VIRTUAL DEAD LOAD METHOD TO CONTROL VERTICAL RIGIDITY OF MULTI-SPAN SUSPENSION BRIDGE WITHOUT PYLONS AND STIFFENING GIRDERS

摘要

The main purpose of this paper is to seek a practical method to reduce too big vertical deformation due to live loads for multi-span suspension bridges without pylons and stiffening girders. Based on the principle of stress stiffening, the virtual dead load method to control vertical rigidity is present. In nature, before the action of live loads, pretension main cables so as to increase vertical stiffness of this kind of bridges, which corresponds to applying virtual dead loads to structures. Virtual dead load takes the same mechanics effects as true dead ones and makes main cables gain bigger initial tension than usual suspension bridges with small and middle span-length, which are not pretensioned. By virtue of the transition equation for flexible suspended cables with small sags, the formulae to compute virtual dead load and the equations to determine initial sags of bearing cables are derived. The determining method of initial pretension is also formulated. The reasonable range of the ratio of deflection due to live loads to span is deducted by means of extensive numeric trial calculations. The theoretical analysis indicates that the effect of virtual load method to control vertical stiffness is significant at little building cost. The routine traffic and emergency relief traffic after huge natural disasters when using them will benefit a lot from the stronger vertical rigidity of this kind of bridges.