Self-anchored suspension bridges differ from the type discussed in Arts. 15.16 only in that external anchorages are dispensed with (see Art. 15.3). Unlike the externally anchored type, self-anchored suspension bridges may properly be analyzed by the elastic theory, since the effect of distortions of the structural geometry under live load is practically eliminated. The structure is also not stressed by uniform temperature change of cables and stiffening girders. The analysis is thus simpler. But the favorable reductions of bending moments that occur with externally anchored suspension bridges are lost. Furthermore, the effect of axial load in the stiffening girder must be considered, as well as the effect of girder camber.
For a symmetrical three-span structure with continuous stiffening girders (Fig. 15.53), a plane system (cable, suspenders, and girder) has three redundants. C. H. Gronquist derived in simple form the elastic-theory equations for determining the redundants for a continuous stiffening-truss system. He took into account camber and its action in reducing cable and truss stress by archlike action. He also demonstrated that the equations for the horizontal component of cable tension Hp under live load for the self-anchored bridge, with girder camber and shortening eliminated, are the same as the elastic-theory equations for an externally anchored suspension bridge.
