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Strength verification of MDF products and fitness

for purpose in structural applications:

Assessment of the structural behaviours and fitness for purpose

of MDF stair solutions

Project number: PNA364-1516 February 2019

Level 11, 10-16 Queen Street

Melbourne VIC 3000, Australia

T +61 (0)3 9927 3200 E [email protected]

W www.fwpa.com.au

Forest & Wood Products Australia Limited Level 11, 10-16 Queen St, Melbourne, Victoria, 3000 T +61 3 9927 3200 F +61 3 9927 3288 E [email protected] W www.fwpa.com.au

Strength verification of MDF products and fitness for purpose in structural applications:

Assessment of the structural behaviours and fitness for

purpose of MDF stair solutions

Prepared for

Forest & Wood Products Australia

by

A/Prof Christophe Gerber & Sean Cowley

mailto:[email protected]

http://www.fwpa.com.au/

Forest & Wood Products Australia Limited Level 11, 10-16 Queen St, Melbourne, Victoria, 3000 T +61 3 9927 3200 F +61 3 9927 3288 E [email protected] W www.fwpa.com.au

Publication: Strength verification of MDF products and fitness for purpose in structural applications: Assessment of the structural behaviours and fitness for purpose of MDF stair solutions

Project No: PNA364-1516

IMPORTANT NOTICE

This work is supported by funding provided to FWPA by the Department of Agriculture and Water Resources (DAWR).

© 2019 Forest & Wood Products Australia Limited. All rights reserved.

Whilst all care has been taken to ensure the accuracy of the information contained in this publication, Forest and Wood Products Australia Limited and all persons associated with them (FWPA) as well as any other contributors make no representations or give any warranty regarding the use, suitability, validity, accuracy, completeness, currency or reliability of the information, including any opinion or advice, contained in this publication. To the maximum extent permitted by law, FWPA disclaims all warranties of any kind, whether express or implied, including but not limited to any warranty that the information is up-to-date, complete, true, legally compliant, accurate, non-misleading or suitable.

To the maximum extent permitted by law, FWPA excludes all liability in contract, tort (including negligence), or otherwise for any injury, loss or damage whatsoever (whether direct, indirect, special or consequential) arising out of or in connection with use or reliance on this publication (and any information, opinions or advice therein) and whether caused by any errors, defects, omissions or misrepresentations in this publication. Individual requirements may vary from those discussed in this publication and you are advised to check with State authorities to ensure building compliance as well as make your own professional assessment of the relevant applicable laws and Standards.

The work is copyright and protected under the terms of the Copyright Act 1968 (Cwth). All material may be reproduced in whole or in part, provided that it is not sold or used for commercial benefit and its source (Forest & Wood Products Australia Limited) is acknowledged and the above disclaimer is included. Reproduction or copying for other purposes, which is strictly reserved only for the owner or licensee of copyright under the Copyright Act, is prohibited without the prior written consent of FWPA.

ISBN: 978-1-925213-88-1

Researcher/s: A/Prof Christophe Gerber & Sean Cowley The University of the Sunshine Coast 90 Sippy Downs Drive, Sippy Downs, Queensland, 4556

Final report published by FWPA in February 2019

mailto:[email protected]

http://www.fwpa.com.au/

PNA364-1516 Strength verification of MDF products and fitness for purpose in structural applications P a g e | 1

Acknowledgement The authors would like to thank the following persons for their contribution and assistance with the producing of this report,

Assistance with the research: Mr Philip Kaeser Assistance with laboratory tasks: Mr Hugh Allan Mr Bernhard Black Technical advice: Mr Dave Gover – EWPAA Mr Andy McNaught– EWPAA


Abstract The project ‘Strength verification of MDF products and fitness for purpose in structural applications’ aims to verify that MDF can be used for structural purposes; verifying in particular, the suitability of existing practice of the structural use of MDF products for stairs and stair elements such as tread-riser systems and stringers by the Australian stair Industry.

The verification of the structural behaviour of MDF staircases has been mostly completed by mechanical testing of stairs and stair elements. Some modelling was also conducted. European guidelines from ETAG 008 (2002) and EN 15680 (2007) procedures were followed to complete the laboratory investigations of the project because no Australian standard details any procedure to investigate stair structures or stair components (i.e. treads and risers, stringers, etc.).

The results and findings of structural investigations of stairs and stair components demonstrate that MDF is adequate and safe to use for structures such as stairs provided geometrical and dimensional limits, as well as construction details have to be adhered to. It is also recommended that MDF products Class (e) (AS1859.2:2004, Cl. 5) or superior be used for the construction of stairs. For use of MDF in structural applications other than stairs, case specific investigations are recommended.

From the results and findings of the project, design commentaries and recommendations have been outlined with view to include them in FWPA guidelines “Design Guide 08: Stairs Balustrades and Handrails”. This inclusion can be in the form of an expansion of the existing sections or section components of the design guide, or an additional Appendix focused on MDF with references to the main sections of the design guide. These guidelines have also been outlined in compliance with the codes (National Construction Code (NCC), Australian Standards, etc.) and industry practice.

.


Contents Acknowledgement .................................................................................................................................. 1

Abstract ................................................................................................................................................... 2

List of Figures .......................................................................................................................................... 4

List of Tables ........................................................................................................................................... 4

1 Introduction .................................................................................................................................... 5

2 Scope of Work ................................................................................................................................. 5

2.1 Purpose ................................................................................................................................... 5

3 Project parameters and methods ................................................................................................... 5

3.1 Actions and Loading Configurations ....................................................................................... 5

3.2 Material Characteristics and Properties ................................................................................. 6

3.3 Mechanical Test Methods ....................................................................................................... 6

3.4 Practice from the stair industry .............................................................................................. 7

3.5 Concluding remarks ................................................................................................................ 8

4 Project synopsis .............................................................................................................................. 8

4.1 General .................................................................................................................................... 8

4.2 Mechanical Properties – MOR and MOE (Project Milestones 1 & 2) ..................................... 8

4.3 Structural Behaviour (Project Milestone 2) ............................................................................ 9

5 Proposed recommendations and design guidelines for the applications of MDF products in stair structures .............................................................................................................................................. 10

5.1 Introductory comments ........................................................................................................ 10

5.1.1 Integration of design recommendation ........................................................................ 11

5.1.2 Material properties ....................................................................................................... 11

5.1.3 Material identification .................................................................................................. 11

5.2 Outline of the proposed design guidelines ........................................................................... 12

5.3 Next steps for the adoption and release of the proposed design guidelines ....................... 15

5.4 Concluding remarks .............................................................................................................. 15

6 Conclusions ................................................................................................................................... 16

Appendix A. References .................................................................................................................... 17


List of Figures Figure 1: Tread and riser housing. ........................................................................................................ 14 Figure 2: Top end landing housing. ....................................................................................................... 15

List of Tables Table 1 - Stair Stringers including MDF (Table 3.4 of Guide) ................................................................ 13 Table 2 - Stair Treads (with open and closed flights) (Table 3.5 of Guide) ........................................... 13 Table 3 - MDF configurations recommended for tread/riser construction. ......................................... 13


1 Introduction The structural uses of medium density fibreboard (MDF) has proven successful over many years in load bearing applications such as furniture and stairs. Conditions of this success have been the material properties of the products, implementation in dry and protected environment and workmanship.

The project objectives are to verify the suitability of use of MDF in structural applications, in particular stairs, and the existing practice of the MDF industry. It is assumed that stairs are structures in the sense of the design codes and must be designed and specified as such, in compliance with AS 1170.1:2002, AS 1170.0:2002 and AS 1720.1-2010.

Knowing the characteristic properties of a material forms a prerequisite to design and an essential part of compliance with the codes. To this end, the properties if MDF products have been investigated to characterise MDF materials to AS/NZS 1859.2:2004 (Milestones 1 & 2). Material testing to an in-grade approach also been undertaken to characterise the MDF stringers to AS/NZS 4063.1:2010 AS/NZS 4063.2:2010 (Milestone 2).

Understanding the construction parameters and system limits are further important aspects to successful uses of MDF in structures. The project has addressed these aspects (Milestone 2) by investigating the span limits for the stringers and tread-riser systems, the detailing of notched assemblies and the most suitable diameter and penetration of screws. The findings of these investigations have informed the design guidelines.

Milestone 3 of the project is merging the knowledge and understanding of Milestones 1 and 2 into design guidelines. These guidelines are required to be compliant with the codes (NCC, Australian Standards, etc), to meet the requirements of FWPA’s design guide “Design Guide 08: Stairs Balustrades and Handrails”, and to be accepted by the MDF industry. This acceptance is essential for the successful implementation of the design guidelines.

This report starts with the definition of the scope of the project (Section 2). It continues with the identifications of the parameters and methods used to complete the project (Section 3) and the synopsis of the project (Section 4). Design recommendations and guidelines are outlined in Section 5. The report ends with some conclusions (Section 6).

2 Scope of Work 2.1 Purpose The investigation aims to fill the gaps of characteristic properties for structural design and application of MDF products, align these properties with MDF industry practice and standard, and outline design guidelines for the use of MDF products in stair constructions.

NOTE: The design guidelines are a series of recommendations and commentaries put forward by the investigators and proposed for inclusions in FWPA guidelines “Design Guide 08: Stairs Balustrades and Handrails”.

3 Project parameters and methods 3.1 Actions and Loading Configurations AS 1170.0:2002 specifies the general procedures and criteria for the structural design of structures and their elements. It provides directives for appropriate actions and combinations of actions for Ultimate Limit State (ULS) the design and Serviceability Limit State (SLS).


The Australian standard for calculation of design actions, AS 1170.1:2002, specifies the type of loads and their intensity to apply on specific structures. For stairs, two live loads (also specified as imposed actions) must be considered: (a) a uniformly distributed load (UDL) of 2 kN/m2 or (b) a concentrated (Point Load) of 2.7 kN. The point load must be placed at the most critical or adverse location of the structures or structural members.

For successful design, the stair structures and their structural members must satisfy both load types and application conditions, incl. relevant load combinations. For the stringers, the concentrated action of 2.7 kN placed at mid span gives the most critical bending moment and displacement, whilst the most critical shear force at the connections of the stringer to the frames (structural supports) are generated by the distributed load of 2 kN/m2.

The load combinations must include the dead loads, i.e. self-weight and permanent actions. In agreement with AS/NZS 1859.2:2004 a density of 850kg/m3 for Standard, General-Purpose MDF was used to calculate the permanent (dead) load. Note this density applies to Class (c) and Class (e) MDF products.

AS 1720.1-2010 sets out the limit state design methods for the structural use of timber products. It covers the design members and connections. Note that Clause 1.4.2.5 of AS 1720.1-2010 allows for a performance design approach of timber structures providing that rigorous structural investigation (load testing) of the ULS and SLS capacities of the elements and systems is conducted.

3.2 Material Characteristics and Properties The AS/NZS 4266.0:2004 series specifies a range of methods to determine a variety of structural properties of reconstituted wood based panels for quality control purposes. Of most importance are the standards which determine the density, moisture content, resistance to axial withdrawal of screws and modulus of elasticity (MOE) and bending strength (MOR). Independent material tests to determine the MOE and MOR allow comparison to the quality control data of the manufacturers and effectively allow the classification of the MDF material. Moisture content and density influence the strength capabilities of members and so will need to be established directly after testing to see how increases in temperature and humidity affect the results. Above all, these tests help to observe the variation which is prevalent amongst batches of MDF even though they are produced to the same specifications.

3.3 Mechanical Test Methods Mechanical testing of stair structures is not detailed in any Australian Standards. AS 1720.1-2010 details the general requirements of testing of timber structures as a whole without any distinguishing between what function these structures have. Two methods are given – “Proof Testing” and “Prototype Testing”. Proof Testing is used for investigating every structure of a population whilst Prototype Testing is used to establish the behaviour of a sample of a population. In each case, an equivalent testing load is used which considers coefficient of variation of a material, the number of samples, the load duration, the type of stress, the type of member and the moisture content. For structural verification and due to the missing characteristic design properties of MDF, whenever MDF is used in structural applications, one of the two afore mentioned tests must occur.

From the Australian Standards it appears that AS/NZS 4063.1 (2010) is the most suitable to establish and compute the MOE and MOR of in-grade or full scale individual timber members. In each case, results can be determined from a simply supported four-point beam test when a load is applied at a uniform rate. In the context of the project, AS/NZS 4063.1 (2010) and its testing procedure will only be used for stringer testing.


In Europe, ETAG 008 has distinct sections setting out the performance requirements for prefabricated stair kits, the verification methods used to examine these aspects of performance and the assessment criteria used to judge this performance for intended use. Topics covered include mechanical resistance and stability, safety in case of fire, hygiene and the environment, safety in use, noise protection, energy economy, heat retention and aspects of durability, serviceability and identifications. Testing stairs in Europe according to ETAG 008 can lead to an ETA (European Technical Approval). Building stairs with MDF in Europe would require an ETA.

Similarly, EN 15680 (2007) provides more specific information about support structures and the test processes for mechanical test methods for stairs and balustrades of prefabricated timber. The tests detailed refer to all components, including handrails and entire flights and refer to load bearing capacity, deflection and resistance against impact loads. For statistical evaluation of results, both ETAG 008 and EN 15680 refer to ISO 12491 (1997). British Standard BS 585.2 (1985) provides similar test methods and requirements.

3.4 Practice from the stair industry Upon consulting several stair-building-and-manufacturing companies it is standard practice to use MDF products for stairs. However, the use MDF for stringers appears to be limited to situations when the stair is built between walls and the stringers can be securely screwed to the wall studs, or can be supported by a wall (stringer installed on top of a wall frame). Companies do not use MDF for the stringers of free-spanning stairs but softwood (often pine) or hardwood. MDF treads and risers are generally housed, glued and screwed (or wedged) into a rebate of 10-12mm, which ultimately does reduce the stiffness and load bearing capacity of the stringer (by removing timber in the area under tension).

The “Stairs, Balustrades and Handrails” design guidelines (FWPA 2012) give recommendations for material, geometry, dimensions and finishes and base this on practical experience without taking structural considerations into account. These apply only to some hardwoods, and sections detailing MDF application do not exist. Similarly, a design guide from the BWF (British Woodworking Federation, 2013) presents information about the construction of hardwood stairs.

Lam et al (2004) performed tests on staircases with cut stringers made from OSB and found that assuming stairs to be simple supported beams significantly over-estimated the deflection of the stair stringer. EN 16481 (2014) models connections with a spring constant but do not provide further information as to why. Similarly, Ehrmann, Nutsch and Siebert (2008) highlighted that it is important to note that connections perform differently absorbing forces both horizontally and vertically and that a systems response to shear forces at the top and bottom ultimately determines its overall integrity.

Stairs must be adequately supported to prevent slip at the bottom end. A simple, cost effective solution is to notch at the top and/or bottom. Henrici (1976) highlighted that the depth of notching has a strong influence on the stress peak in the edge of the nosing – the deeper the notch, the higher the stress concentration. No reference to MDF could be found in the literature however. Ehrmann et al (2008) proposed to cut the stringer at the top to eliminate the nosing and instead effectively lean the stringer against the wall. This means the top is resisting only the horizontal forces and that all vertical forces are dissipated to the bottom connection which in turn is securely connected to the floor. Strong and reliable connections can be achieved with steel fasteners of which there is a broad spectrum of products. However, they are not typically used with MDF.


3.5 Concluding remarks In the context of the project, the Australian Standards have been used to determine the test loads for stairs and to conduct laboratory investigation of some stairs elements. The investigations of stairs and stairs sub-systems have been conducted by considerations the guidelines outlined in ETAG 008 and EN 15680 (2007). Further details about the project findings can be found in 4 Project synopsis.

4 Project synopsis 4.1 General MDF (Medium Density Fibreboard) is an engineered wood panel material with a thickness of 1.5mm or greater, made from wood and other biological fibres bonded together by a synthetic adhesive with the application of head and pressure. Seven classes, made up of 3 grades (Standard, Moisture Resistant and High Performance) exist to ensure production and material standards meet the guidelines of AS/NZS 1859.2:2004.

The species of wood fibre used has a subordinate role influencing the properties of MDF (Deppe & Ernst, 1996) instead the manufacturing process – press method, type of temperature injection, packing pressure and applied adhesive has a strong influence on the density and density profile of the panel. The density and the density profile correlate and determine the mechanical properties of the panel. A high MOE and Bending Strength can only be achieved with a high outer layer density. As there are several parameters to keep constant during manufacture there is always some variation between panels, even those from the same production series.

4.2 Mechanical Properties – MOR and MOE (Project Milestones 1 & 2) The mechanical properties of any type of wood is a measure of a sample’s fitness and ability to resist external forces and applied loads. Knowing such properties enables one to determine the structural use and purpose of specific samples.

Investigations from EWPAA (2014) and USC (Gerber, Cowley & Kaeser 2016) on the bending strength and stiffness of MDF products indicated that MDF products seem to exceed their specific requirements, e.g. Table 7 in AS/NZS 1859.2:2004. The coefficient of variation of the density (1.0 – 2.3 %) and of the MOE and MOR results (1.1 – 7.6 %) indicates that the tested material is very uniform. EWPAA’s samples were extracted from several indistinct production batches, whilst USC’s samples were from two specific production batches.

When comparing the characteristic values suitable for design to overseas specifications (EN 12369.1:2001), the results from the EWPAA report are again higher. Characteristic values taken from EN 12369.1:2001 can only be used when requirements from EN 622.5 are met. EWPAA results meet the requirements for EN 622.5 even though the values have been taken using different test methods. Therefore, for a direct comparison of European Standards and the associated characteristic values, further investigation into Australian MDF would be required focusing on abiding to EN 310.

Table 1 summarises the strength and stiffness properties of STD general-purpose MDF obtained by testing (USC and EWPAA). To facilitate the comparison, the published values from AS/NZS 1859.2:2004 and EN 12369-1:2001 are also listed in Table 1. The test results of the investigations conducted at EWPAA and USC demonstrated that the MDF material supplied for stair construction meets and exceeds the strength and stiffness benchmarks of AS/NZS 1859.2:2004 and EN 12369-1:2001. For information and comparison, the strength and stiffness properties of MR general-


purpose MDF published in 1859.2:2004 and EN 12369-1:2001 are listed in Table 2. This shows that, unlike EN 12369-1:2001, AS/NZS 1859.2:2004 does not differentiate STD and MR panels and specifies the same MOR and MOE values. In EN 12369-1:2001 the characteristics bending and stiffness values are marginally higher for MR panels vs. STD panels.

Table 1: Strength and stiffness properties of STD general-purpose MDF (Class c).

Property EWPAA USC

AS/NZS 1859.2:2004

(Table 5)

EN 12369-1:2001 (Table 10)

Nominal thickness range (mm)

> 12 to 22

> 22 to 33

> 12 to 22

> 22 to 33

> 12 to 22

> 22 to 33

> 12 to 19

> 19 to 30

MOR (MPa) 29 26 34.8 34.2 26 23 211 211

MOE (MPa) 4000 3950 3465 3588 2000 1800 3000 2900 1 Characteristic bending strength of load bearing boards in dry conditions.

Table 2: Strength and stiffness properties of MR general-purpose MDF (Class e).

Property AS/NZS 1859.2:2004 (Table 7)

EN 12369-1:2001 (Table

Nominal thickness range (mm)

> 12 to 22

> 22 to 33

> 12 to 19

> 19 to 30

MOR (MPa) 26 23 221 211

MOE (MPa) 2000 1800 3200 3000 1 Characteristic bending strength of load bearing boards in humid conditions.

4.3 Structural Behaviour (Project Milestone 2) The verification of the structural behaviour of MDF staircases requires mechanical testing and/or structural modelling. Mechanical testing of stair structures or stair components (i.e. treads, risers, stringers, etc.) is not detailed in any Australian Standards. In Europe, ETAG 008 (2002) has distinct sections setting out the performance requirements, verification methods and the assessment criteria used to judge performance. Similarly, EN 15680 (2007) provides information about support structures and the test processes for mechanical test methods for stairs and balustrades of prefabricated timber.

In this project, tests that have been designed with reference to EN 15680 (2007) to investigate the behaviour of treads and risers. This investigation demonstrated that MDF is safe to use for treads and risers in stairs in protected environments. However, geometrical and dimensional limits, and construction details have to be adhered to. It was observed that all samples configurations have significantly higher values for load bearing capacities than the design load of 4.05 kN and the


displacement of each system under serviceability loads of 2.7 kN is less than 2 mm. Impact investigations also showed that the tread and riser construction can easily absorb and dissipate energy and suffer no visual damage or loss of the structural integrity.

Pull-out testing has been performed in accordance to AS 4266-13 to determine the resistance of screws to axial withdrawal. With this investigation, Gauge 8 particle board screws have exhibited the most effective withdrawal strength; most likely situations of stair constructions. As well as this, the thread length is influential for a greater resistance.

Whilst the above tests verify that treads and risers, as currently used in the stair industry, are suitable for use, the following tests are carried out to explore the possibility of MDF being used for stringers as fully free-spanning members.

Specific in-grade testing was also conducted on the stringers. The investigation demonstrated that free-spanning MDF stringers, by extension stairs, of up to span 2.3m and width 1m, are safe build with 32mm thick MDF stringers. The results of the structural tests and the numerical modelling have been compared with the statistical approach of EN 16481 to validate the calculation method for MDF stringers with different locations of the housings.

Investigations of the load distribution demonstrated that both stringers within a system, i.e. stair, carry a proportion of the load regardless of where the load application point is. Therefore, designing stringers as simply supported beams appears to exaggerate the influence of a load in comparison to what actually happens in systems – 100% of the load acting entirely on a stringer in an exception. Neglecting the system load sharing leads to an underestimation of the load bearing capacity of the stringer, treads and risers working as structural systems.

The attachment of the stairs to the top floor (joist or otherwise) are notched configurations in most cases. In agreement with this practice, notched solutions with different depth were tested. It was identified an 80-mm shear block is adequate to meet the design requirements for 32-mm thick stringers.

5 Proposed recommendations and design guidelines for the applications of MDF products in stair structures

5.1 Introductory comments The findings of the project summaries in Sections 4.2 and 4.3 suggest that MDF material is adequate for the construction of stairs under certain construction conditions. However, using MDF as structural material requires that its characterisation complies with the requirements of AS 1720.1-2010. This could impose change to current practice on the MDF industry.

Anecdotal evidence provided by SE Queensland stair manufacturers indicated the need to consider moisture resistance of MDF stair components, as replacement of step treads has been required following the steam cleaning of carpet covering stair treads. It would therefore appear preferable to recommend moisture resistant MDF to avoid premature deterioration of stair structures.

The design recommendations outlined hereafter have been informed by the finding of the projects. They are put forward under the requisite that MDF characterisation satisfies the requirements of AS 1720.1-2010. In addition, the proposed design commentaries and recommendations have been written for prospective inclusion in FWPA guidelines “Design Guide 08: Stairs Balustrades and Handrails”.


5.1.1 Integration of design recommendation There are several options to include and/or integrate MDF application for stairs in FWPA guidelines. However, two of them seem the most suitable:

Option 1: Expansion and amalgamation of MDF design and construction information into the existing section of the guidelines, and

Option 2: Creation and addition of an Appendix about MDF design and construction.

Option 1 may be more inclusive but may require a series of changes to the existing guidelines. Because MDF is only suitable for indoor applications, it is suggested that a swap of Sections 3 and 4 should be considered. This change would allow to include MDF properties in Table 3.4 of the guide.

Option 2 may appear less inclusive but require less changes to the existing guide. With Option 2 it is suggested to incorporate the information related to the use of MDF into Appendix A and to amend the numbering of the current Appendices.

5.1.2 Material properties The availability of characteristic properties is essential to design in compliance with AS 1720.1-2010. Clause 1.3.1 of AS 1720.1-2010 outlines how these properties must be obtained before a material or structural solution can be designed and erected. A fortiori such values must be available when guidelines and/or design recommendations are put forward in industry standards.

Material testing has showed that the material coming out of production exceeded the specific properties of MDF classification, as put forward in Section 7 of AS/NZS 1859.2:2004. An alternative approach may be to declare the values of Section 7 in AS/NZS 1859.2:2004 as benchmark values for MDF products. However, this would not strictly meet the requirements of AS 1720.1-2010 about the completeness of the material properties. It may be therefore recommended (possibly required) that the benchmark properties of MDF used in structural applications be expanded and inclusive of tension, compression and shear strengths.

5.1.3 Material identification A distinct and consistent labelling of MDF panels is a pre-requisite for safe and reliable structural applications of MDF products. It would contribute to clearly specify suitable MDF products in the design phase and to select and verify the products during the fabrication, dispatch, assembly and/or construction of the structures.

The production process of MDF is based on density and resins. The result of this approach is that the products adhere to the requirements of the classification listed in AS/NZS 1859.2:2004 and fit for a scope of purposes. MDF products are generally not graded – grading is not part of the routine quality control of the production – but the classification system of AS/NZS 1859.2:2004 ensure that benchmark properties are met (Tables 3 – 9 in Section 7, AS/NZS 1859.2:2004). These benchmarks can be used as characteristic properties of the products i.e. quality control focusing on density and resin type and content would ensure the benchmarks are met. Investigations on the material properties of MDF materials used in stair construction, conducted at EWPAA and USC laboratories, demonstrated that the strength and stiffness properties of STD general-purpose MDF (class c, AS/NZS 1859.2:2004) meets and exceeds the strength and stiffness benchmarks of AS/NZS 1859.2:2004 (Table 1).


The labelling of the products needs to improve and become more consistent and visible. Adopting the practice from the solid wood industry, e.g. MGP pine used in framing, is a way forward with some adaptation to suit MDF material. Referring to a benchmark approach, the classification of the MDF panels can be provided instead of a specific grade. It is proposed that a labelling solution indicating the supplier, material class (AS/NZS 1859.2:2004), and the moisture resistance capability of the MDF panel should be implemented. The labelling should also be frequently printed to ensure off-cuts remain labelled.

5.2 Outline of the proposed design guidelines In this report Option 1 is presented, outlining how the amalgamation of MDF design information into FWPA current design guide for stairs might be. The sections (in bold hereafter), numbering included, used in Option 1 have been extracted from FWPA “Design Guide 08”. Commentaries are in italic.

Commentary: Inclusion into the Sections of the Design Guide requires some adaptation of the current layout of the Design Guide. A major adaptation is the swap of Sections 3 and 4. This change has been carried out in the design guidelines put forward hereafter.

Introduction

Recommendation:

• In Paragraph 3, inclusion of MDF products as possible material for interior stairs.

1. Types of Stair Construction

Recommendations:

• MDF material application should be limited to closed stair construction. • MDF material is adequate for stairs constructed between walls, or • As free-standing stairs, the span of the stairs (stringers) should not exceed 2.0 or 2.3 m

(Table 3). • For stair between walls, the stringers of MDF stairs should be fixed to the walls at

intervals not exceeding 2.0 or 2.3 mm (Table 3).

Commentary: For best practice, the stringers of stairs constructed between walls should be screwed to each stud of the wall framing.

2. NCC Requirements

Commentary: No amendment required. MDF stairs are required to adhere to all requirements of the NCC.

3. Materials for Internal Stairs – With and Without Riser Boards (Open and Closed Stairs)

Recommendations:

• For durability, MDF material should be protected with coating or covered with a surface finish e.g. carpet1. Coating and finish must comply with the NCC anti-slip requirements.

1 ABCB, 2014-1 Slip Resistance for Stairways, Landings and Ramps: “For finishes such as carpet, tile, slate, vinyl or rubber … the supplier or manufacturer will have the necessary slip resistance information and reports to show their product has been tested in accordance to AS 4586.


Note: Anti/slip is addressed in Section 3.7 of “Design Guide 08: Stairs Balustrades and Handrails”.

• For durability, MDF material of class e (AS1859-2:2004, Cl. 5) 2 or superior should be used.

• MDF material application should be limited to closed stair constructions. • The sizes and spans for stringers and treads for MDF stairs (internal stairs only) shall be in

accordance to the Tables 1-3: Stringers:

Table 3 - Stair Stringers including MDF (Table 3.4 of Guide)

GRADE BENDING STRENGTH

(MPA)

SHORT DURATION AVERAGE MODULUS OF ELASTICITY

PARALLEL TO THE GRAIN (MPA)

MAX STRINGER SPAN (ACCORDING TO DESIGN

GUIDELINES) f’b E 250x50 300x50

F27 67 18500 3900 4500 F17 42 14000 3600 4200 F8 22 9100 3200 3700 F5 14 6900 2800 3400

250x32 300x32 MDF* 23 1800 2000 2300

* MDF Classification (e) (Table 7 in Section 7.9, AS 1859.2 (2004))

Commentary: A note shall indicate that the width of MDF stairs must not exceed 1000 mm. This limit is imposed by structural performance of the steps (and risers).

A visible, consistent and rigorous labelling of the panels is required to during production of the MDF panels/products.

Treads and Risers:

Table 4 - Stair Treads (with open and closed flights) (Table 3.5 of Guide)

TIMBER TYPE GRADE STAIR WIDTH (MM) 750 1000 1200 1500 1800

Minimum Thickness of Tread (mm) … …

MDF* MR** *** *** N/A N/A N/A *Closed flights only. **MDF Classification (e) (Table 7 in Section 7.9, AS 1859.2:2004). **Refer to Table 3 for recommended configurations.

Table 5 - MDF configurations recommended for tread/riser construction.

TIMBER TYPE

GRADE MAXIMUM STAIR WIDTH

TREAD THICKNESS

(MM)

RISER THICKNESS

(MM)

GLUE LINE INCLUDED

GLUE BLOCKS

INCLUDED MDF MR 1m 25 16 Yes No/Yes

1m 18 18 Yes Yes

2 Tests have been performed on material which has been characterised as having a density >660kg/m3 and therefore lies within the range specified in AS 1859.2:2004.


Commentary: In “Design Guide 08: Stairs Balustrades and Handrails” the sizing of the members is provided in Section 3. Section is not applicable to MDF material because of its inappropriateness for external use. This resulted in swapping Sections 3 and 4 when outlining the design guidelines for MDF stairs.

Refer to Commentary in ‘4. Materials for Internal Stairs’, a specific Classification of MDF is imposed with reference to AS 1859.2:2004.

The use of glue in conjunction with screws is recommended as it improves strength capacity of the tread-riser systems significantly.

The mounting of glued blocks is also recommended as it also improves the strength capacity of tread-riser systems.

4. Materials for External Stairs Without Riser Boards (Open Stairs)

Recommendation:

• MDF material is not recommended for external applications.

5. Stair Construction Procedure

Recommendations:

• A margin template to ensure that the housing of the treads and risers are positioned in the centre of the stringer should be prepared for optimum performance. It is not recommended that wedged MDF stringers be used.

Commentary: Failure can occur in compression or tension on routed sections; the housings should be positioned in the middle. This may affect the visual consideration but priority must be given to not compromise the structural integrity.

Figure 1: Tread and riser housing.

• Stringer should only be housed over bearer landing trimmer and dimensionally comply with Figure 2 below, h ≥ 80mm:


Figure 2: Top end landing housing.

• MDF stair can be assembled with screw-gluing connections using water resistant PVA glue and screw size 8G (chipboard screws) with a penetration length tp ≥ 10Ds, instead of tie bolts. Predrilling is recommended to avoid cracking of the MDF material during screw tightening.

6. Balustrades and Handrails

Recommendations:

• MDF material is not recommended for posts and handrails. • MDF can be used for top and bottom rails, and balusters (Ref to Fig 6.1). • Connection of posts into MDF stringers should be carried out as proposed in the guide.

7. Weathering, Finishes and Maintenance

Recommendation:

• Finishes of MDF stairs should comply with the NCC anti-slip requirements.

Appendices

Commentary: No amendment required. Unless a new Appendix is created for MDF stairs i.e. Option 2 is implemented.

5.3 Next steps for the adoption and release of the proposed design guidelines The adopted guidelines need to satisfy the requirements of the NCC and design Australian Standards. FWPA’s requirements for including and publishing the project recommendations as part of “Design Guide 08: Stairs Balustrades and Handrails” represent another condition of the adoption process. However, it appears that the first conditional step to the release of design guidelines may be the acceptance of project recommendations by the MDF industry.

5.4 Concluding remarks As structures, MDF stairs must be designed in compliance with AS/NZS 1170.0-2002, AS/NZS 1170.1-2002 and AS 1720.1-2010. Such design approach requires the availability of the complete set of characteristic properties of products of Class (e) MDF (AS/NZS 1859.2:2004). This recommendation

h


may be implemented by expanded Table 7 in Section 7, AS/NZS 1859.2:2004. The quality control of MDF mechanical properties may be done by the implementation of a benchmark approach i.e. MDF panels of given ‘recipes’ are assumed to meet the benchmark properties of given MDF classes.

MDF stairs must be designed and constructed within all dimensional limits such as width, stringer spans, detailing, etc. The use of moisture resistant MDF, Class (e), is recommended to ensure adequate durability of the stairs. Screwed and glued connections are recommended and should be fabricated with 8G (chipboard screws).

6 Conclusions The project served two main purposes. It verified the suitability of use of MDF in structural applications, in particular stairs, and the existing practice of the Australian MDF industry.

Stairs are structures in the sense of the design codes. They must resist specified loads and load combinations in compliance with Table 3.1 in AS 1170.1:2002 and AS 1170.0:2002 respectively, and be designed in compliance with AS 1720.1-2010. This design compliance requires complete sets of material properties and clear product grade identification.

The project addressed the characterisation of MDF materials by completing investigations in compliance with AS/NZS 1859.2:2004 (Milestones 1 & 2) and with AS/NZS 4063.1:2010 and AS/NZS 4063.2:2010 for the in-grade testing of stringers (Milestone 2). These investigations verified that MDF is suitable for structural applications. However, the availability of declared material properties is inadequate (Section 7 in AS/NZS 1859.2:2004), i.e. the published design values are limited to bending strength (MOR) alone or together with the stiffness (MOE), and is recommended to be completed with tension, compression and shear strengths.

The identification of MDF products should also be improved to meet the industry standard of timber based structural products. It is proposed that a labelling solution indicating the supplier, material class (as per AS/NZS 1859.2:2004), moisture resistance capability should be implemented and printed on the panels with frequency that ensure the labelling of off-cuts.

The findings of the project investigations inform the design guidelines outlined in this report. The NCC, Australian Standards and industry practice are also adhered to. The design guidelines provide directives and recommendations for the constructional aspects such as dimensions (e.g. stair width, stringer span, etc.) and detailing (e.g. housing, notching, etc.), and for the selections of materials. They have also been drafted with consideration to their prospective inclusion into FWPA’s document “Design Guide 08: Stairs Balustrades and Handrails”.

The successful adoption of the design guidelines requires compliance with the codes (BCA, Australian Standards, etc), FWPA’s requirements and acceptance by the MDF industry. The latter is an essential aspect if MDF is to satisfy the requirements of structural applications in the sense of the codes.


Appendix A. References AS 1720.1-2010 – Timber Structures – Design Methods.

AS/NZS 1170.0:2002 – Structural Design Actions, Part 0: General Principles.

AS/NZS 1170.1:2002 – Structural Design Actions, Part 1: Permanent, Imposed and Other Actions.

AS/NZS 1859.2:2004 – Reconstituted Wood Based Panels – Specifications, Part 2: Dry Processed Fibreboard.

AS/NZS 2269.1:2012 – Determination of Structural Properties – Test Methods.

AS/NZS 4266:2004 – Reconstituted Wood Based Panels – Methods of Test.

AS/NZS 4063.1:2010 – Characterisation of Structural Timber, Part 1: Test Methods.

British Woodworking Federation (2013) “Design Guide: Timber Stairs – A guide to Designing and Manufacturing Safe and Compliant Staircases”, British Woodworking Federation.

BS 585.2: 1985 – Wood Stairs: Specifications for Performance Requirements for Domestic Stairs Constructed of Wood Based Materials.

Deppe, H.-J., & Ernst, K. (1996). MDF-Mitteldichte Faserplatten. DRW-Verlag

EN 12369.1. (2001). Holzwerkstoffe - Charackteristische Werte für die Berechnung und Bemessung von Holzbauwerken - Teil 1: OSB, Spanplatten und Faserplatten.

EN 16481: 2014 – Timber Stairs, Structural Design – Calculation Methods.

EN 15680: 2007 – Prefabricated Timber Stairs – Mechanical Test Methods.

Ehrmann, Nutsch and Siebert. (2008). Holztechnik, Der Holztreppenbau. Haan-Gruiten: Verlag Europa-Lehrmittel

ETAG 008: 2002 – Guideline for European Technical Approval of Prefabricated Stair Kits.

Gerber, C., Cowley, S. & Kaeser P. (2016) Strength verification of MDF products and fitness for purpose in structural applications, Milestone 2, FWPA.

Henrici, D. (1976). Spannungsoptische Untersuchung rechtwinkelig ausgeklinkter sowie lochbelasteter Biegetraeger. Holz als Roh- und Werkstoff, pp. 91-100.

ISO 12491: 1997 – Statistical methods for quality control of building materials and components.

Lam et al., F. (2004). Structural performance of wood-based stair stringers. Forest Products journal Vol. 54 No. 4, pp. -44.

McLaughlin, A. (2014) “Report on In-Grade Assessment of Characteristic Strength and Stiffness Properties of Medium Density Fibreboard Manufactured to AS/NZS 1859.2”, EWPAA.

Wood Solutions “Stairs, Balustrades and Handrails” Technical Design Guide, Forest and Wood Products Australia Ltd, 2012.

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