Motivation and Approach
The use of masonry infill and veneer walls, especially in reinforced concrete (rc) framed structures, is widespread in many Countries as natural evolution of the traditional building technique based on masonry walls. The rapid growth in the use of rc elements for creating the bearing structure, transformed the latter into a "wire frame" of negligible volume, mass and stiffness, when compared to traditional masonry walls. The resulting deficits in all the other performances that buildings must provide were compensated by using that material that can naturally offer them, i.e. masonry, for enclosures. Lightened masonry units are able to provide an almost continuous range of density and thus of humidity and thermal performance, including transpiration. Masonry can be constructed to maximize these benefits by reducing to a minimum the mortar joints or by using mortars with suitable characteristics (e.g. lightweight). Masonry units can also be used to cover the rc elements, minimizing the effect of thermal bridges and lower porosity of concrete. They prevent unpleasant micro-environmental phenomena, which cause problems of comfort, aesthetics and durability. Hence, the total cost of the building (initial cost plus maintenance) is reduced. However, the widespread use of non-load bearing masonry enclosures in rc frames is accompanied by a series of drawbacks, including problems related to poor construction and/or poor detailing, excessive settlement or cracking of the non-structural elements. Non-load bearing masonry walls exhibit frequently inadequate performance under serviceability states, being responsible for 25% of the damage in buildings.
The 1971 San Fernando earthquake is considered a milestone in the development of modern earthquake engineering and it has been reported that 1/3 of the reconstruction costs in the fault area were related to enclosure walls and almost 90% of framed building far from the fault suffered damage to non-structural elements. The remarkable progress of seismic Codes achieved since then mainly concerns the structural portions of the buildings. Today, design provisions for ductility and proper detailing can ensure that framed buildings behaves properly under earthquakes. Notwithstanding, the shortcomings of masonry enclosures when subjected to seismic loads, as well as their significant economic impacts are not yet solved, as modern earthquakes confirm. In Lefkada (Greece, 2003, Mw = 6.2) and L’Aquila (Italy, 2009, Mw = 6.3) the dominant type of damage was related to local failures of structural and non-structural components, mainly related to cracking of infill walls and internal partitions, and reported injuries were mostly related to free-falling roof and infill wall clay tiles. The 2011 earthquake in Van (Turkey, Mw = 7.1) demonstrated the highly variable nature of the seismic damage to infill walls in rc frame buildings. In some cases, the infill walls significantly contributed to strengthen the building, while in other situations, masonry infills detached from the structure and/or collapsed due to a combination of in- and out-of-plane demand. This type of non-structural damage can be extremely dangerous for occupants, emphasising the importance of masonry infills in rc buildings and calling for the development of new systems to improve their performance.
It is also worth notice that, in economic terms, the impact of enclosure walls repair can be more relevant than the cost related to purely structural interventions. The FEMA guidelines estimate the structural interventions for commercial buildings counting for approximately 20-25% of the original construction cost, while the other elements count for the remaining 75-80%; values confirmed in many recent studies.
With reference to structural problems, non-structural elements have only recently started calling additional attention and most of the Codes have recognized that also they need to be designed for earthquake actions, in relation to different performance levels. However, by the time being, no sound design procedures exist and no suitable solutions have been investigated and proposed yet in any Eurocode. In the current design practice for new buildings in Europe, rc frame structures subjected to seismic loads are usually examined using linear elastic structural models on which equivalent static or multimodal dynamic response spectrum analyses are performed. The design of infilled rc structures is usually performed on bare frame elastic structural models, where the masonry infill panels are considered in terms of masses and vertical loads only. In this context, the safety verification of rc frames at the ultimate limit state, according to Eurocode 8, has to be accomplished in terms of resistance to seismic action effects for both structural and non-structural elements. For non-structural elements, the verification of resistance for the designed seismic action is foreseen and a simplified procedure is proposed for the evaluation of the horizontal seismic force acting on the non-structural element in the out-of-plane direction. Nevertheless, in Eurocode 8 no recommendation for the calculation of the corresponding resistance of the building enclosures is provided. Moreover, the damage limitation requirements for buildings with non-structural elements are considered satisfied when the induced inter-storey drifts do not exceed certain limits in each storey of the building, defined only as a function of the “ductility” of the infills and on the connection with the surrounding structure, without any reference either to the type of masonry enclosure or the dimensions and amount of infills. Hence, the further development of existing code requirements for seismic design of infilled rc structures, as well as the introduction of practical solutions which allow the compliance with the code, in order to achieve satisfactory levels of damage limitation and life safety, is of primary interest and presents one of the important objectives of this research.
Furthermore, the empirical solutions proposed by the Code are not accompanied by rationales for design, applicable to the various types of masonry enclosures, and not even rules for the use of connectors in composite systems such as masonry veneers are given. The absence of clear performance requirements and design methods and the lack of practical measures can hinder in the long term the further development of masonry construction systems for enclosure walls, also damaging the related industrial sector.