CHAPTER 8 Prestressed Concrete Bridges

7/27/2019 CHAPTER 8 Prestressed Concrete Bridges

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CHAPTER 8a: PRESTRESSED CONCRETE BRIDGES

Prestressed Concrete

- is a combination of high strength concrete and steel strands. This combination

makes a very strong structural material that is used in the building of roof slabs, bridge

girders and railroad ties.

- it can be used to produce beams, floors or bridges with a longer span than ispractical with ordinary reinforced concrete.

Prestressing can be accomplished in three ways:* Pre-tensioned concrete

* Bonded post-tensioned concrete

* Unbonded post-tensioned concrete

Pre-tensioned concrete - is cast around already tensioned tendons. This method

produces a good bond between the tendon and concrete, which both protects the tendon

from corrosion and allows for direct transfer of tension. The cured concrete adheres and

bonds to the bars and when the tension is released it is transferred to the concrete ascompression by static friction.

Bonded post-tensioned concrete - is cast around a plastic, steel or aluminium curved

duct, to follow the area where otherwise tension would occur in the concrete element. A

set of tendons are fished through the duct and the concrete is poured. Once the concrete

has hardened, the tendons are tensioned by hydraulic jacks that react against the concrete

member itself. When the tendons have stretched sufficiently, according to the design

specifications, they are wedged in position and maintain tension after the jacks are

removed, transferring pressure to the concrete.

Unbonded post-tensioned concrete - differs from bonded post-tensioning by providing

each individual cable permanent freedom of movement relative to the concrete. To

achieve this, each individual tendon is coated with grease (generally lithium based) and

covered by a plastic sheathing formed in an extrusion process.

P. H. Jackson patented the concept of prestressing in 1886 and used it for tightening

concrete blocks and concrete arches to serve as floor slabs.

C. E. W. Doehringobtained a patent for prestressing concrete slabs with metal wires.

However, these early attempts were unsuccessful, because the prestressing was lost

through shrinkage and creep of concrete.E. Fressynet a French engineer who successfully develop the modern concept of

prestressed concrete. In 1927, he demonstrated the usefulness of prestressing using high-

strength steel to control prestress losses.

R. E. Dill introduced the practice of prestressing in United States for producing

concrete planks and fence posts.


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Luzancy Bridge(1941-1946) 180-ft, segmentally constructed, two-hinged, portal

frame bridge of arch form over river Marne at Luzancy, France.

Walnut Lane Memorial Bridge(1951)the first major prestressed concrete bridge, the

three span (74, 160, and 74-ft), cast-in-place, post-tensioned bridge in Philadelphia,

Pennsylvania.

Terminology

Anchorage seating deformation of the anchorage, or seating of tendons in the

anchorage device, that takes place when prestressing force is transferred from the jack to

the anchorage device.

Bonded tendona prestressing tendon that is bonded to the concrete, either directly or

through grouting.

Coating material used to protect prestressing tendons against corrosion, to reducefriction between tendon and duct, or to debond prestressing tendons.

Couples (coupl ings) the means by which prestressing force is transmitted from one

partial length prestressing tendon to another.

Creep of concretetime-dependent deformation of concrete under sustained load.

Curvature fr ictionfriction resulting from bends or curves in the specified prestressing

tendon profile.

Debonding (blanketing) wrapping, sheathing, or coating a prestressing strand to

prevent bond between the strand and surrounding concrete.

Ducta hole or void formed in the prestress member to accommodate a tendon for post-

tensioning.

Ef fective stress stress remaining in concrete due to prestressing after all calculatedlosses have been deducted, excluding effects of superimposed loads and weight of the

member; the stress remaining in prestressing tendons after all losses have occurred,

excluding effects of dead load and superimposed load.

Elastic shortening of concreteshortening of a member caused by application of forces

induced by prestressing.

End anchoragea length of reinforcement, mechanical anchor, hook, or combination

thereof, beyond the point of zero stress in reinforcement; a mechanical deviceto transmit

prestressing force to concrete in a post-tensioned member.

End block an enlarged end section of a member, designed to reduce anchorage

stresses.

Fri ction (post-tensioning)surface resistance between the tendon and its duct in contactduring stressing.


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Grout opening, or venttemporary force exerted by the device that introduces tension

into prestressing tendons.

Post-tensioninga method of prestressing in which tendons are tensioned after concrete

has hardened.

Precompressed zone the portion of flexural member cross section that is compressed

by prestressing force.

Prestr ess, loss of reduction in prestressing force resulting from combined effects ofstrains in concrete and steel, including the effects of elastic shortening; creep and

shrinkage of concrete; relaxation of steel stress; and, for post-tensioned members,

friction and anchorage setting.

Prestressed concretereinforced concrete in which internal stresses have introduced to

reduce potential tensile stresses in concrete resulting from loads.

Pretensioninga method of prestressing in which tendons are tensioned before concrete

is placed.

Shear lagnon-uniform distribution of bending stress over the cross section.

Shr inkage of concretetime-dependent deformation of concrete caused by drying and

chemical changes (hydration process).

Tendon wire, strand, bar, or bundle of such elements, used to impart prestress toconcrete.

Tendon stress, relaxation of time-dependent reduction of stress in a prestressing

tendon at constant strain.

Transferact of transferring stress in prestressing tendons from jack or pretensioning

bed to concrete member.

Transfer lengththe length over which prestressing force is transferred to concrete by

bond in pretensioned members.

Wobble fr iction friction caused by unintended deviation of a prestressing sheath or

duct from its specified profile or alignment.

Wrapping, or sheathing the enclosure around a prestressing tendon to prevent

temporary or permanent bond between a prestressing tendon and surrounding concrete.

Materials of construction

The three main materials used in construction of prestressed concrete girders:

a.) concrete

b.) reinforcing bars

c.) prestressing steel

High-Strength Concrete

- it is a type of high performance concrete generally with a specified compressivestrength of 6000 psi(40 MPa) or greater.

- some defined it as concrete having a 28-day of compressive strength of 8000 psi

or more.

*Concretes having compressive strengths higher than 10,000 psi are sometimes referred

to as ultra-high-strengthconcretes. Use ofmicrosilica(also known as silica fume or

condensed silica fume), very-high-quality aggregate, and extremely low water-cement

ratios (less than 0.3) using high-range water reducers (known as superplasticizers) have

made it easy to produce over 10,000 psi concretes.

Advantages high-strength concrete:

improved behavior under overload or partial-prestressing conditions.


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the reduced porosity and permeability of high-strength concrete enhancedurability.

increased compression and flexural capabilities. increased span capabilities of high-strength concrete girders.

Disadvantages of using high-strength concrete:

increased quality control is needed in order to maintain the specialproperties desired.

careful materials selection is necessary. low water to cementitious materials ratios require special curing

requirements.

Structural Lightweight Concrete

- technically referred to as structural li ghtweight-aggregate concrete.

- concrete having a 28-day compressive strength in excess of 2500 psi and a 28-

day air-dried unit weight not exceeding 115 lb/ft3.

Advantages of using lightweight concrete:

its lightness make it ideally suited for bridge superstructures. lightweight concrete is used to reduced deadweight of the superstructure

in cases where normal-weight concrete is to heavy from a practical

standpoint.

useful for multilevel interchange structures, where minimum structuredepths are required and locations for columns are limited.

the reduced deadweight of concrete translates into reduced reinforcingand prestressing steel in the superstructure and reduced reinforcing and

concrete in substructure.

the reduced mass of the superstructure made from lightweight concrete(which typically can be 25 to 30 percent lighter than its normal-weightconcrete counterpart) permits longer spans and deeper sections while

maintaining the same dead load and an increased live load capacity.


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the reduced mass of the superstructure can help minimize earthquake-induced forces.

it makes it economical to transport sizeable precast sections; reduces theneed for extensive falsework; speeds erection; and allows for use of

smaller, lighter, and more economical equipment.

Disadvantage of using lightweight concrete:

it has a lower modulus of elasticity compared to normal-weight concrete.Because of its lower modulus of elasticity, a lightweight concrete member

can produce more than twice the amount of deflection of a normal-weight

concrete member for a given load, consequently requiring a higher

amount of prestressing.

BeniciaMartinez Segmental Bridge

the largest lightweight concrete segmental bridge in California (1.2-miles). The 1962

bridge consists of seven 528-foot (161 m) spans which provide 138 feet (42 m) of

vertical clearance, carrying four lanes of traffic in the southbound direction, as well as a

pathway for pedestrians and bicyclists.

Prestressing Steel

- made from high-tensile steel in form of cables or rods.

- the in. diameter 270-k strand is the most commonly used prestressing

reinforcement for bridge girders, whereas deformed bars are used for stirrups and non-

prestressed steel.

Corrosion of prestressing steels

A serious factor affecting durability of prestressed concrete member is corrosionassociated with prestressing steels. Corrosion is the deterioration of a metal by chemical

or electrochemical reaction with its environment.


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Reinforcing corrosion and concrete deterioration are believed to be initiated by

the penetration of chlorides, moisture, and oxygen.

Corrosion of prestressing steels in prestressed concrete structures can be much

more serious than corrosion if reinforcing steel in conventional reinforced concrete

structures because the prestressing strands have relatively smaller cross-sectional area

under very high stress.

Grouting

The purpose of grouting is to provide permanent protection to the post-tensioning

steel and to develop bond between prestressing steel and the surrounding concrete.

In post-tensioned bridges, the tendons are placed inside flexible, galvanize,

corrugated ferrous-metal ducts and grouted with neat cement grout (a suspension of

water and cement with a water-cement ratio of 0.45 or less) with or without admixtures.

Improper grouting practices and high chloride content in the grout are believed to

be serious sources of corrosion that can trigger a collapse without warning.

The grout itself was found to be highly contaminated with chlorides, up to 8000parts per million. For bridges, AASHTO 10.3.4.3 [AASHTO, 1992] limits chlorides in

admixtures to 0.005 percent, or 50 ppm, and requires that water in grout be potable,

clean, and free of injurious quantities of substances known to be harmful to portland

cement and prestressing steel. As a practical matter, total chloride content in grout should

be limited to 100 ppm.

Ynysygwas Bridge

A 60-ft-long, simpy supported, segmental bridge, built in 1953 in Great Britain.

All nine of the Igirders (each containing of eight precast segmental sections) collapsedon December 4, 1985.

Azergues river bridge

A post-tensioned concrete structure built in 1962. The entire superstructure needs

to be replaced as a result of the serious corrosion of prestressing steel resulting from

chloride penetration.

A prereplacement inspection of this bridge in 1972, prompted by serious cracking

of the girders showed, that of the 144 tendons investigated, 16 were fully grouted, 38

were partially grouted, 80 were ungrouted, and 10 were neither stressed nor grouted.

Advantages of prestressed concrete:

1. Prestressed concrete products are usually produced in plants using high-strength

concrete under controlled conditions, resulting in higher quality products with longer life

expectancy.

2. Tension cracking can be eliminated in a prestressed structure, thereby minimizing the

penetration of water and air, leading to improved durability and enhanced service life of

concrete and reinforcement.


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3. Prestressing permits a more efficient use of concrete as a structural material, because

the entire section, not just the uncracked portion, is made to resist compression.

4. Prestressing reduces the diagonal tension. Use of inclined tendons reduces the shear

carried by the webs.

5. During prestressing, both concrete and steel are proof-loaded, ensuring safety underservice loads.

6. The smaller girder depths that are possible with prestressed concrete are advantageous

under the constraints of limited overhead clearance and free board (for bridges over

waterways).

7. Prestressing greatly reduces (practically eliminates) cracking due to fatigue.

8. When box girders are used, their shallow depths, slenderness, and uncluttered exterior

and underside appearance reflect good aesthetics.

9. Prestressed concrete bridges have relatively longer service life.

10. Cast-in-place post-tensioned construction is adaptable to large interchanges with

complex geometries involving curved, superelevated, skewed, multilevel sections and

sections of varying width.

Disadvantages of prestressed concrete:

1. A major disadvantage of prestressed concrete, compared to steel, is its own

deadweight. Dead load, more than live load, dominates in long-span bridges, resulting insupporting substructures that are heavier, and consequently uneconomical.

2. Prestressed concrete is more sensitive to quality of materials and workmanship.

3. Prestress losses, due to various sources such as creep and shrinkage of concrete or

relaxation of prestressing steel, are an important consideration, which a designer must

consider very carefully.

TYPES OF PRESTRESSED CONCRETE BRIDGES

Generally, the lengths of precast prestressed concrete girders are limited by the

constraints of transportation and handling systems, which dictate the maximum

segment size produced at the fabrication plant. Hauling girders from the casting

yard to the confines of cities is always a tricky problem. A trend toward longer

spans with single- length members has resulted in a need for deeper I- and Bulb T

beam sections.

In the US, some states limit the transportable lengths to about 130 ft.

In Japan the maximum transportable length of a precast concrete girder is limited

to just 71 ft. by transportation authorities.

Factors in selecting the type of prestressed concrete:1. Feasibility of construction2. Economics


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3. Product availability4. Time constraint5. Technical development6. Environment

During the early stages of development, different shapes and sizes of girders were

designed for each new bridge, but the popularity and frequent use of precast

prestressed girders led various states to standardize their own girder shapes.

Various Types of Prestressed Concrete Bridges

1) Solid Slab and Voided Slab Bridges 3 to 8 ft. wide and 10 to 18 in. deep Economical for short spans in the 30 ft. ranges because of their flexibility and

depth limitations.

Deeper slabs are made economical for slightly longer spans (20 to 55 ft.) byproviding longitudinal voids to reduce their deadweight.


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2) T-beam Bridges

Deck Bulb T Beam

4,6, and 8 ft. wide Single-T, Double- T, and Multiple-T sections span ranges 20 to 80 ft. Bulb- T series, developed by Concrete Technology Corporation for increased

span capabilities can span up to 100 ft. These girders are reported to have

withstood more than 5 million cycles of fatigue loading and satisfied all

serviceability requirements.

3) Prestressed Channel Girder Bridges


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Similar to Double- T beam section except for the overhanging flange. Less efficient than a Double- T section and hence uneconomical because of

reduction of concrete area in the compression zone of the section.

4) Box beam Bridges

3 to 4 ft. wide and spans from 60 to 100 ft. Two types of box beam girders The spread box beam bridge

Beams are placed at selected transverse spacing to support a cast-in-placedeck.

Adjacent box beam bridges


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The box beams are design contiguously to provide the desired bridgewidth resulting in a superstructure commonly referred to as a multibeam

deck.

Two advantages: (1) their shallow depths provide easy solutions whereonly limited superstructure depths are possible. (2) 3- and 4-ft-wide

sections can be combined to produce arbitrary deck widths.

Provides a ready-made deck that can be advantageously used as workingspace for other construction work, this elimination the need for costly

falsework.

5) I-beam bridges

AASHTO-PCI I-beam used by many states in the US. Several of the state usetheir own standard I- and box sections.

Thin webs are preferred because they reduce the deadweight of the girders,obviously resulting in increased flexural capacity for the live load. Thin webs,

however, may require extra care during transportation and handling to maintain

stability, and they may be too narrow to accommodate ducts for post-tensioningsteel.

6) Trapezoidal box and U-beam bridges


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Beams having the shape of an inverted channel section, and thus referred to as U-beams or trapezoidal box beams, are also feasible for short-span bridges they may

or may not have cantilevered top flanges extending beyond the webs on each

side.

They are used in Canada and England but have not been popular in the US. Generally referred to as multispine bridges, such bridges consist of precast

prestressed open cross sections and a cast-in-place concrete deck on top, essentialresembling spread box beam superstructures.

U-beam Bridges

In the US feasibility studies about U-beam superstructures had beenconducted and was concluded that this bridge system was uneconomical.

One of the advantages of the U-beam is its adaptability to horizontally curvedbridges: Its webs can be precast with different depths to accommodate thetransverse deck slope required for superelevation. Handling and erection are

consequently easier and thus more economical in terms of equipment and

construction costs.

Trapezoidal box Girders

Suitable for short and medium-span bridges. This system consists or precast prestressed units of standard widths (of top

flange, 6 and 8 ft.) and standard depths (30, 36, and 42 in.) to achieve a deck

of specified width in 20ft increments.

The structural efficiency of these T-box girders varies from 0.515 to 0.56. The stability of T-box girders during handling, transportation, and erection is

not a problem because of the high torsional rigidity. This system can be ideal


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for building short and medium span bridges in congested urban areas in high

seismic zones.

The advantage of T-box girders over U-beam girders is that T-box girdersenhance the durability of the precast deck, leading to savings in the life-cycle

costs of the superstructure.

An apparent drawback of this system is the increased self-weight of themember due to the integrally precast overhanging top flanges; this canincrease hauling and erection costs. This difficulty can be overcome by using

lightweight structural concrete. Another method is to use a drop-in segment

simply supported over the cantilevered end of the side or the end spans; this

method was used in building the Tlalpan Freeway bridges in Mexico City.

7) Box Girder Bridges

Box girders with single or multiple cell cross sections are used for medium andlong span bridges.

Two basic forms of construction are used for box girder bridges For simple and continuous spans, box girders are cast-in-place, often

integrally with the supporting pier shafts, and subsequently post-tensioned. For long spans, the segmental construction technique is used; the single-cell

section is the more common type.

A single cell box section can be used for deck widths of about 35ft. for widerdecks, multiple cell box girders are recommended.

The major advantage of the segmental construction technique is that it does notrequire costly and cumbersome falsework, and it avoids associated problems such

as interfering with existing traffic and creating detours. The method becomes

extremely efficient when precast units are used, resulting in reduced construction

time. Growing experience with segmental construction technique has led to its

adaptation for most new bridge sites. This technique has also made medium-span

bridges more economically feasible where single length girders are not practical

or where site conditions do not permit shoring and formwork.

POST-TENSIONED PRESTRESSED CONCRETE BRIDGES

*BOX GIRDER BRIDGES (CAST-IN-PLACE)

The cast-in-place, post-tensioned, prestressed concrete box girders are typically built on

falsework and are extensively used for medium-span and long-span bridges. Outwardly,

their appearance is similar to that of reinforced concrete box girders except that, in most

cases, the prestressed box girders would be relatively slender for the same span. Forlonger spans, prestressed girders would be the choice.


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*cross-sectional details for both RC and PC box girders are similar.

*RC box girders deck and soffit slab have considerable amount of conventional

longitudinal reinforcement

*PC box girders deck and soffit slab have a large number of prestressing tendons placed

in girder stems.

For Prestressed Concrete:

*strands of tendons are placed in ducts.

To accommodate these ducts, the webs (often referred to as girders or as stems) of

prestressed box girders are made wider than those of the reinforced concrete box girders,

although oval ducts can be used for thinner webs. The design of the deck is the same for

both RC and PC box girders, and the cost due to girder (web) spacing and deck

overhangs (the portion of the deck that extends beyond the exterior girders) is also the

same for all types.

After tensioning the Tendons:

*the ducts are grouted under pressure

The hardened grout serves two purposes: it protects the tendons from corrosion and

bonds them to the ducts to develop integral action with concrete.

Tendons in PC box girders may be internal, that is, embedded in the girder and

the soffit, or external. External tendons are placed in girder cells or even outside theprimary girders, and are not bonded to them. Placement of tendons outside the girders

results in two significant advantages: it permits girders to be thinner, which reduces the

deadweight of the box girder, and it allows tendons to be replaced if they are damaged or

deteriorated. However, the accompanying reduced ultimate load capacity is a

disadvantage. And providing proper protection from corrosion of external tendons is

always a matter of concern.

Internal Tendons

*both bonded and unbounded have some advantages and disadvantages: when subjected

to overloads, post-tensioned box girders with bonded tendons develop clearly spaced fine

cracks that disappear or close completely upon removal of the overload. But when

girders with unbounded tendons are overloaded, widely spaced large cracks appear that

do not close upon removal of overload. This problem can be alleviated by placing

reinforcement in girders with unbounded tendons to reduce the size and spacing of

cracks caused by overloading.

Design Considerations

Design parameters and the proportions of various components of post-tensioned concrete

box bridges, which have evolved from experience in California are discussed by


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Degenkolb (1997) and design manuals (PTI, 1978), and are specified in AASHTO 9.8.2

and 9.9 (AASHTO, 1992).

a) Depth-to-span ratio. The suggested depth-to-span ratios for preliminary design

are shown in the table below.

TYPE OF STRUCTURE DEPTH-SPAN RATIO

One and two-span structures 0.04-0.045

Multispan structures 0.035-0.04

Haunched structures at pier 0.048

Haunched structures at centerline span 0.024

b) Thickness of top and bottom slab of web (girder). Typically the top slab thickness

is kept as the greater of 6 in. or 1/30th of the clear distance between fillets or girders(AASHTO 9.9.1). The overhang is usually nonprismatic, where the minimum thickness

(at the free end) is the same as the top- slab thickness, and where the thickness uniformly

increases toward its junction with the outside girder(web). The bottom slab is kept as the

greater of 5 in. or 1/30th of the clear distance between webs or fillets (AASHTO 9.9.2).

However, the California requirements, which are more stringent than the AASHTOs,

require the minimum thickness of both top and bottom slabs to be 1/16th of the clear

distance between the fillets or girders (CALTRANS, 1993b).

c) Load distribution. Load distribution in a box girder bridge is influenced by the

number and dimension of cells, the depth-span ratio, the width-span ratio, the number ofdiaphragms, and by other factors. AASHTO table 3.23.1 (append A, table A.7) gives the

distribution factor (DF) for a box girder bridge as S/7. Therefore, the live load per girder

is given by:

For cast-in-place box girders with normal span and girder spacing, the slabs can

be considered integral parts of the girders (i.e.webs), and the entire slab width can be

considered to be effective in compression (AASHTO 9.8.2.1). This assumption permits

designing the entire box girder as a unit instead of designing the girder as several

modified T-beams, as in past practice. Therefore, the equivalent DF for the entire box

girder can be expressed as (PTI, 1978).

This distribution factor is applied to either live-load moment due to the truck or to that of

the lane load, whichever governs.

d) Tendon requirements. Graphical design aid for quickly estimating the amount of

post-tensioning steel required is suggested in the design manuals such as

CALTRANS(1993A) and PTI(1978). For a given span length and appropriate depth-to-

span ratio, the approximate amount(in psf of the deck) of post-tensioning steel is

determined from the graphs (see appendix B, figs. B.1-B.6); the required concrete

strength is given by the dashed lines in the graph. Generated by the computer for HS20

loading, these graphs are valid for the zero allowable tensile stress. However, they can


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also be used when the allowable tensile stress is by using 85 percent of the indicated

value for simple spans and 75 percent of the indicated value for multiple spans

(PTI,1978). Typically, in post tensioned construction, several post tensioning strands are

encased in conduit, the diameter of which depends on the number of strands encased.

The minimum duct size is governed by AASHTO 9.25.4 (AASHTO, 1992), which

requires that the duct area be at least twice the net area of the prestressing steel if thetendons consist of several wires, bars, or strands.

Typical strand tendons in galvanized semigrid post-tensioning ducts (PTI, 1978)

Number of size of working force @ approx stress

strands duct (in.) level of 0.6fs (kips)

9-12 223-296

13-18 3 322-446

19-24 271-595

25-31 4 620-768

e) Tendon location. Graphical design aids are used to estimate the eccentricities of

the post-tensioning force for the box girder. Two problems are involved here. First, the

centroid of the group of strands in the duct must be determined. The random position of

the post-tensioning strands in the duct make determination of the centroid of the group of

strands a difficult problem. In practice, depending on the required number of strands and

the size of the post-tensioning duct, the location of the centroid of the group of strands(distance Z between the centroid and the center of the duct) are assumed as shown

below:

Location of centroid of strands in a Post-tensioning duct (PTI,1978; CALTRANS,1993d;

AASHTO,1994)

Duct size (outer diameter)(in.) distance Z (in.)

3 or less

3-4

Over 4 1

f) Friction: straight girders. When strands are pulled through post-tensioning, loss

of prestressing force occurs due to friction between strandsand the surrounding ducts.

Total loss of prestressing force.

g) Friction: horizontally curved girders. Additional friction losses should be

considered for such bridges if the tendons are on a horizontal curve.

h) Anchorage zones. In both pretensioned and post-tensioned beams, the stressing

force is transferred to beams in their end portions known as the end zones or the


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anchorage zones. in post-tensioned beams, the prestressed force is transferred directly on

the ends of the beam through bearing plates and anchors.

Precast Post-tensioned Prestressed Segmental Bridges

Segmental construction technique evolved in Europe in the aftermath of WWII for the

replacement of thousands of war-damaged bridges. The acute shortage of steel in post-war Europe gave the impetus to use prestressed concrete in replacing bridges throughout

the Europe.

Precast segmental construction was used in 1941 by the French pioneer in

engineering, Eugene Fressynet, in constructing the 180ft two hinged portal-framed

bridge over the Marne River near Luzancy, France.

*several schemes of segmental construction are in use. The schemes used

determines both the design and the calculations and forms the basis of classifying

bridgesas follows:

1. Cantilever bridges (bridges made of a succession of cantilevers )

2. Bridges with concrete precast beams

3. Incrementally launched bridges

4. Bridges built of self-supporting and self-launching centering

The most widely used method in segmental construction is the cantilever method, in

which the bridge superstructure is built by a succession of segments.

The first segment of the bridge is supported on a rigid abutment or pier. This supports the

next segment, including the weight of the formwork of the construction equipment, as a

cantilever. After it gains sufficient strength (if cast-in-place), this second segment is

integrated with the first one by post-tensioning, which makes the assemble self

supporting.

Prestressed Concrete Suspension Bridges

Prestressed concrete suspension bridges are generally of the self-anchored type

that uses prestressed concrete girders instead of steel girders i.e. the force on the external

cables is used by anchoring the cables into the concrete girders.

Miscellaneous Prestressed Concrete Bridge Types

Many different types of prestressed concrete bridges, such as stress-ribbon, truss,

through-girder, and inverted suspension bridges have been built. But these are rather

uncommon types that may adapt to special situations. These applications arise from the

fact that the concept of prestressing can be used as a most desirable alternative to carry

loads in tension instead of in flexure.

*stress ribbon bridges


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Construction of these bridges involved simply tying two or more fiber ropes at

each end across a span forming a catenary, which supported an overlaid walkway made

from transversely laid bamboo sticks.

In principle, the stress-ribbon bridge is similar to those primitive suspension

bridges, except that modern construction uses high-strength materials and engineering

technology involving precasting and prestressing. The fundamental idea is to produce a

suspended, but tightly stretched, ribbon of prestressed concrete that is anchored in the

abutments and laid across intermediate supports provided with cantilever arms.

The superstructure of a stress-ribbon bridge generally consists of a prestressed

band attached to rigid end abutments. The deck is formed from precast concrete

segments that are suspended on a high-strength steel bearing cables and then shifted

along the cables to specified position. Joints between the segments are concreted in

place, followed by prestressing the whole deck, this developing compression and rigidity

sufficient to carry the dead and live loads. Generally, high strength cables are passedthrough a series of precast concrete components, the deck assembly of which can be

tensioned from stiff abutments.

The stress-ribbon superstructure differs from that of the conventional suspension

bridge in that both the cable and the deck can be independently tensioned; in a

suspension bridge, the main load-carrying element is the cable, with the deck acting as a

stiffening element.

*prestressed concrete truss bridge

A few examples of prestressed concrete used to build truss bridges are reported inthe literature (Caroll, Beaufait, and Bryan, 1978: Gerwick, 1978; Naaman, 1982). These

bridges can be successfully built from precast pretressed concrete elements, which can be

assembled on site and connected by post-tensioning.

*prestressed concrete through-girder bridge

A prestressed concrete though-girder bridge is characterized by a single open-

section girder of trapezoidal form resembling a U-section with inclined legs. The purpose

of the inclined legs, which act as the load carrying girders, is to reduce the span of the

transverse slab (i.e. the bottom width of the U-girder), thus reducing both slab thicknessand pier widths.

*prestressed concrete inverted suspension bridges

Conceptually, these bridges are similar to to the suspension bridges except that

the cables are used below the deck. This proves that prestressed concrete is one of the

most desirable alternatives for carrying the load mostly in tension instead of in flexure.

Experience from these bridges has led to the development of a self-anchoring inverted

suspension bridge believed to be a low-cost solution for spans ranging from 200-400ft

over deep valleys.


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In the United States, the most commonly built prestressed concrete superstructure for

short and medium spans consists of I-beams and the second most commonly built type is

box girders, as dictated by economics. The use of other types such as various types of T-

beam configuration is limited to only few states. In the northwest and west-coast states,

cast-in-place, post-tensioned box girders are more common than any other type.

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CHAPTER 8 Prestressed Concrete Bridges