When to specify structural timber
Long span GLVL being manufactured in a Steico factory
Engineered timber is being used on a wider range of construction projects. Martin Twamley explains which types are suited to which applications.
Steel and concrete are the dominant structural materials in construction, but they are relatively new technologies. We built for thousands of years with natural materials and it was only the onset of the industrial revolution that saw the wide uptake of man-made alternatives.
However, as raw materials become scarce and expensive to extract, and with concerns about their sustainability credentials, there is an argument in support of switching to timber for more structural applications.
Types of engineered timber
- I-joists and metal web joists – typically used for basic floor, wall and roof construction.
- Glulam – for higher load bearing applications.
- LVL and GLVL (laminated veneer lumber and glued-laminated veneer lumber – where the highest loads need to be accommodated).
- CLT (cross-laminated timber) – for structural walls and floors often in high-rise applications.
Incorporating engineered timber increases the structural possibilities and makes better use of a natural resource. Combining structural timber with natural insulation also means a healthy internal climate with quantifiable benefits for the occupants.
Currently, the majority of structural timber used is in roof trusses and floors. It is common in smaller-scale residential developments and extensions. But engineered timber is also suitable for large-scale commercial projects which use cross-laminated timber (CLT) and glued-laminated veneer lumber (GLVL) (see box: Types of engineered timber).
Long spans can easily be accommodated by increasing the depth of the sections used, with limited impact on weight. This can be illustrated by a like-for-like comparison of steel and timber beams with the same bending strength: a GLVL beam manufactured by Steico, measuring 360mm x 200mm, has a weight of 39.6kg/m. This is barely half that of a standard steel HEM 160 H-beam, measuring 180mm x 166m, and weighing in at 77kg/m.
GLVL was used on the extension of Steico’s offices in Munich. The high loading requirements – up to 5.0 kN/m2
– allowed the long span capacities of the GLVL to be used to their maximum. Spans of up to 7.25m were incorporated with depths of only 240mm. Fire certification up to F90-B was achieved using simple char rating calculations (see box: Key standards for engineered timber).
Steico’s GLVL beams can be manufactured up to 18m length, with depths up to 400mm and widths up to 1,250mm. In practice, the length of the spans depends on loading conditions.
On the practical side, the lightweight nature of timber makes it relatively easy to handle. Conventional tools and techniques can be used with structural timber, so onsite adjustments are straightforward to accommodate.
Structural timber’s sustainability is best assessed by including embodied energy, which measures environmental impact through a material’s entire life cycle, from felling trees through manufacture, delivery, use and disposal.
According to a study by the University of Massachusetts, the embodied energy for timber is 2.0MJ/kg, compared to 10.50MJ/kg for steel and 12.50MJ/kg for concrete. Additionally, wood acts as a carbon store by locking CO2 in its structure. Timber trade body Wood for Good estimates an additional 3.81 million tonnes of CO2 would be locked away each year if UK housing targets were met with timber-frame homes.
Martin Twamley is technical manager of Steico
Key standards for engineered timber
Design of timber structures:
- BS EN 1995-1-1 Eurocode 5, plus the relevant National Annex (UK NA).
- PD 6693-1. Non-contradictory complementary information to BS EN 1995-1-1 Eurocode 5.
- BS EN 1995-1-1 Eurocode 5 also includes structural fire design, including char rating calculations.
Sustainable sourcing certifications:
- Forest Stewardship Council (FSC).
- Programme for the Endorsement of Forest Certification (PEFC).