Simply Supported and Continuous Bridges

“ SIMPLY SUPPORTED AND CONTINUOUS BRIDGES”

TALHA BIN MEHMOOD

SEAT NO: CE-013/2011-12

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 hinge or roller at the other end then such bridges are called

simply supported bridges

KEY POINTS 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 with beam layouts that do not lend themselves to continuity, such as varying beam spacing or splayed framing

KEY POINTS (CONTINUE)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.

KEY POINTS 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 not take place.

KEY POINTS (CONTINUE)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 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 is available and span lengths are greater

BEHAVIOR

(SINGLE SPAN OR CONTINUOUS BRIDGES) AASHTO DESIGN REQUIREMENTS

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 two-thirds (2/3) the slab bearing width but not more than the clear span plus the slab thickness.

DESIGN REQUIREMENTS(CONTINUE)

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 mid-span points shall be detailed as well as any additional points required to meet a maximum spacing between points of 30'-0" [10 m].

DESIGN REQUIREMENTS(CONTINUE)

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

Details for multi-span reinforced concrete slab bridge superstructures are provided 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 SPAN BRIDGES

For continuous Span bridges, the minimum analysisrequirements shall be as specified in Table I in which;

* no seismic analysis requiredUL uniform load elastic methodSM single-mode elastic methodMM multimode elastic methodTH time history method

CONTINUOUS SPAN BRIDGES 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-to-support, abutments excluded.

CONTINUOUS SPAN BRIDGES

ADVANTAGES OF CONTINUOUS BRIDGES OVER SIMPLY SUPPORTED BRIDGES

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, 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

ADVANTAGES (CONTINUE)

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.

ADVANTAGES (CONTINUE)FFFF

ADVANTAGES (CONTINUE)

The depth of decking at mid span is reduced.The expansion joints required will be less. 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 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.

DISADVANTAGES (CONTINUE)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 SUPPORTED BRIDGES IN THE WORLD

Gotteron Bridge Fribourg, Switzerland

Madison Bridge, Ohio River

Missouri River Bridge, Chamberlain

CONTINUOUS BRIDGES IN THE WORLD

Continuous concrete highway bridge, Kentucky unites states

Continuous steel plate girder bridge, Switzerland

Main River Bridge,frankfurt Germany

REFRENCESwww.google.comHighway bridge structuresHighway engineering hand bookAASHTO LRFD Design Specifications 2007

THANK YOU !