ABSTRACT

The design of the Messina Bridge deck has inspired the design of a number of recent long span bridges with two individual box girders separated by

a relatively wide gap. A number of such bridges with dual box decks are currently under construction and these provide valuable information on fab­ rication and construction issues associated with multi box decks. In the design competition for the Stonecutters Bridge in Hong Kong in 1999-2000, the winning design was a cable stayed bridge with a dual box deck.The dual box configuration for the Stonecutters Bridge was also influ­ enced by the decision to form the two 300 m high towers as simple single vertical columns of conical shape on the bridge centreline.The width of the gap between the two boxes was consequently dictated by the tower diam­ eter at deck level and not by aerodynamic considerations. (Figures 8.4 and 8.5) With a different tower design it would have been possible to arrive at a narrower gap or even to have a simple mono-box deck.This was actually later proved by the design of the Sutong Bridge where a mono-box deck was used for a span 70 m longer than that of the Stonecutters Bridge. In the original competition design forthe Stonecutters Bridge each of the two boxes had a continuously curved soffit but during detailed design the box shape was changed so that only the outer parts of the boxes were curved whereas the inner parts were plane.This design change simplified the joint between the longitudinal boxes and the cross beams, as well as improv­ ing the aerodynamic behaviour. With the sharp corners at the transition

between the curved and the plane soffit it proved advantageous to add guide vanes to suppress vortex shedding and avoid the associated vertical vibrations that could lead to user discomfort. Coincident with the design of the Stonecutters Bridge, another major cable supported bridge was being designed in Hong Kong -theTsing Lung Bridge. With a span of 1450 m it would have had the third longest span of any sus­ pension bridge at the time. (Figure 8.6) Due to its location close to Hong Kong's new international airport at Chep Lap Kok there was a height restric­ tion for the towers. As a consequence, the main cable sag to span ratio had to be limited to 1/15 - a smaller value than found in any other major suspen­ sion bridge.