Page 33 - Build 151

DESIGN
Suspended oors
The BRANZ Technical Helpline often receives queries for designing and building suspended timber floors and dealing with uplift. Nine of the most common questions are answered here.
oor joists
solid blocking at mid-span when joist span > 2.5 m
RIGHT
COMMON QUESTIONS ABOUT DESIGNING AND BUILDING SUSPENDED FLOORS USING NZS 3604:2011
TOM EDHOUSE,
BRANZ TECHNICAL ADVISOR
option 1: continuous boundary joist, 25 mm thick and same depth as joists
option 2: solid blocking (at same depth as joists)
a) Suspended oor on piled foundation.
NZS 3604:2011 Timber-framed buildings sets out the requirements for suspended oors.
What’s required to laterally support ends of oor joists?
NZS 3604:2011 has two options for the lateral support at the ends of oor joists for a suspend- ed oor on a pile foundation (see Figure 1a):
● Aboundaryjoist25mmthickandthesame depth as the joist.
● Solid blocking the same depth as the joist with individual blocking pieces xed at 1800 mm centres. Blocking needs to be directly above and xed as required in Table 7.5 to either:
• a bearer connected to sub oor bracing
• a top plate of lower oor braced walls. Bearer plates attached to a concrete foundation acting as a brace require an 1800 mm length of blocking in the line of the foundation wall (see Figure 1b).
1800 mm centres maximum
timber pile
wall plate
bearer
continous solid blocking for 1800 mm when sub oor support is foundation wall
concrete foundation wall
1800 mm
b) Suspended oor on concrete foundation wall.
Figure 1 Suspended timber oor joist layout (redrawn from NZS 3604:2011 Figure 7.2).
What lateral support of oor joists is needed under oor diaphragms? Lateral support to the joists of a oor diaphragm can be provided by:
● full joist height solid blocking between all joists around the perimeter
● a minimum 25 mm thick boundary joist at the full depth of the joist around the perimeter.
The potential loads are greater, so NZS 3604:2011 Figure 7.9, Table 7.5 and clause 7.3 give increased xing requirements for where the oor acts as a diaphragm.
What mid-span lateral support is needed?
Mid-span lateral support is required to be installed:
oor joists
Build 13519—Decemberr201153//JJanuarry201164 — 31