Simply Supported and Continuous Bridges

(1)

“ SIMPLY SUPPORTED AND

CONTINUOUS BRIDGES”



T

ALHA

BIN

MEHMOOD



SEAT NO: CE-013/2011-12



(2)

SIMPLY SUPPORTED BRIDGES

When

When the

the main

main girders

girders of

of a

a

bridge are supported by a

movable hinge at one end and a

fix

fixed

ed hinge

hinge or

or roller

roller at

at the

the other

other

end then such

end then such bridges are called

bridges are called

simply supported bridges

(3)

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

geometrie

geometries with

s with beam lay

beam layouts that do

outs that do not

not

lend themselves to continuity, such as

varying beam spacing or splayed framing

(4)

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

addition of one or more spans.

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.

(5)

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.

.

(6)

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 d

(7)

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

ultimatel

ultimately results in

y results in the economic section

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

when a sound founda

foundation is

tion is avail

available and

able and

span lengths are greater

(8)

BEHAVIOR

(9)

(SINGLE

(SINGLE SP

SP

AN OR

AN OR CONTINUOUS

CONTINUOUS

BRIDGES) AASHTO DESIGN

REQUIREMENTS



 Continuous reinforced concrete slab bridgeContinuous reinforced concrete slab bridge

design shall be in conformance with AASHTO, design shall be in conformance with AASHTO, latest edition.

latest edition.



 For simple span reinforced concrete slabFor simple span reinforced concrete slab

bridges cast in place directly on concrete bridges cast in place directly on concrete

substructures, the effective span length shall be substructures, the effective span length shall be considered equal to the clear span plus

considered equal to the clear span plus

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

(10)

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

crown lines, and phased construction lines

phased construction lines

for the full length of the bridge.



Bearing points, quarter-span points and mid-

Bearing points, quarter-span points and

mid-span points shall be detailed as well as any

span points shall be detailed as well as any

additional points required to meet a

maximum spacing between points of

30'-0" [10 m].

(11)

DESIGN REQUIREMENTS(CONTINUE)



Details for simple span reinforced

concrete slab bridge superstructures are

provided in Standard

provided in Standard Bridge Drawing

Bridge Drawing

SB-6-94.



Details for multi-span

Details for multi-span reinfor

reinforced concrete

ced concrete

slab bri

slab bridge superstructur

dge superstructures are p

es are provided

rovided

in Standard Bridge Drawing CS-1-93

(12)

AASHTO SEISMIC ANALYSIS

SINGLE SPAN BRIDGES

Seismic analysis is not required for

Single span bridges, regardless of

seismic zone.

(13)

CONTINUOU

S S

P

AN

BRIDGES

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

* no no seismic seismic analysis analysis requiredrequired UL

UL uniform uniform load load elastic elastic methodmethod SM

SM single-mode single-mode elastic elastic methodmethod MM

MM multimode multimode elastic elastic methodmethod TH

(14)

CONTINUOUS

SP

AN

BRIDGES



 According to AASHTO, The selection of theAccording to AASHTO, The selection of the

method

method of of analysis analysis depends on depends on Seismic Seismic zone,zone, regularity, and importance of the bridge.

regularity, and importance of the bridge.



 Regularity is a function of the number of spansRegularity is a function of the number of spans

and the distribution of weight and stiffness. and the distribution of weight and stiffness.



 Regular bridges have less than seven spans; noRegular bridges have less than seven spans; no

abrupt or unusual changes in weight, stiffness, abrupt or unusual changes in weight, stiffness, or geometry; and no large changes in these

or geometry; and no large changes in these parameters from span to span or parameters from span to span or support-to-support, abutments excluded.

(15)

CONTINUOUS SPAN BRIDGES

(16)

ADV

ANT

AGES

OF

CONTI

NUOUS

BRIDGES OVER SIMPLY SUPPORTED

BRIDGES



 Where possible, bridges should be madeWhere possible, bridges should be made

continuous. Continuous spans are less

continuous. Continuous spans are less prone toprone to catastrophic collapse from loss of substructure catastrophic collapse from loss of substructure support due to

support due to stream erosion, earthquakstream erosion, earthquake, ore, or vehicle or vessel collision

vehicle or vessel collision



 Aesthetically, continuous bridges are generallyAesthetically, continuous bridges are generally

superior, especially if constant depth is superior, especially if constant depth is

maintained, and do not require the cosmetic maintained, and do not require the cosmetic

plates or other devices that have sometimes been plates or other devices that have sometimes been used to conceal the gaps between simple spans used to conceal the gaps between simple spans

(17)

ADVANTAGES (CONTINUE)



 As the bearings are placed on the centerline of As the bearings are placed on the centerline of

piers, the reactions at piers are transmitted piers, the reactions at piers are transmitted centrally.

centrally.



 It is found that the continuous girder bridgeIt is found that the continuous girder bridge

suffers less vibration and deflection. suffers less vibration and deflection.



 The continuous girder bridge requires only oneThe continuous girder bridge requires only one

bearing at each pier as against two bearing for bearing at each pier as against two bearing for simply supported girder bridge.

(18)

ADVANTAGES (CONTINUE)

FFFF FFFF

(19)

ADVANTAGES (CONTINUE)



 The depth of decking at mid span is reduced.The depth of decking at mid span is reduced. 

 The expansion joints required will be less.The expansion joints required will be less. 

 There is reduction in cost as less quantity of There is reduction in cost as less quantity of

concrete and steel are required because of the concrete and steel are required because of the reduction of mid-span moments

(20)

DISADVANTAGES OF CONTINUOUS

BRIDGES OVER SIMPLY SUPPORTED

BRIDGES



 For a For a composite bridge, composite bridge, in rin regions of egions of negativenegative moment the deck is considered to be cracked due moment the deck is considered to be cracked due to the tensile forces at the top of the section),

to the tensile forces at the top of the section),

however, that the reinforcing steel in the concrete however, that the reinforcing steel in the concrete slab can actually be taken as part of the composite slab can actually be taken as part of the composite section. This means that the

section. This means that the tensile properties of tensile properties of the reinforcing steel can assist in resisting the

the reinforcing steel can assist in resisting the tension which exists in the top flange of the tension which exists in the top flange of the composite section. This is one of the

composite section. This is one of the challengeschallenges faced by the designer due

(21)

DISADVANTAGES (CONTINUE)



 Continuous bridges, however, incur greaterContinuous bridges, however, incur greater

fabrication costs than their simply supported fabrication costs than their simply supported

counterparts, where a simply supported span can be counterparts, where a simply supported span can be designed and erected relatively quickly.

designed and erected relatively quickly.



 The design of continuous spans is more involved as itThe design of continuous spans is more involved as it

is statically indeterminate structure and their is statically indeterminate structure and their

construction impacted by the need for customized construction impacted by the need for customized plate girders, splices, etc.

plate girders, splices, etc.



 The detailing and placing of reinforcements are to beThe detailing and placing of reinforcements are to be

carried out with extreme care. carried out with extreme care.



 The placing of concrete and removal of formwork areThe placing of concrete and removal of formwork are

to be executed carefully in proper sequence. to be executed carefully in proper sequence.



 Continuous span bridges are also more susceptible toContinuous span bridges are also more susceptible to

settlement problems than simply supported bridges settlement problems than simply supported bridges