chapter 8 prestressed concrete bridges
TRANSCRIPT
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
1/19
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.
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
2/19
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.
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
3/19
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.
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
4/19
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.
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
5/19
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.
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
6/19
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.
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
7/19
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
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
8/19
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.
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
9/19
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
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
10/19
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
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
11/19
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
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
12/19
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
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
13/19
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.
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
14/19
*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
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
15/19
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
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
16/19
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
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
17/19
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
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
18/19
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.
-
7/27/2019 CHAPTER 8 Prestressed Concrete Bridges
19/19
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.