Seismic Performance and Retrofit of a Mid-Rise Post-second World War Residential Building Made of Unreinforced Masonry

The housing and economic crisis in Central and Eastern Europe after the Second World War led to construction based on functionalist architectural patterns, which was reflected in the emergence of the first major urban residential agglomerations. Many examples of such buildings have been built also in Slovenia, mostly between 1945 and 1964, when the new building code after the 1963 Skopje earthquake has been issued. The rationale of this paper is to examine the seismic performance and retrofitting possibilities of such residential multi-story buildings, which mostly have regular geometry and are from three to six stories high. The vertical load-bearing structure is usually made of unreinforced masonry (URM) load-bearing walls, and the floor slabs are made of prefabricated reinforced concrete joists with hollow clay brick infills. Such structural systems often do not contain wall ties, and the connection of the walls with the floor slab is often not ensured. This significantly reduces the seismic resistance of such buildings. In the first part of the paper the URM buildings from this era are described based on their architectural, geometrical, and structural features. A general structural resistance to horizontal loads of the buildings has been questioned by comparison with the requirements of modern seismic codes (Eurocode 8). The second part presents numerical simulations carried out on by non-linear static (pushover) analyses. The seismic demand of several analyzed variants has been analyzed by the N2 method with the aid of TREMURI computer program. Various variants with different positions of seismic retrofitting interventions with Fiber Reinforced Cementitious Matrix (FRCM) were analyzed and compared in terms of their strengthening efficiency. Preliminary results show that a combination FRCM and other more invasive retrofitting techniques would be necessary for the examined building to reach the code-based compliance criteria.