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Category 4: Special Projects
Introduction
In the redesign of the central squares “Korenmarkt”
and “Emile Braunplein” and nearby streets in Ghent,
a City Hall had to be designed. The hall is the central,
eye-catching feature of the masterplan.A steel structure
of about 40 m x 15 m, the hall is set on top of four
concrete pylons at the corners. The façade and roof are
covered with wood both externally and internally. The
roofs and the long façades have been given an extra
cover of glass panels above the wood.
The roof consists of two ridges and a gutter in the
middle, but it is slightly diagonal, so the façades at the
front and the back have two asymmetrical tops. This
asymmetry refers to the façade of the City Hall and has
a proportion that is close to the golden ratio of Da Vinci.
Since both of the façades are identical (not mirrored but
rotated), the building has achieved a symmetrical shape
which feels very asymmetrical. This accentuates the
strictness.
Through the wooden surface, small rectangular
windows give out, making for a very strong effect
especially on the inside.
Assignment
Our mission was to calculate the steel structure and
design it through to execution. The mission was later
enlarged to include the calculation of the wood, the inox
glass supports and the glass panels.
When calculating the steel structure, the “fun” lay in the
fact that the asymmetrical structure generated certain
forces that had to be controlled to maintain the straight
line of the long façades.
The ridge-beams and the gutter-beam were a help,
but the frame-working of profiles over the two ridges
and the scupper were essential in the stability of the
structure. The frames are supported by frames in the
long façades containing adjustable cables. The side
forces on the façades were taken by the horizontal
Vierendeel-frames. The front façades were designed as
a close web of profiles, as a solid structure.Given that
the façades are open, there were a lot of cases of wind
in which each had an influence on another part of the
structure.
On the steel structure, on top of steel purlins, wooden
girders were assembled. A multiplex with cutouts for
the 1,550 little square windows was placed on these
wooden girders. On the multiplex, on top of a waterproof
layer, twills in tropical hardwood (afrormosia) were fixed
with afrormosia planks in between, to create a smooth
wooden surface. The calculation of the wood and the
connections was another engineering challenge.
On the afrormosia twills, inox supports were fixed.
These support the glass panels. In the interior of the
hall, oak planks were assembled on wooden girders.
One challenge was to create the bottom of the gutter
like a straight sharp knife. Another was to design
the fixations in a way that achieved fire-resistance.
For all the individual parts - the steel structure,
multiplex, afrormosia, oak, inox supports, and the glass
panels and square windows - we made the stability
calculations, while we also made the production
drawings. By working this way, we were able to adjust
each component to each other.
The fire-resistance of the structure was another
important consideration.
Application of Scia Engineer
A structure like this can only be calculated by a
complete 3D-model. In this way the total influence of all
external loads can be considered and the global stability
can be calculated, with the accompanying deformations,
as also the details of connections.
Because of the very specific and strict architectural
design the freedom in stability-design was very limited
and one of the main challenges was to make the
structure possible to assemble.
The module in Scia Engineer was also very useful to
determine which profiles has to be treated with fire-
resistant paint.
City Hall “Kobra” - Gent, Belgium
Software: Scia Engineer
