The static and seismic retrofitting design, for masonry historical buildings, has to follow a right hierarchy of interventions, taking into account that to improve the seismic behavior of a masonry structure, it is necessary to guarantee a “closed box” behavior for the whole structural body or, in case of complex buildings, to guarantee a closed box behavior for each building's wing. Thus it is fundamental to distinguish the interventions for the global behavior improvement from those related to local reinforcements. In this chapter is then proposed a scheme of interventions hierarchy and, therefore, a related design process road-map together with the explanation of a correct design philosophy for the static and seismic retrofitting of historical masonry buildings. Moreover it is also reported an example of two distinguished levels of intervention, with numerical analyses supporting that solution.
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The author of this chapter has experience in writing retrofitting guide lines for historical masonry buildings of villages damaged by the 2009 L’Aquila earthquake. Experiences reported in: Viskovic (2010).
Reviewing existing books, guide lines and rules, about structural restorations, reparations and seismic retrofitting, it is possible to notice as generally they describe the different typologies and techniques of interventions (often not very up-to-date), sometimes underling the possible reversibility or the compatibility levels, but without to define a clear hierarchical order among them.
The different importance of the diverse intervention typologies is generally not well explained despite the fact that many books, on masonry building seismic retrofitting, clearly underline that a better safety level may be reached with a structural “closed box” behavior.
On the contrary, the attention is generally focused on the single structural elements with their possible pathologies and on local or partial collapse mechanisms, without an overall view of the structural behaviors.
Moreover, many times the attention is “deviated” by an excessive attention given to the possible partial collapse mechanisms, forgetting that applying interventions devoted to reach a correct “closed box” overall behavior, they are avoided nearly all the possible partial collapse mechanisms.
Clearly, designers expert in the field of masonry structures know by themselves the correct intervention hierarchy and are able to follow a correct design process.
But after a big earthquake, when there is a large amount of buildings needing structural reparations and retrofitting, a lot of designers, many times not expert in masonry structures, are involved.
The author have checked several reparation projects and retrofitting projects, proposed to the authorities, showing not only not up to date solutions (often invasive and not compatible with historical masonries) but also wrong design processes. Reading analyses and design reports, it was often evident the empirical process of searching, by numerical analyses, the weaker structural elements and simply to reinforce them.
Some of these typical wrong processes and solutions will be illustrated in the paragraph on “Common errors in seismic retrofitting”.