This project was entered in the LSAA 2007 Design Awards (#3197) "Medium Fabric Structures"
Entrant: Tattersall Engineering
Client: Aquinas College / J A Dodd Architect: Designinc Adelaide (Geof Naim)
Structural Engineer: Tattersall Engineering (See SEMF 2015)
Specialist Consultant(s): Wade Consultants Builder: Tecraft Pty Ltd
Fabricator(s): Horizon Sailmakers / C E Bartlett, Riband (Steel), A Noble & Son (Cables)
Application and Function(s) of Project:
The structure is a pivotal element in an $8m redevelopment of the main Aquinas College campus, which comprised a new library, Year 9 centre, canteen and arts centre as well as refurbishment of the old library as an IT, arts and careers centre. The Forum structure links and protects all the adjoining buildings, proving a substantial undercover area in the middle of the school which serves as a protected function centre, meals and play area, incorporates a stage with a protected auditorium area, and links with the existing gymnasium and sports centre building.
The entrant was engaged as structural and civil consultants for the entire $8m re-development, and because of the firm's experience with designing and building a number of major tensile structures was asked by the Architect to develop a concept design for a linking forum area in the centre of the redeveloped school. A basic plan of an eccentric annulus was provided by the architect, and this was developed by the entrant into the eccentric semi-toroid shape of the final design. The brief was to provide protected paths of access from all adjoining buildings, an auditorium area engaging the stage, control of rainwater runoff, and a central garden area.
The scale of the protected spaces was required to be open but not cavernous, link with adjoining single storey buildings, provide rain protection with sun and light penetration, and achieve an acoustic environment which would complement social functions, concerts and school assemblies.
The design has achieved the brief by providing 8m high spaces as an auditorium with the supporting structure also able to be used to mount sound and light systems, rainwater draining away form external areas with edges of the structure sailing over adjoining buildings, and an innovative system of curved split PVC pipe gutters attached to catenary cable edges with powder coated brackets.
The structural system uses a series of ten radiating trusses, these elements being bow-string trusses with a curved CHS rafter with downward loads of fabric prestress being resisted by cable bowstrings. Since spans of these trusses are up to 28m, a single 273 mm dia. CHS rafter was able to be used by the provision of a central lateral restraint system
incorporating droppers, lateral braces, and a central circumferential tie which runs around the centre of the toroid, rising and falling as the overall structure geometry dictated. This innovative system meant that the roof structure required only 6 kilograms per square metre of steel and the overall structure including rafter, columns and bracing required only 8
kilograms per square metre, even with spans of up to 28 metres.
Around the central internal ring alternate bays of inclined support columns were braced with crossed CHS members,
whilst the external support columns were supported by cable guys (some single and some double as structural
requirements and pedestrian access paths dictated) fixed to ground anchors comprising galvanised reinforcing bars
grouted into drilled holes in clay and mudstone up to 12m deep. Compression loads under columns were supported on drilled bored piers, incorporating end bearing and skin friction for the most economical structural solution.
The membrane system is essentially a radiating barrel vault shape with the fabric pulled down between the curved rafters to provide the sagging curvature around the circumferential direction and the hogging curvature in the radial direction.
Tensioning was provided utilising 4 field joints where the membranes were tensioned using chs pipe in hems and adjustable brackets, the weather protection being provided by rolled angle gutters fabricated with the curved CHS rafters. Catenary edge cables were used around the external and internal circumferences, and at intermediate rafters the membranes and cables were connected to a tensioning assembly able to be tensioned radially using profiled clamping plates and threaded rod, fixed through the rafters.
The fabric chosen was Ferrari 1002T, a PVC-coated polyester with PVDF coating, chosen for a combination of structural capacity, retention of surface gloss, and durability. An extended warranty was purchased to warrant 100% replacement over a 12 year period.
Support steel was structural steel, with a coating system of grit blast, inorganic zinc, epoxy tie coat, and polyurethane top coats, chosen for excellent durability and life, a quality smooth high gloss finish, resistance to site damage, and ability to be touched up and recoated without excessive preparation.
Connections components utilized a mixture of painted structural steel and stainless steel, whilst edge cables and fitting were galvanised steel. Appropriate consideration of galvanic interaction was made when selecting choice and location of differing materials.
Foundations used reinforced concrete, and ground anchors utilizing galvanised steel reinforcing bars grouted into drilled holes with a Densotape anti-corrosion system to provide debonding and protection in areas where corrosion could be critical.
Steel shop drawings were prepared using a three-dimensional X-steel model which covered all the steel and cable components. Major structural steel elements were trial assembled and check measured in the shop prior to delivery to site. Temporary bolted connections were provided so longer rafters could be broken into transportable sections, bolted together and erected, with final joining by site welding and removal of erection assembly components.
Fabric patterns were prepared from a model co-ordinated with the X-steel model, checked in three dimensions, and with flat patterns checked by measuring prior to and after completion of joining.
Foundation elements were set out by a surveyor from setout drawings prepared by the entrant and the final constructed dimensions checked on site by the entrant and the surveyor.
Tolerances were as tight as practically able to be achieved, as proven by the fact that the steelwork, with complex, three-dimensional geometry, was able to be fully erected in three days without any adjustment other than one bracing element which was subject to a fabrication error on the length ( this had not been one of the major elements checked in the shop) .
Construction and Maintenance
Construction involved a documented procedure developed by the designer. The steel structure was erected by the following method
- supporting columns erected with guy cables (bases and foundations were designed for columns to be free standing during the erection process).
- rafters bolted together and erected with two cranes, field joints site welded, temporary lateral restraints provided .
- lateral bracing and support elements installed
- fabric deployed at ground level on clean plastic with clamp plates and tag ropes attached and edge cables inserted,
- supporting rope network installed between rafters
- fabric flaked into a long section ruining transversely across the rafters, tied and lifted using a lifting beam,
spreader bar and laid across the frames
- fabric deployed from boom lifts with tag ropes and tied off at clamping plate connections and field joints
- edge chs and edge connection elements installed and fabric pulled up to field splice rafters
- connections made and fabric tensioned
Maintenance for cleaning and maintenance is readily achieved by the following procedures developed as part of the design
- access up the field splice rafters is readily available using a low level scaffold and a twin lanyard harness system
- access across the fabric surface for inspection and maintenance is achieved using restraint ropes from the field splices and a twin rope system
- access to edges is readily available from a small boom lifts