Seismic upgrading San Benedetto Hospital in Alatri
Location: Alatri
 
Client: ASL Frosinone
 
Year: 2023

INTRODUCTION TO THE PROJECT

St. Benedict’s Hospital, located in Alatri and built in 1978, covers an area of about 33,500 square meters, with a structure consisting of six building bodies, each characterized by two-way flat frames made of reinforced concrete. Structural elements were designed with seismic action in mind, in accordance with the technical regulations of the time of construction.

Nevertheless, recent analyses that The SIA has carried out have revealed major shortcomings in its ability to withstand seismic events, according to current technical regulations.

SIA was also commissioned to design the necessary structural improvements. The assignment involved seismic vulnerability assessment and design of seismic consolidation works for the hospital complex.

The interventions studied by La SIA’s engineering team focused on installing advanced seismic isolation systems and adopting state-of-the-art engineering techniques to increase the building’s seismic resistance, as if it were newly designed.

The biggest challenge of this project was to implement these seismic engineering interventions in such a way as to minimize the disruption of daily activities and ensure seismic retrofitting without negatively impacting the hospital’s operations.

From a structural point of view, the hospital complex consists of a reinforced concrete framed load-bearing structure cast in situ. The original design, dating back to 1978, called for the construction of No. 6 building bodies separated by joints 15 and 30 cm thick and, therefore, structurally independent of each other.

Overall, the building has an irregular “H”-shaped plan and a footprint of about 33500 square meters, with practicable flat roofs.

Figura 3 - Schematizzazione dei corpi di fabbrica della struttura
Figure 3 - Schematization of the building bodies of the structure

Bodies A, B and C have six floors above ground and have, in addition, a basement and a basement.
Bodies D, E and F, on the other hand, are two stories above ground and also have a basement and a basement floor.
All building bodies have bidirectional r.c. frames, concrete-lath slabs with r.c. distribution slab, and curtain walls consisting of double perforated brick liner

Interestingly, at the time of construction of the original structure, the area where it stands had been classified as seismic category two.

Figura 9 - Evoluzione della classificazione sismica del territorio italiano 1937,1975,1984
Figure 9 - Evolution of seismic classification of the Italian territory 1937, 1975, 1984

Thus, the structure is designed to not only support gravity loads (according to Ministerial Decree 16/06/1976), but also to resist seismic actions according to DD standards. MM. 3/3/1975 n° 39.

RESULTS OF VULNERABILITY CHECKS STATE OF AFFAIRS

The structures of the hospital complex were analyzed in detail, and through FEM modeling it was possible to assess both their ability to withstand static loads and seismic loads.
The numerical analysis carried out under the condition of gravitational loads only showed a substantially satisfactory outcome even in relation to the current NTC2018.
With regard to the behavior of the building in the presence of seismic actions, the judgment is far from positive: although the building was designed with horizontal seismic actions in mind, it does not possess sufficient load-bearing capacity to support the horizontal design loads, assessed according to current technical regulations. In particular:
– Despite the presence of bidirectional frames, unconfined nodes are present.
– The existing joints are found to be of sufficient thickness to avoid hammering phenomena, except for two joints that in any case exceed the verifications by a limited percentage and only at 100 percent of the design earthquake.
– The infills on the top floors of all building bodies are vulnerable to the phenomenon of overturning out of plane under horizontal actions.
In light of the above, given the type of analysis performed (linear dynamic with behavior factor), the safety assessment of the building under consideration was evaluated by taking into account the parameter defined by NTC2018, which is the indicative factor for a quick comparison between the maximum action that can be borne by an existing structure and the maximum action that would be used in the case of a design from scratch.
This parameter is defined as follows:

Based on the overview given in the above tables, it is possible to deduce the overall seismic risk index value, related to the whole structure, considered to be equal to the lower of the values relevant to the different building bodies, for each limit state considered:
zE (SLD) = 0.27
zE (SLV) = 0.25

DEFINITION OF SEISMIC CONSOLIDATION STRATEGIES

Given the deficiencies in the state of affairs, there are several strategies for intervention. Based on the assumption that demand is greater than capacity, in general, it is possible to classify intervention strategies according to whether in interventions one acts on reducing demand and/or increasing capacity: one can, on the one hand, intervene by increasing the capacity of existing structures through global reinforcements or by acting locally by increasing the ductility and/or strength of even individual components.

Figura 6 - Consolidamento sismico di strutture esistenti attraverso L’AUMENTO DELLA CAPACITÀ
Figure 6 - Seismic consolidation of existing structures by increasing capacity

On the other hand, it is possible to adopt intervention strategies that reduce the demand, such as reducing the seismic action on the building through base isolation, which increases the period of the structure, or damping the seismic action, with the use of viscous dissipation devices, going to reduce the spectrum.

Given the main need to intervene without service interruption, there was a move toward Passive structural control techniques such as base isolation systems: In the case of seismic stress, the insertion of isolators allows the structure’s own period of vibration to increase, moving it away from the area of the response spectrum with higher accelerations. This creates a dynamic decoupling of the construction from the ground (“filter” effect), so as to reduce the transmission to the superstructure of the energy provided by the seismic action.

As a result of the latter, the foundation-isolator-structure system is able to dissipate the seismic energy of the soil: dissipation is concentrated almost exclusively in the isolation devices, which dissipate the seismic energy transmitted to them from the foundations at the expense of large deformations through large hysteresis cycles.

This allows the superstructure to have a response practically in the elastic range.

This considerably modifies the seismic input, as reducing the accelerations transmitted to the building considerably raises the structure’s responsiveness to ultimate strength and damage limit state.

Among the different types of isolators, the design choice was oriented to the seismic isolation system of the entire structure through the application of Curved Surface Sliding Isolators, with the timely sizing of the characteristics in the study of stresses on individual pillars.

Such systems allow the decoupling of soil-structure motion by relative sliding of two contacting surfaces. This sliding is activated at the moment when the frictional force between the contacting plates is won. Contact surfaces can be flat or curved. In the latter case we speak of pendulum (friction pendulum) insulators.

The advantage of using pendulum insulators is the self-recent capacity of the device, which, on the other hand, is lost in the case of flat surfaces. An important advantage of this family of insulators is the high load-bearing capacity under vertical load, when compared with the elastomeric family of insulators. Also important is the characteristic of maintaining structural performance in case of fire.

From the design point of view, the most important parameter of the isolation system is its displacement capacity, i.e., the maximum allowable displacement before the device’s crisis is triggered. This value must be compared with what is called in technical jargon “seismic displacement demand,” which depends mainly on the seismicity of the site. In essence, seismic displacement demand represents the relative soil-structure displacement demand triggered by the earthquake.

This intervention leads to the need to make additional interventions on the “boundary conditions” of the building, so that the relative displacement between ground-elevation, is not prevented or does not cause damage.

The structure is mainly surrounded by an open air gap, which makes this system as the most cost-effective compared to others: where the gap is covered by sidewalks integral to the earth retaining wall, the demolition of the existing one and the construction of connecting walkways at the building entrances equipped with sliding fixtures at the base are planned.

Figura 11 - Planimetria architettonica piano terra - POST OPERAM
Figure 11 - Ground floor architectural plan - POST OPERAM

The presence of bidirectional beams and rigid floors at the first deck of the structure, in itself, constitutes a rigid diaphragm, so the insulators will be installed below this diaphragm, subject to the cutting of the abutments.

Also in this case, to avoid interference and possible weakening of existing elements, as well as to achieve an isolation plane as uniformly altimetrically as possible, it is necessary to make reinforced concrete pulvini equipped with steel sleeves, preparatory to the proper installation of sliding devices.

Figura 12 - Dettaglio costruttivo tipologico incamiciatura in c.a. pilastri - piano S2
Figure 12 - Construction detail typological encasement in reinforced concrete pillars - floor S2

At the joints between the various building bodies, the adjacent pillars will be re-narrowed in such a way as to obtain a single structural element, equipped with appropriate connecting pulvinus on which the insulation device will be installed. This was done in order to reduce the number of devices, while ensuring sufficient system redundancy, and to avert the occurrence of tensile stresses that could affect the proper functioning of the isolators.

To ensure complete decoupling of the motion of the substructure from that of the superstructure, it is also necessary to demolish the connecting ramps between floors S2 and S1, for the stairwells of bodies A and C. The vertical connection between these floors can then be restored with new steelwork stairs, suitably jointed to the existing structure.

CHOICE OF INTERVENTION CATEGORY

The choice of the type of seismic risk reduction interventions and the level of safety to be achieved were dictated firstly by the reduced resistant capacity of the structural elements (evidenced by the seismic vulnerability analysis); secondly, the current intended use of the building, having the function of a hospital facility and therefore having to guarantee its operation at all times, even during the implementation of any type of work, was also taken into account.

Thus, interventions aimed at seismic upgrading of the structure (as defined by NTC 2018) through a reduction of seismic demand by installing seismic isolation devices were planned.

This choice, together with the intervention technologies adopted, has the following advantages:

– Significant increase in the eigenperiod of vibration of the existing structure, resulting in the abatement of seismic action;

– the use of a seismic isolation system allows the structure to always work in the linear field and therefore induces no substantial deformation in the main elements. This feature preserves the integrity and nullifies damage to all nonstructural elements such as infills; it also provides absolute protection of what is contained in the structure. Structure maintenance burdens after an earthquake are virtually zero;

– intervention limited to only one floor (second basement) of the entire structure: local reinforcement of the pillars and insertion of the insulator affect only that specific level. All levels above the isolation level do not require any intervention as the acting stresses are greatly reduced and thus can maintain the original architectural features. The work of cutting the pillars and inserting the insulators can be carried out without interfering with the activities carried out on the upper floors;

– Choice of low-maintenance devices, such as curved surface sliding pendulum insulators. These are completely made of metal carpentry and synthetic materials and thus are not subject to any deterioration phenomenon. Maintenance can be considered marginal in that a visual check of the corrosive protection is required only about every 5 years. In addition, following a seismic event, none of the constituent elements of the device need to be replaced: only extraordinary supervision is required.

PROJECT METHODOLOGY

The analysis of building response under seismic conditions was performed by means of dynamic modal analysis with assigned response spectrum (spectrum from normalized RSL). The superstructure and substructure were modeled as systems with linear elastic behavior, while the insulation system, in relation to its mechanical properties, with a linear visco-elastic constitutive bond.

In particular, for the purpose of describing the behavior of the isolators, an equivalent stiffness referring to the total design displacement, for each limit state examined, of each device forming part of the isolation system was adopted.
The total equivalent system stiffness (Kesi) of insulation is equal to the sum of the equivalent stiffnesses of the individual devices.

The energy dissipated by the isolation system is expressed in terms of the system’s equivalent viscous damping coefficient (ξesi), evaluated with reference to the energy dissipated by the isolation system, in cycles with frequency in the range of the natural frequencies of the modes considered.
Below are some images of the calculation model adopted.

As can be seen from the images above, the introduction of the isolator system enables the decoupling of superstructure/substructure motion. The period elongation produced by the devices, results in much higher displacements at the isolation interface than in the ante operam displacements, but the motion of the superstructure is almost entirely translational (torsional effects induced almost exclusively by the accidental eccentricities prescribed by the regulations) of a rigid type, confirming the effectiveness of the designed system.

TYPE OF ANALYSIS

To provide an indication of the seismic safety of the seismically isolated building POST OPERAM, as argued in the previous paragraphs, a linear dynamic analysis (RSA) was performed. This analysis was conducted for 100% of the design seismic action.

As can be seen from the spectra above, for periods T ≥ 0.8 Tis (period of the isolated structure), in accordance with current regulations, there is a reduction of spectral ordinates equal to:

formula 1

Where:

formula 2

Is the equivalent viscous damping of the insulation system.

The isolated structure, as a result of the planned interventions, has a significantly higher own period, compared to the fundamental periods of each ante operam building. This, together with the increased damping offered by curved surface slip devices, results in a significant reduction in seismic demand in terms of acceleration.

CONCLUDING CONSIDERATIONS POST OPERAM

For the hospital complex, having identified the most critical issues, ad hoc interventions were designed aimed at seismic upgrading of the garrison and minimal invasiveness.
Analyses performed on the calculation model implementing the previously illustrated interventions showed satisfactory outcome, for all the verifications required by the current technical regulations.
The safety assessment of the building under consideration POST OPERAM showed the following indices:

– ANTE OPERAM zE SEISMIC RISK INDEX (SLD) = 0.27
– POST OPERAM SEISMIC RISK INDEX zE (SLD) = 1

– ANTE OPERAM zE SEISMIC RISK INDEX (SLV) = 0.25
– POST OPERAM SEISMIC RISK INDEX zE (SLV) = 1

The implementation of the insulation system allows the structure to be seismically adapted, in accordance with the requirements of the current NTC2018.

– Project by Chiara Esposito – Structural Civil Engineer