Even artists turn to RWDI for engineering support, and in the case of Christo’s Gates in New York’s Central Park, our support was focused on the wind engineering analyses. The purpose was to ensure the stability of the 7,503 individual gate-like structures that were to be erected for this exhibition in January and February 2005.
Reminiscent of the Japanese torii gates traditionally found at the entrance to Shinto shrines, the 16-foot-tall gates of varying widths were set up along the 23 miles of pathways in New York’s Central Park. Each of the Central Park gates consisted of two uprights and a crossbar, all made of square vinyl tube. A piece of saffron-colored nylon fabric hung from the crossbar and reached halfway to the ground.
All of the gates were individual, free-standing structures and had to remain stable and erect throughout the duration of the erection and exhibition, even in the face of the city’s predictably gusty winter winds. Each vertical piece was attached to a long, narrow steel base plate at right angles to the axis of the gate. But because this was a temporary exhibition, rather than a permanent installation, none of the gate supports could be anchored to the ground. Instead, the mass and geometry of the steel base plates had to be sufficient to resist the wind-induced loads on the gates, such that gravity alone would prevent them from overturning in the event of high winds. We were hired to validate the design concept.
We combined a statistical analysis of historic wind data for Central Park with wind tunnel testing of scale models of the gates to determine how the proposed base structures would perform. By analyzing New York City weather data for the month of February, which was when the exhibition would take place, we determined appropriate design wind speeds for reference in the wind tunnel tests.
For the wind tunnel tests, we built 1:7 scale aeroelastic models of the gates, in three different widths, that were geometrically and aerodynamically similar to the full-size gates. Preliminary tests confirmed to us that the worst case loading occurred with head-on wind perpendicular to the gate and was not directly affected by the presence of adjacent gates.
We subjected each of the models to simulated turbulent flow in our boundary layer wind tunnel, measuring the overturning moment at the base of each leg of the model throughout the test. Finally, by applying appropriate scaling factors, we converted the responses measured on the aerolastic models to full scale responses.
The wind tunnel tests identified the base plate weight that would be required to prevent each of the gates from overturning. Our findings enabled those involved in the project to confidently design for an appropriate level of safety accounting for a high wind event that might have occurred during the installation and exhibition of The Gates.
The Gates project proceeded with full confidence of structural stability, thanks to thorough and complete engineering prior to its execution. Then-mayor Michael Bloomberg officially opened The Gates exhibition the morning of February 12, 2005, by unfurling the first piece of fabric, and by afternoon all the panels were unfurled. Over the next two weeks an estimated 4 million visitors from around the world came to Central Park to view The Gates, which were then removed without a trace.