Cálculo de pilas con aparatos de apoyo tipo POT y tablero con punto fijo

Resumen

In recent years a great number of high speed railway bridges have been constructed within
the Spanish borders. Due to the demanding high speed trains route's geometrical requirements,
bridges frequently show remarkable lengths. This fact is the main reason why
railway bridges are overall longer than roadway bridges. In the same line, it is also worth
highlighting the importance of high speed trains braking forces compared to vehicles.
While vehicles braking forces can be tackled easily, the railway braking forces demand the
existence of a fixed-point. It is generally located at abutments where the no-displacements
requirement can be more easily achieved. In some other cases the fixed-point is placed in
one of the interior columns.
As a consequence of these bridges' length and the need of a fixed-point, temperature,
creep and shrinkage strains lead to fairly significant deck displacements, which become
greater with the distance to the fixed-point. These displacements need to be accommodated
by the piers and bearings deformation. Regular elastomeric bearings are not
able to allow such displacements and therefore are not suitable for this task. For this
reason, the use of sliding PTFE POT bearings has been an extensive practice mainly
because they permit sliding with low friction. This is not the only reason of the extensive
use of these bearings to high-speed railways bridges. The value of the vertical loads
at each bent is significantly higher than in roadway bridges. This is so mainly because
the live loads due to trains traffic are much greater than vehicles. Thus, gravel rails
foundation represents a non-negligible permanent load at all. All this together increases
the value of vertical loads to be withstood. This high vertical load demand discards
the use of conventional bearings for excessive compressions. The PTFE POT bearings'
higher technology allows to accommodate this level of compression thanks to their design.
The previously explained high-speed railway bridge configuration leads to a key fact regarding
longitudinal horizontal loads (such as breaking forces) which is the transmission
of these loads entirely to the fixed-point alone. Piers do not receive these longitudinal
horizontal loads since PTFE POT bearings displayed are longitudinally free-sliding. This
means that longitudinal horizontal actions on top of piers will not be forces but imposed
displacements. This feature leads to the need to approach these piers design in a different
manner that when piers are elastically linked to superstructure, which is the case of
elastomeric bearings.
In response to the previous, the main goal of this Thesis is to present a Design Method for
columns displaying either longitudinally fixed POT bearings or longitudinally free PTFE
POT bearings within bridges with fixed-point deck configuration, applicable to railway and road vehicles bridges. The method was developed with the intention to account for
all major parameters that play a role in these columns behavior. The long process that
has finally led to the method's formulation is rooted in the understanding of these column's
behavior. All the assumptions made to elaborate the formulations contained in
this method have been made in benefit of conservatives results.
The singularity of the analysis of columns with this configuration is due to a combination
of different aspects. One of the first steps of this work was to study they of these design
aspects and understand the role each plays in the column's response. Among these
aspects, special attention was dedicated to the column's own creep due to permanent actions
such us rheological deck displacements, and also to the longitudinally guided PTFE
POT bearings implications in the design of the column. The result of this study is the
Design Method presented in this Thesis, that allows to work out a compliant vertical
reinforcement distribution along the column. The design of horizontal reinforcement due
to shear forces is not addressed in this Thesis.
The method's formulations are meant to be applicable to the greatest number of cases,
leaving to the engineer judgement many of the different parameters values. In this regard,
this method is a helpful tool for a wide range of cases. The widespread use of European
standards in the more recent years, in particular the so-called Eurocodes, has been one
of the reasons why this Thesis has been developed in accordance with Eurocodes. Same
trend has been followed for the bearings design implications, which are covered by the
rather recent European code EN-1337. One of the most relevant aspects that this work
has taken from the Eurocodes is the non-linear calculations security format. The biaxial
bending simplified approach that shows the Design Method presented in this work also
lies on Eurocodes recommendations.
The columns under analysis are governed by a set of dimensionless parameters that are
presented in this work. The identification of these parameters is a helpful for design
purposes for two columns with identical dimensionless parameters may be designed together.
The first group of these parameters have to do with the cross-sectional behavior,
represented in the bending-curvature diagrams. A second group of parameters define the
columns response.
Thanks to this identification of the governing dimensionless parameters, it has been
possible what has been named as Dimensionless Design Curves, which basically allows to
obtain in a reduced time a preliminary vertical reinforcement column distribution. These
curves are of little use nowadays, firstly because each family of curves refer to specific
values of many different parameters and secondly because the use of computers allows for
extremely quick and accurate calculations.