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Atmospheric Boundary Layer Wind Tunnel Experiments at CSTB Nantes

Paris Saint-Lazare train station is getting a lifting as well as a new roof over the large passenger hall.
Figure 1 – Train station reduced model - Photo ©J.-L. Martin
Figure 1 – Train station reduced model - Photo ©J.-L. Martin

For this purpose, AREP & Equilibre Structures’ engineers have dimensioned the load bearing structure. Additional tests were necessary in order to optimize the structural components, especially with regards to wind and snow external loads.

For the sake of the study, a mock-up model was built by Nantes’ CTSB engineers at a 1/200 scale. All the roof panels were very precisely modeled, as well as the train station remaining buildings but with a lower level of definition (see Figure 1). Lastly the rest of the urban surroundings in a 400m radius were built with coarse blocks (see Figure 2).

Figure 2 – Rotating plate within the wind tunnel - Photo ©CSTB

The roof panels on the mock-up are then equipped with pressure gauges, as seen in Figure 3. They enable to precisely measure the maximal and minimal wind loads, either in a direct way (wind compressive forces) or indirect (roof rip-off).

The mock-up is then placed within the wind tunnel on a rotating plate (Figure 2), enabling to test every wind incidence angle.  In order to reproduce the ground roughness as well as the urban environment one, small blocks are placed upstream to yield an equivalent level of turbulence (see Figure 4).

Figure 3 – Pressure gauges - Photo ©CSTB
Figure 4 – Ground roughness and upstream turbulence creation - Photo ©CSTB

Between each experiment, the mock-up is rotated with a 20° angle to reproduce realistically the different wind incidence angles observed through the year. Results are then expressed with various representations, such as maximal or minimal pressures by wind incidence, or the overall ones (across all incidences) as shown in Figure 5, or even the maximal structural effort for a given roof span for a given wind incidence.

Figure 5 - Minimal loads on the roof for all wind incidence angles (Results from CSTB)

Those experiments, alongside some numerical CFD (Computational Fluid Dynamics) simulations and some statistical meteorological data analysis, helped to assess the actual loads on the overall structure and thus optimize for the better the structural elements dimensions.