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Design of anticlastic shells in innovative textile reinforced cement composites

Monday, 13 December, 2010 - 17:00
Campus: Brussels Humanities, Sciences & Engineering campus
Tine Tysmans
phd defence

Even though short fibres are increasingly used in concrete applications, still their maximum inserted amount is limited, and their function is restricted to crack width reduction and durability increase; steel reinforcement is still necessary to provide tensile resistance. With the development of a very fine grained, pH-neutral Inorganic Phosphate Cement (IPC) at VUB, high amounts of dense glass fibre textiles can be inserted in the matrix. As such, a durable cement composite is created which possesses - besides its compressive stiffness and strength - a high tensile capacity without the need for steel reinforcement.

Addressing the renewed design interest in complex curved structural surfaces, this work studies the structural potential of the innovative textile reinforced cement composite applied in small span anticlastic roof shells, and enables their design. Not only does the use of flexible, non-corroding reinforcement facilitate the construction of strongly curved anticlastic shells – increasing the designer’s freedom - , it also has structural advantages. Unlike steel, the glass fibre reinforcement does not require a concrete corrosion cover. Hence, as a span-varying parameter study shows in this work, shells in textile reinforced cement composites can be made significantly thinner and thus lighter than steel-reinforced concrete shells when covering small spans (< 15 m).

In order to take advantage of the composite’s full nonlinear tensile capacity in shell design, its mechanical behaviour is experimentally studied under the biaxial stress states which are experienced by the anticlastic shells. The evaluation and validation of a finite element material model (Smeared crack model) to simulate the experimentally observed behaviour facilitates the finite element analysis and design of shells in this composite.

Finally, an integral methodology to design thin anticlastic shells in textile reinforced IPC is set out. The method first proposes a strategy to determine an initial, structurally efficient, anticlastic shell geometry under self weight (using the dynamic relaxation form finding technique) which exploits both the tensile and compressive capacities of the composite, as illustrated by four case studies. Next, a structural design method is proposed for the shells which follows the Eurocode design principles and integrates the acquired knowledge on the composite’s mechanical behaviour. Application of the design methodology on a 10 m span case study yields an extremely thin (15 mm) anticlastic shell and illustrates the high potential of textile reinforced IPC composites in small span, complex curved, structural surfaces.