Page 121 - THE FIRE STATION

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THE BUILDING’S STEEL SKIN
Ghislain André et Denis Mélotte
The galvanized steel sheeting surrounding the building has two
functions: (i) it protects the building against the sun; and (ii) it regardless of the relative position of the rails and the sheeting. A
acts as a railing for the 4th-floor roof terrace and its perimeter
running track. The sheeting’s 51% perforation rate (12 mm holes
diagonally spaced 16mm from centre to centre) makes it seem
virtually transparent for building occupants. Nevertheless, this
thin (2mm) sheeting offers substantial protection from the sun,
allowing a choice of glazing with average solar performance
(solar factor (g) = 40%). This in turn benefits light transmittance
(LT = 71%) and colour rendering (CRI = 95%).
The perforated skin is 7.584 metres high. In the initial design,
three horizontal support rails were planned. In the course of
a dialogue between designers and builders (see the Chapter 6
on page 179), the number of supports was reduced to two with-
out any impairment to stability or rigidity.
The sheets are fixed to each waved fixing plate (i.e. every 250
mm) on these two rails. The rails are galvanised steel U profiles
(a cross-section 100/60/4 mm for the bottom rail and 100/60/5 mm
for the top rail), cut in 3.00 m sections, curved and fixed to the
facade every 1.50 m by means of lugs with a thermal break dis-
cussed in detail in Chapter 7, p. 202. While the U-profile rails and sheets must be aligned to allow the sheets to be properly fixed.
lugs can be installed simply by nuts and bolts, the sheeting has
to be attached from the outside, without any bolting. The conven-
tional solution in such situations is to use self-tapping screws,
but this method has two important drawbacks, both of which are
unacceptable: first, a screw’s point of penetration would pierce
the galvanisation layer, impairing its anti-corrosion properties;
and, second, the iron filings produced by the screwing are sub-
ject to corrosion and would inevitably cause rusting.
It was therefore important that all machining of the support rails
be done in the factory and that the system’s geometry ensure that,
at each attachment point on the rail, there was always at least one
hole in the sheeting in the right place to accommodate a screw,
simple geometric simulation revealed that a 30 mm circle fulfilled
this condition. The circular holes were thus pierced every 250 mm
in the U-profile rail prior to galvanisation. A 40/20 tube made
of 2 mm thick aluminium was then slid into the inside profile of
the rails and fixed with M8*40 bolts every 250mm, each under a
waved fixing plate. The sheets were then fixed directly to the alu-
minium tube, through 30 mm holes made in the U-profile support
rail, by means of self-tapping screws 6 mm in diameter placed in
the waved fixing plates and equipped with rectangular galvanized
steel plates (50/30/2 mm) increasing the contact surface between
the screw head and the steel sheet. The aluminium tube thus
forms a receptacle for its own filings, and thus not subject to
corrosion.
The fixing system must also take account of major tolerances
(+20 / -5 mm) in the length of the perforated sheets, inherent
to the cutting method used. Two adjacent metal sheets can
thus differ in length by up to 25 mm.
It goes without saying that the pierced holes in two adjacent
While the tolerance is hardly noticeable on the bottom edge of
the facade, at its top edge the non-alignment of two sheets can
reach a maximum of 25 + 8 = 33 mm. A curved galvanised L
profile (80/50(2) covers up these differences, while at the same
time rigidifying the structure. This profile was originally meant
to be attached to the outside of the sheeting. However, the deci-
sion was taken in the construction phase to move it to the inside,
meaning that all horizontal profiles are only just traceable to
someone looking at the building form the outside.
Finally, the sheets are secured at regular intervals by blind
rivet nuts.