Simply Supported and Continuous Bridges

“ SIMPLY SUPPORTED AND CONTINUOUS BRIDGES”

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TALHA BIN MEHMOOD

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SEAT NO: CE-013/2011-12

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CE-512 (BRIDGE ANALYSIS AND DESIGN)

SIMPLY SUPPORTED BRIDGES When the main girders of a bridge are supported by a movable hinge at one end and a fixed fixed hinge or roller at the other end then such bridges are called simply supported bridges

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Generally width of bridge is divided into number of individual spans. For each span, the load carrying member is simply supported at both ends.. Simply supported bridges should be provided where adjacent spans are unavoidably different in length and depth, or where adjacent spans have widely different geometries geometries with beam layouts layouts that do not lend themselves to continuity, such as varying beam spacing or splayed framing

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Simply supported bridges may also be preferable where the bridge is part of a facility, such as an interchange, where stage construction will require future removal or addition of one or more spans. They are suitable at places where uneven settlements of foundations are likely to take place They are generally best suited for short crossings and where speed of construction is an issue.

CONTINUOUS BRIDGES The bridge supported at three or more points and capable of  resisting bending and shearing forces at all sections throughout its length. length.

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In continuous bridges spans are continuous over two or more supports. They are statically indeterminate structures.

They are useful when uneven settlement of supports does does not take place.

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In continuous bridges the bending moment anywhere in the span is considerably less than that in case of simply supported span. Such reduction of bending moment ultimately ultimately results in the economic section for the bridge. In continuous bridges the stresses are reduced due to negative moments developed at pier or supports. Continuous bridges are typically favored when a sound foundation foundation is available available and span lengths are greater

BEHAVIOR

(SINGLE SP SPAN AN OR CONTINUOUS BRIDGES) AASHTO DESIGN REQUIREMENTS 

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Continuous reinforced concrete slab bridge design shall be in conformance with AASHTO, latest edition. For simple span reinforced concrete slab bridges cast in place directly on concrete substructures, the effective span length shall be considered equal to the clear span plus twothirds (2/3) the slab bearing width but not more than the clear span plus the slab thickness.

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The Designer shall include a final deck surface elevation table. Elevations shall be shown for all profile grade lines, curb lines, crown lines, and phased construction lines for the full length of the bridge. Bearing points, quarter-span points and midspan points shall be detailed as well as any additional points required to meet a maximum spacing between points of  30'-0" [10 m].

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Details for simple span reinforced concrete slab bridge superstructures are provided in Standard Bridge Drawing SB-6-94.

Details for multi-span reinforced reinforced concrete slab bridge bridge superstructures superstructures are provided p rovided in Standard Bridge Drawing CS-1-93

AASHTO SEISMIC ANALYSIS SINGLE SPAN BRIDGES

Seismic analysis is not required for Single span bridges, regardless of  seismic zone.

CONTINUOUS CONTINUOU S SP SPAN BRIDGES For continuous Span bridges, the minimum analysis requirements shall be as specified in Table I in which; * UL SM MM TH

no seismic analysis required uniform load elastic method single-mode elastic method multimode elastic method time history method

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According to AASHTO, The selection of the method of analysis depends on Seismic zone, regularity, and importance of the bridge. Regularity is a function of the number of spans and the distribution of weight and stiffness. Regular bridges have less than seven spans; no abrupt or unusual changes in weight, stiffness, or geometry; and no large changes in these parameters from span to span or support-tosupport, abutments excluded.

CONTINUOUS SPAN BRIDGES

ADVANTAGES ADVANT AGES OF CONTI CONTINUOUS NUOUS BRIDGES OVER SIMPLY SUPPORTED BRIDGES 

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Where possible, bridges should be made continuous. Continuous spans are less prone to catastrophic collapse from loss of substructure support due to stream erosion, earthquake, earthquake, or vehicle or vessel collision Aesthetically, continuous bridges are generally superior, especially if constant depth is maintained, and do not require the cosmetic plates or other devices that have sometimes been used to conceal the gaps between simple spans

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As the bearings are placed on the centerline of  piers, the reactions at piers are transmitted centrally. It is found that the continuous girder bridge suffers less vibration and deflection. The continuous girder bridge requires only one bearing at each pier as against two bearing for simply supported girder bridge.

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The depth of decking at mid span is reduced.

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The expansion joints required will be less.

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There is reduction in cost as less quantity of  concrete and steel are required because of the reduction of mid-span moments

DISADVANTAGES OF CONTINUOUS BRIDGES OVER SIMPLY SUPPORTED BRIDGES 

For a composite bridge, in regions regions of negative moment the deck is considered to be cracked due to the tensile forces at the top of the section), however, that the reinforcing steel in the concrete slab can actually be taken as part of the composite section. This means that the tensile properties of  the reinforcing steel can assist in resisting the tension which exists in the top flange of the composite section. This is one of the challenges faced by the designer due to continuous bridges.

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Continuous bridges, however, incur greater fabrication costs than their simply supported counterparts, where a simply supported span can be designed and erected relatively quickly. The design of continuous spans is more involved as it is statically indeterminate structure and their construction impacted by the need for customized plate girders, splices, etc. The detailing and placing of reinforcements are to be carried out with extreme care. The placing of concrete and removal of formwork are to be executed carefully in proper sequence. Continuous span bridges are also more susceptible to settlement problems than simply supported bridges

SIMPLY SIMPL Y SUPP SUPPORTED ORTED BRIDGES IN THE WORLD

Gotteron Bridge Fribourg, Switzerland

Madison Bridge, Ohio River

Missouri River Bridge, Chamberlain

CONTINUOUS BRIDGES IN THE WORLD

Continuous concrete highway highway bridge, Kentucky unites states

Continuous steel plate girder bridge, Switzerland

Main River Bridge,frankfurt Germany

REFRENCES 

www.google.com

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Highway bridge structures

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Highway engineering hand book

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AASHTO LRFD Design Specifications 2007

THANK YOU !