<p>Multi-span suspension bridges have many advantages with respect to constructability, cost and aesthetic aspect, compared to 3-span suspension bridges, because the number of anchorages can be reduced. However, there are not enough design experiences for such bridge type so that various studies for multi-span suspension bridge must be carried out. Especially, the slip between cable and saddle on the top of middle pylon is one of the most important factors in the design phase.</p> <p>In this paper, parametric analyses were performed to examine the variation of safety factor for cable slip according to design variables, such as sag ratio, stiffness of middle pylon and girder for 4-span suspension bridge. Eventually, the followings were obtained.</p> <p>As the sag ratio or the stiffness of middle pylon decreases, the safety factor for cable slip increased. When the self-weight of stiffening girder increased, the safety factor also increased.</p>

<p>A development for the new structural system having a superior wind resistance performance may be the alternative way for ensuring wind stability. The cable stayed-suspension bridge is one of alternative structural types. In this study, a parametric analysis was performed to examine static and dynamic behaviour of the cable stayed-suspension bridge according to variation of girder length supported by stay cable. The analytical result shows that the reasonable span ratio of stayed span over main span (stayed + suspended span) is between the range of 0.25~0.30. Also, structural performance of the cable stayed-suspension bridge with the span ratio of 0.25~0.30 is improved by more than 50% in comparison with a conventional suspension bridge. This improvement can lead to the reduction for the steel quantity of cable and stiffening girder and the size of anchorage.</p>