A question frequently asked by readers is how do you compensate for the bracing that is removed when demolishing an internal wall during a renovation or refurbishment? We have some advice.
WHEN AN INTERNAL WALL of an existing building is to be removed, the bracing capacity of the remaining building is likely to be reduced, whatever the age of the building and whether or not the existing bracing has been formally designed. For this reason, most BCAs will require some assurance that the remaining (or additional new) structure will still comply with the Building Code. To compensate for the loss of the wall, its contribution to the structure must be assessed, and that contribution must be replaced somewhere else.
NZS 3604:2011 bracing basics
To determine what replacement bracing is required, some knowledge of NZS 3604:2011 Timber-framed buildings bracing design concepts is required.
A fundamental principle of wall bracing requirements in NZS 3604:2011 is that lateral loads must be evenly distributed across a number of relatively closely spaced building elements arranged along bracing lines. These are imaginary lines allocated in two directions to run along and across the full length and width of a building parallel to the external walls. Although they have no physical significance, they determine the locations of the bracing.
Bracing unit (BU) is a term for a bracing rating that both quantifies wall bracing elements and provides a measure of the wall bracing element racking performance. A building must contain bracing units allocated in the bracing lines in both directions. How many bracing units are required depends on construction, size, layout and location of the building.
Working out bracing lines for existing construction
Bracing lines can be used to determine where replacement bracing should go if a wall that has bracing is removed from an existing building. Bracing lines should be applied to the existing building following the same criteria as a new building.
They should align with the external walls of the building, then be allocated to align with internal walls and evenly distributed throughout the building. Rules for laying out bracing lines to ensure good seismic performance include:
they must run in two orthogonal directions
they should be as evenly spaced as practical – generally, there should be more bracing lines rather than fewer to make the distribution rules easier to apply
where two walls are parallel to one another and:
up to 2 m apart, a single bracing line may be placed between them and the bracing elements in both walls will contribute to the total BUs on the bracing line
more than 2 m apart, an extra bracing line should be inserted to facilitate the distribution of the BUs – an additional bracing line will not affect the number of BUs required as the bracing demand is not increased
bracing elements can be located anywhere on a bracing line.
Locating new braced wall elements
Once the locations of existing bracing have been identified and bracing lines allocated accordingly, the location of a new braced wall element can be determined. Generally, the new braced wall element should be located on the same bracing line as the bracing element that was removed.
If a new door or window opening is cut into a braced wall and the remaining wall cannot provide the required bracing, additional bracing may be installed elsewhere on the bracing line (Figure 1). If this is not possible, the new bracing element may be able to be located on a bracing line less than 2.0 m from where the bracing element was removed.
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Manufacturer bracing ratings for new work
Manufacturers usually give the bracing rating in BUs for their proprietary wall systems. Ratings are calculated using the P21 test, developed by BRANZ to evaluate the bracing capacity of wall systems.
However, the test was developed for new building work so it is generally not applicable to existing construction and cannot be used in situations where renovation work is being carried out.
Another approach is needed when determining bracing requirements for renovations.
BRANZ help to determine old bracing ratings
The details of an existing wall can be confirmed during the deconstruction of the wall. To assist this process, a BRANZ study determined bracing ratings (BUs) by testing a range of older generic wall construction types including:
lath and plaster
plasterboard (on one and both sides)
tempered hardboard-lined wall (one side only)
vertical and horizontal corrugated steel
different types of let-in bracing.
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