Seismic strengthening and energy retrofit with post-tensioned timber structures and building envelope components

Radical decarbonisation in the construction sector is an urgent need to mitigate climate change. Therefore, new environmentally sound technologies in timber have to be developed and applied for seismic strengthening of buildings in seismic active regions in order to allow them for a second life cycle. In combination with energy retrofit, such seismic intervention backs the EU renovation wave and empowers a clean energy system, because buildings operational heat energy demand will be lowered close to nearly Zero Energy Building (nZEB) level with an improved building envelope. The intervention adapts concepts and principles originally developed for precast concrete construction, post-tensioned (PT) timber frame systems have been developed and tested since 2005 at the University of Canterbury. The technology takes advantage of unbonded post-tensioned steel tendons passing through internal cavities in timber beams or walls to create a moment resisting connection. The seismic demand is satisfied through controlled rocking between structural elements and tendon elongation, which ensures recentering capabilities. Post-tensioned timber frames as two-dimensional or three-dimensional structure are an emerging concept in timber engineering with great potential for the creation of high-performance and cost-efficient joints. The suitability as a retrofit concept is evaluated on two case studies that are part of the ProGETonE H2020 research program, which aims to provide an innovative, and integrated retrofit concept. Both measures serve to prolong the use of the building and avoid its demolition, a reuse case almost at is best, providing ultimate circularity index results. Therefore, only almost, because such a deep renovation intervention consumes new resources to build and also requires the replacement of specific components as windows or even the adaptation of floor plans to today’s needs. Decarbonisation effects are revealed by calculation of Life Cycle Assessment (LCA) indicators for timber-based external retrofit envelope components that are assembled onto existing buildings for stiffer structures and for insulation purposes. The design and technology concept revealed allowing for the combination of strong structural behavior, necessary building physics performance to achieve for nZEB energy demand; with new Cross-Laminated Timber (CLT) components exposed also connection details and construction processes are discussed.