LSAA 2011 Design Award Entry (4210)
APPLICATION OF PROJECT:
Canopy shade structure: The architectural PTFE membrane canopy provides weather protection for an outdoor events plaza and exterior walkway.
A truly stunning piece of fabric architecture was created with this stunning lightweight membrane roof at ITE College (West) in Singapore. Supported only by a complex cable system and large steel wall plates bolted to 4 different buildings, the 3000m2 PTFE Canopy provides shelter to the Events Plaza and exterior walkways below spanning an incredible 75m span in one direction and 45m in the other.
The project presented many extreme challenges that were overcome through innovative thinking and design. The 3000m2 canopy was hoisted into position from ground level and secured in just 16 hrs using 10 electric chain hoists, in conjunction with a 200T mobile crane.
The completed saddle shaped‐membrane structure adds a beautiful yet practical architectural focal point within the development and exceeded the expectations of both the client and the main contractor.
DESIGN / FABRICATION / INSTALLATION BRIEF
The project brief involved the design and construction of an architectural PTFE membrane canopy to provide shelter for an events plaza and exterior walkway. The membrane canopy forms part of a new state‐of‐the‐art tertiary college development in Singapore.
The canopy’s primary design requirements were:
• The design was to be lightweight and minimalist with no large structural steel to provide a free form canopy
• To protect students from the sun’s heat
• To provide weather protection for the events plaza below
• To provide weather protection for an elevated walkway system
• To Naturally ventilate and create airflow
• Low maintenance installation
• Satisfy the strict building codes in Singapore
The 3,000 m2 canopy sits between 20 and 35m above the events plaza below, spans 75m in one direction and
45m in the other, and is supported by 6 different levels of 4 separate high rise buildings. Incredibly the entire system weight was only 6.5kg / m2 due to the efficient use of the surrounding concrete buildings to support the large forces generated by the membrane canopy.
The membrane canopy was designed with an anticlastic form with a very high pre‐stress of 5KN x 5KN (due to the large spans) and is structurally supported by a steel cable system involving both under and over fabric cables, catenary edge cables and large guy cables. Large, high strength bolts were cast‐in during construction into the structural concrete beams of the buildings and were used to secure large, complex steel brackets for supporting the membrane roof. The as‐built location of the cast‐in bolts was surveyed using a combination of conventional surveying and a high tech 3D scanning system.
During the design stage, detailed analysis reports were completed detailing the water runoff design of the canopy, and the thermal effect the canopy had on the airspace below the roof. A water diversion system was used in some areas, as a result of the analysis, to control the water runoff from the roof.
The membrane was fabricated in three pieces joined by detailing the under fabric cables to become part of the filed joint system. The membrane canopy was on the builder’s programs critical path due to all of the finishing trades requiring access to the events plaza below. There was only a four week window to install the membrane and careful planning was required for the lift. The lift was forecast to be completed in 16 hours to a point where it was safely fixed off to the wall plates prior to final tensioning.
The membrane roof covers an area of 3,000m2 between 4 separate ten(4.5m) –storey high rise buildings. The roof also covers the upper level of the innovation walkway which is a concrete walkway structure that connects the four high‐rise buildings at levels 4 and 6. To achieve the required form and cover, the membrane roof had to be fixed to the concrete structures of the four high rise buildings, and the innovation walkway at a total of 24 fixing points. These fixing points range from level 4 to level 10 of the high‐rise buildings.
The membrane roof was designed using a structural support system consisting of high strength Galfan cables, steel membrane clamp plates, structural shackles, and complex structural steel wall brackets. The cable system comprised two 75m long over‐fabric cables, two 45m long under fabric cables, catenary edge cables and guy cables for the four floating membrane plates.
Due to the nature of the fixing points, structural analysis of the membrane roof needed to be completed so that the very high forces generated could be taken into account when designing the high rise structures. In addition,
24 unique sets of high‐strength cast‐in bolts needed to be designed, supplied, positioned, and cast‐in during the construction of the high rise structures.
Prior to final detailing and patterning of the membrane roof, each set of cast‐in bolts needed to be accurately surveyed due to the stringent tolerances required for the structural system to work effectively. Due to the lack of access to the fixing points, coupled with the extreme vertical height, the as‐built survey was completed using a combination of traditional and 3D laser survey methods.
Access for the execution of this project was a major constraint due to the fact that no large and heavy access equipment could be located directly below the membrane roof. This was due to the second floor events plaza area being designed as a suspended concrete slab, with a structural rating of only 5kpa. This 5kpa limit meant that no boom lift that weighed over 5,000kg could be used onsite for the installation. The vertical reach required to access the points ranged from 10m to 40m making it impossible to access the higher points with equipment located at the events plaza level. To overcome this issue a range of lightweight spider lift equipment were used and located at the events plaza level, and crane located outside the plaza lifted units to the 6th floor of the innovation walkway.
Following completion of the as‐built survey, the final patterning and detailing of the membrane roof was completed so that the Galfan structural cables, membrane plates, and steel brackets could be fabricated. The entire 3,000m2 canopy was fabricated and delivered to site in three sections.
The steel wall brackets ranged in weight from 150kg through to 600kg; originally it was intended for these to be lifted using tower cranes, however, due to the program the tower cranes were already dismantled and off‐site. In addition, the low structural rating of the events plaza meant that access for mobile cranes was not available for lifting the steel brackets for most of the fixing points. To overcome this, a complex lifting system was devised by using floor cranes installed at specific locations such as rooftops, and open floors above each fixing point.
Chain block systems had to be designed and integrated into the lifting sequencing due to limited access for cranes on the events plaza, all of the installation systems used had to be signed off by a local professional engineer and certified before use.
The fabric used was PTFE Sheerfill Type II in white translucent by Saint Gobain Performance Plastics. PTFE was selected due to its high tensile strength, pollution resistant Teflon coating, fire resistance, and anticipated design life of 25+ years. German engineered and fabricated Galfan cables where used for all of the cables required of different diameters ranging from 20mm to 45mm
The entire 3,000m2 PTFE membrane was fabricated in 3 sections and assembled at ground level, fitted out with all cables, membrane plates and field joint hardware over a period of approx 2 weeks.
COLLABORATION, CONSTRUCTION AND MAINTENANCE
Installation of the membrane roof was a complex process completed in two parts. Firstly, the steel wall brackets were lifted and fixed in position over a period of 2 months (sequencing was required based on when the builder had finished the cast‐ins for the locations of the wall plates) and then the PTFE membrane and hardware were installed and tensioned over a period of 4 weeks. Installation of the steel wall brackets was a lengthy task that had to be coordinated with many other trades due to lack of access for both cranes and access equipment.
The membrane was then hoisted into position using 10 electric chain hoists rigged into a double stage lifting setup and a 200 tonne mobile crane, the lift needed to be completed in one day so that the membrane was secure and would shed water prior to being left unattended.
Due to the lack of access for cranes and access equipment, a complex and detailed lifting plan was required for the installation and tensioning of the PTFE canopy.
The major constraints that impacted this lifting plan were:
- The only area available for laying and fitting out the fabric was the events plaza
- The membrane roof needed to be lifted vertically between 25‐40 metres simultaneously from at least 10 points
- Mobile crane access was only available for lifting two of the points at one end of the roof
- There was no access for cranes or access equipment to the events plaza area
- At all times during the lift, the membrane roof had to be able to shed water in case of rain
- The entire 3,000m2 canopy needed to be lifted and secured in one continuous process to ensure that it would not pond water or be damaged by winds
The lifting plan involved the following:
- Five supervisors, five local riggers and ten local labour
- All supervisors and crane operator connected via two way radio
- One 200tonne mobile crane connected to lift two points at one end of the membrane roof vertically, and up and over the innovation walkway
- Ten 2‐tonne electric chain motors with integrated control system, to simultaneously lift the ten other points down either side of the membrane roof
- Due to the chain motors only having 20metres of chain, a two‐stage rigging system was devised using cable slings and a midpoint changeover rigging point
- Once the membrane roof lifted into position, each point to be changed over and rigged to shackles and slings for safety (not left on chain motors)
- The lift ran smoothly and was completed in a period of 16hrs exactly as planned
Following the lift, the membrane roof was systematically tensioned to the side and end points using a combination of chain blocks and cable tifors. In order to tension the membrane to the correct fixing point design loads, final tension on the four main guy wires was completed using two 20‐tonne hydraulic rams and pressure gauges. The initial tensioning of the 3,000m2 roof was carried out over a period of 8 days. The membrane was then further progressively tensioned over the next 2 weeks with the final tension on the main guy cables being completed using 2 of 20 tonne hydraulic rams with stress gauges, in order to monitor the tension forces needed for installation.
Project Number: 4210
Title: ITE College West, Singapore
Location: 1 Choa Chu, Kang Grove, Singapore
Entrant: Fabritecture Pty Ltd
Role: Design, fabrication and installation
Client: Gammon Pte Ltd
Architect: DP Architects Ptd Ltd
Structural Engineer: Wade Design Engineers
Others: Ozrig - Installation advice & riggers
Builder: Fabritecture Pty Ltd
Fabricator: Tensile Structures International PVT Ltd (Fabric)
Steelwork etc: TTJ