This volume contains papers of the 9th European Workshop on the Seismic Behaviour of Irregular and Complex Structures (9EWICS) held in Lisbon, Portugal, in 2020. This workshop, organized at Instituto Superior Técnico, University of Lisbon, continued the successful three-annual series of workshops started back in 1996. Its organization had the sponsorship of Working Group 8 (Seismic Behaviour of Irregular and Complex Structures) of the European Association of Earthquake Engineering.

This international event provided a platform for discussion and exchange of ideas and unveiled new insights on the possibilities and challenges of irregular and complex structures under seismic actions. The topics addressed include criteria for regularity, seismic design of irregular structures, seismic assessment of irregular and complex structures, retrofit of irregular and complex structures, and soil-structure interaction for irregular and complex structures. Beyond an excellent number of interesting papers on these topics, this volume includes the papers of the two invited lectures – one devoted to irregularities in RC buildings, including perspectives in current seismic design codes, difficulties in their application and further research needs, and another one dedicated to the challenging and very up to date topic in the area of seismic response of masonry building aggregates in historical centers. This volume includes 26 contributions from authors of 11 countries, giving a complete and international view of the problem.

The holds particular interest for all the community involved in the challenging task of seismic design, assessment and/or retrofit of irregular and complex structures.