1 Study of Bridges with Composite Steel-Concrete Box Girder Decks Manuel Maria Teixeira d’Aguiar Norton Brandão Civil Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Portugal Abstract This dissertation studies the specific phenomena associated with the analysis and design of composite box girder decks, namely under service conditions and during the incremental launching stages of construction. Primarily, the phenomena of shear lag, stiffened plate buckling due to direct stresses, plate buckling due to shear, resistance to Patch Loading and distortion of box girder decks’ cross section are identified and studied, and their analysis and design methodologies defined in the Eurocodes are presented. These methodologies are then applied through a design case of a bridge with a composite box girder deck with typical spans of 63 m. For this example the structural analysis, the verification of the deck’s slab, the global verification of the deck both in service and during the deck’s launching are performed. This study concludes that: a) the effects of shear lag and stiffened plate buckling due to direct stresses are decisive for the design of the bottom flange of the deck, b) the effects of plate buckling due to shear and the local verifications for Patch Loading during the incremental launching govern the web design, and finally that c) the effect of the deck cross section torsional distortion is significantly reduced when diaphragms are used throughout the span. Keywords: Bridge, Composite steel-concrete deck, Box girder deck, shear lag, Plate buckling, Patch Loading, Distortion, Incremental launching. 1. Introduction Bridges with composite steel-concrete box girder deck have been adopted in numerous cases. This type of solution has the benefit of having great resistance relatively to its own weight, resulting from the optimization of steel and concrete properties. Often, the most suitable construction process for this type of deck is the incremental launching method, usually being the steel structure launched in the first place and then the slab concreted with the formwork directly supported on the steel structure. The main goals of this dissertation are: (1) to study the specific aspects of composite steel-concrete box girder deck’s design, namely the phenomena of shear lag, stiffened plate buckling due to direct stresses, web buckling due to shear, web resistance to Patch Loading and evaluate the cross section distortion of a deck under eccentric loads; (2) to present the verification methodologies of the composite steel-concrete deck according to European Eurocodes Standards (EC); and finally (3) to apply this methodologies to a project example on a preliminary level. 2. Deck Description The case study is a road bridge with a typical span of 63 m (Figure 1) and two symmetrical decks 10,5 m wide with a unidirectional slope of 2,5%. The road profile over the deck is shown in Figure 2.
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Study of Bridges with Composite Steel-Concrete Box Girder Decks
Manuel Maria Teixeira d’Aguiar Norton Brandão
Civil Engineering Department, Instituto Superior Técnico, Universidade de Lisboa, Portugal
Abstract
This dissertation studies the specific phenomena associated with the analysis and design of composite box girder decks, namely
under service conditions and during the incremental launching stages of construction.
Primarily, the phenomena of shear lag, stiffened plate buckling due to direct stresses, plate buckling due to shear, resistance to
Patch Loading and distortion of box girder decks’ cross section are identified and studied, and their analysis and design
methodologies defined in the Eurocodes are presented.
These methodologies are then applied through a design case of a bridge with a composite box girder deck with typical spans of
63 m. For this example the structural analysis, the verification of the deck’s slab, the global verification of the deck both in service
and during the deck’s launching are performed.
This study concludes that: a) the effects of shear lag and stiffened plate buckling due to direct stresses are decisive for the design of
the bottom flange of the deck, b) the effects of plate buckling due to shear and the local verifications for Patch Loading during the
incremental launching govern the web design, and finally that c) the effect of the deck cross section torsional distortion is
significantly reduced when diaphragms are used throughout the span.
This work resulted in a series of conclusions that are summarized below:
The shear lag effect has great influence in the design of the compressed plate, particularly in the evaluation of
in service stresses. In ULS, the reduction of effective area of the plate is very low and doesn’t need to be taken
in account in structural analysis , which is the most common case in the design of this type of deck;
Stiffened plate buckling due to direct stresses has great influence in the evaluation of the resistance of the
box bottom in ULS and, like the shear lag effect, it was not taken in account in structural analysis;
Web buckling due to shear has influence in the design for ULS. The Patch Loading effect also has great
influence in web design when the incremental launching method is adopted;
At first the web thickness was chosen to be 40 mm at support sections and 20 mm at span sections. After the
design for web buckling and Patch Loading effect, it was observed that the web thickness was oversized at
support sections and undersized at span sections. Tough less significantly, web thickness also influences the
SLS verifications. Choosing the most appropriate web thickness is of great importance in steel and composite
steel-concrete bridges for there is a great amount of steel in such plates. Therefore, the proposed values for
the new web thickness are 25 mm for support sections and 22 mm for span sections;
The cross section distortion due to eccentric loads such as the road loads established by EC1-2 causes an
increase of direct stresses that must be taken into account in the design of this type of decks, namely when
calculating the in service stresses. When diaphragms are placed along the span, these stresses are significantly
reduced;
During incremental launching of the steel structure it was necessary to use, as usual, a front nose to ensure
that the critical steel fibers did not reach the yield stress limit.
References
[1] SÉTRA – Steel-Concrete Composite Bridges Sustainable Design Guide – Ministère de l’Écologie, de l’Énergie, du Développement durable et de la Mer, May 2010.
[2] Silva, L. S. & Gervásio, H. – Manual de Dimensionamento de Estruturas Metálicas: Métodos Avançados – CMM, Fevereiro de 2007.
[3] Beg, D., Kuhlmann, U., Davaine, L. & Braun, B. – Design of Plated Structures – ECCS, 2010.
[4] Virtuoso, F. – Dimensionamento de Estruturas: Vigas de Alma Cheia – IST, Maio de 2009.
[5] Gozzi, J. – Patch Loading Resistance of Plated Girders: Ultimate and serviceability limit state – Luleå University of Technology. Luleå , Suécia, Junho de 2007.
[6] Pedro, J. – Distorção em tabuleiros de pontes em caixão – IST, Fevereiro de 1995.
[7] Eurocódigo 0 – Bases para o projecto de estruturas – EN 1990. CEN. Bruxelas, 2008.
[8] Eurocódigo 2 – Projecto de estruturas de betão – Parte 1-1: Regras gerais e regras para edifícios – EN 1992-1-1. CEN. Bruxelas, 2004.