Our main area of interest within structural engineering is the use of different materials which, a priori, will not be understood to be suitable for a structural function. An early example of this was glass, even though it is considered nowadays to have a structural purpose on its own merit and is reasonably well understood mechanically. In the past, we have done research as well in terms of methacrylate structural performance, especially for water-retaining structural applications. In recent years, FRP and GRP have attracted our attention, not only as plate or shell structural elements but also for framing applications – especially as this material would allow for bespoke structural shapes – and even foundation design, as a substitute for concrete.
​Currently, it appears that there is no real drive in the plastic manufacturing industry to consider cast-in-place or pre-cast applications for plastic structural frames and floorings but the recent inclusion of this material in some technical publications may well be an eye-opener for the industry.

Although timber has reasonable mechanical strength in terms of structural bearing capacities, it is far from the typical structural steel parameters. Therefore, it makes sense that steel reinforcement can be used to improve timber performance, especially regarding its tensile capacity. In this particular study, steel reinforcement is presented as prestressed cables instead of steel plates, which are normally used in flitched beams, for example.
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There are two proposed alternatives. The first involves prestressing cables to provide a constant balancing moment (CBM) along the beam’s longitudinal axis (elevation on top in the image). The second involves a variable balancing moment (VBM) for a simply supported beam. Although the latter proposal is more efficient structurally, it requires more complex detailing, as explained below. Both systems require end anchor plates and tension cables, which include turnbuckles for tension adjustment. They can be installed both internally (recommended for hollow timber sections) or externally to the beam, which is advisable for solid or mass timber beams. For the CBM option, end plates anchor the prestressed cables running along both sides of the beam if these are to run externally. The VBM configuration, on the other hand, requires not only end anchor plates but also intermediate resin-dowelled connections into the beam, with increased eccentricities at locations of higher bending moments. These dowelled connections have limited capacity and therefore the achievable balancing moment is substantially lesser than CBM beams which relies on the end bearings only.

In the diagram above, a simply supported beam is exemplified, but other arrangements, such as cantilevers and intermediate supported beams, follow the same principles. The main advantages of prestressed timber beams include increased stiffness compared to the same unreinforced timber sections. This results in increased load-bearing capacities, reduced deflections, increased durability (as compressive stresses minimize the occurrence of cracks) and material efficiency.

Vaulted structures, such as masonry or concrete arches and vaults, have been amongst us for several millennia. Their intrinsic structural strength, provided purely by their form, was first exploited by the Romans, who knew few other technologies capable of allowing for large structural spans. Over the centuries, other cultures adopted this strategy, including Victorian structural engineers.
​While this research does not introduce a conceptually new solution, it offers modern analysis, detailing, and manufacturing considerations for this structural system as an alternative to the widely used precast or composite floors in today’s construction industry.

​Our study involved the structural analysis of several mass concrete planks, with spans ranging from 3 to 6 meters and depths between 50 to 150mm at the supports. At the vault crown, the thickness decreases to create the arched intrados, maintaining a minimum thickness of 40mm for practical construction considerations. The analysis considered typical residential and commercial imposed loads. Results showed excellent performance both in terms of deflection criteria and mechanical strength without the need for steel reinforcement if very low concrete strength grades are avoided. Precast manufacturing techniques under controlled environments are expected to negate the need for steel reinforcement in terms of limiting cracking due to shrinkage. For large concrete pours, fibres could also be considered instead of steel reinforcement. In terms of robustness, vaulted mass concrete slabs can be designed to resist tying tensile forces, including at the bearing positions via resin anchors or cast-in-place connectors. In terms of lifting operations, steel reinforcement may be needed for large format units, unless sandwich lifting reinforcement frames are used during installation.

Shell vaulted design leverages the natural strengths of concrete, allowing for a more efficient distribution of material. This design not only enhances structural integrity but also minimizes the amount of concrete required without the need for steel reinforcement, leading to substantial environmental and construction cost benefits.