Scia User Contest 2005 - page 101

Structural Design
Generalities and critical elements of the design
The structural design of the construction
was
made entirely by Besix Design Department. The
high degree of complexity of this design is due to:
• The project has numerous and very critical
structural irregularities in the foundations and
the superstructure.
• The shape, the height as well as the presence
of surrounding high rise buildings
make it
especially sensitive to
wind effects. For these
reasons, a
wind tunnel test
was
made on a
physical
model. These tests have been used to
define the wind load on the structure and to
confirm that local turbulence does not affect
the safety of pedestrians using the adjacent
public spaces.
Moreover the
wind action
induces dynamic effects increasing the forces
in the structure and reduces the comfort of the
occupants.
On this last aspect, the literature
specifies that the level of discomfort is reached
when the horizontal acceleration that users are
subjected to exceeds 0.02g or 0.2m/sec².
Con-
sequently, the design includes a detailed analy-
sis of the
wind effects, both on the induced
forces and the horizontal acceleration.
• The dynamic behaviour of a building as slen-
der as the CBX tower depends also significant-
ly on the soil - structure interaction that has
therefore been thoroughly examined.
• The asymmetrical and hybrid foundation sys-
tem requires a precise calculation of the differ-
ential settlements.
• The average vertical compression stress in the
columns is
much higher than in the core
walls. This means, for high rise buildings, very
significant differential displacements between
the central core and the façade columns due
to the elasticity, shrinkage and creep. The
design includes a detailed examination of
these displacements and required compensa-
tions at each level.
Modelling and structural calculation
A complete 3D model of the tower including all
slabs, beams,
walls, columns, piles and the raft was
made with ESA-Prima Win.
The details of the
model incorporate even the
main openings (up to 2 m²) in the walls and slabs,
the different type of concrete grades, the cracked
inertia of the elements subject to bending and the
springs simulating the soil - structure interaction.
The model includes all loads on the slabs as well as
the wind forces from 36 different angles.
Considering the exceptional size of the
model,
three different users worked, in a first stage, on
three separate parts. These
were subsequently
assembled to constitute one complete model of
the building. For the typical floor, the possibility
to copy and paste groups of elements
was
invaluable for the efficiency of the
model con-
struction.
These
programming
advantages
enabled the engineers to complete a model run-
ning on PC within three weeks.
The size of the mesh used for the general down
load path calculation and the dynamic behav-
iour of the tower was of about 2m. This size was
locally refined to detail the calculation of the
forces in parts of the structure such as on one
floor level or walls on three levels.
The results obtained from the complete 3D
model
were compared
with partial
model and
hand calculations. This has specifically been the
case for the down load path, the transfer ele-
ments and the slabs. These comparisons showed
that the different approaches were either corre-
sponding or complementary. It is also to note
that, for such large model, the various possibili-
ties of graphical viewing are essential for the effi-
ciency or the result interpretation and
model
control.
The calculation of the horizontal acceleration
resulting from the dynamic behaviour of the
tower under the wind effects was deducted from
the methods given in the Eurocode 1 EN 1991-1-
4.6 and the CTICM n°3,
wherein the parameters
of frequency and static horizontal displacement
under peak wind loads,
were calculated with the
complete 3D model
The analysis of the settlements and soil-structure
interaction, necessary to define the stiffness of the
springs to locate at the foot of the
model,
was
made
with the geotechnical computer program
PLAXIS and a hand calculation approach.
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