Issue 47

V. Alecci et alii, Frattura ed Integrità Strutturale, 47 (2019) 161-167; DOI: 10.3221/IGF-ESIS.47.13 163 maximum economical return; a functional factor exists because the buildings are often organized to have parts with a particular orientation also for better enlighten the different spaces; finally, the aesthetical-formal factor depends on the new architectural trends (as better described in the next paragraph). Plan irregularity is characterized by uneven plan distribution of earthquake resistant vertical structures and/or of masses; it results in a dangerous torsional behavior consisting of large floor rotations. A close relationship between mass and stiffness plan distribution reduces eccentricity between mass center CM and rigidity center CR, thus resulting in a regular, mainly translational behavior of the building (Fig. 2). Figure 2 : Plan irregularity (on the left) and regularity (on the right). Plan irregularity is due to a non-symmetrical plan, resulting from a particular functional distribution of the spaces which leads to concentrate stiffness and/or strength on one side of the building plan (stiff side) compared to the other side (flexible side). It may also be due to mass concentration on one side of the building, which becomes the flexible side. In case of seismic actions, building response is characterized by floor rotations (torsional behavior) with increase ductility demands on structural elements often leading to very severe damage or collapse. Vertical irregularity is due to sudden variations in mass, stiffness (and strength) along the building height, which result in formation of soft/weak storeys where an earlier collapse can develop due to concentration in member forces and ductility demands (Fig. 3). It can be very common, in contemporary buildings, that layouts of different levels develop as distinct entities in which even structural elements can change in size, consistency, position. This can occur because of changes in the use of a single level or due to the will of the designer to formally differentiate the façade (facades with large openings alternate to fully “closed” portions). Both plan and vertical irregularities (Figg. 4-5) do not allow uniform damage distribution and, therefore, strength and ductility resources of the entire structure cannot be fully exploited. Building codes, such as Eurocodes and the Italian NTC, penalize design of buildings classified as irregular by prescribing larger design forces. Figure 3 : Vertically irregular (above) and regular (below) configurations.