WAITOMO CAVES VISITOR CENTRE Alistair Cattanach, BE (Hons ...

2-M16X200 COACH SCREWS, SHOP
DRILL SHANK Ø X LENGTH. SITE DRILL
ROOT DIA. TO 220 LONG
4X/ 8MM OUTER
THREADED DIA. 300
LONG SPAX SCREWS
110 40
their perimeter members. With the cushions connected
in a series these forces balance, but still the perimeter
of the overall roof is subject to high (3-8kN/m) pull-in
forces. These are resisted by a complex series of edge
catenaries and tethers as shown in the images below.
3-14g BATTEN
SCREWS, 150 LONG
Figure 7. Typical block detail
90
STRUCTURAL ACTIONS
In the first instance, the timber ribs act as large arches
spanning the 28m across the structure. These arches
are made from two layers of LVL ribs interconnected
with intermittent blocks. By clamping the blocks
between the two layers, Vierendeel action allows
localised loads to be shared out to the greater shell
structure.
Because the arches are arrayed around a circle of
revolution, when wind actions try to rack the structure,
the diagonal nature of the grid causes it to lock up as it
rolls forward, i.e. the arches at the end are skewed to
the arches in the centre, and hence under racking loads
the ends act as diagonal braces.
Because of the unusual geometry, wind actions on the
structure were derived from a wind tunnel test. The two
dominant load cases were maximum uplift from the wind
blowing in the end of the structure, and the forward
racking of the shell from the wind blowing down the hill.
As the structure weighs less than 30kg per square
metre, gravity load cases did not dominate over wind.
Figure 9. Top edge catenaries
The structure needs to allow for any one cushion to
become deflated suddenly, and repaired/replaced. With
the e-plane over 0.5m above the t-plane, the resulting
twisting forces on the intermediate structural members
would usually be immense. To counter this, we
conceived a world-first system where the cushions are
held on a series of flexible poles above the timber
structure. The e-plane is therefore free to move above
the t-plane as unsymmetrical loads are placed upon the
utilising a 20mm pad of neoprene at the fixing point.
Figure 10. Major Edge support
Figure 8. T-plane cables
allowing the two ends of the shell to move towards each
other. Similarly, as the structure eases down when the
wind drops, or under gravity loads, the tips relax
downwards and outwards. To control these tendencies, a
net of cables were included in the t-plane: cables
crossing the diamonds inhibit this scissoring action. The
dominant uplift loads are evident in the greater number
of transverse cables, as shown in the diagram below.
The ETFE cushions resist forces by pulling inwards on
Figure 11. Lower edge catenaries
NEW ZEALAND TIMBER DESIGN JOURNAL VOL 18· ISSUE 3 9