In Knokke-Heist (Belgium) the new health care facility AZ Zeno is being built. Spreading over a 20 hectares ground, it houses a hospital of 360 beds, a rehabilitation center, a care hotel and public event spaces on a surface area of over 48 000 m². The preliminary design stage began with a competition in 2007 and the inauguration is expected for the summer of 2017.
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Hospital AZ Zeno
in Knokke
In Knokke-Heist (Belgium) the new health care facility AZ Zeno is being built. Spreading over a 20 hectares ground, it houses a
hospital of 360 beds, a rehabilitation center, a care hotel and public event spaces on a surface area of over 48 000 m².
The preliminary design stage began with a competition in 2007 and the inauguration is expected for the summer of 2017.
Application of post-tensioning in a concrete structure building
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Architecture and landscape
The building seems to levitate above the landscape which gives
a high level of transparency to the ground floor. Its design
results from the ambition to avoid the feeling of entering a
building when walking on the ground floor towards the hospi-
tal. The graphic design model of the complete building is
shown in figure 2; figure 4 gives the starting structural concepts.
The plan view of this low-rise building consists of a central
zone, four wings and five patios (fig. 3). The building covers
overall dimensions of approximately 200 m by 150 m. Above
the open space at street level, the building consists of one tech-
nical level and three storeys. A basement is placed underneath
the main part of the complex.
At the ground floor, the centrally located reception zone is
surrounded by a 300 m glass façade that creates a seamless
transition between the inside and outside, and is favorable to
the integration of the building in the rural environment.
The same transparency and pleasant environment is pursued in the architectural configuration of the four hospital wings rising
up from the ground and covering the outdoor space with its
curve shaped undersurface. Each wing has a central patio; the
corridor and rooms situated around the patios benefit from
views on the landscape and abundant day light.
Structural conception
In the early stages of conception, several structural solutions
were considered (fig. 4). Finally, the solution of a concrete box
slab, positioned on a limited number of supports, is chosen to
create the separation between the supporting deck and the
upper structure, while the shape of the structural concrete
directly stands out and does not require a non-structural skin.
This solution is applied for the three almost identical wings A,
B and C, and for wing D and the connection zones AB, BC and
CD that are disposed around the central zone E (fig. 3). Each
wing is composed of a double slab structure supported by three
ir. Vincent Servais,
ir. Axel Rémont,
ir. Stephanie Pareit
Bureau Greisch
1
The new health care facility AZ Zeno in Knokke
2 Graphic design model: bird's-eye view on the hospital AZ Zeno
credits:: BURO II & ARCHI+I - AAPROG ? BOECKX3 General floor plan of the structure at the technical level
circular beam on
column basket connection zone BC
central zone on
column grid
thermal expansion
joint
peripheral wall supported by
façade columns AB
CD
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2
3
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4 Sketches and 3D simulation of the structural design in an
concept stage
5 Simplified model used for shape determination and
complete calculation model through wing D
6 Cross section through wing A
In the central zone E, it was important to provide free space in
the reception area. Therefore a one-storey-high steel truss
structure has been created at the first floor to support the
regular column grid of the levels above. Only the vertical shafts
and few columns remain on the ground floor. The truss level
has a horizontal floor slab and offers the space to accommodate
large technical installations. Technical ducts are connected to
this equipment and branch out to the wings of the hospital
passing through the hollow space between the cloud's slabs.
The building is founded on large concrete slabs, reinforced by
foundation piles. Under the concentrated loads, mainly below
the baskets, the foundation piles prevent excessive deformation.
Under the basement, located under the central zone and wings
B and D, these piles are loaded in tension when the groundwater
level is high.
Shape optimization
The shape of the lower plate is determined by both architectural
and structural constraints. Indeed, architecture imposes
geometric and aesthetic constraints such as:
- the small height of the concrete slab on its external border;
- the height of the concrete slab its transition to the central
zone;
or four baskets of columns. These baskets are composed of 18
to 20 inclined steel tubes with outer diameter 298.5 mm that
are arranged in two concentric circles (fig. 5).
A grid of vertical walls, varying in height from 0.70 m at the
outer perimeter to maximum 3.30 m at the supports, connects
the double slab structure consisting of two 250 mm thick plates
and gives shear resistance to the structure. The walls are situated
along the bearing directions and follow the disposition of the
upper structure's columns, at its turn defined by the architectural
grid of rooms and corridors with a typical span of 8 m (fig. 6).
The upper plate of the box slab is horizontal and supports the
three levels of hospital rooms with a classical column and beam
structure. The lower plate is curved to create a cloud-like shape
that enhances the lightweight appearance of the building. It is an
aesthetically attractive solution that also reflects the structural
behavior: additional height and structural capacity is created at
locations where bending moments are important (fig. 5). The
complex geometry, the 30 to 40 m spans and the limited height
of the structure have led to a cast in situ prestressed post-
tensioned solution in C40/50 concrete.
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The general structural concept is to set the height of the double
plate slab proportionally to the bending moments. For this
purpose a first 3D finite element model was built. In this
model, the double plate with vertical walls is simplified to one
equivalent plate with variable thickness. Internal forces are
analyzed and the thickness of the plate is defined for every
- an identical height for all supports;
- a minimum height of the structure to provide the space needed
for the technical ducts between the upper and lower plate;
- a minimum height below the lower plate to allow fire rescue access.
5
6
roof
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Hospital AZ Zeno in Knokke 3 2017
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7 Drawing of the reinforcement of
the ring beam
8 Transversal and longitudinal cross
sections of wing A
point of the surface. This is an iterative process since modifica-
tion of the plate thickness causes stress redistribution. This
process is carried out by an algorithm assigning, for each finite
element of the slab, the optimum height regarding the bending
moments in both principal directions. A manual correction is
completed to avoid too many and too abrupt variations of height.
Based on the modeling results, a precise 3D draft model of the
structure is created. It is used to build a precise finite element
model, including the representation of the upper plate, lower
plate, and vertical walls. This is the final model used for further
calculation.
Structural model
The 3D finite element plate model allows a realistic simulation
of the overall structural behavior including effects of prestressing,
creep and shrinkage, foundations and soil stiffness, and finally
phasing stages of the construction. This model creates the
possibility:
- to compute the stresses in all the different walls and plates;
- to determine the locations, the amount and the trajectories of
prestressing cables;
transversal bearing axis
longitudinal bearing axis
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7
D7
D7' D9' D10
longitudinal cross section
cross section at mid-span
transversal cross section
9 Complex steel reinforcement of the bottom plate of box slab
and the ring beams
credits: JL DERU 10 Posttensioning cables on the structural bearing axes of wing A
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Hospital AZ Zeno in Knokke 3 2017
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- to evaluate posttensioning losses due to restraints at supports and long term effects;
- to study the interaction between the different zones A, B, C, D and E, for example differential deformation at thermal
expansion joints, and forces acting between neighboring
wings;
- to evaluate displacements and their evolution over time and during the stages of the construction, taking into account
creep and shrinkage effects;
- to define pre-cambers of the upper plate and the edge line of the double slab in order to respect limited tolerance for the
disposition of the upper structure and the windows.
Conception and details of posttensioning
The vertical walls on the main bearing axes (fig. 8 and
photo 10), are each provided with two ducts containing a
number of strands varying between 19 and 37, introducing a
compressive force of 170 kN per strand after losses. One of the
design criteria to define the posttensioning forces in each axis
is to avoid cracking of the concrete in serviceability state, for
durability reasons and to limit deformations. To reach this goal,
the trajectories of the cables are adjusted to compensate tensile
stresses that correspond to the bending moments. Thus, the
compressive posttensioning force is applied in the upper plate
above the supports, and in the lower plate at mid-span.
As the global 3D finite element model does not suit to analyze
local behavior, additional models are created to study details
and connections. An example of this type of detail is the ring
beam between the box slab and the supports. At the top of each
basket structure, a concrete ring beam makes the interface
between the lower plate of the concrete cloud and the steel
tubes (fig. 7 and 12, photos 9 and 11). To obtain minimal visual
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10
Hospital AZ Zeno in Knokke 3 2017
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11 Slab mould and reinforcement of the ring beam
12 Model of the bottom slab and the ring beam
structural behaviors and interactions. Combined with
advanced execution techniques like posttensioning and
precise realization of the complex geometry, this project
successfully integrated architectural idea with the optimized
bearing structure.
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impact, both the ring beam and the column heads are sunk
into the cloud. The ring beam's most important function is the
redistribution of the vertical loads transmitted by the shear
walls into the different columns. The ring beam also allows
resisting the horizontal component of the compressive forces
arriving on one side through the inclined lower plate, and on
the other side by the tilted steel columns. Given the eccentric
position of the beam compared to the lower plate, torsion had
be taken into account. This results in highly steel reinforced
zones.
Conclusion
The realization of the new hospital in Knokke-Heist is a one of
a kind in health care construction. The structural concept is
remarkable due to its high level of transparency throughout the
building and its imaginative geometry. It requires a structural
design that includes unusual techniques for both calculation
and construction of a low-rise building. Realistic and complete
3D modeling allows to take into account all
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? PROJECT DETAILS
developer vzw Gezondheidszorg Oostkust
architect Temporary association BURO II & ARCHI+I -
AAPROG - BOECKX.
stability Greisch ? structural design of the ground level
and technical level; SCES ? foundations and
prefabricated structure of the upper levels
technical equipment Ingenium
contractor BAM contractors
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