Ultimate Flexural Strength Of Composite Steel-Concrete .

Ultimate Flexural Strength of Composite Steel-Concrete DecksConsidering Construction StagesGeraldo António Lopes Costa*Instituto Superior Técnico, TULisbon, PortugalAbstractThis paper focuses on the study of the rupture in plastic flexure of composite decks, simulating construction stages. Sincethe cross-section may consist of several types of materials with different mechanical properties, the analysis in time throughthe consideration of a constructive procedure leads to a redistribution of stresses that may worsen the structural system. Inthis context, an assessment is made on the design of composite bridges, identifying the key features and structural solutions.Likewise, it will be described the most usual erection methods used in bridge construction. In order to establish the influenceof constructive phases, the stress-strain relations for structural analysis that were used on the developed models are exposed,to prove the suitability and validity determining the plastic hinge mechanism of composite bridges. In addition to commonfeatures, there is the modeling of two composite decks in the automatic calculation program SAP2000 , which allows theintroduction of FE with different stress-strain relations at specific timings. The specific behavior of each material is studiedseparately, in order to identify the occurrence of yield and rupture on each one. Therefore, the modeling adopted can coverrange of use that is not restricted to this work. Results indicate that the consideration of construction stages does not influencethe flexural rupture of composite decks. Nonetheless, some general grounds are set out for decks that don’t present a plasticbehavior.Keywords: composite bridges; construction stages; utlimate flexural strength; composite steel-concrete decks; pushover analysis.1. Introduction2. Composite Steel-Concrete DecksBridge projects are intrinsically linked to the concept ofwhat is designed. The design phase is essential in thedevelopment of an optimized and suitable bridge to itslocation. During this initial stage, without making any typeof complex analysis, it is necessary to assess the constraintsimposed by the bridge site and define the most appropriatestructural solution, but also ponder the economic andaesthetic factors.The general design is usually based on rules ofpreliminary sizing. After outlining an initial design plan,more sophisticated models should be developed to analyzeand verify specific problems and security aspects of thestructure.Composite bridges’ technology has had significantdevelopments since the first solutions were constructed inthe 1950s. These have evolved from simple structurescomposed of parallel girders over small spans, to longcable-stayed bridges.The appropriate mix of key building materials, such asstructural steel and reinforced concrete or prestressedconcrete, allows the construction of more efficientstructures, compared to the application of these materialsindividually. The benefits of using composite solutions aresignificant, particularly when it comes to short constructiondeadlines, quality and durability assurance. The use of steelgirders allows the concreting in situ of reinforced concreteslabs, speeding construction planning,———**Correspondinge-mail: geraldo costa@hotmail.comauthor. Tel.: 0-000-000-0000 ; fax: 0-000-000-0000 ; e-mail: author@institute.xxx .1

even when it is at stake complex geometries as curves withsmall radius or cross-sections that vary widely.Composite solutions for main decks, compared tosolutions completely made of steel, leads the decks withmore inertia and resistance to compression, because of theconcrete slab, which reduces the amount of steel needed,mainly due to decrease of bracing systems. In comparisonto structures made only of reinforced concrete orprestressed concrete, composite decks allows the increasein slenderness, as well as the reduction of the bridge selfweight.should be considered, since it can have a significantinfluence on main girders design. Currently, the threetypical methods that can be applied or combined to erectcomposite decks are: Assembly in situ; Cantilevering; LaunchingThe choice of a particular constructive method resultsfrom the economical analysis of many factors. Basically, itis associated with the constraints imposed by the particularlocation of the bridge, available equipment andconstruction deadlines.Assembly in situ span after span involves constructing abridge from its individual components in its final position,usually over formwork or some other form of temporarysupport. This may temporarily block a road, railway orriver over which a bridge is built.Cantilevering involves assembling a bridge, usuallycontinuous over several spans, progressively from one orboth abutments, by attaching portion of sections to the endof the ones already erected. The position of the site and theaccess to it will determine the size of the pieces erectedtrough lifting from ground level or running along the deck.Launching involves building a bridge on rollers on itsfinal alignment, but at the side of the river, road or railwayto be crossed. When complete it is pushed or pulledforward to cross the obstacle and land on bearings on thefar side.After the assembly of the steel structure through one ofthe methods described, it supplies the basic platform that isneeded for the concreting in situ of the reinforced concreteslab. As soon as the concrete begins to dry, the compositesection begins to function for all future loads, eliminatingthe possibility of local instability of the top steel flanges.Furthermore, adequate bracing must be provided atabutments and piers near the bottom flanges, to increase thestiffness of the cross-section, in order to preventdistortional buckling of the decks. At mid span regionssubject to positive moments, the bottom flanges should berestraint, now against lateral-torsional buckling rather thandistortional buckling.2.1. Preliminary Sizing of Composite DecksSolutions with continuous main decks are normal, ratherthan the use of simply supported girders on bridges. Thisimproves the structural efficiency and is generally moreeconomical, taking into account that there is no need forexpansion joints within the deck, which reduces the longterm costs associated with maintenance. In Fig.1 areindicated some criteria for preliminary sizing of continuousbridges.Fig.1. Typical slenderness for road bridges [1]It should be noted that these indications are for roadbridges. As for railway bridges, the slenderness of the maindeck should be reduced, taking into account the higherdemands and requirements of less deformability.3. Structural Analysis2.2. Construction MethodsThe global method of analysis adopted in this study isthe .elastic-plastic analysis, .taking .into .account .the .non-linear material properties, which involves considering for(break)At the design state of a bridge, or even at the time ofconceptual choice, the method by which it will be erected2

all the materials constitutive relations with both elastic andplastic stages. Thus, the principle of superposition ofeffects does not apply, so the software SAP2000 it usedfor the simulation of construction stages and incrementalloading, until it s reach the ultimate flexural strength ofcomposite steel-concrete structures. In parallel, ananalytical study was made on plastic mechanisms of simpledecks. The plastic bending moments for each cross-sectionthat belongs to a deck was determined on an iterativeprogrammed worksheet, in order to verify the stresses anddeformations of each material at every increase in loading.It should be noted that the study of decks using elasticplastic global analysis is mentioned on Eurocode.4(EN1994-1-1), but with several restrictions. Moreimportantly, it is recommended that regions where plastichinges are formed must be class 1 sections, to ensure thenecessary rotation capacity that enables the progression ofloading up to the rupture point.The elements which constitute a co

effects does not apply, so the software SAP2000 it used for the simulation of construction stages and incremental loading, until it s reach the ultimate flexural strength of composite steel-concrete structures. In parallel, an analytical study was made on plastic mechanisms of simple decks.