# Seismic Design of Timber Buildings: Highlighted Challenges and Future Trends
## 3 Timber Buildings--Future Trends and Challenges in Seismic Design
### Seismic Retrofitting with Timber  
Seismic retrofitting can be defined as the modification of existing structures to make them more resistant to seismic activity, ground motion or soil failure due to earthquakes. Seismic retrofit strategies have been developed in the past few decades following the introduction of new seismic provisions and the availability of advanced materials. Different techniques and methods are applied for a seismic retrofitting of different structures (e.g., concrete structures [97, 98, 99], timber structures [100, 35, 101], historical and heritage structures [102, 103, 104] and especially masonry structures [105, 106, 107, 3, 108, 109, 110, 111, 112, 113, 114,  
Extensive research on seismic strengthening of different types of existing buildings with CLT panels was performed by [PERSON] and [PERSON] [112]. The experimental work and numerical study were focused on typical non-earthquake-resistant older masonry and concrete structures from Southern European countries. A new outer shell is added onto existing buildings (Figure 5). It is made from CLT plates that serve as the load-bearing layer and are attached to the existing buildings with special connections. The basic idea of the proposed CLT outer envelope is that it is light (it does not contribute much to seismic forces and is easier to install), strong (timber has one of the best strength:weight ratios) and can be produced as a prefabricated system so little on-site work is necessary. Depending on the building type (concrete or masonry), the elements are anchored into buildings on floor levels, either into concrete slabs or masonry walls. The CLT is connected to the existing building with special steel brackets that offer high strength and stiffness and have a \"control fuse\" that enables a brittle yet predictable failure of the connection. For the unreinforced masonry walls, the most efficient mechanically-attached strengthening panel resulted in a 33% increase of idealized strength and a 166% higher idealized ultimate displacement. A substitute frame model, with concentrated plasticity for modeling URM and linear elastic shell elements combined with nonlinear springs for modeling the CLT strengthening system, could satisfactorily describe the behavior of the tested wall specimen. Finite element models of the tested RC frame, with or without masonry infill, showed that the damage in both RC frame and masonry infill is lower at equal ground acceleration when the CLT strengthening plates are installed on the structure.  
The state-of-the-art research have; therefore, shown that a CLT seismic strengthening system has the potential to be used in design practice, but currently there is still a lot of \"unknowns\" and further research is needed. A holistic approach should be applied--seismic and energy retrofitting--as both are very important. In addition to the currently existing regulation on energy certificates of buildings, which are informing the general public, owners and potential buyers as to which condition a building is regarding the energy consumption, a similar concept shall be established for seismic evaluation of buildings--\"seismic certificates\". This will, on one hand raise awareness of current owners about the seismic health of the buildings where they live, and on the other hand additionally define and position a real estate worth on the market, and; therefore, influence the demand on the seismic retrofit of existing buildings in order to be competitive on the market.