Engineered wood products now included

This Issue This is a part of the NZS 3604:2011 and more feature

By - , Build 124

The new NZS 3604 includes the use of laminated veneer lumber (LVL) and glued laminated timber (glulam), now common in residential construction.

Over the past 10 years, LVL and glulam products (collectively termed engineered wood products or EWPs) have become more prevalent in residential construction, particularly in lintels, beams, rafters and floor joists where larger section sizes of solid timber have become less available or are unable to span the distances required. Currently, it is estimated that over a third of lintels used in prenailed wall frames use LVL or glulam timber.

Standard has restrictions on use

Because EWPs are mainly proprietary in nature (that is, grades are particular to one manufacturer), their inclusion within NZS 3604 needed to:

  • fit within the scope of the existing standard
  • not create situations where resultant loads may compromise solid timber components able to be selected from existing tables
  • recognise the specific engineering design required to produce selection charts or software.

NZS 3604 is a prescriptive standard so it is necessary to provide restrictions in order to remain within the scope of the document. Clause 2.3.9 does this by restricting EWPs to only include LVL and glulam products manufactured using radiata pine or Douglas fir and nominating their respective manufacturing standards (AS/NZS 4357 Structural laminated veneer lumber and AS/NZS 1328 Glued laminated structural timber).

Timber treatment of EWPs

For timber treatment, NZS 3604:2011 calls up NZS 3602:2003 Timber and wood-based products for use in building.

Although NZS 3602 allows untreated LVL for subfloor framing, several of the NZS 3602 tables do not have LVL and glulam options. Where the level of treatment for EWPs is not listed in NZS 3602, a conservative approach has been taken in NZS 3604:2011 by requiring the same level of treatment that would be required for kiln-dried radiata pine to comply with NZS 3602.

When read in conjunction with the recent amendment to B2/AS1, it would appear that a minimum level of H1.2 would apply for LVL in many cases. Specifiers and builders should note, though, that few producers of LVL provide an H1.2 option at present, so the H3.1 LOSP treatment is most likely to be provided when treatment is required.

Use by substitution or specification

EWPs can be used in two ways within NZS 3604:2011 – as a direct substitute for solid timber grades like SG 6, 8 or 10 (clause 2.3.9.5) or as a proprietary grade (clause 2.3.9.6).

DIRECT SUBSTITUTION

When using the direct substitution method, EWPs must:

  • be the same finished size as the member being substituted
  •  have the grade verified as meeting the requirements of an SG grade and be marked in the same manner as an SG grade in accordance with NZS 3622.

While few manufacturers of EWPs are likely to do this, there is one example in the marketplace of LVL studs provided as being SG 8.

Back to top

PROPRIETARY GRADE

Otherwise a proprietary grade can be used. This can be a different size from the solid timber member being substituted and have different engineering properties (usually better) as long as:

  • it is a framing member for a building within the scope of NZS 3604:2011
  • any loadbearing reaction from the member is not greater than 16 kN in either an upwards or downwards direction
  • the selection charts or software used have been engineered as a minimum in accordance with B1/VM1.

The loadbearing reaction figure is a conservative approach – 16 kN was near the upper limits of loads generated by the solid timber members already selectable in NZS 3604 and likely to be substituted with EWPs, even though other framing members generate higher resultant loads. It was also the figure recommended for the loadbearing reaction limit for roof trusses.

Documentation will need to be provided to support the charts or software used to select proprietary grades (see commentary clause 2.3.9.6). This should include a producer statement (design) from a chartered professional engineer covering the engineering basis of the selection charts or software used.

EWPs for future revisions

The introduction of EWPs into NZS 3604:2011 provides more options for users of the standard. Future revisions may seek to clarify or add to the use of EWPs and possibly include composite EWPs such as ‘I beam’ joists.

Back to top

Download the PDF

More articles about these topics

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

Advertisement

Advertisement