1,721,262 research outputs found
La variazione di schema statico nei ponti in cemento armato precompresso durante le fasi costruttive e l’influenza dei fenomeni lenti
No full textThe 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 provisional supports occur till the final configuration is achieved. The study is based on analytical methods and creep models available in the literature, discussing the implications on long-term behavior of bridges, both in terms of deformation and variation of the stress state. A practical application is also presented
Prevenzione del degrado dei ponti in c.a.p. con la precompressione bilanciata. Il ponte funicolare
No full textLa trave e l'arco hanno comportamenti strutturali completamente differenti. La trave infatti, sotto l'effetto dei carichi applicati, ha un comportamento tipicamente flessionale mentre l'arco, grazie alla spinta e alla sua sua curvatura geometrica, può essere progettato con un comportamento funicolare a prevalenza assiale. Le strutture precompresse possono combinare intelligentemente questi due diversi comportamenti e la precompressione può essere favorevolmente utilizzata per trasformare la trave in una struttura funicolare, analoga ad un arco, con comportamento prevalentemente assiale. In questa nota viene presentata una filosofia di progetto per lo studio dei ponti a piastra precompressi, basata sul “Load Balancing Method”, con la finalità di eliminare le deformazioni di tipo flessionale dovute ai carichi permanenti e limitare le deformazioni differite dovute alla viscosità nel calcestruzzo. Gli aspetti relativi alla evoluzione nel tempo delle deformazioni e delle sollecitazioni dovuti ai fenomeni viscosi sono analizzati e discussi applicando i principi della visco-elasticità lineare. La precompressione viene valutata e dimensionata come carico equivalente che controbilancia i carichi permanenti applicati, rendendo nulle le deformazioni elastiche flessionali prodotte dai carichi, eliminando in tal modo anche le deformazioni differite dovute alla viscosità. Viene proposta una ottimizzazione del comportamento strutturale nei ponti a piastra, tramite l'uso della precompressione bilanciata nelle due direzioni. La determinazione della forma più conveniente della struttura viene ricavata considerando la variazione dello spessore nei ponti a piastra e del layout dei cavi di precompressione, garantendo la realizzazione del comportamento funicolare e l'eliminazione delle deformazioni differite di tipo flessionale sotto l'azione dei carichi fissi. Questi concetti vengono utilizzati quali criteri di “Conceptual Design” per ottenere un comportamento ottimale e per prevenire l'azione del degrado nelle strutture di ponti in c.a.p. Il “caso studio" relativo a un ponte a piastra, progettato dall'autore e costruito negli anni '80 a Jeddah in Arabia Saudita, viene presentato e discusso. Ciò allo scopo di mostrare la utilità e convenienza dell'approccio proposto, alla luce del tempo di esercizio trascorso
Unified theory for analysis of curved thin-walled girders with open and closed cross section through HSA method
No full textThe behaviour of thin-walled structures is deeply influenced by non-uniform torsion and cross section distortion. In this paper the extension of the Hamiltonian Structural Analysis (HSA) Method to thin-walled straight and curved beams is presented. The proposed method solves the structural elastic problem of thin-walled beams through the definition of a Hamiltonian system composed of 1st order differential equations. The method allows engineers to solve the elastic problem by introducing the degrees of freedom and the corresponding compatibility equations, founding equilibrium equations in the variational form. The methodology is explained in the framework of the so-called Generalized Beam Theory, considering beams on elastic foundation and thin-walled structures with non-uniform torsion and distortion in a unified theory for open and closed cross section considering the shear deformability of the wall midline. The exact solution is found for straight and horizontally curved beams and distortion of box section is directly solved avoiding the analysis through the Beam on Elastic Foundation (BEF) analogy. Numerical applications are given in order to show the wide range of applicability of the proposed method and to validate it through comparisons with literature data
Applicazione delle Linee guida italiane alle valutazioni di sicurezza strutturale del ponte Corleone sulla tangenziale di Palermo
No full textThe "nominal life" of a large part of existing reinforced and prestressed concrete bridges, in Italy, is expiring, often in the absence of ordinary or extraordinary maintenance. Consequently, the Italian infrastructural network is in a critical phase, followed by the alarmed awareness of the public opinion, especially after the collapse of the Polcevera viaduct. Specific guidelines on risk classifi-cation, management, safety assessment, monitoring of existing bridges have been issued by the Superior Council of Public Works in 2020, to address the emergency. Between ‘50s and ‘70s of the last century, important examples of reinforced and prestressed concrete bridges were built in Italy by the most famous Italian designers, and they have to be safeguarded by finding the appropri-ate strategies for their preservation. Among these, the Corleone bridge, an arch bridge on the Palermo ring road, designed in 1958, finds its place both from the historical design point of view and critical state of degradation. The Municipality of Palermo requested an accurate safety check of the bridge, in the light of the indications of the Ministerial Guidelines, to evaluate its operational per-formance and the conditions of its transit ability and/or its permanence in service, with indications on the retrofitting interventions. The working group investigated the shortcomings due to natural aging and lack of maintenance, highlighting the critical aspects connected to the current situation, with the presence of Gerber saddles in the deck and the necessary limitations to transit before the rehabilitation
Influence of secondary torsion on curved steel girder bridges with box and I-girder cross-sections
No full textSteel curved girder bridges are largely used today in motorways and railways. They are often composed of thin-walled crosssections, entirely made of steel or with an upper concrete slab. The deck may have I-girders or box cross-sections: in any case curved girders are subjected to twisting moment, associated with bending, even for dead loads. Moreover, in thin-walled sections the influence of non-uniform torsion becomes sizable with respect to Saint Venant torsion, modifying the state of tangential stresses in the section and introducing axial stresses due to warping being prevented. Open sections of I-girder bridges are especially subject to these phenomena and warping can be significant not only for curved bridges but also for eccentrically applied traffic loads in straight ones. In this paper a method for the evaluation of the effects of non-uniform torsion, based on an energetic approach, is proposed; the method is simple and fast, with a reduced computational burden with respect to finite elements. The solution of curved girder bridges is performed by the Hamiltonian Structural Analysis method, which is implemented for straight and curved girders with thin-walled cross-sections. A parametric analysis is proposed for single and multi-span bridges, with variation in type of loads applied, crosssection parameters, geometric curvature of bridge deck and stiffness ratios of the cross-section. The parametric study is presented through dimensionless diagrams of internal forces for the evaluation of the global behaviour of curved girder bridges together with indications on the stress state of the cross-section. The interpretation of the results of the analyses performed can be useful to designers for the conceptual design stages and for optimization of the geometric and mechanical parameters of I-girder and box girder cross-sections
Symplectic analysis of thin-walled curved box girders with torsion, distortion and shear lag warping effects
No full textShear lag effect and torsional and distortional warping can significantly affect the performance of thin-walled curved box girders used in modern bridge engineering. The structural behaviour of these bridges exhibits complexity due to coupled bending and torsion together with warping effects of non-uniform torsion, distortion and shear lag. A practical method of analysis, based on the symplectic approach, with the same perspective as the Higher Order Beam Theories, is presented for overcoming the difficulties of numerical approaches via the Finite Element Method. In this paper, the Hamiltonian Structural Analysis method implements the analysis of the shear lag effect together with non-uniform torsion and distortion, for curved box girder bridges. The examples given can allow engineers to evaluate the effects of the negative shear lag phenomenon and the distribution of axial stresses in box slabs and webs, due to different warping effects, which strongly influence thin-walled box sections subjected to point loads and restrained areas
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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