Issue 28 – June 2011
For some years, spans given in NZS 3604 Timber-framed buildings for timber floor joists have been the maximum span for the specific timber size and spacing, based on the ultimate strength, deflection or stiffness and (for internal joists) floor vibration. Where noticeable vibration does occur, it is likely to be as a result of the way the floor was designed and built, such as:
- floor joists supported by a beam or lintel
- shrinkage of solid blocking
- the use of trimmer and trimming joists within the floor
- going beyond the maximum spans/spacing of members.
It is more likely to be an issue in buildings built before the mid-1980s. However, there have been calls to the BRANZ helpline about buildings built during the 1990s where the floors have a noticeable vibration during normal household activities. Where floor deflections are particularly critical in new construction or renovation work, designers should discuss with owners the design options available, such as:
- increasing member size or reducing spans or spacing of joists – maximising spans given in the current NZS 3604 may mean that deflection in the floor will be felt by some people
- not taking beam and lintel spans to their maximum
- ensuring framing is dry enough when installed
- ensuring blocking is used and properly installed.
As said in the April Guideline, attempting to remedy noticeable floor deflection once a building is completed, particularly for upper floors, can be difficult and costly.
Information supplied by BRANZ.
- NZS 3604:2011 Timber-framed buildings