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Hoofdkantoor KPN Rotterdam
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DRAAGCONSTRUCTIE
The
inclination
The 6
degrees inclination of the building, or better to say of its eastern facade
close to the Erasmus bridge, was the first challenge to the structural engineer
to solve. The position of the columns (vertical supports), in relation to the
inclined side of the building, influences the horizontal force components of
this inclination. Namely, the bigger the span, the bigger the vertical floor
support reaction to be carried by the inclined side and the bigger the
horizontal load caused by this inclination on the stabilising structure (Fig.
10). However, even with the best possible position of the columns in the given
architectural design, this horizontal force component was still too big to be
taken by conventional structural measures like cores or shear walls only. As an
excellent architect also Renzo Piano had the same feeling and has drawn already
in his first sketches on the back side of his cigar box a reversely inclined
compression strut to compensate and support the leaning side of the building.
This synergy of form and structure and the chemistry of understanding in
between the different disciplines had a big potential in it to come to a strong
architectural and structural concept for this building.
10
11
The
static scheme
The
reversely inclined strut, depending on its angle of inclination, the magnitude
of the vertical load assigned to it, the place where it is attached to the
building and the way how it is designed and detailed to function in the total
structural scheme can:
-Compensate
(counterbalance) the horizontal forces caused by inclination of the building
-Form
part of (and function as) an outrigger
-Perform
both
An
analysis of different structural schemes revealed that the most economical
solution was the counter balancing only, in combination with structural core
for overall stability (Fig. 11).
12
13
For
counter balancing the central column in the inclined facade does not continue
to the foundation but stops at the level +10.50m above ground level. The total
force in this column is transferred to the strut and together with the tuned
angle of the strut it counterbalances from the level +46.55m the influence of
the slope of the building.
The
structure
Once
established, this static concept appeared to be a very strong one. Different
alternatives in different structural technologies could be realised. Out of
these alternatives two economically equal ones were considered and maintained
up to the tender stage. Both alternatives were alternatives with hybrid
structures:
-Alternative
1
Cast in
situ core and load bearing facade in grid 8 with precast prestressed hollow
core slabs in between grids 6 and 8. The inclined part of the building in
between grids II and 6 being made of structural steel with composite steel
concrete floors.
-Alternative
2
Cast in
situ core and load bearing facade in grid 8 with precast prestressed hollow
core slabs in between grids 6 and 8, the inclined part of the building in
between grids II and 6 being made with two storey high precast concrete columns
and composite precast concrete beams bearing precast prestressed hollow core
slabs (Fig. 12 + 13).
In both
alternatives the compression strut has been designed as steel concrete
composite member with structural steel tube diameter 800mm at the ends (2000mm
in the middle) partially filled with concrete over a length of 5 m at both
ends. The floor at level +46.55m where the horizontal balancing force from the
strut is transferred to the structure is designed as massive 260mm thick cast
in situ concrete floor.
Finally
on grounds of contractors experience and preference, as there was no difference
in price, the alternative 2 was chosen.