  All attempts of aeroelastic explanations for the torsional instability of
suspension bridges have been somehow criticised and none of them is unanimously
accepted by the scientific community. We suggest a new nonlinear model for a
suspension bridge and we perform numerical experiments with the parameters
corresponding to the collapsed Tacoma Narrows Bridge. We show that the
thresholds of instability are in line with those observed the day of the
collapse. Our analysis enables us to give a new explanation for the torsional
instability, only based on the nonlinear behavior of the structure.
