FAIR-CT95-0091 Round Small Diameter Timber for Constructions

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FAIR Area 1.3 - Forestry-Wood Chain : Solid Wood Products : Wood (Lignocellulose)

	Proposal No:	FAIR-CT95-0091
	Date Prepared:	September 1999
	Source:	Final report 1998

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Final report 1998

Introduction The final report of the above project has been published in book form by the Technical Research Centre of Finland, from whom copies may be obtained. The publication, edited by Alpo Ranta-Maunus, reports the results obtained in a three-year European project on the possibility to use small-diameter round-wood in construction. Seven organisations from 5 countries worked together carrying out research that covered a wide range of subjects: forest thinning methods, economy of harvesting, wood drying, strength testing and grading, feasible use in construction, and joints of load-bearing structures. This publication summarises and discusses the scientific results. Other illustrative documents are being published for guidance of architects and builders.

Abstract The use of small-diameter timber in construction has been investigated. The aim of the work is to increase the use of the wood harvested in forest thinning in construction applications. The work has covered a wide range of aspects, from availability of the material to design of the structures. This publication summarises the results in the following areas:

• availability, dimensions and quality of conifers harvested in forest thinning
• cost of harvesting and woodworking
• comparison of three drying methods:
  • seasoning
  • warm-temperature kiln-drying
  • high-temperature kiln-drying
• improving durability
• strength of round small-diameter conifers
• potential types of structures to be built from round timber
• new mechanical joints

The tree species that were included in the study are:

• Scots pine
• Norway spruce
• Sitka spruce
• Larch
• Douglas fir

The main reasons why round timber is rarely used in construction can be summarised as:

• the material is not available via the normal commercial routes
• the roundness requires special methods and systems that are not known by architects and carpenters
• the strength values of timber connections are not available for engineers
• there is a lack of standards and models

This research aimed to produce information needed in the use of small roundwood in load-bearing structures in order to remove the obstacles mentioned above.

Results concerning availability of construction-quality round timber in the first commercial thinning reveal that the resource itself is vast: millions of cubic meters in Finland alone. The yield per hectare is, however, limited and dependent on the dimensions required. When the diameter of the final product is adequate at less than 100 mm, the first commercial thinning is also economic for the harvesting of construction timber. The economics of manual and mechanical harvesting have been compared. When larger dimensions are needed, the second thinning is more likely to produce the required material.

The cost of producing round timber is primarily dependent on the surface quality needed: timber peeled cylindrical is twice as expensive as material that is only debarked. Both of these have their own market. The cost of construction is dependent on labour costs, which at the moment is higher for round timber than for sawn timber because conventional systems are not suited for the use of round timber. Additional costs may arise from the deviations of cylindrical form.

Drying is a critical phase of production, which determines how much checking is required. In this respect, high-temperature drying gives much better quality than normal, commercial warm-temperature kilning or natural seasoning. Accordingly, the drying method should be chosen based on the surface requirements. End-cracking also affects the capacity of joints.

The strength of small-diameter timber was observed to be higher than expected. Characteristic values are presented as well as a proposal for visual strength-grading. A method for non-destructive mechanical strength-grading based on X-ray analysis is also proposed. A statistical analysis is presented that relates the dependence of strength and stiffness to various factors such as density, knots, moisture content, diameter and age.

New mechanical connections have been designed and tested. For engineered structures, a round form enables the use of steel lacing around the wood, which considerably increases the load capacity of the joints.

The largest quantities of round timber are used, and can be used, in non-structural applications and in small, traditional-type buildings. Smaller in volume but important for the image of roundwood is its application in the architecture of medium-sized leisure industry buildings in which the load-bearing structure is visible. As part of the project, designs for a footbridge and a watchtower have been made.

Contents The book is divided into five chapters, with a summary, and three appendices as follows:

• Abstract
• Preface
• 1. Introduction
• 2. Harvesting, quality and manufacturing
• 2.1 Economy of harvesting
• 2. 1.1 Introduction
• 2.1.2 Material and methods of time and productivity study
• 2.1.3 Results
• 2.1.4 Conclusions
• 2.2 Yield and quality in the first thinning
• 2.2.1 Introduction
• 2.2.2 Material
• 2.2.3 Methods
• 2.2.4 Results
• 2.2.5 Discussion
• 2.3 Drying
• 2.3.1 Seasoning
• 2.3.2 Kiln-drying at 70 degrees C
• 2.3.3 Discussion on drying
• 2.4 Oil impregnation and drying

• 3. Strength of small-diameter timber
• 3.1 Testing methods
• 3.1.1 Bending tests
• 3.1.2 Compression test
• 3.1.3 Buckling test
• 3.1.4 Tension test
• 3.1.5 Dynamic modulus of elasticity
• 3.1.6 X-ray density measurement
• 3.2 Materials and statistical methods
• 3.2.1 Sampling
• 3.2.2 Measured features of material
• 3.2.3 Statistical methods
• 3.2.4 Hypotheses of the mechanical properties of round timber
• 3.3 Strength results
• 3.3.1 Overview of results
• 3.3.2 Correlation analysis
• 3.3.3 Effect of machine rounding and sawing
• 3.3.4 Differences between species
• 3.3.5 Effect of diameter
• 3.3.6 Buckling strength
• 3.4 Strength-grading
• 3.4.1 Visual grading
• 3.4. 1.1 Determination of visual grades
• 3.4.1.2 Characteristic values for grades and species
• 3.4.1.3 Strength classes for grades
• 3.4.2 Dynamic modulus of elasticity as a grading parameter
• 3.4.3 Grading by X-ray

• 4. Mechanical connections
• 4.1 Potential roundwood connections
• 4.2 Laced joints
• 4.2.1 Description of laced joints
• 4.2.2 Estimation of load capacity of wire-laced joints
• 4.3 The Block Joint
• 4.3.1 Description of the joint
• 4.3.2 Test results
• 4.3.3 Design criteria for the joint
• 4.4 Forced plate joint
• 4.4.1 Introduction
• 4.4.2 Calculation

• 5. Roundwood Structures
• 5.1 Introduction
• 5.2 Historical influences
• 5.3 The present situation
• 5.4 Market study
• 5.4.1 Overview
• 5.4.2 Results by country
• 5.4.3 Conclusions
• 5.5 Structure of programme
• 5.6 Overview of uses
• 5.6.1 Playground and leisure equipment
• 5.6.2 Commercial landscaping
• 5.6.3 Loghouses
• 5.6.4 Pole buildings and pole barns
• 5.6.5 Space frame and specialist roofs
• 5.6.6 Towers and domes
• 5.6.7 Bridges
• 5.7 Case studies related to round-pole group visits
• 5.7.1 Holland
• 5.7.2 England
• 5.7.3 Finland
• 5.7.4 Austria

• 6. Summary
• REFERENCES
• APPENDICES
• Appendix A: Voluntary Product Standard VPS-SRT-1
• Appendix B: Voluntary Product Standard VPS-SRT-2
• Appendix C: Voluntary Product Standard VPS-SRT-3