21 August 2006
Springy floors, sagging lintels, and walls moving excessively in high winds should be problems of the past with the introduction of Amendment 2 to NZS 3604:1999 – Timber framed buildings.
NZS 3604 provides practical guidance on how to design and build houses to meet the requirements of the Building Code. Amendment 2 introduces new timber grades so designers can now choose to use higher grades than No.1 Framing. Regardless of which timber grade is used, the new design tables will ensure that the building will perform well. This should increase home-owners’ confidence in the building process.
Benefits to home owners, designers and builders
“Home owners, designers and builders can be confident that using this Amendment will result in better quality houses, fewer complaints from homeowners and less chance of call backs,” says Mark Batt, Business Relationships Manager at Standards NZ.
“Amendment 2 is already being widely used in the building industry since its publication in June. Standards NZ strongly recommend that designers who have not already done so, start using this Amendment immediately, as we believe houses built to the existing Standard may not perform as well as they should.”
Using the amended version of NZS 3604 should significantly reduce problems caused by using weak timber, such as overly springy floors, sagging beams (lintels), and walls moving excessively in high winds – problems that can result in considerable remedial work and dissatisfied home owners.
What has changed?
NZS 3604 provides details on how to design and build a house and is used by a wide range of people in the building industry such as designers, builders and engineers. The Amendment revises the existing tables for No.1 Framing and introduces new timber grades.
Tim Barton, an architect in Blenheim, experienced practical problems with sagging lintels made under the old Standard and says the Amendment helps to solve these problems. “We were receiving many complaints about the aluminium-framed doors we had specified. These doors are difficult to ‘ease’ and are sensitive to any bowing of the over-head beam, to which they’re attached.”
“This caused arguments between the door suppliers and the carpenters who had installed the timber beam, concealed immediately above the doors – it’s difficult to see whether this beam (the lintel) had bowed under the weight of the building above or not. I’m sure many of the defects in the doors had been caused by lintels we had specified from the old Standard and neither the aluminium window suppliers nor the carpenters were to blame.”
The existing Standard was largely based on a single timber grade – No.1 Framing. However No.2 Framing was allowed for non load-bearing walls. No.1 Framing is graded visually but is not checked to confirm the timber’s engineering properties (strength and stiffness).
In Amendment 2:
Due to the lower design properties of the unverified No.1 Framing, some of the spans and spacings allowed have been reduced. This has been done because of evidence that some of the pine being milled today has significantly lower strength and stiffness than in the past. Reasons for reduced strength and stiffness include early milling and use of fertile ex-farm land for forestry.
A number of new timber grades have been introduced. This timber has been through a verification process to confirm the engineering properties (strength and stiffness). Two additional sets of tables are included in the Amendment, providing information on the use of new timber grades.
The Amendment will better link the properties of New Zealand’s wood resource to engineering requirements of our houses, which will result in better performing houses and should increase consumer confidence in timber houses.
Citation in the Building Code
Until the Department of Building and Housing has made its decision about citing this Amendment, it will need to be treated as an Alternative Solution. However, because spans and spacings have been reduced for No. 1 Framing, and properties verified for other grades, we expect territorial authorities (councils) will have no problem accepting the use of this Amendment.
Timber grades now included in NZS 3604
After extensive industry consultation, the Standards NZ committee settled on the following grades:
Machine stress grades (MSG)
“In the Timber structures Standard (NZS 3603), the ‘F’ grades have been deleted, as these historic grades were not seen as relevant,” says committee member Doug Gaunt from SCION. “A set of new, properly verified MSG grades have been introduced, based on Australian MGP grades. The new MSG grades in NZS 3603 range from MSG6 up to MSG15 and cover the range in properties of NZ’s forest resource.”
MSG grades that have been introduced into NZS 3604 (note: MSG12 and MSG15 exist but are not covered in 3604) are:
MSG 8 – broadly equates to the properties of the old No. 1 Framing grade.
Visual stress grades (VSG)
“The big change for the visual grades has been the introduction of engineering property verification, which has resulted in the new VSG10, VSG8 and G8 grades,” says Doug Gaunt. “Each of these grades, like the MSG grades, has regular checks (verification) on bending strength and stiffness.”
“For those producers who are not in a situation to undertake verification, the old grade of No. 1 Framing still exists, with lower design values. Span tables are included in Amendment 2 for this grade. No. 2 Framing is still allowed in non load-bearing internal walls.”
VSG 8 – broadly equates to the properties of the old No. 1 Framing grade.
Committee member Roger Shelton, Structural Engineer at BRANZ, says “given the new timber grading regime described above, Standards NZ decided to take the opportunity to develop completely new engineering models to derive the various selection tables.”
“The models developed were easily able to accommodate these grade variations, as well as the changes in timber properties caused when timber graded and supplied dry is allowed to become wet in service. The models also incorporate some systems effects, such as stiffening by linings and claddings, member end conditions, and limited load sharing.”
A notable departure from previous versions of the Standard is the change from ‘call’ sizes, as defined in the Standard for Metric dimensions for timber, NZS 3601:1973, to finished timber sizes. References to timber sizes that are not commonly available, such as widths of 125 mm and thicknesses of 70 mm, have also been removed.
Designers will need to make sure they have clearly specified which grade they are designing to and builders need to ensure they are using the grade of timber specified. Timber merchants will not necessarily stock all grades, so designers should check before specifying.
As well as ensuring that the correct strength grade is used, it is also critical to ensure that when treated timber is required, the appropriate level of timber treatment is used. The Standard for Timber and wood-based products for buildings, NZS 3602:2003 specifies the full requirements for timber treatment in buildings.
New research investigates maximum bracing capacity
For houses, a vital element of the overall bracing system is bracing walls, which are built to resist the forces of wind and earthquakes. Over many years, findings from BRANZ Ltd research projects have provided input to the development of documents such as NZS 3604:1999 Timber framed buildings, to make NZ homes safer.
BRANZ has a research project underway that is investigating the seismic performance of ‘whole’ houses, to determine how all the parts of a house contribute to overall bracing performance. One part of this project is to determine the maximum strength of individual bracing walls for a typical house that was designed and built using NZS 3604. Analysis of the initial results indicates that the strength of typical suspended timber floors, to which bracing walls are connected, may be less than previously thought.
On this basis, BRANZ is undertaking additional research to investigate maximum bracing capacity in homes. The project, to be completed by the end of this year, may lead to some recommendations for changes to bracing detailing. The Department of Building and Housing and Standards NZ will be monitoring the project’s progress.
For further information about this project, please contact Greg Baker, Manager of Fire and Structural Engineering, at BRANZ Ltd, e-mail email@example.com, phone 04 238 1365.
Copies of the following Standards are available for purchase on this website or by calling 0800 782 632: