A Ministry that resists wind

Architect: Pierre Fakhoury - Photo credit: PFO

"The awning that will cap the Ministry of Finance looks like a bird getting ready to take off.  It is important that this resemblance should not become reality! " This is how Patrick Philippon, design engineer at Eiffel Construction métallique, describes the new Ministry project in Libreville.  The state of Gabon hired Pierre Fakhoury (the architect who designed Yamoussoukro basilica and is currently finishing rehabilitation of Abidjan's famous Ivory Hotel in Ivory Coast) to design a modern and functional complex to accommodate Ministry of Finance staff and the Minister himself.  The architect let his imagination run free and designed an aerated project with pure lines.  "There are actually three buildings, above which there is an enormous 45 m high 100 m x 100 m awning", says Isabelle Metais, Project Engineer at Eiffel Construction métallique.  It is certainly magnificent, provided that it stays in place.  Although winds are not particularly strong on the Gabon coast, there is no question of taking the slightest risk.  The awning is in an open environment not far from the sea, and large uplift and drag forces that are difficult to estimate on paper could be applied to it.  Dynamic forces and vibrations of the very slender support columns also need to be measured.  Eiffel Construction métallique, that assists the architect in the preliminary design of the steel structure, asked CSTB to carry out a wind expertise.

Wind tunnel tests

Model tests at CSTB

Jérôme Vinet, CSTB's climatological and aerodynamic engineer, explains: "We created a plexiglas model on a rotating table at a 1:150 scale.  The environment within a radius of about 270 m was included.  We needed to take into account of existing constructions and vegetation, and we did so using polystyrene projections".   730 measurement points were placed on the roof and on the three buildings supporting it, and on walkways, some glass walls and the Auditorium, to determine local loads. Global or semi-global loads were obtained by integrating synchronous pressures.  Finally, the dynamic behaviour was studied by making wind speed measurements at the columns.

The model was tested.  For two weeks, the wind blew hard on the small scale model of the Ministry of Finance in CSTB's Jules Verne wind tunnel in Nantes.  Experts carefully evaluated its effects on the structure.  "We did not have much climatological data for this area that we could use to simulate the wind speed and direction, says Jérôme Vinet.  Therefore we applied Eurocode standards that are very safe in this context."

Libreville: model in its environment
Measurement at mid-height of columns

CSTB submitted its report to Eiffel Construction métallique at the beginning of April, and Eiffel Construction métallique will study it and will no doubt ask for some points to be refined.  "We will see whether or not a specific aspect needs to be clarified after looking at the test results, says Patrick Philippon. CSTB has been very attentive to our needs in continuing the studies until they are complete so that it can provide us with reliable and complete wind design information that we will use to design the roof."

Although it is too early to give a final opinion, it can already be said that the Ministry of Libreville structural project will resist wind forces. No major modifications need to be made to the building. Finances will be firmly anchored to the ground.