True to Form

A look at flexible formwork research at the Centre for Architectural Structures and Technology (C.A.S.T.) in the Faculty of Architecture at the University of Manitoba.

C.A.S.T., the Centre for Architectural Structures and Technology at the University of Manitoba’s Faculty of Architecture, is a unique research and teaching laboratory dedicated to the exploration of new ideas and methods for the design and construction of architecture. The $1.2 million C.A.S.T. laboratory building (see CA, February 2003, p. 20) has been built with government grants from the Canada Foundation for Innovation, Western Economic Diversification Canada, and the Manitoba Innovations Fund, and with nearly half a million dollars in donations from the Canadian construction industry. C.A.S.T. combines speculative research with design and technical education in a wide range of materials, tools, and technologies. It brings together architects, engineers, builders, and industry in a unique environment of exploration, construction, and testing. Research at C.A.S.T. is currently concentrating on flexible fabric formworks for concrete structures.

Fabric formwork

The natural tension geometries given by formwork fabrics simplify the production of lightweight, high efficiency structural shapes. The formworks themselves are extraordinarily light and very inexpensive. The flexibility of a fabric formwork membrane makes it possible to produce a multitude of architectural and structural designs from a single, reusable mold. The use of permeable formwork membrane fabrics produces improved surface finishes and strength as a result of a filtering action allowing air bubbles and excess mix water to bleed through the formwork membrane.

Methods for casting columns, walls, panels, beams, and slabs in both cast-in-place and precast applications have been developed. Cast-in-place and precast fabric-formed columns have been used structurally in Canada and internationally. Fabric formwork products for casting foundation footings and small columns are being manufactured and marketed by Fab-Form Industries of Surrey, B.C. The Centre for Architectural Structures and Technology at the University of Manitoba is the first academic research laboratory engaging in fabric formwork research. Research on fabric formwork technologies invented at C.A.S.T. are being carried out in Chile at its Catholic University of Valparaiso and the “Open City,” and in Scotland at the University of Edinburgh. Fabric-formed construction projects have also been carried out in Japan by architect Kenzo Unno, inventor of an elegant cast-in-place fabric-formed wall system.

Fabric-cast beams

The world’s first reinforced concrete beam cast from a pre-stressed fabric mold has been produced at the Con-Force Structures factory in Winnipeg by myself, Christopher Wiebe, Phillip Christensen, and Fariborz Hashemian (figures 2 and 3). This prototype beam was formed from a single flat rectangular sheet of inexpensive geotextile material. The ability to easily form complex curves from a single fabric sheet provides a method of construction that places material only where it is needed. This method produces concrete structures that are both more efficient and more beautiful than conventionally formed concrete. The longitudinal shape of a fabric-formed beam can easily follow the bending moment diagram for its loading envelope and support conditions, and the transverse sections can vary to reduce concrete in the tension zones, distributing concrete to the compression zones of the beam (figure 4).

The complex three dimensional curves of this prototype beam were achieved quite simply and rapidly by stretching a fabric sheet in a basic wooden framework made of 24s and plywood. Figure 5 shows the wooden framework “tables” used to support the fabric sheet with the fabric sheet stretched in place and ready for concrete placement.

Although load tests have not yet been performed, the structural efficiency of these beams promises to be far greater than that of conventional prismatic, rectangular beams. The 12-metre prototype shown (figures 2 and 3) uses half the concrete of an equivalent rectangular beam, and uses 40% less reinforcing steel. Compared to conventional reinforcing designs, the reinforcing steel fabrication and installation for this beam is simple, involving far fewer pieces of steel.

The same flat sheet of fabric can be used to form a wide range of efficient beam geometries. The same formwork rig can form individual beam shapes configured to follow the bending forces produced by specific support and loading conditions. In early 2004 with my students I will begin casting a series of large scale working models formed to a variety of efficient beam shapes, while our engineering partners at the University of Manitoba will begin structural tests on these models to test both structural behavior and engineering analysis methods.

Fabric-formed precast concrete panels

With my students at the University of Manitoba, during the winters of 2002 and 2003, I developed and produced full-scale prototypes for a wide variety of precast panels formed in flexible fabric molds. This work was accomplished with in-kind support from the Canadian Precast Concrete Institute (Manitoba chapter). These 812 panels were cast in a simple reusable pre-stressed fabric sheet. Some of these panels are illustrated in figures 7 and 8.

Fabric-cast panels are of two basic types: “direct-cast” panels and “invert-cast” panels, as described below.

Direct-cast panels

Fabric-cast precast panels begin with the following method of construction: the rendering in figure 6 shows a typical precast panel formwork rig. The bottom layer consists of a lower frame that establishes the boundaries and dimensions of the panel to be cast. Inside this lower frame is placed a set of intermediate supports in a pattern that sets the design of the panel’s surface (figure 10). A fabric membrane, placed inside a steel self-stressing frame, is placed over the lower frame and the intermediate supports. Finally an upper frame, matched to the lower frame, is placed on top of the fabric to contain the wet concrete and set the edge thickness of the panel (figure 11). The weight of the concrete placed on top of the fabric membrane causes the fabric to deflect downwards between the intermediate supports. These deflections can be controlled by the design and placement of the intermediate supports, and by the tension levels exerted on the fabric membrane. The left-hand panel in figure 9 shows the “direct cast” panel produced by this formwork rig. The right-hand panel is an invert-cast panel produced from the left-hand panel which was used as its mould.

Invert-cast panels

When a direct-cast panel is used as a mold, it produces a panel which inverts the geometries produced by the original fabric formwork. Figure 11 shows two different panels in production: the bottom of the image shows a completed direct-cast panel formwork ready for concrete placement, while the top of the image shows a direct-cast panel being prepared as a mould for an invert-cast. Figure 1 shows a detail of its inversion. The process of invert casting transforms the three dimensional tension geometries originally produced by the fabric formwork into three-dimensional compression shell or vault geometries. This structural transformation results from the fundamental natural law that tension and compression are physical and geometric inversions of each other.

Because fabric formworks will automatically produce pure tension geometries for any boundary and loading condition, a simple method now exists for producing funicular compression shell and compression vault geometries for any boundary and loading condition.

Mark West, the Founding Director of C.A.S.T., has invented numerous techniques for forming concrete structures in flexible fabric molds, including columns, beams, slabs, and precast wall panels. With his students and the kind support of the Manitoba chapter of the Canadian Precast Concrete Institute, new methods for casting concr
ete panels and beams have been developed and tested recently in full-scale construction demonstrations.

Students: Kenneth Borton, Spencer Court, Andrea Flynn, Colin Grover, Charlie Hoang, Amy Johnston, Jeffery Machnicki, John Melo, Michael Monette, Brian Pearson, Dave Thomas, Jonathan Trenholm, William Vivas, Chris Wiebe, David Wittman, Deanna Yereniuk.