Safety assessment of an ancient masonry structure by means of a homogenized non-linear approach

In the paper, both a simplified at hand and a FE numerical analysis on an ancient masonry building called “Siloteca” [1] in Milan, Italy, is discussed. The so called Siloteca building belongs to a more complex built aggregate, originally conceived by the French Napoleonic army as riding stable during the Cisalpina Republic. At present, the building belongs to the Science and Technology museum and it is utilized as archive. The aggregate may be regarded as subdivided into two separate blocks, each one further sub-divided into eight isolated buildings. Nowadays, only six stables of one of the blocks are still present. Two of the six structures serve now as a Museum deposit and are the object of the present study, whereas the other four are in worst condition, partially roofed and collapsed and in a general decayed state. Siloteca exhibits meaningful crack patterns and an active overturning mechanism of the main façade. Aim of the present paper is to proposed simplified and sophisticated models, either analytical or numerical, for the interpretation of the present state of degradation of the building. On the bases of such models, a rehabilitation intervention aimed at avoiding possible partial collapses and at restoring the original undisturbed stress state will be planned. The masonry face texture is relatively regular and well organized; its section is constituted by three leaves of header bricks, one leaf being alternatively constituted by one-half brick. A quite large sub-vertical crack is present in the central long wall, at a distance of about 10 meters from the main façade, which is progressively rocking. The reason at the base of the façade movements is probably due to differential settlements of the foundation, consequence of a large excavation realized some decades ago to install large gas oil tanks for the museum. In the paper, for a direct mechanical interpretation of the reasons at the base of the formation of the crack pattern, a simple but effective fully equilibrated model is discussed, able to predict quite accurately the position of the cracks. The model is also utilized to estimate soil elastic vertical stiffness –within a Winker approximation- to be used in a second phase with a fully non-linear FE code. Once soil constants are at disposal from such a procedure, a homogenized non-linear FE code recently proposed by the second author [2][3] is utilized to have an insight into the state of mechanical degradation of the structure. A hypothesis on foundation settlement is provided to justify crack maps exhibited by the structure.