Timber iQ February - March 2020 // Issue: 48 | Page 39

FEATURES
rigidity for the panel is obtained in both directions, similar to
plywood but with thicker components. In this way, the panel has
great tensile and compressive strength. These panels can be
used to form wall, floor and roof panels. The panels are quick to
produce in a factory environment and they can be machined to
very high tolerances. Panel sizes are typically 16 metres by 2.95
metres but can be larger. The majority of CLT producing factories
are located in Europe, where the raw material softwood timber
resource is located. This softwood resource is plentiful and
relatively inexpensive.
HARDWOOD AS AN ALTERNATIVE TO
SOFTWOOD
In the last few years the American Hardwood Export Council
(AHEC) has explored the possibility of using hardwood as an
alternative fibre resource to softwood for CLT manufacturing.
AHEC has collaborated on a number of landmark CLT projects
utilising American tulipwood and each project has been
undertaken with an element of research and development to
expand the knowledge around American tulipwood as a raw
material for potential commercial CLT production.
The first demonstration of the laminating potential of American
tulipwood was the Endless Stair in 2013. This structurally
challenging installation designed by dRMM architects with the
help of Arup structural engineers was a three-storey network of
stairs based on the concept by the Dutch graphic artist M. C.
Escher. Initial testing at Trento University prior to the Endless Stair
project revealed significant strength benefits of American
tulipwood, including being three times stronger in rolling shear
than softwood.
The knowledge gained from the Endless Stair project was taken
further with The Smile, a project designed by Alison Brooks
Architects in collaboration with Arup for the London Design
Festival in 2016. This showcased the enormous structural
potential for tulipwood CLT, emphasising its strength-to-weight
ratio by reducing the amount of fibre required to achieve the same
strength performance as the equivalent in softwood timber. The
Smile represented the first ever use of industrial-sized hardwood
CLT panels.
The year 2017 saw the completion of the first permanent building
made from American tulipwood CLT. Maggie’s Oldham, designed
by dRMM Architects, selected American tulipwood for its
potential to deliver large panels and, considering the function of
the building, for its visual beauty and warmth. Maggie’s Oldham
represented a milestone in the development of hardwood CLT,
showing that American tulipwood CLT could be manufactured on
a commercial scale for a project with a very tight budget.
The most recent demonstration of the structural capability of
tulipwood CLT was in the 2018 London Design Festival project
MultiPly – a three storey, three-dimensional maze structure
designed by Waugh Thistleton Architects. The complete structure
comprised 17 interconnecting modules made from a total of 10
260mm and 100mm thick x 2.6m square CLT panels. This
modular structure with its flexible design and connection detail
allowed it to be re-erected in a different configuration as part of
2019’s Milan Design Week.
During the MultiPly project a comprehensive testing regime was
conducted at Edinburgh Napier University in order to examine all
of the structural properties of American tulipwood CLT and to
enable a comparison with published data for softwood CLT. This
www.timberiq.co.za
The Endless Stair project revealed significant strength benefits of
American tulipwood.
information will be published in a Tulipwood CLT Technical
document which will cover all aspects and findings of the
production of the MultiPly project.
THERMALLY MODIFIED TIMBER
Thermal modification is now globally recognised as a low
environmental impact method of preserving timber for use in
outdoor environments. Originally developed in Scandinavia as a
method of preserving softwoods without the need for chemicals,
it soon became apparent that some American hardwood species
have the structure to enable them to withstand the intense kilning
process that is employed to modify the timber. The North
American temperate hardwood forest resource does not offer
durable timber species in any real commercial volume and so to
be able to thermally modify hardwood species such as ash,
tulipwood, soft maple, yellow birch and red oak, allows North
American hardwoods to be utilised for external applications with
confidence. The three main benefits of thermally modified timber
are increased durability, increased dimensional stability and a low
thermal gain, meaning it is comfortable to touch or walk on in hot
temperatures. The modification process also treats the wood
through the whole cross section of the timber.
Europe is still very much the hub for thermal modification of
timber with the majority of large producers in this region.
However, increasing investment in the technology is being made
in North America and elsewhere across the globe, as the demand
for sustainable outdoor timber products increases.
AHEC has pioneered a number of showcase projects using
thermally modified hardwoods in recent years, beginning with
Martino Gamper’s Infinity Bench for the London Design Festival in
2011. Since then many other global projects have shown just how
versatile thermally modified American hardwoods can be. From
David Trubridge’s Aleni reclining chair in thermally modified
American ash in New Zealand to the Houtlander’s extraordinarily
complex thermally modified red oak bench project in South Africa.
These projects are helping to develop awareness of these products
and are, undoubtedly, having a positive impact on the market for
thermally modified American hardwoods in these regions.
Two permanent examples of the use of thermally modified
American tulipwood can be seen in the United Kingdom. These
are Asif Khan’s Room on a Hill - an outdoor play and learning
environment made from thermally-modified tulipwood slats in a
galvanised steel frame, which has stood the rigours of a
succession of enthusiastic schoolchildren since 2015 – and the
acclaimed fluted tulipwood cladding that envelops the exterior of
Maggie’s Oldham and is weathering well in the harsh climate of
northwest of England. Both projects are being monitored for their
long-term performance.
// FEBRUARY / MARCH 2020
37