3.7 Crossing stay cables
3.7.1 Design solutions for a multiple main span cable stayed bridge
The problems of a cable-stayed bridge with three or more towers are well recognised . The central tower cannot obtain direct support by being tied back to anchor piers and out-of-balance live load on only one of the main spans would cause a significant sway of the central tower , resulting in large deflections and large bending moments in the tower and deck .
A number of parametric studies were carried out to investigate tower and deck stiffness . The method finally adopted in the specimen design was to overlap or cross the stay cables over approximately 25 % of each main span . By overlapping the stay cables in this way , a virtual truss system is developed which provides overall global stiffness , improving both the static and dynamic performance .
When out-of-balance live load is applied to one main span , the tower movement causes the stays to lift the opposing main span . Over the region of the crossing stay cables a decompression is developed in the stay cables connected to the far flanking tower , which in turn is tied back to the far anchor piers .
3.7.2 Performance of the crossing stay cable system
The deflections of the bridge were considered when only one of the main spans is loaded . The specimen design and an identical bridge without crossing stay cables were compared ( load model 1 was applied in accordance with the UK national annex to Eurocode 1 ( BSI , 2008 )). The orthotropic deck variant is generally more flexible than the composite deck because the lighter construction means reduced stay cable quantities with a consequent reduction in global stiffness . For this variant , the deflections increased if crossing stay cables are not provided , this would cause concerns over serviceability performance .
Deflections alone do not demonstrate the need for crossing stay cables for the composite deck variant . However , their benefit is more clearly revealed when the bending moment in the deck is considered . Figure 4 shows the live load bending moment envelope for the case with and without crossing stay cables . Both main spans are shown with the central tower located at x 5 0 m . Significant moments are developed in the deck when crossing stay cables are not provided . The virtual truss system is effective at reducing these moments , allowing savings in structural steel and preventing crack control difficulties in the concrete slab of the composite variant .
Figure XXX : Live load moment envelopes in the cable-stayed bridge main spans
Another important factor is longitudinal overturning at the base of the central tower . Although Beamer Rock provides a sound foundation , the contours of the rock mean that if the footing is larger , it must be founded at a lower level , which requires expensive rock excavation below mean water level . Compared with an identical solution without them , the crossing stay cables reduce the overturning moment and allow a more compact footing .
Figure 4 : Crossing stay cable system : cable tensions due to out-of-balance loading
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