strength.
A new build, which inadvertently passed its inspection by council officers who failed to correctly identify the large cut-out in the Strandboard lining for a water toby and shut off valve; allowing for the potential torsional buckling of the element in a significant seismic event.
Good building inspectors aren’t born, they’re trained and at NZ House Surveys our staff are the best and receive regular ongoing training as a commitment to the service we offer our customers.
Don’t leave your next purchase to chance – call our friendly team today to ensure your next purchase is safe for tomorrow - Phone 0800 487 884.