Progettazione di ponti a graticcio con sola precompressione esterna

Italian Concrete Days.jpgAbstract: 

Nel presente lavoro gli autori descrivono la progettazione strutturale di un ponte con sola precompressione esterna, una soluzione semplice ed economica per ponti di media luce.

Si fa riferimento ad un viadotto caratterizzato da campate lunghe 45 m, schema statico di trave continua, e impalcato composto da quattro travi post-tese a doppia T.

Si svolge la progettazione del sistema di precompressione e degli elementi strutturali; si studiano con particolare attenzione lo stato di pressoflessione delle travi (considerando in primo luogo sia armatura passiva sia precompressione esterna, e in secondo luogo solo armatura passiva), e la progettazione dei blocchi di ancoraggio dei cavi esterni di precompressione.

Articolo presentato in occasione degli Italian Concrete Days 2018 di aicap e CTE


Design of grid deck bridges with external prestressing only

Progettazione di ponti a graticcio con sola precompressione esterna

INTRODUCTION

1.1 Description of the bridge

This paper proposes an extremely simple and eco-nomical solution to deal with the construction of medium span bridges using techniques that are easily available in road construction sites.

The viaduct studied is 150 m long and has 4 spans: two central spans of 45 m, and two outer spans of 30 m. The bridge is made of reinforced and prestressed concrete, and the deck is composed of four post-tensioned girders with double-T section. Girders are 43.3 m ad 28.3 m long, and are continu-ous at piers.

The bridge deck (Figs 1-2) has a total width of 12 m. The structural slab has a constant thickness of 0.25 m and the transverse slope is created using a layer of asphalt concrete binder with variable thick-ness.

Girders are produced directly inside the construc-tion site, as they are ordinary reinforced concrete el-ements; external prestressing is applied before lifting girders on piers. The deck is supported by 3 piers and 2 abutments.

progettazione-di-ponti-a-graticcio-con-sola-precompressione-esterna-001.jpg

progettazione-di-ponti-a-graticcio-con-sola-precompressione-esterna-002.jpg

2 MATERIALS

 

Materials used are: concrete C45/55 for girders, con-crete C32/40 for structural slab, steel B450C for pas-sive reinforcement, and Dywidag threaded bars 36WR for external prestressing. Permanent corro-sion protection of external prestressing bars is made of PE protection tube and grouting with cement grout after stressing.

3 LOAD ANALYSIS

Loads included in the model are:

  • Permanent actions: structural and non-structural permanent loads;
  • Impressed deformations: creep and shrinkage (automatically taken into account by MIDAS Civil), thermal actions (ΔTN,con = 23°C, ΔTN,exp = 29°C, ΔTM,heat = ΔTM, cool = 7°C);
  • Live loads (according to italian NTC 2018);
  • Wind load (wind pressure p = 1.82 kN/m2);
  • Prestressing load (4 threaded bars 36WR Dywidag for each girder, A = 1018 mm2, fjack = 840 MPa).

4 MODELLING

4.1 Description of the model

Time dependent material properties (creep, shrink-age and the variation of modulus of elasticity with time) are automatically applied by MIDAS Civil to concrete. Girders, slab, and transverse diaphragms are modelled using beam elements. The bridge deck is composed of 4 girders in longitudinal X direction (including longitudinal structural slab) and structural slab in transverse Y direction.

The transverse structural slab allows the collabo-ration between longitudinal girders. Each beam in longitudinal direction represents the precast girder and the collaborating structural slab.

Girders are continuous and simply supported on piers. The structural continuity is obtained through the realisation of pier transverse diaphragms after the placement of precast girders over temporary sup-ports.

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L'ARTICOLO COMPLETO E' DISPONIBILE IN ALLEGATO


KEYWORDS: external prestressing; post-tensioning; deviator; anchor block; double-T section / precompressione esterna; post-tensione; deviatore; blocco di ancoraggio; sezione a doppia T.


Nel 2020 si terrà a Napoli la terza edizione degli Italian Concrete Days organizzati da aicap e CTE. Per saperne di più collegarsi a questo LINK