21 design of long-span bridges with conventional reinforced concrete decks

8/11/2019 21 Design of Long-span Bridges With Conventional Reinforced Concrete Decks

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IBSBI 2011, October 13-15, 2011, Athens, Greece

DESIGN OF LONG-SPAN BRIDGES WITH

CONVENTIONAL REINFORCED CONCRETE DECKS

Ioannis A. Tegos1 and Stergios A. Mitoulis21,2Aristotle University of Thessaloniki, Dept. of Civil Engineering, Greece

e-mail: [email protected], [email protected]

ABSTRACT: This paper investigates the applicability of conventionally

reinforced concrete decks in long-span bridges, i.e. in bridge decks withoutprestressing tendons. A bridge actually built almost a decade ago by the Egnatia

Highway SA, which lies along the Northern Part of Greece, was used as the

benchmark bridge for the investigation. The analysis and design of the real

bridge was performed according to the codes existing during the bridge final

design. he bridge was re-designed according to Eurocodes utilizing a

conventionally reinforced concrete deck by adopting Eurocodes class D. The

study verified that the concrete sections with only ordinary strength steel can be

utilized in bridges with span lengths up to 46 m.

KEY WORDS:Bridge; Exposure Class D; Eurocode; Ordinary Strength Steel;

RC deck.

1 INTRODUCTIONIntegral abutment and integral pier bridges are jointless bridge structures, whose

deck is rigidly connected to both the abutments and the piers. They improve

aesthetics and earthquake resistance towards the traditional systems with

expansion joints, which permit thermal expansion and contraction, creep, and

shrinkage. The increased cost of maintenance or replacement [1] of these faulty

expansion joints, along with the initial cost of their design, manufacture, and

installation, led to the advancement of the case for integral abutment bridges,

which are compatible with conventionally reinforced concrete decks.

The final design of bridges takes into account the influence of the bridge

classification, as defined by the exposure classes A, , C and D of Eurocode 2Part 2 [2]. Bridge design and is strongly related to the classification used during

analysis as it reflects on the bridge structural cost and the long-term condition,

namely the durability, of the bridge.

The use of ordinary steel for reinforcement for the design of long span bridges,

without prestressing tendons, which leads to the compulsory adoption of an

exposure class A or B, was also studied, in terms of constructability and cost-

effectiveness. An investigation was conducted in order to identify the


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2 Proceedings IBSBI 2011

applicability of conventionally reinforced concrete decks in long-span bridges,

i.e. in long bridge deck spans without using prestressing tendons. The selection

of the bridge exposure class is strongly affected by the serviceability needs of

the deck that is related to the allowance for deck cracking or not. It is noted that

the erection of bridges with conventionally reinforced decks, i.e. without

prestressing, longer than 20m is a relatively demanding construction. This is

due to the fact that the ratio of the longitudinal reinforcement at splices results

high and additionally the depth of the deck cross section, which is needed in

order to control the decks deflection, is typically large.

2 DESCRIPTION OF THE BENCHMARK BRIDGE GENERAL

The investigation utilised an as-built bridge as benchmark. The bridge ofKleidi-Kouloura belongs to Egnatia Motorway that runs across Northern

Greece. It is a cast-in-situ structure with a total of three spans and a total length

equal to 135.8 m. Figure 1 illustrates the longitudinal section of the bridge and

the cross sections of the box-girder deck, the pier and its foundation. The deck

of the bridge has a constant height of 2.18 m, while prestressing consists of

tendons 20x19T15 (20 tendons of 19 wires with diameter 15mm each) with a

parabolic geometry. The bridge has a seat-type abutment on which the deck is

supported through two sliding bearings, while is rigidly connected to the piers.

The clearance between the deck slab and the backwall is bridged by an

expansion joint with a movement capacity of 100 mm. The bridge has an agle

of skew equal to 63.4o, as shown in Figure 2.

2.18

6.00

0.60

13.50

A1 P1 P2 A2

KOULOURA KLEIDI45.10

135.8045.60 45.10

9.50 9.50

2.0m

Deck at the midspan

PierFoundation

3.00

7.50

2.00

1.00 22.50

2.00

Figure 1. Geometric layout of Kleidi-Kouloura Bridge.

Figure 2. Plan view of Kleidi-Kouloura Bridge.


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I.A. Tegos and S.A. Mitoulis 3

3

DECK DESIGN WITH ORDINARY STRENGTH STEELThe last decade an intense effort and research was conducted to utilize ordinary

strength steel in bridge decks. This is due to the need for rigid deck to abutment

and deck to pier connections that allow the dissipation of part of the induced

seismic energy through hysteretic behavior of the abutments and the piers.

Design of conventionally reinforced bridge decks also eliminates the

disturbance caused by the prestressing tendons during construction. In this

framework an investigation was conducted to identify the constructability of

long spans, up to 46.0 m, by utilising only ordinary strength steel, i.e. without

prestressing tendons. In order to achieve such a design alternative, the cross

section of the bridge deck was selected to be a void-slab, as shown in Fig. 5.

0.456.805.75

14.45

1.75

2.60 2.409.70

0.35

1.25

0.30

1.25

0.30

1.25

0.30

1.25

0.30

1.25

0.30

1.25

0.35

2.40

1.80

2.40

Figure 3. Cross section of the void-slab deck at the mid-span.

The use of conventional strength steel reinforcement for the erection of long

span bridge spans is restrained due to the reasons underlined in the introduction.

However, the preliminary design of the reinforced concrete deck that was

attempted in this study was based on the codes prescriptions [2][3]. The

selection of the void-slab bridge gave an acceptable longitudinal reinforcement

ratio at splices, as shown in Figure 6 and 7. The scaling and the layout of the

reinforcement with bars of 14.0m long, as shown in figures 8 and 9, and the use

of bundled bars was found to facilitate the cast of concrete. The shear action

was receiver by appropriate ratio of shear reinforcement. The control of the

deflection was achieved by the resulting hyperstatic bridge system.

4 THE BRIDGE DECK

The bridge deck is connected rigidly to the piers. The cross section of the deckis solid over the piers, which means that is has no voids. Solid deck sections

extend to a distance equal to 2d=21.75 =3.50 m at both sides of the piers,

where d is the depth of the decks cross section. The cross section of the deck is

also solid over the abutments and extends to a distance equal to 1.5d=2.65 m

from the support. The reason why the deck cross section was designed to be

solid over the supports (i.e. abutments and piers) is that the punching shear


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loading of the deck, due to the concentrated loads, was found to be more critical

than the corresponding shear loading. This is due to the fact that the piers

diameter that is 2.0 m, is smaller than 3.5 times the depth of the decks cross

section, which is equal to 3.5d = 3.5d = 6.15 m. Additionally, the typical

favorable influence of prestressing is not developed, due to the fact that only

ordinary strength reinforcement bars were used for the deck.

5 MODELING

Modeling of the deck of the bridge was attempted by shell elements of SAP

2000 [4].

piers fixed at foundation

solid deck section void-slab

deck sectionsolid deck section

piers

cantilever slabs

1

Figure 4. The model of the bridge with the shell elements.

6 BENDING DEFLECTION AT MID-SPAN

The calculation of the decks vertical deflection was performed assuming that

the bridge deck is cracked, i.e. the effective stiffness of the deck was calculated.

It was found that the high ratio of the longitudinal reinforcement is favorable for

the stiffness of the long span of the deck. The stiffness of the deck is equal to

KII,eff= 0.69EcmJc. The deflection of the deck was then calculated equal to 170

mm namely equal to L/264=45.6m/264, which is acceptable.

7 DESIGN OF THE BRIDGE DECK

The deck belongs to class D according to Eurocode 2 Part 2 [2] in both the

longitudinal and the transverse direction. Table 1 shows the bending moments

and the total reinforcement requirement of the deck against the ultimate limitstate loading.

Table 1. The bending moments and the longitudinal reinforcement at the mid-

span and at the support.

Bending Moment (MNm) Longitudinal reinforcement area (cm2)

Central span 61.88 882.34

Support -65.50 970.26


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8

REINFORCEMENT SCALING AND LAYOUT

The reinforcement of the bridge deck was designed in a manner to avoid

longitudinal reinforcement splicing at deck flanges that are in tension, i.e.

splicing of the reinforcement bars is avoided were the bending moments are

positive at the spans and negative at the supports. Single or bundled bars

appropriately scaled were utilised under the following concept:

(a) The reinforcement laps were set in a manner that the maximum length of the

reinforcement bars, that is 14.0m, is a multiple of the required lap lengths of the

20mm bars. This was achieved by determining the maximum allowed ratio of

the longitudinal reinforcements, which is the upper bound set by the code [5]

and is related to the reinforcement to concrete bond conditions, the diameter of

the bar and the grain size, which is the nominal maximum aggregate size. Then

the n parameter was defined, which was the integer part of the maximum bar

length (14.0m) divided by the required lap length. Then, ngroups of bars were

utilised, which were set in the transverse direction of the deck cross section

according to Figures 5 to 8. For example the in case of 20 reinforcement bars

and for a maximum grain size 16 mm, groups of 20 bars, from which 19 are

effective as shown in Figure 5, can be utilised, according to Figures 5 and 6, for

the bottom flange of the deck, where the bond conditions are good. For a slight

over-reinforcing of the deck, i.e. the use of a longitudinal reinforcement ratio

only 5.3% greater than the one allowed by Eurocode 2 Part 1 [5], it was found

that reinforcement splices can be avoided.

8.1 Single bar reinforcementSingle bars can be used for the reinforcement of the top flange of the deck, i.e.

at decks supports, where the bonding conditions are unfavorable and hence the

reinforcement lap lengths are large. The groups of bars consist of 14

reinforcement bars, from which 13 are effective, Figures 5 to 8. This led to a

7.7% overdesign. It is noted though that this over-reinforcing is much smaller

than the one that would be needed in case the codes splices were applied.

8.2 Bundled bars(b) The use of bundled bars according to Eurocode 2 [5] and EKOS 2000

section 17.12 [6] is deemed to be a design alternative in case of more

demanding deck reinforcements. The spacing between the bar bundles and thelap lengths are determined by the equivalent diameter nof the bundled bars.

Figures 9 to 12 illustrate the reinforcement scaling and layout in case bundled

bars 220 are used for the reinforcement of the deck at the mid-span and at the

supports.


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1 to 20

e

e-Obar>Ograin + 5mm

20mm

x2x

3x

23

45

67

89

10

11

12

13

14

15

16

17

18

19

20

laps lp=700mmmid-span

L=14 m

1

transverse axis at the

Figure 5. Reinforcement scaling with single bars 20 of the deck at the mid-span (the scale isdistorted) (where xis the reinforcement lap and eis the spacing of the bars).

O10/150 O10/150

O20/4318O20 16O20 14O20 O14/150

O10/150

Figure 6. Cross section of the deck with the single-bar reinforcement layout at the mid-span .


1 to 14 L=14 m

b

ee

x2x

3x

x2x

3x

11

2

43

9

10

56

78

1 L=14 m

14

13

12

Figure 7. Reinforcement scaling with single bars 20 of the deck at the support (the scale is

distorted), (where xis the reinforcement lap and eis the spacing of the bars).

O20/43O20/43 16O20 16O20O20/43 6O20

O14/150O14/150

O14/150

O10/150 O10/150O10/150 O10/150

Figure 8. Cross section of the deck with the single-bar reinforcement layout at the support.


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2

3

4

L=14 m1

5

6

7

8

9

10

11

12

13

14

L=14 mto1 14

x

2x

3x

mid-span

e

e

e-2O>grain

20 2 mmlaps lb=x=1.0m

x

2x

3x

Figure 9. Reinforcement scaling with bundled bars 220 of the deck at the mid-span (the scale isdistorted) (where xis the reinforcement lap and eis the spacing of the bundled bars).

O10/150

20O202O20/75 6O20 2O20/75 2O20/756O20

O10/150 O10/150

6O20O14/150

Figure 10. Cross section of the deck with the bundled bar reinforcement layout at the mid-span.


to1

10L=14 m

x

2x

3x

e

e

laps lb=1.40m

1 L=14 m

2

9

8

7

6

5

4

3

10

Figure 11. Reinforcement scaling with bundled bars 220 of the deck at the support.

O10/150

O14/150 2O20/758O20

2O20/7518O20

2O20/756O20 O14/150

Figure 12. Cross section of the deck with the bundled bar reinforcement layout at the support.


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9

CONCLUSIONS

The applicability of conventionally reinforced concrete decks in long-span

bridges, i.e. in bridge decks without using prestressing tendons, was studied

utilising a benchmark bridge actually built along the Egnatia Highway. The

study came up to the following conclusions:

The deck of the bridge can be reinforced with bundled bars. It was found that

14x220 and 10x220 were found to be adequate for the decks mid-span

and supports correspondingly. The checks showed that concrete with a

maximum grain size 31 mm can be used. The over-reinforcing steel is up to

8 % at the mid-span and 11 % at the support.

An alternative reinforcement layout is the use of bar groups of 20 bars and

14 bars for the decks mid-span and support respectively. In that case

concrete with a maximum grain size 16 mm can be used, while the over-

reinforcing steel is up to 5 % and 8 % for the decks mid-span and supports

correspondingly.

As far as its concerns the deflection of the deck it was found that the decks

deformation is acceptable, despite the fact that no prestressing was utilised

and due to the use of high longitudinal reinforcement ratios.

The use of more steel bars aiming at avoiding reinforcement splices, was up

to 11%. The last overuse of steel was found to be less than the corresponding

overuse that would be needed in the conventional design case with

reinforcement splices.

This paper shed light on the reinforced bridge and showed that the use ofonly ordinary strength steel can be a design alternative for long span bridges.

REFERENCES[1] Mitoulis SA, Tegos IA, Stylianidis K-C., Cost-effectiveness related to the earthquake

resisting system of multi-span bridges, Engineering Structures, Vol. 32, Isuue 9, pp. 2658-

2671, 2010.[2] EN 1992-2 Eurocode 2: Design of concrete structures-Part 2: Bridges, 2004.[3] DIN-Fachbericht 102, Betonbrcken, DIN Deutsches Institut fuer Normung e.V, 2003.

[4] Computers and Structures INC. SAP 2000. Nonlinear Ver. 11.0.4. Users Reference Manual,Berkeley, California; 2002.

[5] EN 1992-1 Eurocode 2 - Part 1: Design of concrete building and civil engineering structures,2004.

[6] Ministry of Environment, Land Planning and Public Works of Greece. Greek code for thedesign of reinforced concrete structures, (EKOS-2000), Athens, (In Greek), 2000.

21 design of long-span bridges with conventional reinforced concrete decks

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