Times are a’changing for hardwoods
TRADA has made major changes to its Wood Information Sheet Structural Use of Hardwoods to reflect new and updated standards.
Hardwoods have a number of benefits over structural softwoods and yet, when it comes to specification in construction, they command a small share of the market.
One of the most significant changes is to strength grading, where the grade designation HS has been updated to STH, which is harmonised across Europe. The new and updated standards include BS EN 350, BS 5756, BS EN 16737, BS EN 14080 and BS EN 14081-1.
The WIS also details the advantages that hardwoods can deliver as well as more practical advice on storage, how to specify and specialist hardwood design.
It’s a timely update. Andrew Lawrence, Arup timber specialist, says: “Hardwoods represent 30% of the forest cover in Europe and North America, and include some of our strongest and most beautiful timbers, yet we barely use them structurally. They therefore represent a hugely underutilised resource. With new engineered hardwood glulam, LVL and CLT now becoming available, this is all changing.”
Meanwhile, the section on the marking of structural timber has also been revised to include the process of CE marking as required by the Construction Products Regulation 2011. Updates have also been made to the strength classes section to reflect changes in the standard BS EN 338.
Choosing hardwoods may allow a designer to use timber throughout a structure without the need to switch to other materials, such as steel or concrete, for long spans or heavy loadings. They can be used for typical beam and column applications, such as joists, lintels or purlins, as well as parts of built-up components such as portals or trusses.
The hardwoods preferred for structural use generally have greater natural durability than the commonly used softwoods. They do not therefore normally require preservative treatment, provided that the vulnerable sapwood is excluded.
Service conditions and durability are important factors when specifying the species. Risks include attack by fungi, boring insects and marine borers. Meanwhile, the available sizes might not match what is in the British Standard, hence the need to consult suppliers.
Hardwoods are often provided by specialist suppliers who will saw the wood to order, especially wood from temperate sources. Therefore, the range of sizes may be negotiated with potential suppliers.
Structural design using hardwood is little different in principle from design using softwoods. To design with Eurocode 5, you need the characteristic values of strength, stiffness and density. This relies on knowledge of several British Standards
There is a three-step process for determining structural design values. Step one looks at strength grade, step two looks at strength class and step three examines characteristic values.
Meanwhile, the importance of hardwoods is reflected in the updated version of TRADA’s National Structural Timber Specification.This much-awaited update includes added coverage of hardwoods for the first time.
Developed by TRADA experts the NSTS has been written to cover information exchange, materials, fabrication, erection, protection, and quality assurance within timber design, specification and use. It provides a definitive, comprehensive, stand-alone specification for structural timber in building construction.
View our full list of Wood Information sheets here.