design of a composite road bridge with high strength steel

8/7/2019 Design of a Composite Road Bridge With High Strength Steel

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EUROSTEEL 2008, 3-5 September 2008, Graz, Austria

DESIGN OF A COMPOSITE ROAD BRIDGE WITH HIGH STRENGTH

STEELS AND ULTRA-HIGH PERFORMANCE FIBER REINFORCED

CONCRETE

Aude Petela

, Ludovic Picardb

, Florent Imbertya

, Jol Raoula

aSETRA, Large bridge division, Bagneux, France

bDREIF, Bridge and tunnel division, Versailles, France

INTRODUCTION

In France, steel S460 is regularly used in bridges since about 15 years. S690 is defined in French

rules since about 30 years but has never been used in bridges; as French design rules do not allow

any steel yielding, hybrid girders have never been used in bridges too. Eurocodes now allow the use

of such advanced design, so this paper describes the complete design according to Eurocodes of twovery innovative twin girder bridges, made of high strength steel (up to S690) and of hybrid girders.

Another innovation proposed here is a ultra-high performance fiber reinforced concrete slab

(UHPFRC).

The aim of this theoretical study is to demonstrate the competitiveness of such design.

These bridges have a symmetrical composite two-girder structure with three spans of 64 m, 88 m

and 64 m for the first one, and 95 m, 130 m and 95 m for the second one. The total slab width is21.5 m, corresponding to four traffic-lanes.

The concrete slab and the design of the two bridges are presented here. Finally an economical

balance sheet is drawn up.

1 ULTRA-HIGH PERFORMANCE FIBER REINFORCED CONCRETE SLAB

1.1 Presentation

The concrete slab is inspired by the ribbed slab of the French national project MIKTI [1]. In this

project, the slab was 12 m wide and was divided in 2.5 m elements for possible truck delivery. Ribs

spacing was 0.6 m from axis to axis in longitudinal and transverse directions. The total slab

thickness was 0.38 m, composed by 0.33 m high ribs and 0.05 m high slab. Ribs were 0.07 m wide

at the bottom, and 0.10 m wide at the top.

0.

40

21.50

0.

615

14.303.60

0.60

Fig. 1. Transverse profile of the deck (with strands)

In this paper, the deck width is much larger, as it is designed for four traffic lanes. With 21.5 m

wide, a 2.0 % transversal slope either side of the bridge is necessary. The centre-to-centre spacing

between main girders is 14.3 m and the slab cantilever either side is 3.6 m long. The prefabricatedslab segments are now 21.5 m long and 2.5 m wide. The MIKTI slab has been adapted to these

constraints by rising the ribs height. The total slab thickness linearly varies from and 0.615 m at its

axis of symmetry to 0.40 m at its free edges, including the constant 0.05 m high top slab. The other


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dimensions have been kept, the width of 0.07 m at the bottom of the ribs being necessary for the

strands cover, the width of 0.10 m at the top of the ribs and the depth of 0.05 m of the top slab

having been successfully tested in laboratory for local resistance during the MIKTI project. The

transverse profile of the deck is presented in Figure 1. Figures 2 shows a detail of the dimensions on

main girders.

0.07

0.10

0.

42

0.

05

2.50

Fig. 2. Transversal cut of the UHPFRC slab on the main girders

1.2 Transverse bending and prestressing

The loads applied on the concrete slab for transverse bending study are selfweight, non structural

equipments, and traffic loads LM1 (UDL and TS). The non-structural equipments are composed of

a 6 cm high asphalt layer, two lateral safety barriers, two cornices, and a central safety barrier, i.e. anominal load of 19.25 kN/m.

In the transversal direction three internal strands T15S are put at the top of each rib (see Figures 1

and 2) all along the width of the slab. Two additional strands are placed at the bottom but are not

grounded in the outside cantilever parts (see Figure 1).

The strands have a cover of 3.5 cm. As the strands are horizontal, the minimal cover for the upper

strand UHPFRC is on the extremity of the cantilever parts. The distance between the axis of the

strands is 3.75 cm.

An evaluation of transverse bending efforts in the concrete slab has been made using a complete

finite elements model associating wired and shell elements. It showed that, due to the unusual

shape of the slab (thicker in the center than above main girders, quite short cantilever) internal

forces and moments are slightly lower than in the abacuses [2] that were primarily used, which are

designed for classical twin-girder bridges.

1.3 Fabrication and transportation

The slab elements should be concreted upside down, as UHPFRC is self-compacting concrete. The

weight of each 21.5 m x 2.5 m slab element is about 27 tons. These weight and dimension allow a

truck transportation, even if the weight is rather bigger than for bridge classical slab elements. Note

that the slab could be provided in 1.90 m wide elements instead of 2.5 m wide elements.

2 BRIDGE 1: 64 M 88 M 64 M

2.1 Description

A three spans composite twin-girder bridge, carrying four traffic lanes, has been studied. The span

lengths are 64 m for the end spans and 88 m for the main span.

The concrete slab has been described in paragraph 1. In order to prevent any concrete

decompression in the slab at characteristic SLS, the slab is longitudinally prestressed by twelve

12T15 external tendons crossing the transversal ribs at the locations of the slab centroid, and a

jacking down of 0.8 m high at the internal supports, providing a 8.3 MPa compression in the slab

after creep and shrinkage.

Steel girders are 3.5 m deep and the flanges are 1.2 m wide. They are hybrid as the steel grade of

the lower flange is S460, whereas the web and upper flange are in steel S355. The web and flangesyield strength ratio is inferior to 2.0, in accordance with to EN 1993-1-5. Steel grade S460 is

necessary at mi-span because of high stresses due to the different loads and the prestressing by


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imposed deformation, and at intermediate supports in order to justify the lateral torsional buckling

with reduction curve d (See 2.4). Table 1 presents the steel distribution along the bridge.

Vertical web stiffeners are present at each transverse frame (see 2.4), and additional vertical

stiffeners are placed near the intermediate supports at 1.8 m in order to justify the shear resistance.

Table 1. Structural steel distribution for a main girder

Supports location

Zone length (m) 47.5 10.5 12 10.5 8 39 8 10.5 12 10.5 47.5

Upper flange thickness

(mm) - S35535 50 70 50 35 35 35 50 70 50 35

Lower flange thickness

(mm) - S46035 50 75 50 45 60 45 50 75 50 35

Web thickness (mm) - S355 20 22 22 22 20 18 20 22 22 22 20

The construction is similar to the one of a composite bridge with prefabricated slab elements, with

modifications due to the longitudinal prestressing. The different phases are the launching of the

steel girders, the placing of the 2.5 m long and 21.5 m wide slab elements, the tensioning of the

longitudinal tendons, the connection of the concrete slab to the steel structure (by concreting some

recesses in the concrete slab), the application of a jacking down at the internal supports, and at last

the installation of non-structural equipments.

2.2 Stresses justifications

Stresses in the sections have been justified under SLS and ULS combinations, according to

Eurocodes. As girders are hybrid, the class of the web is determined using the lower flange yield

strength (EN 1993-1-5).

2.3 Fatigue verifications

Fatigue load model 3 of EN 1991-2 is used for this verification. The loads are supposed to be

centered on each of the two slow lanes of the road.

The road is supposed to carry medium flow rates of lorries which corresponds to traffic category

n2. So the indicative number of heavy vehicles expected on each slow lane is 0.5 x 106.

The following categories of detail have been verified:

Lower flange: splice zones and weld of vertical stiffeners

Upper flange: splice zones and weld of studs

Web: shear stressThe prestress in the slab is designed to avoid tensile stresses at characteristics SLS. The neutral axis

of the composite girder is always close to the upper flange. Thus, there is significant normal stressrange only in the lower flange of the girder, and this one remains quite low. Besides, shear stress

range is not significant.

Fatigue was never crucial for the design, with a margin of at least 15% regarding fatigue resistance.

2.4 Transverse frames Lateral torsional buckling (LTB)

Each transverse frame is composed of a T-vertical stiffener (web 400 x 20 mm and flange 450 x

20 mm) and a brace girder located at mid-depth.

In span, they are spaced out by 8 meters. Additional bracing frames are located at 4.0 m and 12.0 m

from each side of the intermediate supports.

The general method of the EN 1993-1-1, 6.3.4 is used because the lower flange of the bridge is not

a uniform section subjected to a uniform compression. LTB is analysed considering the transversalbuckling section composed of the lower flange and a part of the web supported by springs located at


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each transversal frame. LTB justification implies at first the determination of the buckling modes

of the lower flange and the correspondent minimum critical amplifiercr,op:

Shape of the first mode (cr,op = 7.20)

The reduction factor is equal to op = 0.825 (curve d).

The maximum normal stress in the lower flange is 306 MPa, and the yield strength of the lower

flange is 410 MPa (steel grade S460 ML, considering the thickness of 75 mm of the flange). It leads

to a minimum amplification factorult,k= =306

4101.34.

Finally the criteria of stability is verified : ==1.1

34.1*825.0

1

,

M

kultop

1.005 1.0

3 BRIDGE 2: 95 M 130 M 95 M

3.1 Description

Several twin-girder bridges with a main span bigger than 120 m have been built in France since a

few years (Jassans, Centron, Triel), so it is interesting to try to improve the design of such bridges,

with advanced materials. The second bridge studied here has three spans of 95 m, 130 m and 95 m.

Like the previous bridge, this bridge carries four traffic lanes, and his slab is the one described in

paragraph 1. As the spans are much longer than in the previous bridge, steel grade S690 is

necessary in the lower flange near the intermediate supports and in the middle of the main span. In

order to prevent any concrete decompression in the slab at characteristic SLS, the slab islongitudinally prestressed by thirty 12T15 tendons and a jacking down of 1.2 m high at the internal

supports, providing a 14.3 MPa compression in the slab (on the intermediate supports) at infinite

time. Steel grade of the webs and of the upper flanges is S460 along the whole bridge.

Steel girders are 5.0 m deep and the flanges are 1.3 m wide. The flange width is rather small in

order to avoid large reductions in resistance of class 4 sections.

Table 2 presents the steel distribution along the bridge.

Table 2. Structural steel distribution for a main girder

Supports location

Zone length (m) 70 16 18 16 21 38 21 16 18 16 70Upper flange thickness

(mm) - S46030 35 40 40 35 30 35 40 40 35 30

Lower flange thickness

(mm)35 45 60 40 40 40 40 40 60 45 35

Lower flange steel grade S460 S460 S690 S460 S460 S690 S460 S460 S690 S460 S460

Web thickness (mm) - S460 20 22 22 22 20 18 20 22 22 22 20

The construction phases of this bridge are the same as for the first bridge.

3.2 Stresses justifications

As for the previous bridge, stresses in the sections have been justified under SLS and ULS

combinations. Where steel grade S690 is used, the resistance of the composite cross-section is

limited to the elastic resistance because the scope of EN 1994-2 is limited to S460.


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This decrease of weight and increase of rigidity of the slab allows a significant structural steel

saving: 38 %. On the whole bridge deck, the weight saving is about 25%, which will then have an

influence on the piers and footings dimensions too.

4.3 Bridge 2

For this second innovative bridge, the comparison has been made with a theoretical bridge with

steel S460 along the whole bridge. Table 4 summarizes the results.

Table 4. Weight comparison Bridge 2

Advanced bridge 2Reference bridge

(S460)Weight

reduction

Concrete 3460 tons 4280 tons

Concrete for connexion (filling ofsome recesses)

180 tons

Transverse strands 69 tons

Concrete slab

Longitudinal tendons 138 tons

10%

Steel for I-girders 990 tons 1390 tonsStructural steelTransverse cross-bracings 192 tons 652 tons

42%

Non-structural equipment Non-structural equipment 1270 tons 2130 tons 40%

Bridge weight 6290 tons 8450 tons 26%

Slab share 61% 51%

Steel share 19% 24%

Slab + structural steel +non-structural equipment

Non-structural equipment share 20% 25%

The use of high limit strength steel S690 and of UHPFRC slab allow a significant weight reduction

of the structural steel of 42%, all the more so as the test bridge comprise high limit strength steel

S460.

5 SUMMARY

Two examples of very innovative twin-girder bridges with high limit strength steels and a UHPFRC

slab have been presented. Stresses, fatigue and lateral torsional buckling have been checked

according to Eurocodes. The economical balance sheet shows a significant steel weight reduction of

about 40%, and an overall superstructure weight reduction of about 25%.

REFERENCES

[1] Toutlemonde F. et al, Fatigue performance of an UHPFRC ribbed slab applied as a road bridge

deck verified according to the Eurocodes, CONSEC'07 Tours, France, 2007

[2] Brisard S., Abaques pour la flexion locale de la dalle dun bipoutre entretoises, Bulletin

ouvrages d'art du Stra n54, 2007

[3] Ultra High Performance Fiber-Reinforced Concretes, Interim Recommendations, AFGC-

SETRA Publication, 2002

[4] Johansson B. et al, commentary and worked examples to EN 1993-1-5 "Plated structural

elements", prepared under the JRC-ACCS cooperation agreement for the evolution ofEurocode 3, 2007

[5] Bitar D., Rsistance la flexion des poutres hybrides section en I, Construction mtalliquen2, 2003

[6] EN 1993-1-12, Application rules for the extension of EN 1993 up to steel grades S700, 2007[7] Guidance book on Eurocodes 3 and 4, Application to steel-concrete composite road bridges,

SETRA, 2007

design of a composite road bridge with high strength steel

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