190
EUROPA
Transfer trusses
The development of the Brussels metro
network required the completion of a new
connection between the Schuman and
Josaphat stations located on two different
lines. However, the plan for this new line,
defined well before the project for the new
headquarters of the Council of the European
Union, crosses the floor space of Block A
of the
Résidence Palace
, from the top of
the first basement to the floor of the fourth
basement, over a width of approximately
25 metres. This tunnel required the comple-
tion, in the existing building, of a bridging
structure transferring the loads on either
side of the tunnel. The structure studied
before the designation of the project team
for the headquarters of the Council of the
European Union was a concrete one over
two floors whose large dimensions greatly
altered Block A’s listed sections due to their
presence.
From the beginning of the project study,
Philippe Samyn suggested replacing the
concrete structure by a lighter metal struc-
ture but distributed over three floors.
From the first feasibility study, it appeared
that this metal version allowed for nearly
full respect for the listed sections. Both the
Régie des Batiments and the Council were
rapidly convinced by this version and its
study was entrusted to the Study Team.
This metal structure was made up of four
trusses, A, B, C and D, with the same external
dimensions, but whose drawing varies
depending on the window or door bays
in the existing structures adjoining them.
They are made up of reconstituted metal
profiles up to 1.20 metres high and made
up of flat metal bars up to 120 mm thick.
Trusses A and B are built within the building,
which required the demolition of the floors
in this zone over three levels. These trusses
suspend two highly reinforced concrete
slabs and 86 cm thick, one for, trusses A and
B, the other for trusses C and D, the space
between trusses B and C being taken up by
the listed corridors whose floor slab, which
could only be preserved, is supported by the
extremities of slabs AB and CD. These con-
crete slabs replace the existing slabs on the
ground floor and extend to the listed façade,
the internal load-bearing walls and the walls
of the historic stairwell. The installation
of these slabs under the works required
that they be propped using channels ap-
proximately 60 cm wide, as is common for
underpinning works.
Whilst, in the final phase, these slabs are
suspended to the trusses by flat metal rods,
highly resistant steel bars are installed ini-
tially to allow for the various jacking stages.
The first jacking stage is the one that, once
the work is completed, will allow for the ten-
sioning of the trusses by gradually lowering
the load towards the ends of the tunnel. This
phase slightly lifts the building to “unstick” it
from its foundation walls. Jacking phases are
also planned subsequently as the planning
for the completion of the tunnel requires the
underpinning of the building very early on
in the process, whilst demolitions still have
to be carried out before starting the actual
construction. These successive demolition
reconstruction phases, after the loading of
the trusses, significantly alter the stress on
them, hence the variation in their distortion,
which must be compensated by the length-
ening or shortening of the bars in order to
maintain the levelness of the concrete slab
whose structures it supports would not
tolerate the distortion.
Floors in the existing building
The floors in the preserved section of the
historic building are made up of ribbed
concrete slabs. They had been produced
with the help of a construction process
innovative for the era, consisting of placing
preformed inverted U-shaped steel sheets
on the deck support systems whose bases,
placed side-by-side, after removal of the
24-cm thick formwork, make up the tops
leaving a thin 5-cm thick slab.
Boring surveys show that these floors are
covered with a large screed whose thickness
varies from 4 to 14 cm. Calculations based
on these observations determine an overall
load-bearing capacity at 200 kg/m², which
does not correspond with the programme
of requirements. In addition, these floors
do comply with fire resistance imposed by
regulations.
This led Philippe Samyn to replace the screed
with a subfloor allowing to recover a useful
excess load on the order of 150 kg/m², and
fire resistance is provided by flock-coating
the lower side, which allowing to preserve
these ribbed slabs in the new configuration.
It also leads to cost reduction as well as
significant savings in terms of waste and
materials.
Jacques Schiffmann,
engineer, Prof., subsequently partner
of Philippe Samyn, within Setesco,
from 1986 until his retirement
WORK IN PROGRESS
