Dissipative properties of timber diaphragms strengthened with plywood panels

In the field of seismic retrofitting, a common intervention to improve box-like behaviour in an existing building is the strengthening and stiffening of existing timber floors and roofs. However, these retrofitting methods should be carefully applied, because they change the static scheme and the buildings' response to earthquakes. Strengthening solutions with high reversibility and light weight have therefore to be preferred, and in this context the overlay of plywood panels on existing floors can improve their characteristics in terms of strength and stiffness, but also enhance their energy dissipation, already at a very limited deflection. This strengthening technique was adopted and tested within an experimental campaign aimed at assessing the seismic response of timber diaphragms with typical characteristics from the Groningen area, located in the northern part of the Netherlands. In that region, human-induced earthquakes take place due to gas extraction and the existing buildings are not suitable to safely withstand these seismic events. This paper presents a summary of the results of the experimental campaign on as-built and retrofitted timber diaphragms, and evaluates the beneficial damping properties of floors strengthened with plywood panels, connected only to the underlying planks and not directly to the joists. The results are compared with the data available in literature and provide new reference values for the coming version of the Dutch seismic standard.