Codes and Standards
As yet there are no Australian or NZ standards specific to the manufacture and use of CLT. However, CLT is very similar to Glue Laminated Timber in that it is manufactured from finger jointed, dried timber using glue lamination under pressure. XLam manufacture complies with Adopted Standards which govern the design, preparation, finger jointing, planing, lamination and verification of XLam CLT products:
|AS/NZS 1328 Glue Laminated Timber Parts 1 & 2|
|AS/NZS 1491:1996 Finger jointed structural timber.|
|ISO/TC 165/SC N695 (Working Draft): Timber Structures – Cross Laminated Timber – Part 1: Component Performance and Production Requirements|
CLT buildings need specific engineering design and certification by a chartered structural engineer. A brief introduction to engineering considerations in the use of XLam CLT is given below. The XLam Design Guide provides information necessary to support structural design consultants.
CLT has a high strength-to-weight ratio, allowing long spans which greatly simplify building structure. Commercial floor spans of up to 7m are attainable without the need for intermediate support. For a residential loading, indicative spans are 3.4m for a 3-layer 105mm floor panel and 5.1m for a 175mm panel. Spans are governed by both deflection and vibration.
The structural performance of a CLT building is reliant on how the panels are connected together. Loads must be effectively transferred from panel to panel. In particular, horizontal seismic and wind forces must be dealt with through effective bracing connections. Long engineered timber screws are the simplest and most commonly used means of connection, supplemented by a range of visible or concealed steel fasteners for specific applications. The use of CNC-machined keyed connections may effectively transfer shear loads and reduce the need for mechanical fixings
The recent Canterbury earthquakes in New Zealand have brought seismic design into sharp focus. For the scale of damage experienced, a relatively low loss of life was largely due to the predominance of timber buildings. In the aftermath, many standing concrete buildings had to be demolished because it was impractical or uneconomic to repair them. In contrast, a CLT structure is safe, and importantly, relatively easy and quick to remediate after an earthquake which is a huge benefit to building owners.
CLT has a high strength to weight ratio which lends itself to seismic design. Performance is very dependent on connections. XLam has developed reference connection details to assist with structural engineering design.
Humans are very sensitive to low frequency vibrations transmitted through a floor. The low damping ratio and fundamental natural frequency of vibration of CLT require careful consideration during design and construction. Effective damping of excessive vibration is achieved through smart design and construction. Floor spans are considered in relation to stiffness of the supporting structure, and the density and thickness of floor coverings.
Creep is primarily dependent on the type and duration of the applied load, the level of stress in the timber, and changes of in-service moisture content. The orthogonal arrangement of layers within CLT make it more prone to time-dependent deformations under load than aligned linear engineered wood products such as glulam. XLam CLT span calculations assume dry service conditions and adopt a creep factor of k2=2.0 for Serviceability Limit State deflection.
|Timber:||Australian and NZ grown Pinus Radiata|
|Glue:||Purbond clear polyurethane glue.|
|Maximum panel size:||15m × 3.4m|
|Panel thickness:||60mm up to 405mm or more in 3, 5, 7 or 9 layers.|
|Treatment:||Untreated, or preservative treated to H1.2 or H3.2 hazard class.|
|Appearance:||Non-visual, visual and premium visual.|
|Surface protection:||Optional factory-applied surface sealer and/or site-applied wrap.|
|Other XLam products:||XLam shear wall.||AirStair CLT commercial stair.||Concealed string residential stair.|
|Certifications:||Fire Testing certification, Durability Statement, Quality Assurance.|
The selection and specification of XLam product is governed by a range of design considerations. For further guidance and support contact XLam.
The sound insulation of CLT building elements relies on appropriate design and construction detailing.
There are two sources for sound transmission through a wall or floor: airborne sound such as speech and music, and impact sound through direct contact with the building structure, such as footsteps and door slams.
Airborne sound transmission loss is designated by a weighted Sound Reduction Index (Rw) in Australia, and in NZ by a Sound Transmission Class (STC). The rating values for each are very similar. Impact sound transmission is defined by a designated Impact Insulation Class (IIC).
The BCA and NZBC set standards for building elements which prevent undue noise transmission to habitable spaces of household units from other occupancies or common spaces.
Compliant ratings can be effectively achieved by wall and floor assemblies incorporating mass (CLT), de-coupled structure through separation, resilient connections, air gaps, and absorbent insulation within cavities. Designers should also consider and avoid ‘flanking’ sound transmission via alternative paths, eg through services ducts or where carried along an exterior wall which is common to two adjoining tenancies.
The good news for designers is that XLam will now be able to provide test results to substantiate theoretical acoustic predictions. Testing of XLam CLT panels by the Timber Development Association is due for completion at the University of Auckland in June 2016. This will be the most comprehensive acoustic testing program for CLT ever carried out anywhere in the world, entailing over 130 tests on 80 different CLT wall and floor applications, both as bare panels and in combination with various lining systems. The test results will provide consultants with hard evidence to substantiate the acoustic performance of CLT construction.
Approvals and Consents
Under both the Australia and NZ Building Codes the use of CLT currently requires project-by-project justification as an Alternative Solution. Alternative Solution proposals must be shown to comply with the performance requirements of the Building Act.
A building permit application is assessed by the RBS (Relevant Building Surveyor) who will consider whether documentation supporting an Alternative Solution for XLam CLT demonstrates compliance with performance requirements under the Act and Regulations. In issuing a determination, the RBS must consider the assessment methods set out in part A0.9 of the BCA. The documentary evidence to support compliance with a Performance Requirement is set out in Clause A2.2 of Volume One or Clause 1.2.2 of Volume 2 of the BCA.
Application for an Alternative Solution in NZ is made under Section 33 of the NZ Building Act. An applicant must satisfy the BCA (Building Consent Authority) that the proposed use of CLT will comply with the performance requirements of the NZBC. The process is outlined under http://www.dbh.govt.nz/blc-alternative-solutions and guidance is given under http://www.dbh.govt.nz/establishing-compliance-alternative-solutions.
XLam will assist building approval and building consent applications with detailed evidence, including an Alternative Solution document to substantiate compliance with performance requirements of the BCA and NZBC.
As at 2017, New Zealand Building Consent Authorities have accepted over 500 Alternative Solution applications for the use of XLam CLT. None have been declined.
Popular perception is that wood burns fast and wood buildings are at high risk and unsafe in fire. The truth is quite the opposite. Certainly kindling burns quickly. But mass timber takes a long time to ignite, and once it does, is protected by the build-up of charcoal which occurs over its face. On its own, timber performs much better in fire than does steel.
The Fire Codes of Australia and NZ are formulated to permit time for occupants to safely leave a burning building before structural collapse or succumbing to heat or smoke inhalation. A code-stipulated safe evacuation period (Fire Resistance Rating/Fire Resistance Level) of up to 90 minutes in Australia and 60 minutes in NZ will cover the vast majority of building types and uses. (Walls at a site boundary may require 120 minutes).
The char rate of Radiata pine is accepted 0.65mm per minute. For XLam CLT this char rating is correlated to allow for glue-line failure above 200 degrees Celsius and a zero strength/stiffness heated zone protruding from the char front. Full scale laboratory testing verifies that, provided the CLT element is specified to retain sufficient strength beyond the char layer, it will continue to perform its structural function.
XLam CLT panels are specifically designed to comply with fire resistance ratings stipulated by the Fire Codes. Full scale fire tests have been carried out and independent test laboratory certifications can be made available to support building consent applications.
Delivery and Protection
As far as is practicable, XLam will arrange delivery of the panels in a sequence to suit the intended lifting program on site. Prior to manufacture, the installation sequence should be planned and advised to XLam.
Depending upon crane capacity and available space at the site, panels may be craned into their final positions directly from the delivery truck, or unloaded in pre-determined packs for subsequent placement. For larger projects and/or where site space is limited, there may be a need for off-site storage and panel retrieval to suit the construction program. If panels are to be unloaded before assembly, they must be laid flat, evenly supported clear of the ground and protected from weather and sun.
Assuming timely closing in of the building, exposure to the elements is normally of no structural consequence. Immediately after fixing, panel joints should be taped and flashing tape applied to exposed edges of openings to cover end grain timber. Ponding water should not penetrate the panel edges.
The need for panel surface protection depends on whether panels are to be visually exposed or lined over. The usual causes of damage to visual surfaces are water staining and yellowing from ultra-violet light exposure which occurs very quickly in Australia and NZ. In general, a well-sequenced construction plan which achieves rapid closing in will lessen the risk of surface disfigurement.
An understanding of moisture control is important for the use and continued durability of XLam CLT. The correct application of building physics will assist hygrothermal performance and prevent condensation occurring within the building envelope.
Building Physics (or Design) Considerations
Panels must be protected from permanent exposure to rain. In the majority of cases this will mean cladding. The cladding system should be fully weathertight and backed by a drained and vented cavity.
Good natural ventilation and/or appropriate air exchange and extraction systems should be installed to minimise the build-up of internal moisture. High levels of internal humidity for extended periods will also create a risk of interstitial condensation.
Thermal insulation should be installed against the outer face of panels to keep the timber warm. CLT panels themselves offer good thermal resistance, however additional insulation will usually be necessary to achieve code compliance. If rigid insulation is used (EPS, XPS or PIR), the cladding system can be attached to cavity battens screwed to the CLT through the insulation.
An air barrier should be installed against the outside face of the panels. The boards within the panels are not edge-glued, and therefore without an air barrier, some leakage may occur. Some sheet insulation boards may perform as an air barrier, provided the joints are sealed.
CLT can buffer variations of indoor humidity by absorbing high humidity indoor moisture and desorbing when the humidity drops. Leaving panels unlined, or applying vapour-permeable linings and finishes, will help facilitate the natural dehumidifying affect, which improves human comfort and is one of the big benefits of XLam CLT construction for residential and health care buildings.
- Panels should be protected from moisture during construction and allowed to sufficiently dry before the application of linings. This includes adequate protection from rain and ground water on site. High initial moisture content of the timber elements — typically through exposure to weather during construction — creates a potential for interstitial condensation.
Due to a greater thermal mass than timber framed construction, XLam CLT has more ability to retain heat. Assisted by passive solar gain, solid wood panels are able to collect and store energy for emission later in the daily cycle, evening out internal temperature fluctuations.
XLam CLT has the same thermal performance as solid pine. The conductivity (U-Value) of Radiata pine is 0.120 W/mK at 12% moisture content which is the expected condition of CLT in use.
The thermal resistance (R-value) of XLam CLT is calculated from dividing the thickness in metres by conductivity. Thus some for some typical XLam panels -
|60mm CLT:||R = 0.50|
|105mm CLT:||R = 0.875|
|175mm CLT:||R = 1.46|
To obtain the Construction R-value for a roof, floor or wall assembly, the R-value for the XLam panel is added to the R-values of other envelope components to achieve a total thermal resistance.
CLT makes a healthy contribution to insulating the building envelope. Thermal performance is boosted through adding a proprietary sheet insulation product on the outer face of the wall panels. For XLam TwinSkin floor and roof cassette panels, insulation is incorporated within the cassette itself during manufacture.
Good planning and organisation is key to the speed of prefabricated construction. XLam works with the contractor to coordinate delivery with the contract program, identify the best assembly sequence, supply panels in prearranged order, and advise on site management. Checklists include site access, crane weight, lifting procedure, building tolerances and weather protection. Fixings can be scheduled and supplied by XLam. On-site support is given as required.
Radiata pine is suitable for use without preservative treatment in protected locations within a dry service environment with no risk of water incursion. Where necessary, XLam CLT can be supplied treated to H1.2 or H3.2 durability standard to meet the required hazard class. Treatment may be incorporated in all layers, or alternatively in one or both outer layers only, depending on the source of risk. XLam does not recommend the use of exposed CLT in external locations.
In parts of Australia additional treatment is necessary to comply with the requirements of H2 class for protection against termites and borers.
XLam CLT is first and foremost a structural system but many designers and building owners want to see the wood!
Radiata pine has knots and defects according to its location within the tree, the way it is cut, and the specified grade of material. Visual and structural defects of every board are assessed by XLam during manufacture, and selectively removed to the extent required for end use. Most people prefer to see some knots remain as a natural characteristic of the wood. The unobtrusive finger jointing process using clear polyurethane glue enables the outer face layers to be made almost knot-free if required. The required appearance grade should be agreed with XLam on ordering, and thereafter steps must be built into the construction plan to ensure visual surfaces are not compromised by inappropriate handling, prolonged UV exposure or rain.
Chain of Custody
In order to continue to deliver on our commitment to CofC certification, Xlam is committed to CofC certification and the following:
Ensuring that all resource is from legal and known origins, by maintaining systems and processes to demonstrate the traceability of all resources inputs.
Provide adequate administrative arrangements and support to deliver the necessary CofC control systems.
Maintaining documentation and control systems to enable trusted and correct use of certification and logos on wood products.
Provide ongoing training and development of personnel to ensure adequate knowledge to allow for efficient and accurate implementation of CofC systems.
Regularly engaging an independent third party to review systems and ensure ongoing compliance.
Ensuring that the facilitates and processes meet or exceed all applicable workplace health and safety and environmental laws or regulations.
Chain of Custody Complaints
As part of XLams commitment to ongoing improvement a company complaints procedure has been implemented. All complaints shall be assessed and reviewed within 2 weeks from date of registration.
Site visits shall be conducted as required within 2 months of registration, if required to verify claims made onsite.
A complete account of the registration through completed action shall be documented.
All non-complaint material deemed to breach the relevant guidelines and shall be reported to the relevant party located within Australia. A thorough risk assessment shall be conducted should the risk be low with respect to frequent non-conformance issues.