structural elements Using oak, with confidence
Oak-framed buildings are enjoying a renewed popularity due to the material’s strength characteristics. Ed Hamilton of Oakmasters explores why green oak has a place in modern construction and addresses structural questions.
classes from very durable (1) to not durable (5) based on a number of biological tests, oak gets a score of 2, surpassed only by a handful of tropical hardwoods. In bending tests, steel proved to be three times stronger than
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oak, however, it is also ten times heavier, which works in favour of oak in terms of its weight/strength ratio. Other tests looking to establish comprehensive and shear
strength place oak in the medium to high strength category. It is notable that oak’s strength increases as it dries.
Strength-graded timber
It is important to use strength-graded timber in order to ensure its structural suitability. TRADA, the leading authority on wood, allows timber to be graded visually by certified organi- sations. Many oak-framing companies are certified to stress grade oak during the production process. There are four stress categories – TH2, THB, TH1 and THA – in increasing order of strength. Main oak frame beams and posts will typically be in the THB category while THA is usually applicable to oak flooring.
Moisture and drying
As oak dries, it shrinks and cracks until it reaches its equilibrium level of moisture content. The majority of shrinking occurs in two directions. Typical shrinkage values from green to 12 per cent moisture content are as follows:
•longitudinal shrinking (along the grain) ≈ 0.15 per cent •radial shrinking (across the grain) ≈ 4.5 per cent •tangential shrinking (parallel to growth rings) ≈ 7 per cent
Fissures tend to appear only from the centre to the outer edge of the log, which means that any beam will retain a minimum of half of its structural integrity at any one time. Skilled craftsmen take this into consideration, while advances in design technology for oak framing mean that allowances are made for loading stress and shrinking very early on in the process.
Air-dried vs green
This age-old conundrum still seems to create confusion but it all comes down to the environment for which the oak is intended.
Typically, oak placed in an indoor heated environment
will eventually reach a moisture content equilibrium of approximately 7-9 per cent. Oak placed in an indoor unheated environment will stabilise at around 12-14 per cent and where exposed to sun and rain, it will vary between 12 and 20 per cent. In short, an oak frame will carry on shrinking and
expanding whether it is green or air-dried. However, the combination of hardness, drying time, drying movement and pre-existence of fissures makes air-dried oak a less viable commercial proposition. Provided that shrinking and cracking are taken into
consideration when designing the frame, the main difference is that air-dried oak takes longer to produce due to storage requirements; is harder to work with due to existing fissures and hardness and is ultimately more expensive, while offering the same structural guarantees as green oak.
Creating an energy efficient building envelope
In creating a seamless envelope, conforming to tightening Building Regulations is the first requirement. While an oak frame can be combined with conventional wall build up methods including masonry, render or cladding, more sophis- ticated methods using structural insulation panels (SIPs) are gaining popularity. The combination of the SIP system and oak frame provides
the structural strength and aesthetics of the oak superstructure with the speed, rigidity and thermal advantages of SIPs. The use of SIPs can also avoid the need to use softwood studwork
enturies-old barns, houses and churches all prove that oak can withstand the test of time. According to European standards, which classify timber into five
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