Chapter 8b Precast Prestressed Concrete Composite Deck Panels

7/27/2019 Chapter 8b Precast Prestressed Concrete Composite Deck Panels

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CHAPTER 8

PRECAST PRESTRESSED CONCRETE COMPOSITE DECK PANELS

The cast-in-place reinforced concrete deck supported by precast prestressed concrete I-

beam is the common type of prestressed concrete bridge built in the United States. Thebeams used in this type of bridge have stirrups projecting from their top flanges that are

embedded in the cast-in-place concrete for the deck to develop composite action.

FIGURE 7.70Typical details prestressed deck panels girder (top), with prestressing strand projections

(center), and on steel girder (bottom).

AASHTO 9.23 covers general requirements for reinforcement for deck panels. Itmandates

1. Prestressing of deck panels in a direction transverse to the beams on which theyare supported.

2. Roughening of the top surface of the deck panels to ensure composite action withcast-in-place concrete , and

3. Providing reinforcing bars to equivalent mesh, at least 0.11in2 / ft. of deck panel,transverse to the strands.

AASHTO 9.25.2.2 requires that the prestressing strands be placed symmetrically

and uniformly across the width of the panel, with their spacing limited to the

smaller of 1 times the composite slab thickness or 18 in

TRANSVERSELY AND LONGITUDINALLY PRESTRESSED DECKS


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In prestressed concrete bridges, the term prestressing force generally refers to

longitudinalprestressing force that is applied through tendons present in the bottom

flanges and in the webs of girders. In addition to this longitudinal force, the deck, which

forms the top flange of these girders, may also be prestressed in transverse or

longitudinal directions or both. The reason for this prestressing is simply to introduce

precompression to counteract tension cracks caused by loads or any other sources.

A transversely post-tensioned deck will remain uncracked under the action ofnormal loads, and any cracks that might form will remain tightly closed because

of the post-tensioning force. Transverse post-tensioning enhances durability of

the deck against temperature and shrinkage cracks by providing precompression

in the top face of the deck, a desirable approach for the design of bridge decks

exposed to an aggressive environment.

A transversely post-tensioned deck provides greater load capacity of the top slabin the transverse direction. The deck slab thickness is reduced, leading to reduced

concrete quantities and reduced dead-load moments and shears.

Transverse prestressing of decks permits longer spans of slab between girders(webs), which reduces the number of webs required in wide structures. This

reduces forming costs and concrete quantities. The cost savings for multiweb box

girder bridges cast on falsework results mainly from a thinner top slab and fewer

webs.

Transverse prestressing of decks is particulary advantageous for box girders withlarge overhanging flanges because it permits longer lengths of the overhangs.

For wide segments, transverse post-tensioning usually results in reduced overallstructure cost.

FIGURE 7.72

Cross section of Eel River Bridge, California. Redesigned transversely post-tensioned

superstructure (top), and original design without transverse post-tensioning (bottom)

[Tadesko, 1976]


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FIGURE 7.73

Transversely post-tensioned cross section of the Denny Creek Bridge, SnoqualmieNational Forest, Washington [PTI 1978].

FIGURE 7.74Transversely post-tensioned cross section of the box girder for the Biloxi Viaduct,

Biloxi, Mississippi [Phipps and Spruill, 1990].

Most of the transverse post-tensioning of a deck slab is accomplished in one of two

ways: either by 270-ksi in.-diameter strands or by bar tendons. Note that transverselypost-tensioned deck slabs also normally have longitudinal and transverse non-prestressed

reinforcement near the top and bottom of the slab. This reinforcement serves two

functions:

1. It provides the necessary flexural capacity to permit removal of the section fromthe forms and to allow handling prior to transverse post-tensionin, and

2. It contributes to the flexural capacity of the slab at the ultimate conditions.CONTINUITY IN PRESTRESSED CONCRETE BRIDGES

Methods of Achieving ContinuityIn prestressed concrete superstructures, continuity is generally accomplished by post-

tensioning at the site. Methods of achieving continuity, i.e.,the type of tend on layout and


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the method of framing that should be used, in prestressed concrete superstructure

construction. A brief description of these schemes for achieving continuity follows. Note

that, in general, the tendon profile follows the deflected shape of the girder, or the

moment diagram due to uniform load plotted positive downward.

Monolithic continuity - Monolithic continuity involves cast-in-place construction

FIGURE 7.81Tendon layout in continous beams with monolithic continuity: (a) prismatic girder, (b)

Nonprismatic girder, and (c) prismatic girder with overlapping tendons [Nawy, 1989].

Nonmonolithic continuity - Nonmonolithic continuity involves precast pretensioned

elements


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FIGURE 7.82Methods of achieving continuity using precast pretensioned girders

(a)Post-tensioning in the negativemoment region of a two-span girder,(b)Full length post-tensioning in a two span girder,(c)Post tensioning using couplers,(d)Nonprestressed steel, and(e)Post-tensioning in nonprismatic beams

OPTIMIZATION OF PRECAST PRESTRESSED CONCRETE BRIDGE

SYSTEM

Selection of a particular superstructure types depend on several factors:

Feasibility Span Capability Limitation of depth-to-span ratio System availability from precasters or suppliers Performance history Engineers own experience and preferences Regional trend Construction time constraints Aesthetics Cost

In general, structural problems of optimal design may be identified by:

a. Uncertainty consideration


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b. Design variables, such as materials, geometry, and loading

c. Problem formulation, which involves objectives, limit states, and constraints.

Level of Optimization:

Level 1. member optimization

optimization of girder cross section of specific structure types.Examples:

Optimization of concrete slabs Single-span steel girders Single-span Prestressed concrete girders Single-span Box girders Single-span multibeam girders Multispan simple girders Continuous Prestressed concrete Box girder Frame bridges

Level 2. Configuration or Layout optimization

Determining the best combination of longitudinal and transverse memberarrangement within the given bridge system.

Level 3. System optimization

Overall features of the structural systems. Various parameters include materials,structural type and configuration, member cross-section and span length,

combined longitudinal and transverse member arrangement etc.

PRINCIPLES OF PRESTRESSED CONCRETE DESIGN

Prestressing

a process of imposing a state on a structural body prior to its placement in

service, to enhance its load-carrying capability and service performance.

Two forms of prestressing:

Pretensioning tendons are being stretched and anchored to abutments beforeconcrete is poured.

Post-tensioningis the process of imposing prestress by stressing and anchoringtendons against already hardened concrete,

Classification of tendons:

Bondedrefers to tendons that are substantially bonded to the concrete . UnbondedForce is applied only at anchorages.

FLEXURAL ANALYSIS

Members can be analyzed as homogeneous and elastic under service loads, that allassumptions of simple bending theory and Naviers formula can be applied:


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R

E

y

f

I

M

The stress due to the transverse loads can be calculated by the simple flexure formula:

S

M

I

Myf

t

tS

Pe

A

Pf

tS

eA

A

P1

b

bS

Pe

A

Pf

bS

eA

A

P1

Fiber stresses:

Permitted Moment capacity, or the design strength:


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Where:

The quantity q is called the reinforcing index.

For the rectangular beams with reinforcement index greater than 0.361

, the design

flexural strength is limited to,

2'211

08.036.0 bdfM cn

For the value .85.01

2'25.0 bdfM cn *

2.1crn

MM

1

/

*

b

cncdcpcrcr

S

SMSffM

Where:

The average unit stress on concrete area Ac due to these forces can be expressed as

If the average fiber stress in Ac is assumed to be 0.5fc, i.e., half the maximum allowable

compressive stress fc then it can be expressed as


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flexural cracks - which are almost perpendicular to the longitudinal axis of the beam,

develop in the regions of large moments

web - shear cracking - the magnitude of principle tension is relatively high compared to

flexural stresses

the net shear in a pre-stressed concrete beam is

the net shearing force to be carried by concrete is reduced by Vp:

The shear strength of a prestressed concrete member is taken as the sum of the shearstrength provided by the concrete and by the web reinforcement. For design purposes

this relationship is written as

The shear Vci is computed from AAHSTO Eq. 9.28:


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Composite Construction involves construction in which precast member acts in combination with

cast-in-place concrete, that is poured at a later time and bonded to the

member.

Advantages and disadvantages of composite construction:

A majoradvantage of a composite construction is the economy achieved through

combining precast and cast-in-place concrete while retaining the continuity and

efficiency of the monolithic construction.

Another, is that product of superior quality are obtained through precasting;plant fabrication is under good condition, forms are reused, strands are tensioned long-

line.


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A disadvantage of the composite construction is that the restraint provided by the cast-

in-place slab causes an increase in the prestress losses of the precast element and thus

influences the time-dependent deflection.

As shown in Fig. 7.96, in multiple-span structures, the cast-in-place slab can be poured

continuously over the supports of precast beams placed in series to develop continuity.The cast-in-place slab also offers an effective means of distributing loads laterally.

Chapter 8b Precast Prestressed Concrete Composite Deck Panels

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