1 INTRODUCTION

1.1 The Savona-Ventimiglia Highway

The A10 Savona-Ventimiglia is an important highway that connects Italy to France. It has a length of 113.3 km crossing the territory through the prov-inces of Imperia and Savona in the Liguria Region, in the north west of Italian peninsula.

Figure 1. A10 Savona-Ventimiglia Highway panoramic view.

The route is for the 60% in tunnel or on viaduct,

including 67 twin-tube tunnels and over 90 viaducts.

The highway was builded during the sixties from 1962 to 1966, while in 1971 it was fully opened.

The Autostrada dei Fiori company, that manages the A10 highway, has undertaken since several years ago the seismic vulnerability assessment together with the seismic retrofit design and in the last period they are carrying out of the first works.

In the next part of the paper a detail of the seismic structural assessment approach and two case studies of bridge retrofit, to the A10 structures developed by Sineco S.p.A., are presented.

1.2 Notes on seismic classification

The Italian territory was exposed along the years to several earthquake of different importance. The first seismic classification of Liguria territory was presented in 1927 with a Regal prescription of 13th march 1927, n. 431.

A more detailed classification was presented in 1982 (DM LL.PP. of 27th july 1982), with a division of the territory into 3 different seismic zones, em-ploying an “S” coefficient (first category S=12, second S=9, third S=6). With this division a lot of municipalities were classified in second category.

Seismic Risk Assessment and Retrofit Design of Existing Concrete Bridges for the Italian Highway Savona-Ventimiglia

C. Bafaro, G. Massone & G. Pasqualato SINECO, Milano, Italy

G. Massa & F. Lenti Autostrada dei Fiori, Imperia, Italy

ABSTRACT: The seismic behaviour of existing bridges can be assessed with different tools which range from simplified linear elastic calculations to more refined 3D linear or non-linear finite element analyses. The paper describes the seismic analysis and structures vulnerability of a important number of bridges and viaducts on the highway from Savona to Ventimiglia in Italy, placed over a high seismic hazard area. The activity per-formed over the last ten years, was the understanding the bridges seismic behaviour and the road safety in an earthquake scenario. The method adopted for the seismic risk assessment is the classic Response Spectrum Analysis and in some particular cases the 3D non linear; regarding the Eurocode 8 (2005) and the new Italian seismic design guidelines (NT 2008). All the structural elements were analysed: from the bearing to the pier, abutment and the foundation system, in order to apply a real risk analysis (the probability of failure) and the corresponding vulnerability of the highway bridges. The retrofit design were also described for the more rele-vant and typological structures. Full details of the retrofit given in the paper, basically consisting in the repair of local structural damages and the bearing substitution. The seismic isolation was also consider as a solution that could bring the viaducts to acceptable levels of seismic safety. Finally in the paper, some case studies are presented during the construction phases.

In 2003 the seismic classification was changed af-ter San Giuliano di Puglia earthquake (Province of Campobasso, 2002).

It was introduced the OPCM 3274 standard, by which the whole Italian territory was classified as seismic and divided into 4 zones (zone 1 maximum value of Peak Ground Acceleration).

Figure 2. Seismic classification of Italian territory presented in 1927.

The Region of Liguria lies in zones 2, 3 and 4, the

maximum degree, the zone 2, is in west part (Impe-ria area) where is located the A10 Savona Ventimig-lia highway.

Figure 3. Seismic classification of Italian territory presented in 2003.

The last update was the introduction of new Ital-

ian technical standards (Nuove Norme Tecniche per

le Costruzioni, Ministerial Decree 14th January 2008) with an update of the o earthquake zones on the territory of the Liguria Region ..

The seismic construction code standards in Italy started in the seventies with the first edition (lex n° 64 by 1974). Before that the design method was oriented to the normal construction standard without specific seismic criteria.

Today the used reference is the Code NTC 2008 (Norme Tecniche per le costruzioni).

From the 2003 after the OPCM 3274 started the duty of the analysis of the existing strategic struc-tures (highway bridges, hospital, schools etc.) in all the Italian territory, this activity has the aim to iden-tify the vulnerability of each structures reaching the seismic assessment of the whole Italian territory.

2 SEISMIC RISK ASSESSMENT: GENERAL OUTLINE In order to choose the design retrofit solutions, it

is important to define seismic risk assessment of a highway system, its level and its mitigation. So, a viaduct have to be placed in a general contest, as a part of a system. The seismic risk assessment is an important concept, because a seismic scenario is de-fined related to it.

Seismic risk assessment can be defined as: “…the probability and relative severity level, in dependence of a determined period and the total possible effects of a seismic event […] In general, a seismic risk is determined as a combination of hazard, vulnerability and exposure and its measure of failures can be at-tempted, in relation to seismic level and strength of structures and systems” (GNDT–CNR group 1999). The seismic risk assessment is characterized by sev-eral important concepts: − seismic hazard, which is given as seismic level of

a specific zone; − structural vulnerability, which is intended as the

level of seismic structural effects due to a seis-mic event of a specific intensity. The vulnerability is a intrinsic characteristic of a structure indepen-dent to site;

− exposure, where social and economical effects are included. The exposure concerns the site where a structure is built and it considers seismic event in-tensity.

3 SAVONA- VENTIMIGLIA STRUCTURAL VULNERABILITY ASSESSMENT

The Autostrada dei Fiori Company started before the 2003 with a campaign of analysis in order to under-stand the capability of the bridges to overcame an earthquake, this work identified the first priority of bridge retrofit needs.

Since 2003, in agreement with a Government re-quest (OPCM 3274), a systematic study was under-

taken. The main task was to achieve the Seismic vulnerability assessment.

It was performed the seismic analysis for the 39 viaducts along the A10 highway located in the max-imum exposition area (area n° 2 of the OPCM 3274).

This work began with the definition of a guide line that contains the procedure for doing the analy-sis, the main steps were:

• Collecting of the historical data of the bridges (design report, technical drawings, plans, geotechnical data);

• Visual inspection of all the structure parts (from the deck, the joint-bearing to the piers) using Sineco method S.I.O.S.;

• Identification of the structural analysis me-thod (3d linear finite element method analysis)

• Verification of main structural element; • Reporting of the analysis results (identifi-

cation of the most important problems) The study was completed at the beginning of

2009 reporting a huge number of information that produced a first vulnerability assessment scenario.

After this part started the retrofit design, it was performed for 8 of those viaducts whereas for 3 via-ducts was carried out.

Figure 4. San Lorenzo Viaduct.

3.1 Typologies of viaducts analyzed

The main superstructure typology is a simply supported grillage formed by 3 or 4 reinforced con-crete or precast prestressed concrete beams, with 2 to 5 cast in situ transversal diaphragms and concrete slab. In particular cases deck is a simply supported hollow concrete slab. In other cases superstructure is a concrete cast-in-place balanced cantilever box-girder with multicell varying-depth section. The number of spans ranging from 3 to 22, while the length of spans ranging from 28 m to 82 m.

Bearing system consists in elastomeric pads with-out connecting devices.

Main pier typologies are hammerhead piers with single-cell or multicell box section. In particular cas-es piers are solid wall. In some cases both carriage-ways are supported by unique pier. Piers height rang-ing from 4 m to 80 m.

Abutments are reinforced concrete wall diaph-ragms.

Piers and abutments foundations are both shallow and deep (both piles and caissons). In some cases separated piers have the same footing.

Figure 5. San Martino Viaduct.

All viaducts had been designed and builted with

no seismic standards.

3.2 Description of seismic analyses and verifications

After collecting all the information about the structures, the first step was defining design seismic action with procedures described in the standard. In order to analyze the structure, the second step con-cerns the construction of a mathematical model with FEM procedures. The possibilities of modelling the structure with a 3D model represents an important instruments to understand real structural behaviour under seismic action. In order to investigate stresses, displacements in structural elements, the main seis-mic analysis adopted is a linear dynamic analysis with spectral response.

The last phase was the verification of each struc-tural elements as follows (semi-probabilistic ap-proach of limit states were used): − deck joints: relative displacement in longitudinal

and transversal direction; − deck beams and other deck’s structural elements:

strength and deformability; − bearings: strength and deformability; − piers:bending and shear strength at the base; − piers foundation: footing (strength and stability),

soil (bearing capacity) piles (bending and shear strength);

− abutments elevation: strength of vertical and lat-eral wall

− abutments foundation: footing, soil and piles.

As detailed all the main part of the bridge were con-sidered.

3.3 Main seismic vulnerability assessment results

As described above the structures were designed with no seismic criteria, as the following result out-line.

In particular the structural details, in regard to seismic standard, as bearing system and joints are inefficient towards seismic actions for all the bridges.

In the next paragraph it is shown a summary of the results based to the critical level. We assumed as critical level the problem found related to the solu-tion that could be adopted to retrofit the bridge.

Main structural problems can be summarized as follows and in the next picture:

Figure 6. Diagram of the critical of viaduct critical level.

− piers showing shear deficiency while lack of flex-ural capacity at the base of piers is less common (medium critical level).

− Piers foundations showing lack of footing stregth, footing settlement and bearing capacity. Lack of pile strength is less frequent (high critical level). Problems on foundation catalogued as critical could be solved with detailed getecnical analysis that define the correct soil-structure interaction.

− Abutments shows mainly lack of settlement (me-dium level).

− Bearing and joint as detailed before are without connection to the beam an pier and for this reason inadequate (low critical level). The low level is due to the easy way to replace this elements. The seismic analyses allows to collect the sche-

dules required by Civil Protection Department about seismic vulnerability assessment of bridge struc-tures. In these schedules are presented also a valua-tion of the principle design solutions in order to rea-lized a seismic retrofit of the structures analyzed. The schedules are divided in 3 principal parts due to different information levels as follows.

Part 1, general/descriptive: − bridge identification;

− dimensional data; − structural and material type; − […]

Part 2, input/analysis: − seismic classification; − soil factor; − regularity; − knowledge level; − material strength; − analysis; − modeling;

Part 3, results/retrofit: − analysis results; − reference parameters; − risk indicator (IR); − possible design retrofit solution;

In order to verify and to know the seismic vulne-rability level of a structure, the definition of a risk indicator (IR) is very important, because it’s defined in function of critical structural aspect. The values of IR parameter are related to the minimum value of Peak ground acceleration (PGA) that causes first structural failure. Values of IR equal to 1 means no risk whereas values of IR close to 0 means maxi-mum risk.

The maximum IR value obtained for the struc-tures analyzed is equal to 0.8 and only for 5 bridges the value is in the range of 0.2.

The Ir parameter is in agreement to the critical level described before.

3.4 Retrofit design strategy outline

Based on the structural vulnerability and seismic risk assessments of the all viaducts analysed, the Au-tostrada dei Fiori company has developed a retrofit-ting strategy to maximize the benefits in connection to financial commitments. This strategy is based on the following points: − priorities to a single carriageway for carrying out

retrofit solutions (carriageway direction to France);

− replacement of existing bearing system with a new one with appropriate connecting devices that ensure the correct transmission of the horizontal seismic actions from superstructure to substruc-ture and to avoid the unseating problems;

− widening in the expansion joint openings to ac-comodate cyclic movements due to seismic ac-tions. New bearing system in some cases can improve

the structural behavior of the substructures, thus re-ducing retrofit needs.

In the next case study is presented the design de-tail of these solutions.

4 CASE STUDY: RIO GOZZO & PRINO VIADUCTS In this part of the paper the seismic retrofit of two

different applications are described. Both viaducts were improved towards seismic ac-tions by replacement of existing bearing system and in widening expansion joint openings. An appropriate lifting system of decks allow to work in presence of traffic at almost all stages.

The following paragraphs will describe the design features planned for the two viaducts.

4.1 Rio Gozzo Viaduct

The Rio Gozzo viaduct is located along the A10 Highway in the San Remo municipality.

The carriageway to Italy has 10 spans of 41 m in length, with the exception of the first span (France side) that is 20.5 m in lenght. The carriageway to france has 9 spans of 41 m in length.

Figure 7. Longitudinal section of the Rio Gozzo viaduct.

The superstructure typology is a simply supported

grillage formed by 3 precast prestressed concrete beams. The overall width of the deck is of 10.00 m.

Piers have a rectangular hollow section. Abutments and 3 piers have caisson foundations.

All other foundations are shallow.

Figure 8. Gozzo Viaduct.

Existing bearing system consists in elastomeric

pads without connecting devices. The overall height of existing pads is about 5.1 cm.

The project involves the replacement of all exist-

ing bearing system with elastomeric pads with ap-propriate connecting devices that ensure the correct

transmission of the horizontal seismic actions from superstructure to substructure. The new bearing sys-tem has the total thickness greater than the existing one. The difference in elevation due to the new bear-ings was recovered with appropriate connections with the existing road surface before and after the viaduct.

The replacement of existing bearing system was performed in stages by lifting both decks supported by the same pier.

The lifting operations were carried out using syn-

chronous lift systems with 8 cylinders per piers, 4 cylinders for the abutments. Synchronous lift sys-tems was necessary to obtain an absolutely uniform lifting without abnormal stress in the deck. Given the geometry of the structures and the space re-quirements for the replacement of the bearings it was chosen to place the cylinders under the transversal diaphragms.

Figure 9. Gozzo Viaduct lifting system and new bearing under construction.

During the lifting phase the traffic was diverted

on the other carriageway. With the cylinders locked, traffic was reported on carriageway affected by the works, opening a single lane and imposing speed re-strictions for vehicles.

Access to the piers was ensured through the use of a by-bridge platform.

The expansion joint openings were not sufficient to permit the development of maximum displace-ment calculated from the seismic analysis of the via-duct. To ensure the expansion joint opening of 9 cm the curb has been demolished to a thickness ranging from 3 to 5 cm, respectively for abutments and piers using hydraulic cutting machine. The reconstruction of concrete rebar was done using special episodic motar.

Figure 10. Gozzo Viaduct hydraulic cutting system. The old pavement transverse joints were removed

and replaced.

4.2 Prino Viaduct

The viaduct is located along the highway A10 near the exit of Imperia Ovest and has manly a recti-linear planimetry.

Figure 11. Prino Viaduct.

In both two roadways the bridge extended for a

length of approximately 860 m and consist in 22 spans of 39 m of length.

The piers have a rectangular hollow section

(16,90x3,60) and support both roadways. Only the piers n°1, 2, and 3 are independent. The piers height is variable from 14m to 52m

Figure 12. Prino Viaduct planar view .

The deck is constituted by three precast

prestressed beams for each roadways to obtain a total road width of 8,5 m. To place the acceleration and deceleration lanes of the highway exit some spans have four beams. The beams are support by rubber devices, forty years old.

The abutments and 10 piers are on shallow foun-dations, the others on pile foundations.

Figure 13. Prino Viaduct geological section The first calculation analysis has shown strength

problems of a high number of foundations. To avoid structural works on deep foundations with consider-able operating difficulties, it was decided to perform a seismic-geology survey for assessing the seismic input of the actual place. The survey requires the execution of 5 geological drilling and geophysical down hole tests. At the same time a seismological study has identified the most dangerous earthquake zone and defined actions in terms of seismic accel-erograms and response spectrum to the bedrock. These accelerograms were integrated with geological data obtained from local geological and geophysical survey.

This study has allowed to perform a dynamic

analysis nonlinear with natural recorded accelero-grams. The results of this analysis showed a reduc-tion of approximately 25% of the actions in the structural elements compared to those calculated us-ing the elastic spectrum of the standard code.

Figure 14. Prino Viaduct existing bearing detail The purpose of retrofitting design was to : - replace the rubber bearings to reduce the stiff-

ness of the bond and secure the transfer of the ac-

tions and movements between the beams and pre-vent loss of the seat;

- enlarge of the joint openings between decks to avoid the impact;

The replacement of all bearings was realized plac-ing hydraulic jack under the transverses head of beam like in the Rio Gozzo viaduct.

Figure 15. Prino Viaduct lifting system and new bearing In the transitional span n°18 and 19 the external

beams have a terminal bearing very close to the other one of the next beam. To contrast the effect to the asymmetry of the loads, it was necessary to rein-force the transverse beam. This reinforcement will consist of a temporary external prestressing, carried out with 2 bars Ø32 stressed to 40 tons each, placed at about 9 cm above the upper slab. A special metal structure anchored to the slab allow the transfer of prestressing action.

In correspondence of this reinforcement and of the shoulders the lifting required the closing of traf-fic.

To allow the development of maximum dis-placement, calculated seismic analysis of the via-duct, the transverse joint openings between adjacent slabs are widened for a thickness of about 4 cm by cutting with disc system. Existing transverse joints of pavement was removed and replaced with new pavement joint.

To avoid the impact along the longitudinal joint between the two carriageways, the installation of "shock transmitters" devices between the heads of the beams was designed. These devices operate only

during seismic events and, rigidly tying the spans of the two roadways, avoid the impact of structures.

The shock transmission devices, anchored to the structure by spherical hinges, provide a very stiff dy-namic connection while their reaction to low veloc-ity applied displacement (thermal change) is negligi-ble.

Figure 16. Prino Viaduct transversal shock transmitters devices

5 CONCLUSIONS The paper outlines the methodology adopted by

the Autostrada dei Fiori and SINECO companies for structural vulnerability, seismic risk assessment and retrofit design strategies of existing viaducts along the A10 Savona-Ventimiglia highway. Two case studies of retrofit design and Its carrying out has been also presented.

Figure 17. A10 Savona-Ventimiglia Highway panoramic view

The evaluation of structural vulnerability of the

viaducts on a large scale has been conducted mainly with linear methods of analysis, which provides a first screening. In special cases non-linear analysis has been also carried out at this stage. More specific analysis were developed during the retrofit design phase.

Structural vulnerability results are summarized in appropriate form provided by Italian’s Civil Protec-tion Department. Together with considerations on seismic hazard and exposure, structural vulnerability

results allows seismic risk assessment, which per-mits the award of priority in retrofit design choices.

As shown in the two case studies the main retrofit strategy consists in the replacement of existing bear-ing system with a new one with appropriate con-necting devices and in widening expansion joint openings or installation of special devices such as shock transmitters units.

6 ACKNOWLEDGEMENT The authors are grateful to Mr Giorgio Nicolini,

Sineco structural engineer consultant, and Mrs. Marilena Manna for their cooperation.

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