THE ISOLATED SCHOOL IN BOJANO: how to satisfy people safety expectations

SAFETY EXPECTATION FOR SCHOOL BUILDINGS

During the October 2002 earthquake, which struck the Molise region, 27 children and their teacher died in the collapse of the primary school in San Giuliano di Puglia: they were the only victims of the earthquake.

The event clearly showed the needs for safer school buildings, and, as a consequence the Italian Department for Emergency Management (Civil Protection) issued prescriptions for assessing the seismic risk of damage and collapse of all the buildings being "characterized by a risk of collapse with relevant consequences", including the schools.

For reducing the risk against extreme loads, such as strong earthquakes, the only viable way consists in reducing their vulnerability by increasing their performance level .

The application of new design philosophies in earthquake-resistant structures, like base-isolation or energy-dissipation strategies, allows for high increment of safety, up to the full protection against the maximum credible earthquake, at a compatible cost and sometimes even without extra-cost.

Rubber cover Lower plate Upper plate (not visible) Lead core Rubber layers Steel reinforcing plates

Seismic isolation of building and typical isolator device Front view of the main façade of the building

THE BUILDING AND THE ISOLATION SYSTEM

Architectural and structural configurations

The school complex is characterized by an irregular layout from either the architectural and the structural point of view. The plan could be roughly inscribed in a rectangle 50 m long and 26.5 m wide. A portion of the building has two stories, covered by an accessible terrace, while a second portion has three stories and is partially covered by a light roof. External large false columns, pinned at the top and bottom ends, are located along the two main lateral views along the perimeter roads. They give a symbolic representation of the "mechanical" behavior of the isolated building performing as a rigid body pinned on the ground.

The adoption of the base isolation allowed for a strong mitigation of the negative effects derived from the incorrect distribution of the lateral force resistant structures. The isolators are located below the ground level floor, at the top of the foundation beams. In order to optimize the isolators behavior, avoiding vertical load concentration, some modifications to the original structural scheme were introduced.

Isolators characteristics and location A dual isolation system has been used, consisting of Lead Rubber Bearings (LRB) coupled with low-friction Sliding Devices (SD).

The location of the isolators has been defined by taking into account the required vertical load capacity and the need of "centrifugating" the stiffness and centering their resultant , so as to reduce the eccentricity from the mass centre and increase the rotational stiffness inertia, thus minimizing the torsion response. The nominal eccentricity in the longitudinal direction is practically negligible and that in the transversal direction turned out to be 110 mm, i.e. 1/240 of the building’s width. The torsional radius of the isolation system is ry = 20.2 m, while the radius of gyration of the total building mass at the isolation level is approximately ls = 16.2 m, with a ratio between the two values equal 0.80.

Trans.

Long.

Tors.

T

(s)

i

2.269

2.269

1.928

P(T i

95.2

95.1

---

) T *

(s) 2.24

2.24

---

T i /T

1.01

* T i /T

4.86

o T o

(s) 0.467

p(T o

30.6

)

3D virtual views of the building

1.01

5.00

0.454

43.6

-- -- 0.401

14.6

Table

Dynamic characteristics of the building Plan of the first level floor

A B B B A A A A A B A A D

BASE ISOLATION STRATEGY AGAINST STRONG EARTHQUAKES

Base isolation is a reliable technology to protect a variety of structures, either new and old, that have the requisite dynamic characteristics, from the destructive effects of the earthquakes. Unlike conventional design schemes which rely on inelastic action of various structural elements to dissipate earthquake energy, the seismic isolation alternative reduces the force demand on the structure and thereby limits inelastic deformation , thus providing a level of performance well beyond the normal code requirements with potential for substantial life-cycle cost reduction. It consists of the insertion of laterally flexible supports at the foundation level for increasing the fundamental period of the building in the range of the lower energy contents of the seismic input. The difference between the responses of isolated and conventional buildings is showed in the above figure.

B B A B C A C B C C D C C Dynamic characteristics of buildings

The school building has a total mass Mt = 2440 t, while the isolation system has a total stiffness Kt = 41031 kN.

Table reports the natural periods, Ti, of the fundamental modes of the isolated structures and their participation factors, p(Ti). The natural periods, To, and participation factors, p(T0) of the corresponding fixed-base structure are also reported. The high values of the ratio Ti/To (



5.0) and the values of the ratio Ti/T* put in evidence the effectiveness of the isolation system.

A A A A A

Modal shape of the first mode

A B B C A C A D D A B B B A A B A A B D A B A

Layout, typology and details of the isolators. A=LRB



700, B=LRB



600, C=SD P2200, D=SD P600 General views of the numerical models Longitudinal cross sections of the building

DESIGN EARTHQUAKE

Elastic response spectrum

The design of the isolated building has been carried out using the elastic response spectrum provided by the current Italian guidelines for constructions located in Seismic Zone 1 on soils type C, with a PGA = 0.525 g.

Simulated accelerograms

Due to the lack of studies on the site seismicity allowing for the definition of specific accelerograms, dynamic numerical analyses have been carried out using artificial spectrum-fitting accelerograms. 15000 10000 5000 0 0 5

Design elastic response spectrum

10 15 20 Input Isolators 25 Damping Time 30

Input and dissipated energy time-histories

SEISMIC RESPONSE ANALYSES

Numerical analyses have been carried out using ETABS NL on two models differing in the modeling of the isolating devices . A first model, with elastic elements simulating the isolators, has been used for the design evaluation based on response spectrum modal analyses. Dynamic analyses have been then carried out on a second model where the elements modeling the isolating devices perform as non linear, with an equivalent elastic-plastic behavior adopted for reproducing their hysteretic dissipation. In both models a linear behavior of the elevation members (beams and columns) has been considered. Resistance check of devices and structural elements The check of devices is based on the values of forces and displacements computed from the combination of the maximum values corresponding to the application of the seismic action on one direction with the 30% of the maximum in the perpendicular direction and assuming an accidental eccentricity of the masses equal to the 5% of the in-plan building dimensions. The design displacement results to be 275 mm.

The r/c elements of the elevations have been designed reducing the seismic forces by the allowed structure factor q = 1.495.

The maximum inter-storey drift of the elevation turned out to be lower than 0.17%.

Dynamic analyses using generated accelerograms Five dynamic step-by-step analyses have been carried out by applying, contemporarily in the two directions, pairs of the generated accelerograms. Maximum values of the devices' displacements equal to 144.2 and 135.1 mm have been calculated, for the longitudinal and transverse direction respectively, under the ultimate limit state intensity level. The corresponding average values are 122.7 and 117.5 mm. The maximum value of the base shear, in a single direction, resulting from the dynamic analyses, is 6106 kN while that computed from the response spectrum is 9360 kN, thus confirming the precautious resulting values given by the response spectrum modal analysis used in design and due to the underestimation of the energy dissipation capacity of the devices given by the global equivalent damping factor with respect to the actual hysteretic dissipation. Figure shows the time history of the input and dissipated energy for one of the dynamic analysis. Isolators dissipate more than 83% of the input energy, 17% is dissipated by structure damping.

COMPARISON BETWEEN ISOLATED AND FIXED-BASE BUILDING

For the sake of comparison, a fixed-base buildings have been also modeled, analyzed and designed in accordance to the current Italian seismic code. The dynamic parameters of the fixed-base solutions are reported in the previous table.

The response of the fixed-base building model has been computed using as seismic input the design response spectrum of the current Italian seismic code for ordinary buildings having a maximum reduced intensity of 0.32 g.

The reduction of the seismic forces in the isolated building is evident by referring to the total shear force at the first level of the building. In the base-isolated building, a value of 5900 kN, in each of the horizontal directions, has been computed by the response spectrum analysis, which becomes 6100 kN if dynamic analyses, with generated accelerograms applied in the two directions contemporarily, are carried out. The shear forces, computed for the fixed-base building are 7077 kN and 7878 kN, in the transversal and longitudinal direction respectively. As a consequence, the fixed-base building would nominally require about twice the reinforcement of the isolated one. This would lead to over-reinforced sections and therefore the fixed base building should be redesigned using larger section. In spite of this over-strength, the fixed base building, designed with a structure factor greater than 4, has to undergo plastic deformations of the structural elements, and consequently structural damage, starting from quakes with PGA greater than 0.131 g, while the isolated building, having structural elements with smaller resisting sections, is integrally protected against the maximum conventional expected earthquake with PGA equal 0.525 g. The inter-storey drifts of the fixed-base solution, computed for the conventional serviceability limit state earthquake having a PGA of 0.21 g is greater than the allowed limit of 0.4 percent and thus also this check confirm the need for a larger and stiffer section with respect to the isolated solution.

CONCLUSIONS The pilot realisation shows the effectiveness of the seismic isolation technique in improving the seismic protection of school buildings. Base isolation represents the only practical way for achieving the high safety level required to buildings having strategic relevance or being at risk of relevant consequences to the collapse. The cost of the isolation, in this case of school building, represents less than 10 % of the financing cost of the construction. The seismic risk of the building with respect to the expected earthquake intensity has been cancelled: in fact the building performs below its strength limit even for the maximum credible earthquake having an actual return period at the site of about 1000 years.

PRO.MA ingegneria PROVINCIA DI CAMPOBASSO ASSESSORATO AI LL.PP. EDILIZIA SCOLASTICA DIRIGENTE ARCH. GIOVANNA IANNELLI

Marco MEZZI Alberto DUSI

Associate ProfessorUniversity of Perugia, Perugia, Italy Italy[email protected] Civil Engineer, Numeria Consulting srl, Cremona, Italy

[email protected]

Gianpiero DI STEFANO Giovanni PORCARELLI Nicola DI RENZO

Civil Engineer, Provincia di Campobasso, Campobasso, Italy

[email protected]

Civil Engineer, Provincia di Campobasso, Campobasso, Italy

[email protected]

Civil Engineer PROMA Ingegneria snc, Campobasso, Italy

[email protected]