advanced building skins 14 | 15 June 2012 - lamp.tugraz.at - Graz ...

Advanced Building Skins
Figure 6: Curved secondary beams with embedded seat cuts for the oak cladding boards (designtoproduction)
4 Timber for Structural Elements
While – as shown in the previous section – geometrically timber performs much better than steel,
structurally the comparison is more ambivalent. Timber generally requires larger cross-sections, while
on the other hand it weighs much less. Letting aside the details of structural calculation, there are two
main concerns when applying timber structurally – its anisotropic nature and its flammability.
4.1 Fiber Cutting Angle
Wood is an anisotropic material, meaning that its load bearing capabilities (or general properties) are
depending on the load direction. It has by far the most strength in its fiber direction. This is –
alongside the aim to minimize cutoff – the reason for using curved timber blanks (which are gluelaminated
from thin lamellas in a bending mold) instead of milling curved beams out of much cheaper
straight blocks.
From a structural point of view, the optimum would be a curved blank that exactly follows the
direction of the beam it is supposed to become. But in most cases – including the Kilden Façade with
its nearly 1,800 individual curved beams – it is not feasible to order blanks with batch size one, due to
the time-consuming lamination process. So at this point a nonlinear optimization process is applied,
weighing material cut-off against timber blank diversity to assign as many different but similar beam
geometries to the same blank geometry. In the case of the Wave Wall, it was possible to mill up to 13
different curved beams out of identical blanks without exceeding acceptable cutting angles for timber
fibers.
Figure 7: One common timber blank geometry for nine similar, but different beams (designtoproduction)
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