PSI - Issue 29

A. Boostani et al. / Procedia Structural Integrity 29 (2020) 79–86

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Boostani et al./ Structural Integrity Procedia 00 (2019) 000 – 000

2. Strategies for the strengtheningof perimeter walls 2.1 Conservation state and risk evaluation

As previously introduced, consolidation of perimeter wa lls was not only necessary as a strengtheningaction able to preserve the masonry from damages and decay, but such intervention must be taken to improve the safety conditions, taking into account the weakening of the building aga inst earthquakes that will be exacerbated by the removal of the debris and theconsequent loweringof the floor level. Two ma in weaknesses affect perimeter wa lls. First, as a consequence of the different construction techniques involved in the building, a lmost complete separation of the parts built in different ways occurred. Such separation increases the risk to overturning, since the wa ll, divided into three different layers exhibits three different rotation axes. A re-stitching of these different layers is required. The second weakness concerns the mechanica l consistency of some part of the wa lls that once were buried. In particular, the niches of the South wa ll show an inadequate conservation state due to the unreliable construction materia l. The need for serious improvement of such weaknesses requires the development of targetedconsolidation strategies. 2.2 Assumptions about the intervention criteria Conservation of this important site will need to closely adhere to methods and practices prescribed by internationa l charters. First, the interna l architectura l elements of the monument need to be preserved entirely (or as much as possible) and in their current form without additiona l or disturbing interventions. Second, the externa l architecture of the building should be left largely in its present form even if more significant intervention may be possible. The above restrictions make it difficult to make the structure fully resistant to earthquakes, but any structural consolidation measures should aim to improve the seismic performances. Concerning consolida tion issues, various methods may be applied towards the stitching of perimeter wa ll layers to provide connections among the different masonries. Stitching bars could be adopted, a lthough identifying solutions that guarantee chemica l compatibility, durability and mechanica l appropriateness, is crucia l. The compactness and continuity of the existingmasonry should be restored through a focused and extensive intervention filling fractures and gaps found in many areas of the masonry by pouring grout and, where appropriate, by replacement of select areas of brickwork. Targeted research, here exposed, was focused on the strengthening of the earth-based materia l. Laboratory ana lysis on existing materia ls were carried out in order to identify suitable and effective materia ls to be used in this activity. Moreover, in order to reduce overturning vulnerability, it was accepted to place outside the monument simple buttresses, followinga nacknowledged tradition. 3. Vulnerability assessment of perimeter walls in the local seismicscenario 3.1 Overturning damagemechanisms Loca l mechanisms of damage, investigated within the framework of limit ana lysis, provide a conservative estimation of the behaviour typica lly exhibited by complex structures in case of an earthquake, i.e. a by-parts response, with minor computationa l effort, (Jorquera et a l., 2017, Misseri et a l., 2018, Pa lazzi et a l., 2019). Four possible configurations were assumed to eva luate the corresponding load factors λ ci imposing equilibrium on the acting forces. Computing the participant masses to each mechanism, the corresponding spectra l acceleration a 0 * is then identified and, assuming spectra l acceleration a t site S a0 =3.67 m/s 2 , the corresponding safety index I S = a 0 */ S a0 is a lso ca lculated (Table 1), as suggested in by Italian standards, (MIT, 2008). The configuration C0 represents the condition of perimeter wa lls prior to any intervention. The wa ll is assumed to be composed of three layers and the level of the debris is that connected to the ha lf-height of columns. The overturning mechanism can be modelled as a set of three layers rotating independently around a cylindrica l hinge placed at the foot of each layer, Figure 2 The configuration C 1 represents the condition of perimeter wa lls after the remova l of the debris level and when the complete height of the wa lls is unrestra ined. The wa ll is assumed to be composed of three layers, i.e. no consolida tion of the layers is assumed , and the mechanism can be modelled