What do the 2018 NTCs predict
The NTC2018 dedicate the chapter 8 at the safety assessment and interventions on existing structureswhich must take into account the following aspects of construction:
- «it reflects the state of knowledge at the time of its creation;
- There may be inherent, but not obvious, defects in its design and implementation;
- it may have been subject to actions, even exceptional ones, the effects of which are not completely evident;
- its structures may present deterioration and/or significant changes compared to the original situation.»
Calculation resistors
To get to calculate a seismic vulnerability indexthe structural engineer needs to delve deeper into the real resistance of the materialsit armor scheme in the case of reinforced concrete buildings, the presence or absence of effective connections between the load-bearing walls, a precise geometric relief of the structures. Not everything can be detectable visually or through timely removal of plaster.
The estimation of the resistance of the materials and the precise survey of the reinforcements require diagnostic techniquessomeone non-destructive or partially destructive. The value of compressive and shear resistance of masonry and concrete are fundamental parameters to be included in the calculation model, and their estimate influences the result of the safety assessment.
The technical regulation, through the following calculation resistance formula:
fd = fm/ (FC •ϒm)
rewards a higher level of knowledge with a FC confidence factorwhose value decreases from 1.35 for a knowledge level basic LC1, to 1.2 for an extended knowledge level LC2, up to 1 for an exhaustive knowledge level LC3. The lower the level of knowledge, the higher the confidence factor FC will be, which will penalize the estimate of the resistance parameters, resulting in underestimated seismic safety values and consequent more expensive and invasive seismic improvement works.
Semi-destructive testing
The estimation of the resistances can be carried out using the following methods semi-destructive testing :
- carrot extraction from the load-bearing elements in reinforced concrete to be compression tested in the laboratory (Figs. 1-2);
- extraction of tiles inserted into the reinforced concrete elements to estimate the compressive strength of the concrete – pull-out (Fig.3);
- extraction of reinforcing bars to be tensile tested in the laboratory and certify its resistance (Fig.4);
- flat jacks for estimating the compressive strength and elastic modulus of load-bearing walls (Fig.5),
- horizontal jack for estimating the shear strength of the masonry – shove test (Fig.6).
Non-destructive techniques
It is advisable to also tile non-destructive techniques, whose qualitative values will be correlated and calibrated with the quantitative ones offered by the more destructive tests. For example, it is possible to estimate the compressive strength of concrete with the rebound hammer in conjunction with ultrasound Sonreb method (Figs.7-8), or identify the position of the reinforcements using the pacometer (Fig.9). Just as it is possible to investigate the quality of the wall texture through endoscopies (Fig.10) or sonic tests (Fig.11), and estimate the mechanical performance of the mortars with penetrometric tests (Fig.12).
Conclusions
The numerical and qualitative results emerging from the cognitive phase implement the modeling of the entire structural skeleton of the building followed by calculation software, from which a safety index will emerge ζΕ as relationship Between maximum horizontal action bearable by the existing structure (capacity) And seismic action design for a new construction (application).
For new buildings ζAND = 1while for existing structures it is less than unity but with considerable margins for growth if at one greater economic burden invested in the initial knowledge phase, corresponds to a planning of the intervention cheaper and more effective seismic improvement (Fig.13).
