Bracing ratings

By and - , Build 141

Recent BRANZ testing has quantified the bracing ratings of some common older generic bracing systems. These ratings will be useful during repairs or renovations of older buildings.

The Brace 3 specimen – double diagonal braces cut between studs.

FOR NEW HOUSES, manufacturers generally provide wall bracing ratings for their proprietary systems based on results of the BRANZ P21 test method. Designers then ensure that the demand wind or earthquake loads at each level and in each direction are less than the sum of the resistances of the bracing elements.

For renovations or repairs of older buildings, however, the bracing strength of existing construction is often not known. What should be used in the bracing calculations required by building consent authorities?

BRANZ tested older systems

In a Building Research Levy-funded project, BRANZ tested a range of older bracing systems (see Table 3) to provide wall bracing ratings.

In most cases, 2.42 m high timber frames were constructed using 90 × 45 mm kiln-dried MSG 8 radiata pine timber with plates nailed to studs with two 90 × 3.15 mm power-driven glue-shank nails. Although these differ from the original timber and nails, the difference in performance is considered small.

The bottom plates of the walls were fixed to the foundation beam using pairs of 100 × 4 mm hand-driven galvanised nails at 600 mm centres starting 150 mm from the outside stud.

Nogs, where used, were at 800 mm centres except for system Brace 4, where they were at 600 mm centres.

Studs were at 600 mm centres (although in practice they were often at 450 mm centres) except for Lath 1 where they were at 400 mm centres.

Each specimen was subjected to three cycles of in-plane displacement at top plate level to each of +/-8.5 mm, +/-15 mm, +/-22 mm, +/-29 mm, +/-36 mm, +/-43 mm and +/-65 mm.

Back to top

…and established bracing ratings

The proposed bracing ratings for existing and renovated walls based on the BRANZ testing are in Table 1.

The Brace 3 specimen – double diagonal braces cut between studs.

Budgetary constraints meant that it was not possible to test three replicates of each system but the bracing contributions are generally quite low, meaning that any variations in actual strength compared to the tested strength would not influence the overall resistance of the structure markedly.

Back to top

For more

See updated Table 1 at Bracing ratings (Build 144, page 24) and BRANZ Study Report SR305 Bracing ratings for non-proprietary bracing walls can be downloaded from www.branz.co.nz.

Download the PDF

More articles about these topics

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

The Brace 3 specimen – double diagonal braces cut between studs.

Advertisement

Advertisement