projecto base de uma ponte ferroviária com tabuleiro de
TRANSCRIPT
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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.