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

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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 TIM<strong>BE</strong>R DESIGN JOURNAL VOL 18· ISSUE 3
Figures 19. Entry toilets and cave path accurately to start and end the rib alignments. After significant cross-checking the adjustment to the alignment of each rib, all ribs were positioned well within tolerance of 40mm in the length (over 28-34m). Figure 18. Erection photo courtesy Hawkins Once heights were re-checked, end fixings and the fitting of node joints and cables could begin. The end fixings were generally site-drilled 14-gauge self-drilling screws through the steel bunny ears to achieve the 40mm tolerance in length. Once all hardware was in place, the scaffolding was removed and the deflections of the tips monitored. A tip deflection of 376mm was recorded against a predicted 377mm, although there was a subsequent settlement of another 10-15mm after the structure was oscillated to check the dynamic tip frequency. Figure 20. Cave exit path with Visitors centre beyond The ETFE cushions were then erected in their extrusions and subsequently inflated. The pull-in force at the perimeter from this inflation causes the tips of the gridshell to rise approximately 350mm, countering the deadload deflection. Figure 21. Four-way cable node NEW ZEALAND TIM<strong>BE</strong>R DESIGN JOURNAL VOL 18· ISSUE 3 11
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WAITOMO CAVES VISITOR CENTRE Alistair Cattanach, BE (Hons ...

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