BRE DG416 PDF

Introduction

Timbers that carry the dead and imposed loads of buildings or other types of load-carrying construction are said to be ‘structural'. As such, there needs to be proof of design to show that they are capable of carrying the loads as specified in terms of strength and defects (ie serviceability). All timber used for structural applications should be assessed for strength in order to provide the designers with the basic stresses and moduli that they require to make the calculations to prove their design.

There are thousands of species of trees and their wood varies in strength between and within species. Although timber is one of the oldest structural materials, currently very few species have been assessed for structural applications and fully codified, primarily due to cost. In the UK, timber has been strength-graded since the introduction of British Standard Code of Practice 112 (Structural use of timber) in 1952^{[1, 2]}. CP 112 covered both the strength-grading rules for timber and the permissible stress design code. As the process of structural design in timber evolved, the grading rules were separated from the design code: in 1978, BS 4978 (Specification for visual strength grading of softwood)^[3] was introduced along with BS 5268-2 (Structural use of timber – Code of practice for permissible stress design, materials and workmanship)[4], the permissible stress design code. Subsequently in 1979, BS 5756 (Visual grading of tropical hardwood)^[5] was introduced.

Though the grading standards have continued to be developed, the major recent change is European integration of structural codes. The UK has moved away from BS 5268 towards Eurocode 5 (Design of timber structures)^[6] and PD 6693-1:2012 (UK national supporting document to Eurocode 5)^[7] and limit-state design. The basic principles of structural design and fundamentals of strength grading remain constant, but the means by which structural design is achieved have changed. There are now many more methods of machine grading timber, based on a variety of technologies, than only a few years ago, but visual grading is still an accepted and viable method. Strength grading is still based on the statistical distribution of a number of key parameters, and these are increasingly being expressed in terms of a strength class rather than a grade stress.

The specification of structural timber should consider the following points:

• strength classification
• moisture content
• target size and tolerance class
• preservative treatment
• finger jointing
• green gluing
• sourcing and supply