۱- Introduction

۲- Shake table tests

۳- Numerical model

۴- Measured and numerical results

۵- Numerical study on seismic response of inclined tower legs

۶- Influence of torsional stiffness

۷- Conclusion

References

Abstract

More than half of cable-stayed bridges adopt the tower with inclined legs, which can be loaded with a combination of bending, torsion, shear and axial force under earthquakes. To study the seismic response of inclined tower legs, shake table tests were conducted on a 1/20 scaled cable-stayed bridge model with an inverted Y-shaped tower. A description of the model design was introduced and observed damages including horizontal and diagonal cracks at inclined tower legs were presented. A numerical model, considering reduction of torsional stiffness of inclined tower legs after diagonal cracking, was established. The feasibility of the numerical model was validated by a comparison of numerical and test results, which showed good correlation in displacement response at tower top and deck end, and cable force. Based on numerical results, the crack torsional moment of the inclined tower could be easily reached at small peak ground acceleration (PGA), leading to a substantial reduction of torsional stiffness of the section. This reduction helped alleviate the torsional moment demand at larger PGAs and delay the torsional failure of the tower legs. Numerical results also revealed that the bending moment is the primary factor to cause concrete cracks at the lower regions of inclined tower legs whereas complex interaction of large bending moment and torsion results in flexural and torsional damage near the intersection. Conventional ways, which adopt an elastic behavior of torsion, either using stiffness prior to or after diagonal cracking, will lead to intensive overestimation or underestimation of torsional response of the inclined tower legs.

Introduction

In recent decades, with the continual development in design methodology and construction technology, cable-stayed bridges have gained worldwide popularity in spanning large distance, probably up to 1000 m. The ability of a cable-stayed bridge to span such large distance owes much to its supporting system: the decks are supported by cables that are diagonally resisted by strong stiff towers. Acting as the main load-bearing element, the tower is of critical importance to a cablestayed bridge. In general, the tower should be designed to vertically resist large gravity load and also accommodate an amount of lateral loads associated with live loads, wind and seismic actions, and possible others [1]. The tower shape, by affecting load transmitting mechanisms, to some extent determines the success of a tower design. There are several tower shapes that have been successfully applied to practical engineering, namely H-shaped, A-shaped, inverted Y-shaped, single column, diamond-shaped and so forth. Fig. 1(a) shows the typical tower configurations of cable-stayed bridges. A detailed analysis of the bridge tower shape inventory of 70 existing cable-stayed bridges in China shows that approximately 1/3 of cable-stayed bridges utilized H-shaped tower and another 1/3 adopted inverted Y-shaped tower while the rest occupied the remaining 1/3 [2], as shown in Fig. 1(b). Also note that more than half of the bridges utilized the tower with inclined legs (including Y-shaped, A shaped, and diamond-shaped). The preference of the tower with inclined legs owes to the aesthetical appearance, efficiency and stability of the structure.