Seismic retrofitting of the school building “Manlio Rossi Doria” in the municipality of Torella dei Lombardi (AV)
Seismic improvement and/or retrofitting of buildings that are not recently constructed are necessary, in accordance with the Technical Standards for Construction (NTC-2018), to ensure safety and stability in case of seismic events, which are an ever-present risk. Older buildings often do not meet current regulations regarding earthquake resistance, as design criteria and construction techniques in the past did not provide the same safety standards. In addition, over the years, the structural condition of buildings can deteriorate due to factors such as aging materials, wear and tear, or changes in the use of spaces, so improvement and/or retrofitting interventions are essential to reduce the risk of significant damage and to ensure that the building meets ever-present safety requirements.
An example of an improvement-adaptation intervention is the one carried out for the “Manlio Rossi Doria” Comprehensive Institute, in the municipality of Torella dei Lombardi, since, following a structural verification conducted, very low seismic risk indices emerged.
School buildings, being frequented by a high number of people, including children and staff, must ensure particularly high levels of safety in the event of seismic events. Low seismic hazard indices indicate that the structure is not sufficiently resistant to seismic attack, with potentially serious consequences in the event of earthquakes. According to NTC-2018, buildings must meet specific seismic safety requirements, and when inspections show a structural deficiency, it is essential to proceed with improvement and/or retrofitting.
The educational institution consists of 4 structural blocks seismically joined in elevation and with reinforced concrete framed load-bearing structure the A-Body, the B-Body, the C-Body which includes the structural bodies of the gymnasium and the attached grandstand.
Project outline – Istituto Comprensivo “Manlio Rossi Doria”
The improvement-adjustment project, considering the structural deficiencies found, includes two types of intervention:
- Seismic isolation at the base of Bodies A and B, achieved by means of an isolation interface at the intrados of the first deck, by means of isolation devices allocated at the head of the pillars;
- Structural consolidation with the introduction of stiffening walls for structural body C, gymnasium and bleachers
BLOCKS A AND B
Block A is on 4 levels with a ca-framed structure and floors made of prefabricated joists and bricks. The roofing is partly ca-slab and partly pavilion. Body B, on the other hand, is on 3 levels also with a ca-framed structure and floors made of prefabricated joists and bricks. The roof has a solar slab made of ca. Following an extension, a metal carpentry roof with sloping pitches was built on the terrace of the first deck.
The improvement project – seismic adjustment includes the construction of an isolation interface at the intrados of the first deck, by means of isolation devices allocated at the head of the pillars and, in order to allow relative movements between the superstructure and the foundation structures, a horizontal joint capable of guaranteeing the calculation displacements under SLC conditions is provided.
Structural model of Blocks A and B, with isolation plan
At the isolation devices, the pillars are to be encased by means of steel collars and, where necessary, supplementary casting with SCC-type controlled-shrinkage concrete.
At the structural joint between bodies A and B, there is provision for the coupling of beams with steelwork collars and completion pour in order to create a continuous and homogeneous connection between the two buildings.
The insulation system consists of low-friction, curved-surface sliding insulators of the FIP-FPS type (§ 11.9.1 and 11.9.8 – NTC/18), arranged in plan according to the following diagram.
Disposizione in pianta dei dispositivi di isolamento
Design criteria for the intervention on Blocks A and B
For bodies A and B, isolation is provided at the base by means of devices to decouple the seismic motion of the building from that of the foundation soil. This is done by realizing a layer characterized by low transverse stiffness, which allows raising the fundamental periods of vibration and bringing the building into the range of lower spectral accelerations.
To achieve the isolation, a structural discontinuity (isolation interface) of the soffit of the first deck is provided such that large relative horizontal displacements are allowed between the top (superstructure) and bottom (substructure) of the building itself in the horizontal directions. The connection between the superstructure and the substructure is made by means of insulators, i.e., special support apparatuses characterized by low stiffness against horizontal displacements and high stiffness against vertical displacements. This system essentially decouples the motion of the superstructure from that of the substructure, concentrating deformations at the isolation plane. In this way, during the seismic event, the superstructure behaves as a rigid body oscillating on the isolation level.
The insulation project of the Torella dei Lombardi School, represents an ideal case as it has a basement floor with a height of 2.70 m, intrados of the first deck, in which to carry out the intervention. A great advantage of this solution is that it can be implemented on existing buildings without disrupting their functionality, since the work can be carried out either in phases, progressively isolating structural portions without compromising the operability of the entire building, or using hydraulic jacks that allow the building to be temporarily supported without the need for evacuation.
Photographic documentation of the foundations of the school building
The isolation devices in the design feature two concave sliding surfaces with the same radius of curvature; both allow both horizontal displacement and rotation. In this case, each individual curved surface is designed for only half of the horizontal displacement, so that the plan dimensions of the devices can be significantly reduced. What is more, this type of insulators allows the eccentricitỳ of the vertical load to be halved, with a consequent decrease in the P-Δ effect.
Insulator model adopted in design type FIP-FPS
BLOCK C
Block C houses the gymnasium and spaces related to sports activities, including the grandstand. The gymnasium area alone is developed at full height and has a floor slab resting on load-bearing masonry walls and roof slabs made of prefabricated reinforced concrete joists resting on beams cast-in-place with base 30cm and height varying between 50cm and 120cm. The remaining portion of the structural structure is developed on two levels, the second of which was partially covered with an extension later by means of a metal carpentry structure with inclined pitches. Adjacent to the gymnasium, and simically joined to it, is located the grandstand, which is developed on two levels above ground connected by a reinforced concrete staircase.
The improvement project – seismic adjustment for Block C, involves the insertion of reinforced concrete walls and, at the bottom level below each wall, the construction of micropiles. Finally, the elimination of the structural joint in the roof between the gymnasium and the grandstand is planned. Block C, moreover, is seismically jointed from Blocks A and B by means of horizontal and vertical joints designed to absorb displacements at the base of the isolated structural Block (A+B).
Modello strutturale Corpo C
Design criteria for the intervention on body C
In the case of existing buildings, the current technical standards allow a lower level of safety against seismic actions in view of the requirements of the case and the functions
taking place in the construction. In the case of body C, an improvement – adaptation intervention is planned, which aims to increase the pre-existing structural safety. Given the particular intended use, the improvement/adaptation project was conducted so that a value of the ζE parameter (ratio of capacity to seismic demand) of 1.0 was achieved.
In detail, the project consists of:
- introduction of reinforced concrete walls 30cm thick and for the entire height of the structure both in the transverse and longitudinal direction of the building for a length equal to about 2.00 mt;
- construction of micropiles of length 6.50 mt below each wall of diameter Ø200 reinforced with tubular profile Ø127 having thickness 5mm arranged with spacing equal to 50-60cm;
- consolidation of the transverse beams in the roof with carbon fiber interventions at their ends for a length equal to 1.50 mt;
- restoration of degraded areas;
- reconstruction of the emergency staircase in metal carpentry.
The design and verification of the proposed interventions were described with finite element models using Edilus software (ACCA software).
Bibliography
[1] PALAZZO – L. PETTI” Aspects of passive control of structural vibrations”, pp.529-544. In International Journal of MECCANICA vol. 32 n.6 Dec.1997. ISSN:0025-6455
[2] Legge 5 novembre 1971 n. 1086 (G. U. 21 dicembre 1971 n. 321)” Norme per la disciplina delle opere di conglomerato cementizio armato, normale e precompresso ed a struttura metallica”
[3] Legge 2 febbraio 1974 n. 64 (G. U. 21 marzo 1974 n. 76)” Provvedimenti per le costruzioni con particolari prescrizioni per le zone sismiche”
[4] M. Infrastrutture Trasporti 17/01/2018 (G.U. 20/02/2018 n. 42 – Suppl.Ord.) “Aggiornamento delle Norme tecniche per le Costruzioni”
[5] Circolare 21 gennaio 2019, n. 7 C.S.LL.PP. Istruzioni per l’applicazione dell’«Aggiornamento delle “Norme tecniche per le costruzioni”» di cui al decreto ministeriale 17 gennaio 2018 (19A00855) (GU n.35 del 11-2-2019 – Suppl. Ordinario n. 5)
Isolator devices: https://www.fipmec.it/it/prodotti/dispositivi-antisismici/