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Seismic-resistant Highly Deformable Structures

Periodic Reporting for period 1 - SHDS (Seismic-resistant Highly Deformable Structures)

Reporting period: 2015-05-01 to 2017-04-30

The earthquakes that have recently struck Europe, L’Aquila (2009), Lorca (2011) and Amatrice (2016) have shown that even for a moderate seismic hazard level the seismic action can cause huge personal and economic losses. Traditional force-based seismic resistant strategies rely on providing sufficient lateral strength and ductility for a structure to resist an earthquake. The strength of the structures depends on the hazard level at the site (ground accelerations) which can be significant, hence leading to the design of highly uneconomical structures. Existing codes, however, do allow reducing such large lateral forces in favour of more cost-effective structures, provided that the resulting structures can endure a certain amount of damage before collapse. This traditional design philosophy is straightforward, easy to use for practicing engineers, and can be effective at avoiding potential loss of life. However, force-based methods do not allow engineers to decide the way in which a structure can adapt to seismic actions and can lead to undesirable failures. Alternatively, the most advanced philosophies in seismic design target not only the prevention of casualties but also aim to design structures that can ”resist earthquakes of different severity within specified limiting levels of damage” (Performance Based Design -PBD). That would enable the development of optimal structures, maximise use of resources, minimise costs, yet yielding acceptable levels of safety, hence resulting in safer, more resilient and sustainable structures. Such a novel design methodology would also empower designers to experiment with new design solutions, innovative materials and structural components so as to achieve the desired performance levels. This action focuses on a very specific structural component: coupling beams in coupled wall systems. In this system two or more structural walls are linked through beams in regular pattern over the height of the structure. Coupling beams improve the seismic performance of each individual wall and provide a very stable source of energy dissipation. However, the overall behaviour of the system depends heavily on the deformation capacity of the coupling beams. In general, coupling beams are deep elements and their structural performance under seismic action is highly shear-dominated, with very limited deformation capacity. The main objective of SHDS is to develop innovative solutions for the construction of coupling beams with unparalleled deformation capacity so as to improve the performance of more traditional design solutions and guarantee higher level of safety and resilience. The coupling beams being developed in this action work as “fuses” and are the first elements to attract considerable damage during an earthquake, in turn protecting the majority of the remaining structural and non-structural components. Much of the deformation capacity of such innovative structural elements is made possible through the use of a newly developed Highly Deformable Concrete (HDC). HDC is a new material developed within the EU-funded project Anagennisi led by the University of Sheffield. Anagennisi aimed at finding ways of using all components of post-consumer tyres in high value concrete applications. One of the research lines in this project focused on replacing the mineral aggregates in concrete with rubber particles so as to increase the deformation capacity of traditionally brittle concrete. However, high deformability can only be achieved with high rubber content, which in turn can drastically reduce its compressive strength (up to 90%), thus making it unfit for structural use. The innovative HDC uses externally applied advanced composite jackets to enhance the compressive strength to structural grade while keeping the desired large axial deformation capacity of rubberised concrete. A mere 1.6mm thick Aramid jacket wrapped around rubberised concrete columns can lead to extraordinary strength and deformabi
The action comprised both experimental and numerical work. An experimental characterization of reinforced HDC coupling beams was carried out based on multi scale testing at material and structural level. At the material level, more than 300 cylinders/cubes were tested, in collaboration with Anagennisi (Project ID: 603722), to analyse the fresh and hardened properties of rubberised concrete. Different mixes and rubber quantities were tested to examine the effects of key parameters on the overall deformation capacity. The result of this study was the development of an optimised mix with 60% rubber replacement of the coarse and fine mineral aggregates. This optimised mix was adopted for the final development of HDC. Different types of advanced composite jackets were trialled to provide the optimal combination of deformation capacity and strength, including Aramid and Carbon FRP. The developed HDC can achieve compressive strengths suitable for structural purposes (40-120 MPa) with maximum axial deformations 20 times larger than possible with traditional concrete. At the structural level, SHDS has examined the seismic performance of large coupling beams made with HDC. The results obtained during the action are very encouraging and show that HDC coupling beams can resist similar loads as their traditional counterparts while exhibiting two times more deformation capacity and enhanced energy dissipation capacity. Based on the experimental results, analytical and finite elements models were developed to analyse the overall seismic performance of the new system and it was shown that a better performance could be achieved in terms of lateral response and energy dissipation capacity.
The results of SHDS has led to some very positive outcomes that proved the feasibility of HDC at both material and structural level, showing improvements in terms of deformation and energy dissipation capacity of the coupling beams. The proposed system allows engineers to design predetermined and well-engineered components that can attract damage during earthquakes. As a result, post-earthquake repairs in PBD-designed structures would focus on such components rather than on the whole structure, thus minimising costly repairs, and limiting the impact on the community by reducing the time required for repairs. The proposed coupling beams also use materials recovered from waste tyres thus assisting in: achieving a greener economy; reducing waste production and pressure on raw materials; improving resource efficiency; reducing environmental impacts. The advancements resulting from this action will also contribute to the development of further seismic-resistant structural components, such as short columns and high deformability plastic hinges in moment resisting frames. Many of the technological and scientific challenges related to HDC and its applications have been addressed within SHDS and Anagennisi. However, a number of issues still need to be addressed on both the physical and structural behaviour of HDC to reach higher TRL. For example, further research is needed to understand the long term performance (including creep and fatigue) and response to accidental actions (such as fire and impact) of this novel material. Finally, design guidelines need to be developed to enable engineers and contractors to specify and design such novel materials and solutions.
Cyclic test on HDC coupling beam