e-mosty March 2019 Long Span and Multiple Span Bridges - Page 49

Part of the cable stiffness problem is due to the longitudinal movement of the cable, the same problem occurs on the single span suspension bridge at the mid span - with about half of the span loaded. The optimum mix of stays and suspension structure is with stays covering 25% to 30% of the span from each tower. Hybrid stay-suspension bridges have been proposed (Figure 10), particularly for long spans where the stays relieve the cables of a significant proportion of the dead load. This is often solved by clamping the cable to the deck at mid-span (Figure 8f and Figure 9), thus limiting the longitudinal movement. Most modern suspension bridges since the 1930’s have been pure suspension structures. Recently the 1600m span Sultan Selim Bridge (3 rd Bosphorus Bridge), using a hybrid cable stay-suspension form, was completed 4 . Cable clamps were used on the 3-tower Taizhou Bridge (Figure 1). Negative stays clamped to the main cables at intervals along its length and anchored to the tower at deck level (Figure 8g) serve a similar purpose to the cable clamp. Another cable stiffening system is the use of inclined hangers to form a truss arrangement (Figure 8i) or a cable net if the hangers cross. The inclined hangers working with the deck and cables to form a composite system carrying imposed loads and limiting deflection. The amount of load that can be carried depends on the load in the hangers; the additional load should not be such that the hangers become slack. These inclined hangers often have increased fatigue issues. From the analysis of multi-span suspension bridges large movements of the towers are associated with the reduced stiffness. Figure 9: Cable clamp on the Runyang Yantze Bridge 3 Conventional stays radiating from the tower top (Figure 8h) were often used to stiffen early suspension bridges. Limiting the tower movement is key to stiffening a multi-span suspension bridge. Providing a more rigid tower (Figure 8j) significantly influences deflections. A-shape towers or wide braced towers have been proposed. The A-shape tower is effective in limiting deflections. Roebling’s bridges such as the Brooklyn Bridge are particularly noted for the use of additional stays. However, research in China for the Taizhou and Maashaan bridges has shown the stiffness of the The stays cannot extend far into the span unless the towers are made taller. Figure 10: A long span, deep-water, hybrid multi span suspension bridge proposal 1/2019