Search results for "Prestressing"

Influence of bond deterioration on shear-flexure failure of prestressed girders with post-tensioned tendons

2021

Internal prestressing with bonded tendons suffers corrosion of wires and bond deterioration due to the bad infill of grouted sheaths. Many bridge girders show this state through cracking, deflections and in the worst cases, sudden failures. In Italy these are common situations, and the internal tendon corrosion is among the major causes of bridge degradation and damage, especially for grouted post-tensioned tendons. This paper presents the analysis of failure of prestressed cross-sections considering shear and flexure in the case of bond deterioration due to corrosion of internal post-tensioned tendons. The structural behaviour is analysed for different typologies of bridge deck composed of…

Bending-Shear interaction domains for externally prestressed concrete girders

2013

In prestressed concrete structures, the evaluation of the safety level is generally carried out by separating the bending moment strength and the shear force capacity. Actually interaction between bending moment (M) and shear force (V) can have significant consequences on evaluations in service life, especially when the ultimate limit state (ULS) is considered. In this paper, the M-V interaction is addressed for prestressed concrete girders, in the cases of both bonded and unbonded prestressing tendons. It can be demonstrated, by drawing the interaction domains (M-V), that a significant reduction of the safety level has to be considered when shear force is evaluated together with bending mo…

The Rehabilitation of Segmental Bridges Subjected to Time-dependent Deflections Through Additional Prestressing

2022

Segmental bridges built by cantilevering with in-situ casting in the ‘60s-‘80s of the XX Century present a central hinge on the main span or a central simply supported span between the two cantilevers (drop-in span). Due to time-dependent phenomena like creep of concrete and to unexpected loss of prestressing, several bridges of this typology show an abnormal deflection at hinges causing discomfort of traffic as well as insufficiencies of checks at Service and Ultimate Limit States. Moreover, these bridges are today at the end of their nominal service life, impacting significantly on Italian road network. A methodology for retrofitting these bridges by partial recovering of deflection and o…

Influence of axial force and corrosion on failure of prestressed concrete structures considering M-V interaction

2021

Abstract Existing prestressed concrete structures suffer from degradation of prestressing for two main reasons: a defect of the sheath infills (cement grout) of bonded tendons, and corrosion of steel strands. These two causes, often concomitant, lead to a reduction over time of the effectiveness of initial prestressing, in addition to the ordinary tension losses due to concrete shrinkage and creep, and to steel relaxation. In many cases a consequence of bad grouting in bonded tendons is that the structure shows an intermediate behavior between that of external prestressing and that of internal bonded prestressing. This, added to damage to wires due to corrosion, makes it essential for the d…

La variazione di schema statico nei ponti in cemento armato precompresso durante le fasi costruttive e l’influenza dei fenomeni lenti

2018

The construction of prestressed concrete bridges often presents different construction stages in which the structural static scheme changes many times, together with the characteristics of the construction elements. In fact, prefabrication and in situ assembly involve a multiple variation of the scheme with the introduction and removal of restraints, especially when temporary sup-ports are used. These transitional phases can strongly modify the structural behavior, in relation also to the deformations due to shrinkage and creep. In this study an evaluation of the effects of time-dependent phenomena is presented, for the case of complex sequences in which delayed restraints and removal of pr…

Adjustable prestressing for construction stages of incrementally launched bridges

2014

Incrementally launched bridges, which are generally built with the static scheme of a continuous beam on supports, change their scheme several times during launching. In these different configurations, internal forces vary and bending moments of opposite signs occur in the same sections for different positions of deck during advancement. In these bridges, two different kinds of prestressing are necessary: a temporary one during launching and a definitive one in service life. In this study, the possibility of implementing automatic adjustable prestressing for the launching stages is investigated; the launch prestressing is partially supplied by a system which can change the tension value in …

Fenomeni lenti nelle fasi di costruzione ed esercizio degli impalcati da ponte precompressi

2010

Assessment and strengthening of reinforced concrete bridges with half-joint deterioration

2022

Existing Niagara-type concrete cantilever bridges, which in Italy are called Gerber bridges, suffer from degradation due to reinforcement corrosion and a consequent reduced load-bearing capacity. The assessment of these structures is therefore an important part of the procedures for analyzing the state of existing bridges and any interventions required for their retrofitting. In particular, safety against failure of the half-joint must be evaluated with respect to the behavior of the entire bridge and its state of conservation, determining what the conditions are that lead to failure, in terms of loads, state of stress, and possibly cracking state. This evaluation has to be carried out cons…

Pannello precompresso di vetromattoni assemblati a secco per la realizzazione di involucri traslucidi

2012

L’invenzione riguarda la realizzazione di un pannello di vetromattoni montati “a secco” -da impiegare per la realizzazione di involucri edilizi traslucidi anche nel caso di edifici alti- che è in grado di rispondere adeguatamente alle sollecitazioni orizzontali (quali quelle dovute a vento e sisma) grazie all’applicazione della tecnica della precompressione mondirezionale. Per offrire una risposta ai limiti imposti dalla tecnica di assemblaggio tradizionale dei vetromattoni (“ad umido”), l’applicazione della precompressione consente, infatti, di dare ai pannelli preassemblati “a secco” una rigidezza adeguata anche in direzione normale al loro piano di posa, sfruttando le buone caratteristic…

Stressing sequence of steel cable-stayed bridges built by cantilevering

2015

The construction of cable-stayed bridges by cantilevering implies several changes of geometry, stress and strain patterns during the assemblage of segments. The main target to be satisfied in the construction process is the achievement of the required final geometry and of a convenient state of stress for self-weight and sustained loads. The sequence of stay stressing and the values of prestressing forces at each stage of segment assembling have the main role for reaching the desired result of design, due to the large redundancy of cable-stayed structures.Among the different procedures proposed in the literature for initial cable force determination, the Partial Elastic Scheme (PES) Method …