EVOLUTION LAW OF AERODYNAMIC STABILITY OF LONG-SPAN SUSPENSION BRIDGES WITH INCREASING SPANS

Aerodynamic instability is one of the significant concerns in the design of long-span suspension bridges. To provide reference for the determination of decks and spans length in the preliminary design stage of suspension bridges, the aerodynamic stability analysis of suspension bridges with spans from 1000 m to 5000 m was carried out. Based on dynamic characteristics of existing suspension bridges with spans from 888 m (Humen Bridge) to 1991 m (Akashi-Kaikyo Bridge), the developing trends of natural frequencies were discussed. Following developing tendency, finite element models (FEMs) of suspension bridges with spans from 1000 m to 5000 m were established, and the sag to span ratio was 1/11. Four commonly used forms of bridge decks were selected, i.e., single box section (SBS), latticed truss section (LTS), narrow slotted section (NSS) and wide slotted section (WSS). All the widths of decks were adjusted to 36 m with influence on aerodynamic stability excluded. 3-D nonlinear aerostatic instability analysis considering structural geometric nonlinearity and aerodynamic load nonlinearity and 3-D frequency domain flutter analysis were carried out, in which the aerodynamic parameters such as static aerodynamic coefficients and flutter derivatives were obtained from wind tunnel tests. Aerostatic instability wind speeds and flutter critical wind speeds of suspension bridges at 0° and ±3° angle of attack were calculated. The results show that aerostatic instability wind speeds have a decrement with spans from 1000 m to 3000 m. However, aerostatic instability wind speeds rise up with spans increasing between 3000 m and 5000 m. Flutter critical wind speeds continuously decrease with spans growth, similar to the decrement of torsional natural frequencies of main girder. Moreover, a comparison of aerodynamic stability by sections were studied. It shows that the minimum aerostatic instability wind speeds of suspension bridges with SBS and LTS decks are lower than the maximum gust wind speed of 80 m/s in measurement. It is also found that bridges with all four forms of sections have flutter critical wind speeds less than 70 m/s when spans are longer than 2000 m. The study indicates that the aerostatic instability is possible for SBS or LTS suspension bridges modeled in this paper with spans about 3500 m. Flutter will always be the control factor in wind-resistant design of super long-span suspension bridges, and it will be more severe with spans extending.