Volume of a fabric-cast beam compared to a conventional rectangular beam.
BEAMS
Fabric-cast beams
Reinforced concrete beams can be cast in very light fabric formworks, for either precast or cast-in-place construction. The simplest way to do this is to simply hang a flat fabric sheet horizontally as a long open trough. CAST beam research has concentrated on developing construction methods that take advantage of the flexibility of fabric to form more efficient (and beautiful) beam geometries that can reduce concrete volume and dead weight.
The reduction of a structure’s self-weight is of critical concern in long-span structures. As a result, most long span constructions are shaped to follow specific curves dictated by the structure’s natural “force path”. In beams, these curves are found by graphing the bending moment (or bending work) of the beam across its span. The curves of a suspension bridge, or an arch are common examples.
Concrete has been cast in rigid molds since its invention in antiquity, and since the invention of reinforced concrete over 100 years ago, it has been cast, primarily, in rigid rectangular boxes. Since mold-making costs dominate the economy of cast concrete construction, beams have been cast into the most economical shapes available for rigid panel construction – boxes.
Fabric, however, naturally produces curved containers (and using hundreds of times less material in the process). Curved beam forms are a natural fit for fabric membranes, and they can be built quite simply. Flat sheets of fabric can be held and pulled in ways that produce lightweight molds with precise and efficient bending moment geometries. No tailoring, sewing or special tools are required. CAST research has developed several simple techniques for constructing these kinds of beam molds. We are also researching the precise structural behaviour of curved beams in order to determine the best methods for their structural design and detailing.
The self-weight of reinforced concrete structures is high compared to wood or steel structures, and the reduction of concrete dead weight enhances both sustainability and construction economy. If concrete is placed only where it is needed, less cement will be consumed (cement has high embodied energy and its production releases high levels of greenhouse gasses). Simplification and reduction of reinforcing steel (see below) has similar advantages. Finally, dead weight reductions in beams ʻcascadeʼ through the rest of the structural system, reducing design loads on supporting members and providing further material and embodied energy savings throughout the rest of the structure.
Beam Structure Research
A reinforced concrete beam with a depth that varies in proportion to its own bending moment does not behave in the same way as a rectangular (prismatic, uniform-section) beam. For example, so-called “shear” stresses (diagonal tension stresses developed in the “web” portion of a beam near its supports) are an important consideration in rectangular beams, requiring the fabrication and installation of many reinforcing “stirrups” to resist these tension forces. A beam shaped to follow its own bending moment curve has no such tension stresses in its “web” area. Instead, all primary tension forces are naturally “channeled” along the curved edges of its tension zone(s). So, for example, a simply supported concrete beam would not require “shear” stress resisting stirrups, but only require a single line of tension reinforcing along the bottom of the beam.
As structures become more efficient they also become more flexible (as more material is strained more equally). These more efficient beams are no exception, and are more flexible than their rectangular counterparts. This makes deflection a limiting factor in their structural design. Structural engineering research at CAST is aimed at producing simplified reinforcing designs, and practical structural analysis and design tools for these more efficient, and beautiful, beams.
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