Wall bracing to NZS 3604

The steps

1. 1

   Know the site and the building

   The wind zone, earthquake zone and soil class; the roof's and walls' weights; and the building's length, width, floor areas and two heights for each level, H from its floor to the apex and h, the roof's height above the eaves.

   Why Every figure below is read from these, so a wrong zone or a missed level carries through to every line.

   NZS 3604:2011 1.3, 5.2, 5.3, Figure 5.3

   How each is decided, on the overview

2. 2

   Find the wind demand, along and across the ridge

   For each level, read the bracing units per metre for its H and h, wind along the ridge and across it, and multiply by the wind zone's factor. Along the ridge it is per metre of the building's width, across it per metre of its length. Up to 25° the walls' length and width are used; steeper, the roof's. A hip roof takes the across figure both ways, a mono-pitch roof the greater of the two.

   Why Wind pushes on the face square to it, so the wider that face and the taller the building, the more it pushes, and the bracing that resists it runs the same way.

   NZS 3604:2011 5.2.6 to 5.2.8, Tables 5.5 to 5.7

3. 3

   Find the earthquake demand

   Read the bracing units per square metre for the level from the table for its foundation, storeys, claddings and roof pitch, multiply by the factor for the earthquake zone and soil class, then by the level's floor area. A part storey in the roof space adds 4 BU/m². Floors of 3 kPa read Section 14's tables.

   Why An earthquake shakes the building's own mass, so heavier claddings and roofs, softer ground and a more active zone all ask for more. It is the same both ways.

   NZS 3604:2011 5.3, Tables 5.8 to 5.10, Section 14

4. 4

   Take the greater of the two, each way

   Along the ridge and across it, separately, the level's bracing demand is the greater of its wind and earthquake demand.

   Why Wind and earthquake do not act together, so the bracing need only meet whichever asks more, and each direction is braced on its own.

   NZS 3604:2011 5.4

5. 5

   Lay out the bracing lines

   Run lines along every external wall, and through the building each way so that no two are more than 6 m apart (5 m in a subfloor; 7.5 m where dragon ties support the external wall). Lines less than 1 m apart count as one. A wing reaching more than 6 m out is braced as a building of its own, and a step of more than 100 mm in a floor or ceiling takes a line of its own.

   Why The lines carry the floor and ceiling back to the ground at intervals. Too far apart, and the ceiling or floor between them bends; a wing or a step breaks the plane that would carry the load across.

   NZS 3604:2011 5.1.5, 5.4.6, 5.5.2.1

6. 6

   Give each line its least

   Each line carries at least 100 BU, and at least 50% of the direction's demand shared among its lines. Each external wall carries at least 15 BU for every metre of its length.

   Why No line can leave its share to the others, and an even spread keeps the building from twisting about a strong side.

   NZS 3604:2011 5.4.7, 5.4.7.2

7. 7

   Choose and place the elements

   Use elements whose rating comes from a maker's tested system, or plasterboard to the Building Product Specifications at 25 BU/m a face. Ratings are for walls 2.4 m high, and no element counts for more than 120 BU/m on a timber floor or 150 BU/m on a slab. A concrete masonry element is at least 1.5 m long. On an internal line, a pair of elements either side of it may be up to 2 m apart.

   Why A rating is only as good as the test behind it and the fixings that hold it down, and the floor it stands on caps what it can pass to the ground.

   NZS 3604:2011 5.4.2, 5.4.7.1, 8.3.1.4, 8.3.2.2; Building Product Specifications 3.5.3.3

   The makers' systems

8. 8

   Check every line and each direction

   Each line's elements add up to at least its least, and each direction's lines add up to at least its demand. A floor or ceiling diaphragm no longer than 12 m and no more than 2 times its width needs no lines within it, and each line at its edge carries at least 15 BU for every metre across it.

   Why Meeting the total but not a line leaves a weak line; meeting the lines but not the total leaves the building short.

   NZS 3604:2011 5.4.7, 5.6

9. 9

   Show it on the drawings

   A bracing plan for each level with every line named, each element's system, length and rating, the hold-downs, and the demand beside what each line and direction provides.

   Why It is what the building consent authority checks the design against, and what the builder builds from.

   NZS 3604:2011 5.4

The terms

Bracing unit (BU)

The measure of bracing: 20 BU is one kilonewton. NZS 3604:2011 C5.1.2

Bracing demand

What the building asks of its bracing, from wind and earthquake, in BU. NZS 3604:2011 5.2, 5.3

Rating

What an element provides, per metre or each, from its tested system. NZS 3604:2011 5.4.2

Bracing line

A line on plan along which elements are placed to carry the demand to the ground. NZS 3604:2011 5.4.6

Bracing element

A length of wall lined and fixed to a tested system, which resists racking. NZS 3604:2011 5.4.2

H and h

H, from the floor of the level being braced to the apex; h, the roof's height above the eaves. NZS 3604:2011 Figure 5.3

Along and across the ridge

The two directions every building is braced in, named for the wind's direction against the ridge. NZS 3604:2011 5.2.6

Wing

A part of the building reaching out from the rest; past 6 m it is braced on its own. NZS 3604:2011 5.1.5

Diaphragm

A floor or ceiling stiff enough to carry the load across to the lines at its edges. NZS 3604:2011 5.6

Hold-down

A fixing at an element's end that keeps it from lifting as it resists racking.

The makers' systems

Each system's rating is its maker's, from its own test, for an element built as its literature sets out. The table gives each family's range; the calculator holds every system.