Analysis and strenghtening

of existing bridges

PIETRO CROCE

Department of Civil and Industrial Engineering Structural Division - University of Pisa

Leonardo da Vinci Assessment of existing structures Project number: CZ/11/LLP-LdV/TOI/134005

Prague, 26th September 2013

An existing structure requires structural reassessment when:

- its reliability is inadequate, also due to misuse or human errors;

- the structure is modified and/or enlarged;

- the category of use of the structure is improved and/or its

design working life is increased;

- the structure has been damaged or deteriorated by environmental, chemical or biological, attack or by more general time dependent effects;

- the structure has been damaged by accidental loads (earthquake, explosion) or by settlements or by other unintentional events like impacts, vibrations, water losses, floods, bridge scuor, debris accumulation and so on.

According to the flow charts reported in ISO 13822, the investigation process involves the acquisition of all relevant information concerning: the original design and structural conception of the structure, as well as the reference structural codes, if any; the sequence of structural modifications during its life, addition or demolition of structural parts and/or deep maintenance interventions; actual material properties; soil and foundations; actual damage and/or crack patterns and/or failure; required performance level.

General flow of the assessment of existing structures according to ISO 13822

CASE STUDY N. 1

STRENGTHENING OF THE VARA-VIADUCT (Carrara – I)

General view of the Vara bridges

Bridges were built in 1887-1890 as railway bridges linking the Carrara marble caves and the Marina di Carrara harbour.

History of the bridge

and collection of available documentation

Original drawing of the bridge signed by the designer (Eng. D. Zaccagna)

The bridge during the construction

Evolution of the damage

and history of the strengthening

and repair intervention

In 1913 several cracks appeared in the 4th piers and the bridge was propped by strong timber beams.

In 1913 several cracks appeared in the 4th piers and the bridge was propped by strong timber beams shown in the photo. Reasons of the failure were probably mainly poor quality workmanship of the masonry and lateral settlement of the piers due to earth (marble gravel) pressure.

A Carrara travel guide issued in 1931 attributes the failure to pier mining (sabotage), but there is no evidence in 1913 newspapers. Other folk sources dates sabotage to 3rd April 1899 due to loss of work of cartwrights due to train transportation, but again there is no documentation about.

In 1932 a first retrofit was performed substituting the timber props with slender masonry arches connecting the lower parts of the intermediate piers

In 1932 a first retrofit was performed substituting the timber props with slender masonry arches connecting the lower parts of the piers. In 1960 the railway was dismantled and the bridges were reconverted to road bridges. In 2001 the 4th failed again

DATA ACQUISITION Geometrical surveys and crack pattern detection In situ tests: endoscopic inspection of the masonry; core sampling of masonry and infill; visual direct surveys of pier foundation dynamic identification of the bridge

Crack at the extrados of the arch crown

DYNAMIC IDENTIFICATION

FINITE ELEMENT MODELING OF THE BRIDGE

EXAMPLE OF MODE SHAPES

1

mode: f=2,24 Hz 2

mode: f=3,53 Hz

DYNAMIC TESTS

20

HAMMERING THE BRIDGE WITH A LOADED LORRY

DYNAMIC TEST ARRANGEMENT

TYPICAL RECORD OF A TIME HISTORY

RESULTS OF THE ANALYSIS NATURAL FREQUENCIES (FFT OR WAVELET TRANSFORM) MODE SHAPES DAMPING RATIO

FE MODEL CALIBRATION

LOGARITHMIC DECREMENT

RETROFIT DESIGN

LOW ARCHES:

STRENGTHENING OF THE MASONRY REPLACEMENT OF THE R.C. STRINGCOURSE

4TH PIER FOUNDATION: THE FOUNDATION IS STRENGTHEND THROUGH A R.C. RING, FOUNDED ON MICROPILES AND CONNECTED TO THE EXISTING WALLS

DECK: REMOVAL OF PART OF THE INFILLING;

ERECTION OF NEW R.C. SLAB (300 MM THICK) CONNECTED TO ARCHES, INFILLING AND LATERAL WALLS

THE BRIDGE DURING THE RETROFIT

CASE STUDY N. 2

REPAIR AND RETROFIT OF THE MAGRA BRIDGE IN VILLAFRANCA (I)

CASE STUDY N. 2

REPAIR AND RETROFIT OF THE MAGRA BRIDGE IN VILLAFRANCA

(I)

25th and 26th October 2011 due to a large flooding of the Magra River in the North Tuscany (I) several masonry, concrete and composite bridges collapsed mainly due to pier scour.

The bridge during execution

The bridge considered here is a 9-span continuous masonry arch bridge built around 1876 The chord of each arch is 16 m about) In 1964 it was enlarged using r.c. beams During the flooding two spans collapsed due to scouring of the 1st pier The two spans were substituted by a single span Bailey bridge

Collection of available documentation Design of similar bridges Eyewitness evidences of modifications and or failures

Active crack patterns A significant crack pattern opened at the beginning of the XXI century in the arches that collapsed. This crack pattern was monitored. Monitoring 2009

Monitoring 2010

SURVEYS Bridge geometry Material Foundations Soil

NON-DESTRUCTIVE METHODS OF INVESTIGATION

GEORADAR (GPR)

NON-DESTRUCTIVE METHODS OF INVESTIGATION

GEORADAR (GPR)

NON-DESTRUCTIVE METHODS OF INVESTIGATION

3D ELECTRIC TOMOGRAPHY

Resistivity

Voids in the soil

NON-DESTRUCTIVE INVESTIGATION CHART

SOIL STRATIGRAPHY

PILE SECTION

SEISMIC ANALYSIS

t [hr]

Q [

m3/s

] Flood Hydrograph (TR=200 years)

TR=30, 100, 200, 500 years

TR=30 Q=1589 m3/s H=118,78 m a.s.l. TR=100 Q=2199 m3/s H=119,63 m a.s.l. TR=200 Q=2627 m3/s H=120,18 m a.s.l. TR=500 Q=3301 m3/s H=120,95 m a.s.l.

TYPICAL CROSS SECTION

FRONT VIEW

ACTIONS EN1991-2 MATERIALS EN1992-2 TESTS

REPAIRED STATE

FRONT VIEW

LONGITUDINAL SECTION

REPAIRED STATE

CROSS SECTIONS

REPAIRED STATE

EXISTING PIERS

REPAIRED STATE

NEW PIER

REPAIRED STATE

NEW GIRDERS

REPAIRED STATE NEW DECKS

TEMPORARY STEEL TRUSSES

REPAIRED STATE

RENDERING