When it comes to bracing, you are most likely to think of a house’s piled foundations, walls and roof. But if you’re following NZS 3604:2011 Timber-framed buildings to demonstrate Building Code compliance, you must meet requirements around earthquake bracing for large decks as well.
Details for bracing requirements for decks are in NZS 3604:2011 section 7. Where a deck projects more than two metres from a building, it must have anchor and/or braced piles (7.4.2.2).
These must be designed at half the bracing demand shown in Table 5.8 in the standard. Look in the “subfloor structure” column of the table and find the bracing unit figures (BU sq m) that apply to light roof cladding, light single-storey cladding, light subfloor cladding and a roof pitch in the 0–25° category.
Multiply the figure shown in the table by a factor based on the soil class and earthquake zone (found at the base of the table). If the subsoil classification is unknown, use soil class E.
Finally, halve the figure to reach the required bracing demand for the deck. In practical terms, the figure from the table in these circumstances will always be 15.
If you use the multiplication factor for subsoil class E (1.0) and the deck is 20 sq m, 15 x 1 x 0.5 x 20 = 150 bracing units.
These calculations can be used for timber decks supported from the building and where there is no more than three metres between the top of the deck and the lowest cleared ground level (7.4.1.1).
Decks at a greater height require specific design. There are specific requirements around the distribution of bracing units (5.5) — 50% of the total bracing demand divided by the number of bracing lines in the direction being considered (along or across).
External subfloor bracing lines (5.5.2.2) require not less than the greater of the two above from paragraph 5.5.2.1, or 15 BUs multiplied by the length in metres of the external wall.
Bracing units must, as far as possible, be evenly distributed along lines of bracing. Bracing lines must generally be at no more than five-metre spacing — where they do not exceed five-metre spacing, bracing is required along external lines only (see above graphic).
There are no subfloor bracing requirements for decks where the deck projects no more than two metres from the building and there are stringers and/or joists bolted to the building (7.4.2.1).
The standard also makes it clear that wind bracing demand for decks can be ignored (5.2.9).
