CIB W107 Construction in Developing Countries International Symposium
“Construction in Developing Economies: New Issues and Challenges”
18 – 20 January 2006, Santiago, Chile
SPECIFICATIONS TO ACHIEVE QUALITY IN B.O.T. PROJECTS
The Vasco da Gama Bridge Example
F. A. Branco, J. Ferreira and M. M. Branco
Dept. Civil Engineering, IST – Technical University of Lisbon,
Av. Rovisco Pais, 1094-001 Lisboa, Portugal. [email protected]
Abstract
Governments, all over the world, are using the BOT (Build, Operate and Transfer) solution to finance
large infrastructure projects, when public budgets are not available. In this system, the technical
aspects of design, construction, operation and maintenance (during the concession period) are
developed by the concession company, after the signature of the contract, so the quality of the final
infrastructure needs to be very well specified during the initial tender phase. In fact, afterwards, all the
missing aspects will definitely lead to the less expensive solution, without care to achieve quality.
In this paper, illustrated with the BOT tender process of the Vasco da Gama Bridge (one of the longest
in Europe), the special technical aspects that must be considered in the tender specifications, to achieve
quality in BOT projects, are presented. These include the discussion of the service life, the
implications on safety and durability, the definition of the construction quality procedures, the
monitoring of the operation, the implementation of inspection and maintenance plans and the reception
conditions at the end of the concession.
Keywords
BOT, tender, quality, bridge, service life.
INTRODUCTION
Due to the permanent shortage of public funds, governments, all over the world, are using the BOT
solution (or other similar public-private systems) to finance all types of large infrastructure projects. In
this solution, with the initial financing of private banks, a concession company designs, builds and
operates the infrastructure. The private investment is recovered during the concession period with the
operation of the infrastructure. At the end of the concession the infrastructure is transferred back to the
Government authority. Examples of this financial idea can be found in history since many centuries
ago but it began to be currently used in the last decade of the XX century to finance the construction of
roads, bridges, airports, hospital, power plants, etc, due to the significant profits that can be obtained
from these public infrastructures.
As the technical aspects of design, construction, operation and maintenance (during the concession
period) are developed by the concession company, after the signature of the BOT concession contract,
the quality of the final infrastructure needs to be already very well defined during the initial tender
phase when does not yet exist any design. This is a very important issue of quality control in BOT
projects. In fact, afterwards, all the missing aspects will definitely lead to the less expensive solution
and the final quality of the infrastructure will be reduced.
In this paper, illustrated with the tender process of the Vasco da Gama Bridge (one of the longest in
Europe), the special technical aspects that must be considered in the tender specifications, to achieve
quality in BOT projects, are presented. These include the discussion of the service life, its implications
on safety and durability, the definition of the construction quality procedures, the monitoring of the
operation, the implementation of inspection and maintenance plans and the reception conditions at the
end of the concession.
TENDER AND TECHNICAL SPECIFICATIONS
To prepare a BOT tender, the initial studies must clearly define the associated financial model which is
usually related to:
a) Definition of the parameters of operation – ex: cars passing on a road or a bridge, patients
attended in a hospital, etc.
b) Definition of the concession time, by volume or by time – the first solution ends the
concession when a certain number of cars pass (lower risk, better profit control); the second defines a
fixed time for the concession (higher risk). The first, due to the lower risks leads to lower costs of the
infrastructure, but needs a careful control of the operation by the authorities. The second leads to
higher costs of the infrastructure and less control of the operation.
After the definition of the financial model the technical specifications for the infrastructure must be
developed for the tender (Branco 2004). The main problem to prepare the tender of a BOT project is
that the technical specifications must be written defining the quality to be achieved during design,
construction and service life of an unknown design of the future infrastructure.
Presently this is usually implemented by imposing only the application of existing technical codes
during design and construction. Unfortunately this is not enough in this type of projects, because codes
are mainly prepared for design phases controlled by the owner, where it is easy to impose changes. To
guarantee quality in BOT projects, the following technical specifications need to be additionally
considered in the tender documents:
1. Related to the design phase:
a) Definition of the structural service life - The expected structural life of the infrastructure must be
specified. This means not only the definition of the years (ex: usually 100 or 120 years for bridges),
but also the definition of the meaning of the end of the structural life (ex: level of material
degradation). Specifications should also impose the development of a durability design considering the
environment conditions and the definition of material properties, showing that the structural life will be
achieved.
b) Definition of functional service life – The functional capacity of the infrastructure (ex: maximum
volume of traffic) should be specified, defining the functional service life. Based on these elements and
on the expected traffic evolution the designer will adopt an architecture (ex: number of lanes in a
bridge) that will support the expected level of functionality during the functional life. This life may be
smaller than the structural life, meaning that a functional upgrading (ex: widening of the road, building
a new bridge) may be needed after some time, what should be analysed in the proposals;
c) Definition of structural actions – Design code actions are usually defined for current structures with
expected life of around 50 years. For important structures it must be specified that action values have
to be updated for the specified service life (ex: increase seism and wind values) and special actions
may also be considered (ex: heavy load trucks, fire accidents), achieving an increased reliability of the
structure.
2. Related to the construction phase;
a) Definition of structural monitoring – Specifications should impose the implementation of
monitoring equipment to control the structural behaviour during all the construction phases (ex:
evolution of structure and material characteristics) and of final control tests at the end of construction.
b) Definition of durability monitoring – Control of initial durability properties of materials in lab and
in situ, should also be imposed, to check the specified structural service life.
c) Construction procedures – imposition of the implementation of a quality control system where all
the major anomalies and correction procedures are previously defined are also the key stone of the
construction quality.
3. Related to Operation and Maintenance
3.1 Economical aspects
a) Definition of the traffic measurement control – As the concession time and financing of the
infrastructure may depend on the functional volume (ex: traffic) it is necessary to have, during
concession, an independent control of this volume by the authorities. The definition of this
independent system must be specified from the tender phase.
3.2 Technical aspects
a) Definition of the monitoring plan – Specifications must impose the development of monitoring
plans for structural behaviour and durability to control the behaviour of the infrastructure during the
service life.
b) Definition of the maintenance / repair plan – The implementation of a maintenance plan defining
the inspection methodology, the levels of degradation and the associated maintenance/repair
procedures, must also be specified.
c) Definition of the reception conditions at the end of the concession – The accepted levels of
deterioration for the infrastructure at the end of concession must also be specified at tender. This is
usually done based on the maintenance plan, and imposing an additional period (5 - 10 years) after the
concession end, without major repair costs.
CASE STUDY – THE VASCO DA GAMA BRIDGE
The Bridge
In 1995, the 30 years old Lisbon suspension bridge was the only existing fixed crossing over the Tagus
River in Lisbon. The high increase in road traffic, saturated the 2x2 lanes of the existing bridge, what
led to the government decision of building a 2nd crossing, the Vasco da Gama Bridge. This project
was envisaged within a BOT financial solution and a special government office (GATTEL) was
created to prepare the tender, control the design phase, perform the follow up of the construction and
keep the control of operation and maintenance during service life.
An international tender, including a special set of technical specifications, aiming to achieve quality
during service life, was prepared for the new bridge design, construction, maintenance and operation
(with toll payment during the concession period), after which the bridge operation will be transferred
back to the Portuguese Government. The tender proposals were financially evaluated based on the toll
levels and on the total volume of vehicles expected during the concession period, solution that led to
lower financial risks, better evaluation of the investment and a variable concession period (around 30
years), function of the traffic evolution. The tender was won by an international consortium
LUSOPONTE which after finishing the construction of the bridge in 1998, is presently performing its
operation and maintenance.
The Vasco da Gama Bridge consists of a 12km crossing of the Tagus River in Lisbon, being one of the
longest bridges in the world. The bridge is located in the mouth of the Tagus River which is a sea type
salty environment, subjected to tides and waves, in a high seismic zone and subjected to ocean winds.
The crossing is composed of several substructures, named sequentially as: North Viaduct, Expo
Viaduct, Main Bridge, Central Viaduct and South Viaduct (Branco 1999). The Main bridge is located
close to the North embankment, over the main navigational channel, and presents a cable stayed
solution with 420m central span (Fig.1).
Fig.1 – Vasco da Gama Bridge
Traffic and Deck Width – The Functional Life
The bridge functional life is mainly associated with the traffic volume that it can support with the
associated design speed (120km/h). Function of the expected traffic evolution, the tender specifications
defined a deck width (30m) allowing for an initial period with 2x3 lanes and a large shoulder. When
this solution begins to show functional obsolescence (traffic saturation value) the existing width allows
for the implementation of 2x4 lanes, with a smaller shoulder. This is an easy upgrading solution, when
envisaged from the tender phase.
Based on traffic predictions this solution allows a good functional level during the LUSOPONTE
exploitation period and based on a monitoring of the real traffic evolution a decision on the
construction of a 3rd crossing will be defined in the future, keeping the maximum traffic level in
Vasco da Gama Bridge for the remaining structural life.
Structural Service Life and Durability
The bridge is located in the mouth of the Tagus River which is a sea type salty environment. The
tender specifications imposed a structural service life of 120 years for the bridge. For this service life
it was also imposed, in the specifications, that no corrosion initiation begins in concrete main
reinforcing bars and no section reduction occurs in steel structural elements, considering current
maintenance procedures. This imposition was theoretically too conservative, but the variability of the
environment factors and of the present existing simulation models led to this option.
With this imposition, the global service life is expected to be achieved in most of the construction
elements, namely the structural ones, with minor maintenance costs. Of course several other
components of the bridge will have shorter service lives and will have to be substituted during the 120
years within the maintenance plan.
All these aspects are fundamental for the durability of the bridge and the implementation of the above
tender specifications, led, in practice, to the following measures during design:
a)Definition of special geometries and materials - This was performed in terms structural safety and
of physical deterioration based on local environment conditions and using mathematical models for
concrete deterioration (chlorides). In steel structures an additional steel thickness was considered to
take into account the corrosion rates estimated, what was important namely for the steel piles length
located in the inter-tidal zone.
b) Design with flexibility - The components that will need repair or replacement during the service life
were conceived to allow that their replacement/repair will be performed with minor effects on the
bridge operation. This was analysed namely for the bearings and dumpers, expansion joints, stay
cables, external prestress cables, etc.
c) Definition of a durability monitoring system - A plan with periodic in situ measurements and
testing of samples was defined at design stage, to confirm, during construction and service life, the
deterioration rates assumed in design.
Structural Service Life and Safety
For service lives different from 50 years, the characteristic values of code actions need to be changed.
For the Vasco da Gama bridge the basic design actions were defined considering the Eurocodes 1
(bridge actions) and 8 (seismic design), but they were adapted for the tender specifications to consider
a 120 years service life (Branco 2000). These changes led to some special studies here referred.
Environment thermal actions were adapted considering specific studies to define the gradient
temperatures, as a function of the bridge geometry and environment conditions, obtained from a
statistical analysis of temperature and solar radiation at the bridge location.
Wind actions, defined for the service life, imposed wind tunnel tests to check the deck stability for
250km/h wind speeds. The study of dynamic behaviour of the deck under wind gusts was also
imposed. The study of wind vibration of cable stays and the adoption of dampers was also considered.
The adaptation of the seismic actions for the service life led to the use of seismic dampers in the main
bridge (Fig.2). Due to high seismic actions, besides the ultimate limit state, it was considered a special
“trafficability limit state” for a reduced action (75%) where damages allow conditioned traffic
circulation. This was controlled mainly by maximum joint opening or pavement discontinuities.
Fig. 2 – Seismic dampers at main bridge
Construction Procedures
During the construction stage a good quality control is the best way to obtain a high level of structural
safety and durability during the planned service life. The tender specifications imposed that
construction quality control management should be performed by a quality team independent from the
contractor. Their activities were also complemented by periodical checking by the bridge authorities.
Besides the current construction procedures, the following main activities were also imposed at the
tender specifications to achieve construction quality and guarantee the defined service life.
a) Initial characterization of materials properties - Before any construction begins, the contractor had
to study the concrete compositions to achieve the mechanical and durability characteristics defined in
design. This is particularly important because durability tests take time to provide results and if they
are not performed in advance, the construction had to undergo some delays (ex: for the study of
chloride attack, durability tests were defined as initial references to check the concrete characteristics).
Besides these initial studies, a control was also specified at the reception of all the materials, and in
particular for the structural ones, whose composition was periodically checked with mechanical and
durability tests to guarantee their compliance.
b) In-situ control of properties – Tender also specified the implementation of in-situ measurements
(after application in the structure) of the mechanical and durability characteristics of the materials, to
guarantee the technical specifications. This quality control is one of the most important activities and
was imposed in a systematic way during all the construction stage. In fact the durability and
mechanical characteristics of materials, in situ, are always different from the ones obtained from lab
samples and this correlation had to be achieved to have a good simulation of the service life.
c) Construction technology – The control of the construction methods was also specified at tender,
imposing that they were studied, approved by authority, and implemented to guarantee the best
procedures in terms of achieving good quality for the materials and structure. This was complemented
with the quality control system where all the major anomalies and correction procedures were
previously defined.
d) Reception load test – Load tests were also specified to be implemented at the end of construction.
The analysis of the bridge performance, under these tests, is very important to check the models that
have been used at design and to define a reference state that can be used along the service life. The
tests are performed in the bridge under static loads allowing for the determination of the static
behaviour (deflections and strains) of the main spans along the crossing. The dynamic tests were also
imposed to obtain the dynamic characteristics of the bridge and of the forces in the stays.
Operation and Maintenance
The tender specified that, since the opening of the bridge, a structural monitoring plan, a durability
monitoring plan and an inspection and maintenance plan should be implemented to control the
evolution of the main characteristics of the bridge.
a) Structural monitoring - The structural monitoring plan was imposed to analyse some of the main
parameters that control the structural behaviour of the bridge during its life. It considers the automatic
measurement of displacements, rotations, strains, structural temperatures and vibrations in some predefined sections. The seismic accelerations on the soil are also monitored.
The structural measurements are complemented with environment measurements (air temperature,
humidity, rain fall, visibility and wind speed and direction). These measurements are associated with
an action plan where, function of the measured values, pre-defined actions are defined, allowing quick
decisions in the case of an accident like a seism or strong wind. All these values are on line monitored
and results are periodically analysed by designers, or when predefined levels are over passed.
b) Durability monitoring - The durability monitoring imposes a plan that defines periodic testing of
material sample. The corresponding durability values are then used to reanalyze the design estimation
of the service life. If deterioration is higher than previewed at design, repair measures are
implemented. The tests are, in average performed, at 0.5, 1, 2, 5, and all 5 years, after the opening.
Related to concrete elements the tests consider depth of carbonation, chloride path, diffusion
coefficient, permeability, porosity and electrical potential. Related to steel elements corrosion (steel
piles) the steel thickness is also monitored with periodic measurements.
c) Inspection and Maintenance Plan - The imposed inspection and maintenance plan defines all the
inspection points, the periodicity of inspections, eventual anomalies to be detected and maintenance
procedures. These activities are essentially based on a visual inspection complemented with simple
measurements. Particular situations of this plan were already specified from the tender phase:
c.1) Scour - Periodically the depth of the river bed is measured close to each pier. In the navigation
channels several points are obtained to draw the river bed plan. This allows the checking of the
evolution of scour problems close to the piers and if an important evolution is detected a rock scour
protection is pre-defined. This is important because the present theoretical models have very low
precision.
c.2) Cables retention - Due to the time dependent variation of the material properties (namely creep of
the deck) the retention of the cables of the main bridge was pre-defined. This operation envisaged from
tender, considered the following steps: a pre-measurement of the deck profile and of the cables tension
(by dynamic behaviour); and a numerical analysis to compute the new tensions to be implemented to
obtain the ideal profile.
c.3) Drainage system cleaning - A deficient drainage may be the cause of important deteriorations. To
prevent this, the cleaning of the road and drainage pipes was specified to be performed periodically.
Due to environment restrictions it was also specified that the drainage water from the South Viaduct
should be collected in water treatment facilities.
d) Bridge quality at the end of concession – This is a very important issue as high costs may be
involved in the rehabilitation of the bridge at the end of the concession. To prevent this it was specified
at the tender phase that the degradation level of the bridge, at the end of the concession, will be
evaluated and it must be according to the design previsions. If lower levels are found, repair measures
will be implemented according to the maintenance/repair plan, paid by the concessionaire, to achieve a
period of at least 5 years without major repair costs.
CONCLUSIONS
In this paper, the technical main aspects that must be considered in the tender specifications of a BOT
project, to achieve quality, were presented, illustrated with the case study of the Vasco da Gama
Bridge. These aspects included the discussion of the service life, its implications on safety and
durability, the definition of the construction quality procedures, the monitoring of the operation, the
implementation of the maintenance plan and the reception conditions at the end of the concession.
REFERENCES
Branco, F.; Machado,L. (1999). “Search for Quality in the Vasco da Gama Bridge”. Structures for the
Future - The Search for Quality. IABSE - International Association for Bridge and Structural
Engineering. Conf. Report. Vol.83. Zurich.
BRANCO, F.; Mendes, P.; Guerreiro, L. (2000) – “Special Studies for The Vasco da Gama Bridge”.Journal of Bridge Engineering, ASCE - American Society of Civil Engineering, vol.5, nº3,
pp.224-232, Reston.
BRANCO, F.; Brito, J. (2004) – “Handbook of Concrete Bridge Management”. Edition of ASCE Press American Society of Civil Engineering. Reston.