Project: Hoofdkantoor KPN Rotterdam
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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.
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
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).
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.
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:
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.
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.