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

WAITOMO CAVES VISITOR CENTRE 

Alistair Cattanach, BE (Hons), MIPENZ, CPEng IntPE, Mem ACENZ 

Director, Dunning Thornton Consultants Ltd, Wellington 

Email: Alistair.Cattanach@dunningthornton.co.nz 

INTRODUCTION 

Gridshell = a grid-like structure which follows a two-directionally curving surface, with sufficient grid elements that it 

acts structurally like a shell. 

per annum. One of three major caves, the original glowworm cave site is operated by Tourism Holdings Ltd. The design 

team of Architecture Workshop, Dunning Thornton and eCubed were selected in a design competition run by THL. 

Because the site is on Department of Conservation land, owned in conjunction with the local iwi, an environmentally and 

culturally sensitive design was essential, and its iconic image is a key part of continuing marketing of this attraction. 

CONCEPT 

The concept for the timber gridshell roof came from 

Architecture Workshop as a way of creating a stopover in 

the bush on the busy tourist trail between Auckland and 

Rotorua. The centre was originally conceived as a 

freeform organic gridshell, but we quickly established 

the difficulty of constructing such a form on this very 

steep site whilst maintaining the flow of tourists into the 

caves. 

In order to fit in with the topography of the bend in the 

Waitomo River, it was decided to form the geometry of 

the roof from a surface of revolution (see below). 

Figure 1. Image courtesy Architecture Workshop 

The existing topography was terraced between the upper 

cave entry and the lower cave exit, with the road/tunnel 

access midway between. The terraces carry a series of 

timber platforms, whose lightweight nature treads 

lightly on the poor ground, and which were easy to erect 

with a minimum of crane access. Much of the floor 

finishes and cladding to the platforms is timber, ranging 

from traditional rough-sawn decking through to 

Blackbut hardwood-faced ply in the heavy traffic areas. 

Figure 2. Main platforms with Blackbutt ply and pine 

decking 

ROOF GEOMETRY 

The roof is cut from a section of tauroid. This form was 

created by generating a cross-section on a rib-line as a 

perfect circle and then arraying this around the centre of 

NEW ZEALAND TIMBER DESIGN JOURNAL VOL 18· ISSUE 3 7

the tauroid to create the form. Whilst the ribs are 

generated from this circle, their form is much more 

complex. 

and by back-checking in the 3D CAD geometry 

(measuring each side of each rib: equal lengths = no 

secondary bending). 

Figure 3. Image courtesy Architecture Workshop 

The orientation of each rib is set to always face the 

Figure 4). To do this the rib must curve and twist as it 

crosses the roof. A simple way to picture this might be to 

imagine laying a ribbon across the curve of a tyre tube. 

As the rib takes this form, it not only wants to bend about 

its major axis and twist, but also to bend about a 

secondary axis. In order to fabricate ribs that were bent 

only about one axis (and twisted), we allowed the ribs to 

-shape in plan. This removed the 

secondary bending. This process required extensive 

structural form-finding, which was done by a 

mathematical process, a structural modelling process 

Figure 4. Multiple grid geometries (lighter and darker grey) 

on the same shell surface: 

Figure 5. 

object. 

The S-curve formed by bending and twisting an 

The cladding comprises ETFE pillows: inflated cushions 

of high-strength plastic, seen recently at the Watercube 

at the Beijing Olympics. These cushions could be 

patterned to the two-directional curvature of the 

gridshell. However, if they were to be broken into 

diamonds as in other similar previous projects, their 

cladding would be significantly more expensive - the 

majority of the cost coming from the aluminium 

extrusion seaming and clamping at the perimeter. Lifting 

- 

- 

cushions, the belly of which cleared the timber 

structure. 

Rib centres were selected to optimise spans for 

commercially available thicknesses of ETFE foil at an 

average of 4.25m. The line of the cushion edges follow 

over the ribs in their own similar S-curve: the edge 

extrusions also being bent only about one axis and 

twisted. 

Figure 6. Roof geometry courtesy Architecture Workshop 

8 NEW ZEALAND TIMBER DESIGN JOURNAL VOL 18· ISSUE 3

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 


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. 


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 


Figure 22. The finished structure 

SUMMARY 

The structure is unique, and has been interpreted as 

symbolising a traditional Maori eel net with its open 

weave. The extremely complex geometry pushes the 

limits of our three-dimensional modelling tools: there is 

a complex procedure to set up a pure cross-section at 

any one part of the 3D model. However, this complex 

geometry allows an exceptionally light structure, with 

timber only 316mm deep at 4.25m centres to span 

almost 30m across the roof. 

The writer would like to acknowledge Matthew Davies 

and Anthony Gardiner from Dunning Thornton for their 

tireless dedication in engineering and drawing the roof, 

Gareth Alley and Martin Williams for their hard work on 

the platforms, Architecture Workshop for their vision 

and creativity, Nelsonpine Industries for manufacturing a 

non-standard 39mm product for this job, Hunters for 

their collaboration and feedback in the timber 

fabrication, Vector Foiltech in pushing the boundaries of 

ETFE detailing, and Hawkins for their endless hours on 

the tools and commitment to accuracy.

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

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