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

Structurally, this allowed us to limit forces to acting only
along the major axis of the principal members.
FABRICATION
The timber ribs were each fabricated in two layers, so
they could be interwoven to create layers 1-4 as shown
below. Each layer is made from three layers of ex170 x
39mm LVL.
Figure 12. Jig Shop Drawing
together in special jigs to form the curve and twist. The
jig layout, shop-drawn by Dunning Thornton, allowed for
an over-bend and over-twist such that the ribs would
spring back to their ideal shape after the glue had set.
Initial layout in the jigs showed the ribs curving up and
away from the bed. Measurements of this were taken,
and twist tests were made on the prototype to ensure the
correct torsional modulus (G) was being used. Changing
this required re-casting the whole gridshell geometry to
remove the resulting secondary bending stress.
Modification of the jigs to this revised shape indicated a
perfect fit of the timber to the bend and twist required.
Fabrication of the ribs was carried out in winter 2009:
the cold temperatures allowing 4-8 matching ribs to be
glued in the same jig, because the slower glue-setting
time in the cold weather allowed extra time for
manipulation.
All blocks, bolt-holes and other hardware were fitted to
the ribs before H3.1 treatment and the first of four coats
-
34m ribs were formed using high-tensile epoxy rods
shopsite
joint as below.
Figure 13. Rib manufacture
Figures 15/16. Rib splice
Figure 14. Completed half ribs
Each LVL was pre-planed by 1.5mm to remove the wood
compressed during manufacture and allow a fullstrength
glue joint. The three layers were laminated
ERECTION
The gridshell was erected on a birdcage scaffold. The
contact points on the scaffold were difficult to define and
were eventually set as horizontal circular contours
around the structure. The ribs, each weighing
approximately 150kg, were layered over the scaffold
structure with a small long-reach road crane.
10 NEW ZEALAND TIMBER DESIGN JOURNAL VOL 18· ISSUE 3