i

Projecto Base de uma Ponte Ferroviária com Tabuleiro de

Betão Armado Pré-esforçado Executado por Lançamento

Incremental

Preliminary Design of a Railway Bridge with a Prestressed Concrete

Deck Executed by Incremental Launching

Rui Pedro Carrasco Pãosinho

“Extended Abstract”

March 2011

INSTITUTO SUPERIOR TÉCNICO

Universidade Técnica de Lisboa

1

Projecto Base de uma Ponte Ferroviária com

tabuleiro em Betão Armado Pré-esforçado

Executado por Lançamento Incremental

Preliminary Design of a Railway Bridge with a

Prestressed Concrete Deck Executed by

Incremental Launching

Rui Pedro Carrasco Pãosinho

IST, Technical University of Lisbon, Portugal

Key words: Railway Bridge, Prestressed Concrete Box Girder,

Incremental Launching, High-Speed, European standards

1 Introduction

This dissertation aims the preliminary design of a railway bridge,

with a prestressed concrete box girder deck erected by the

incremental launching method. A structural analysis of the

construction and in service stages is carried out, together with the

required safety verifications.

The design solution and construction method are presented, taking

into account the various constraints of the project and the

materials adopted. The design criteria defined in the structural

Eurocodes and the actions are established for both the construction

stages and the in service phase.

An additional special analysis was performed, evaluating the

dynamic response of the deck to the passing of high-speed railway

trains as defined in Eurocode 1. It is concluded that the circulation

of this type of railway traffic does not represent a particular

concern in terms of deck deformation and acceleration.

A cost evaluation based on the general definition of the structure

included in the design drawings is also presented and compared

with a cost evaluation made for a steel-concrete composite deck

solution proposed for the same bridge, in the frame of another

dissertation.

2 The base case design solution

After a careful study of the site constraints, it was decided to

choose a seven span deck with five 51 m typical spans, and two

37,5 m lateral spans (Fig. 1). The design solution adopted for the

deck was of a prestressed concrete box girder, due to its high

stiffness and relative low cost. The 12,3 m width cross section has

a constant depth of 3,5 m (Fig. 2). Due to the distance between the

two vertical webs of the box girder, the piers have a “Y” shape at

the top, followed by a rectangular geometry down to the

foundation.

The deck is supported by the abutments with guided sliding

bearings, in contrast to the fixed bearings placed between the deck

and the piers. In addition, there two seismic dampers were

considered between the deck and each abutment due to the high

horizontal forces produced during the seismic evens. This design

solution produced the best results when compared with the other

possibilities of fixing the deck two one of the abutments. In this

scenario the transverse forces applied on the bridge are absorbed

mainly by the piers, while the longitudinal forces are distributed

between the piers and by dampers to both abutments, ensuring that

all of the different structural elements can safely withstand the

seismic forces.

Figure 1 – Longitudinal layout for the preliminary design

Figure 2 – Deck box girder cross section

2

Figure 3 – Pier top frontal and lateral views

There are two distinct geological areas, with two different load

bearing capacities. On the left side of the longitudinal layout, the

soil mechanical properties are quite favorable, presenting STP

values of 60, ideal for shallow foundations. On the other hand, the

right side of the longitudinal layout displays much lower values

during the SPT test at a shallow depth, only reaching an SPT of 60

at a depth of around 25 m. Taking into account these conditions,

the decision was made that the E1 abutment and the P1 through P4

piers would have strip footing foundations, while P5 and P6 piers

and the E2 abutment would need pile foundations.

The materials used in all of the structure are:

Concrete class C40/50;

Reinforcement rebar’s grade A500NR;

Prestressing tendons of steel grade A1670/1860.

3 The Incremental Launching Method used for

the Construction of Concrete Decks

Taking into consideration the 30 to 35 m distance from the deck to

the ground, the deck spans and cross-section, and the not very

long length of the deck, the incremental launching method was an

appropriate option for the construction of the deck.

The incremental launching method (ILM) consists of building the

bridge deck behind one or both of the abutments, and by means of

hydraulic jacks, pushing it incrementally to its final position.

This method is used when the valley below the deck is deeper than

25 m and the spans are between 40 m and 60 m long. It is widely

used in Europe due to its competitiveness and overall quality, as

well as its capacity to overcome specific constraints, such as:

Very deep valleys;

Deep and/or wide rivers;

Steep slopes where machinery is inaccessible;

Environmentally protected areas where it is not

convenient to place temporary structures to support the

deck during construction.

The implementation of the ILM has many advantages both to the

Contractors and the Owners of the project, namely:

Reduced construction yard areas;

Reduced man-power;

Minimal disturbance of the surrounding environment;

Increased worker safety, since the majority of the

construction is done behind the abutment, on solid

ground;

Greater construction speed, due to the fact that the deck

and piers can be built simultaneously by different

specialized teams;

Light weight equipment that can be reutilized on other

projects of the same method.

The design of the cross section is of great importance, since the

deck must be stiff enough to withstand the launching process, but

at the same time as light weight as possible. The slenderness ratio

(relation between the typical span and the deck depth) is a good

measure of this performance. Box girder decks usually have a 20

to 25 slenderness ratio, which is the span-to-depth ratio, values

that on the ILM drop to a much lower 13 to 17 ratio. Various

slenderness and span lengths of concrete deck bridges using the

ILM were researched, later being combined in a graph (Fig. 4).

The ILM when applied to a prestressed concrete deck, consists of

casting in-situ a segment of the deck, generally 50% of the length

of the longest span, behind one or both of the abutments, and after

the curing process is completed, pushing it forward so that another

segment can be cast behind it. For box girders the casting process

is generally done in two phases, the first casts the lower flange and

the webs, and the second finishes by casting the top flange, this

process is done on a weekly cycle. This procedure occurs because

the formwork for the top flange has to be supported by the bottom

flange and the cooling process of the top flange and webs happens

at different paces (Fig. 5).

Figure 4 – Slenderness ratios for numerous prestressed concrete

incremental launched bridges.

0

5

10

15

20

25

30

20 40 60 80 100 120

Sle

nd

ern

ess

Rat

io

Length of the longest span L (m)

slenderness ratio

slenderness ratio (with the use of many temporary towers and/or long steel noses)

3

Figure 5 – Different cooling rates of the top flange and web (Octávio

Martins, 2009)

A prestressed concrete deck has de advantage of being a cost

effective solution when compared with other possibilities, such as

a steel-concrete composite deck. On the other hand, the concrete

decks higher dead load produces equally higher bending moments

and shear forces during launching. To solve this problem, it is

generally adopted, either a lightweight steel launching nose fitted

to the cantilever end of the deck, which reduces the dead load at

the front, or temporary steel piers at half span that reduce the

bending moments by the same amount.

4 Actions and Design Criteria

All the actions and design criteria are in accordance with the new

structural Eurocodes. Beyond the permanent loads of the self

weight, super imposed loads and prestressing, the live actions

considered in this study are the following:

Traffic vertical loads – the LM71 load model;

Dynamic factor ф;

Nosing Force;

Actions due to traction and breaking;

Actions on non-public footpaths;

Geotechnical static equilibrium;

Wind actions – on the deck and piers;

Variations of temperature, shrinkage and creep – these

three factors were combined in one single equivalent

temperature variation;

Seismic action – quantified in two different directions

through the use of the appropriate response spectrums.

The launching phase required special attention, due to the

uncommon restrictions it put on the entire structure. During

launching the deck is subject to cyclic changes in both bending

moment and shear force, fact that doesn´t allow the use of

parabolic final tendons, since their configuration would not always

be favorable. The adopted solution involves applying uniform

compression in both flanges while guaranteeing decompression of

the bending moments. The shear force resistance was evaluated

with a safety factor of 1,35 to ensure adequate security. Another

concern was the position of the casting joints; after being

determined that each section would have half the length of the

longest span, their position in the final configuration was of great

importance. The solution to place the casting joint at quarter span

length from the piers ensured that they would more or less be

located where the bending moments were equaled zero.

The launching of the deck also induces a great amount of friction

on top of the piers, causing bending moments at the bottom

sections. This event is especially important in tall piers, which due

to their height, suffer on a larger scale. Additional small

eccentricities of the deck vertical load can also cause large

bending moments in the base due to the large dead load of the

bridge deck.

The in service phase was subject to all verifications in accordance

with the structural Eurocodes, using the following design

combinations of actions and design checks:

The General Combination of Actions:

-

;

Seismic combination:

;

Ultimate Limit States (ULS) – utilizing the appropriate

partial factors;

Serviceability Limit States (SLS)

a) Stress limitation through:

i. Characteristic Comb

ii. Frequent Comb

;

b) Crack control;

c) Maximum concrete compression;

d) Maximum vertical deflection.

5 Structural Verifications During Launching

Using a launching nose with a length of 60% of the longest span

(Gohler & Pearson, 2000), the structural analysis of the launching

stages yielded the envelope of bending moments presented on Fig.

6. From these results, it can clearly be seen that there exists an

area where the bending moments (both negative and positive) are

much higher than on the rest of the deck. The area corresponds to

the position over a pier, when the front of the deck is in a

cantilever, right before reaching the next pier. And the same

section, more or less, corresponds to the mid-span position, when

only the launching nose is in cantilever.

4

Figure 6 – Deck bending moments envelope during launching [kNm]

In the design of the prestressing, two bending moments situations

were considered; the first of the maximum bending moments (both

negative and positive) and the second of the maximum bending

moment excluding the area where the peak values occurred. It was

quickly realized that it was unfeasible to prestress the entire deck

to overcome a peak value of bending moment. The adopted

solution consisted in prestressing the deck to overcome the more

regular values, adding external temporary prestressed bars, on a

16 m strip, to counteract the peak values. The prestressing chosen

solution is presented in Fig. 7.

Figure 7 – Cross section featuring the chosen prestressing

solution during launch

Coupled straight tendons were chosen, each consisting of 19

strands of 0,6``. Six tendons were applied in the bottom flange,

and eight on the top flange, while twenty 50 mm diameter

prestressed bars were assigned externally in the center of the top

flange to overcome the additional negative bending moment near

the front nose.

An alternative launching solution using temporary steel piers at

the middle of the spans and suppressing the launching nose was

also analyzed, but was discarded since it required more coupled

straight tendons than the adopted solution and the cost of the steel

piers surpassed that of the launching nose.

During construction, the ultimate limit states (ULS) either for the

bending moments or the shear force were verified by a wide

margin in comparison with the ultimate resistance values.

The bearing friction during the launching process was also

examined in some detail. The maximum friction observed would

crack the base of the two tallest piers, P4 and P5. To solve this

problem, temporary prestressed stay cables composed by only 2

strands were positioned from the top of one pier to the foundation

of the previous one (Fig. 8). By doing this, a bending moment was

created, that counteracted the one produced by the friction

generated during the launching operations.

Figure 8 – Temporary stay cable schematic for piers P4 and P5

6 In Service Structural Verifications

Using the previously defined combinations, an analysis of the

service phase yielded the following results:

108990 76626

125070 81394

71470 55421

57712 37854

The values obtained from the frequent combination were used in

the verification of the serviceability limit state of decompression,

while the most conditioning characteristic combination was

utilized in the verification of the cracking Service Limit State. A

quick analysis of the obtained bending moments shows that these

surpass those of the launching phase, requiring the use of

additional prestressing.

The adopted prestressing solution consists of both external

tendons and mid-span section internal tendons tensioned after

launching operations end. The tendon schematic is as shown in

Fig. 9.

-100000

-80000

-60000

-40000

-20000

0

20000

40000

0

18

36

55

73

91

11

0

12

8

14

7

16

5

18

4

20

2

22

0

23

9

25

7

27

6

29

4

31

2

5

-160000

-120000

-80000

-40000

0

40000

80000

120000

160000

0 50 100 150 200 250 300

Figure 9 – In service external and internal added prestressing

The external prestressing consists of 4 tendons with 22 0,6``

strands, while the additional internal prestressing is composed by

4 tendons with 19 0,6`` strands. This prestressing scheme proved

to be extremely effective, although the average compression

applied to the deck cross section is very high, about 7 to 8,3 MPa.

The ULS verification was made using the following combination:

The following graph presents the comparison with the resistance

bending moment’s values, being visible that the deck bending

safety is assured.

Graph 1 – Deck acting and resisting bending moments [kNm]

The same combination was used for the shear force ULS,

producing the following graph. The shear reinforcement was

evaluated using this data in accordance with EN 1992-1 to

guarantee shear safety.

In service conditions, for the verifications of the piers and

foundations, the most conditioning combinations were the seismic

comb and the wind plus LM71 comb. All code verifications were

successfully assured, guaranteeing structure safety.

The abutment stability was assured, as well as their foundation

safety. The abutments safety verifications involved the

equilibrium, the foundation resistance and the reinforced concrete

checks of important parts of the abutments.

Graph 2 – Deck shear force at ULS [kN]

7 Deck Behavior for High Speed Circulation

High speed railway bridges are an ever-growing occurrence,

especially in recent years due to their energy and economical

stability, environmental and mobility concerns for the future. With

the implementation of this new kind of transportation in Portugal

in the near future, dynamic analysis like the one presently

conducted are of growing importance.

According with the EN 1991-2 the structure response is function

of several parameters needed for a dynamic analysis of high speed

railway traffic, these are:

Speed of railway traffic;

Span length;

Structure mass;

Natural frequencies of the whole structure;

Number of axels, their load and relative spacing;

Structure damping;

Vertical irregularities in the track;

Vehicle mass and suspension characteristics;

Vehicle imperfections;

Existence of ballast.

According to the EN 1990 A2.4.4.2, the maximum vertical

acceleration on a ballast track is of 3,5m/s2, for 10 different load

model trains (High Speed Load Models type A - HSLM-A)

travelling at different speeds. In this dynamic study several time

history analysis were performed between the traveling speeds of

40 m/s and 120 m/s (144 km/h to 430 km/h).

For these ten different HSLM-A the dynamic deck response was

evaluated during time. The maximum vertical acceleration occurs

in the lateral spans, with a value of 1,562 m/s2, less than half of

the maximum permitted value, which proves the deck responds

well to the circulation of high speed trains.

-25000

-20000

-15000

-10000

-5000

0

5000

10000

15000

20000

25000

0

37

,5

49

,5

88

,5

10

0,5

13

9,5

15

1,5

19

0,5

20

2,5

24

1,5

25

3,5

29

2,5

30

4,5

Vsd

Vrd1

6

40,8%

35,5%

15,8%

7,8%

Deck

Infrastructure

Miscellaneous

Construction Process

22,7%

17,0%

27,3%

33,0%Concrete

Formwork

Rebar

Prestressing

8 Cost Evaluation

A cost evaluation based on the general definition of the structure

in the drawings was conducted, and compared with the cost of a

steel-concrete composite deck solution for the same bridge also

utilizing the ILM. This evaluation was made by multiplying the

foreseen material quantities by their unit costs. Pie charts were

elaborated illustrating the different costs associated with each

section of the bridge and each component of the deck.

The estimated total cost of the bridge is 4.200.400 €, or in other

terms, 1035 €/m2 of deck overview area. The steel-concrete

composite deck bridge had an estimated cost of 4.223.272 €, more

or less a similar value, even though the deck cost represented a

much higher 64,5% of the overall cost. The cost difference is

made up by the infrastructure, which in the present design is of a

bigger portion due to the high dead load of the concrete deck.

Figure 11 – Bridge cost division

Figure 12 – Deck cost division

Figure 10 – Vertical deck accelerations for the 10 HSLM-A trains traveling at speeds between 40 and 120 m/s

7

9 Conclusions

From the preliminary design of this prestressed concrete bridge

built through the incremental launching method, the following

conclusions can be taken:

i. The use of a launching nose is more favorable than temporary

steel piers;

ii. Even though the deck is very stiff, it is equally heavy, which

requires the use of a great quantity of prestressing;

iii. The use of internal straight tendons during launching proves

to be a good solution;

iv. During launching there is a 16 m area where the bending

moments are higher than everywhere else on the deck;

because it is an isolated situation, 20 high yield prestressed

50 mm external bars were added to guarantee decompression;

v. The passage of the deck over P4 and P5 during launching

creates high bearing friction forces that required the use of

prestressed stay cables, to prevent cracking form occurring in

the base sections of these piers;

vi. The in service loads are higher than those during the

launching stages, which requires the use of additional

prestressing made up of external cables and internal span

cables;

vii. The final prestressing solution led to ultimate resistance

values that were higher than the active ULS values;

viii. In the resistance to seismic actions, various possibilities were

envisaged, and the most effective was chosen. It consisted in

applying seismic dampers in both abutments, since a solution

with only one fixed abutment yielded too greater forces;

ix. This solution allowed all the piers to have fixed bearings,

since the center of rigidity is more or less at the center of the

deck;

x. The high speed circulation analysis, with velocity´s ranging

from 144 km/h to 430 km/h and the regulation high speed

load models, is not governing the design in terms of forces or

deflection when compared with the combined action of two

LM71 freight trains;

xi. The maximum vertical acceleration observed during the

circulation of 10 different HSLM-A was of 1,562 m/s2, less

than half of the maximum allowed value of 3,5 m/s2;

xii. The cost evaluation leads to a total cost of 4.200.400 €, which

corresponds to 1035 €/m2 of deck overview area; 40,3% of

the total cost is due to the cost of the deck, from which 33%

corresponds to the cost of only the prestressing;

xiii. This cost evaluation reveals to be more or less the same as

the one obtained for a steel-concrete composite deck solution

for the same bridge. Even though the prestressed concrete

deck has a much lower cost, the cost difference between the

two designs is made up by the cost of the infrastructure,

which is of a bigger portion due to the large dead load of the

concrete deck.

10 References

CEN Eurocode 0 - Basis of structural design. - 2005.

CEN Eurocode 1 - Actions on structures - Part 1-4: General

actions. - 2005.

CEN Eurocode 1 - Actions on structures - Part 1-5: General

actions. - 2003.

CEN Eurocode 1 - Actions on structures - Part 2: Traffic loads on

Bridges. - 2003.

CEN Eurocode 2 – Design of concrete structures – Part 1:

General rules and rules for buildings . – 2004.

CEN Eurocode 7 - Geotechnical design - Part 1: General rules. -

2004.

CEN Eurocode 8 - Design of structures for earthquake resistance

- Part 1: General rules, seismic actions and rules for buildings. -

2004.

CEN Eurocode 8 - Design of structures for earthquake resistance

- Part 2: Bridges. - 2005.

Regulamento de Segurança e Acções para Estruturas de

Edifícios e Pontes. PORTO EDITORA – Junho 2007.

Prof. Manfred Theodor Schmid - A Construção e o Lançamento

de Pontes pelo processo dos segmentos empurrados, Rudloff

Industrial Ltda. – 2005 – Accessed on 11th March 2010 -

http://www.rudloff.com.br/conteudo/texto/tx_lanc_pontes.htm.

VIADUC DES BERGERES - Accessed on 24th July 2010 -

http://www.amikpon.net/A89/bergeres.html.

Octávio Martins - Modelo virtual de simulação visual da

construção de pontes executadas por lançamento incremental –

2009 – [Dissertação de Mestrado].

Reis A. J. - Pontes. Folhas da Disciplina. AEIST – 2006.

Association Française de Génie Civil - Guide des ponts pousses,

Presses de l´école nationale dês Ponts et chaussées – 1999.

Bernhard Gohler, Brian Pearson – Incrementally Launched

Bridges. Wiley, 2000.

Rosignoli Marco - Bridge Launching – Parma, Italia: Thomas

Telford Ltd, 2002.

Rosignoli Marco – Prestressing Schemes for Incrementally

Launched Bridges – Journal of Bridge Engineering, May 1999.

VSL International Ltd. – The Incremental Launching Method in

Prestressed Concrete Bridge Construction – April 1977.