There were problems with the innovative use of blue reflective glass in a steel tower: entire 4′ × 11′, 500-lb (1.2 × 3.4 m, 227 kg) windowpanes detached from the building and crashed to the sidewalk hundreds of feet below. Police closed off surrounding streets whenever winds reached 45 mph (72 km/h). Under the direction of Frank H. Durgin of MIT's Wright Brothers Wind Tunnel a scale model of the entire Back Bay and an aeroelastic model of the John Hancock Tower were built and tested in the wind tunnel to identify the problem. The research raised questions about the structural integrity of the entire building (due to unanticipated twisting of the structure), but did not account for the loss of the glass panels. An independent laboratory eventually confirmed that the failure of the glass was due to oscillations and repeated thermal stresses caused by the expansion and contraction of the air between the inner and outer glass panels which formed each window; the resilient bonding between the inner glass, reflective material, and outer glass was so stiff that it was transmitting the force to the outer glass (instead of absorbing it), thus causing the glass to fail.