Tower geometry, aesthetics and practicality The two 157-meter high main span bridge mono towers are a dominant aspect of the project’s visual impact. Tower geometric form is critical. Once the project team decided on a single-shaft tower design, the tower’s geometric form began to take shape. A conical form was originally developed, because the circular form is visually pleasing and responds well to the pattern of the stay cable anchorages and governing seismic demands, which can be of the same order of mag- nitude in any direction. The team reviewed a similar conical geometry adopted for the 308-me- ter tall Stonecutters Bridge towers. In that case, the conical form was constructed with a self-climbing formwork system designed to adapt to the reducing radius of the section. However, given the shorter height of the Gerald Desmond Bridge Replacement Project towers, such a specialized formwork system would not be economical. The conical form was not developed further. Two further sections were contemplated: • A modified cone with constant radius corners and a tapering flat section • An eight sided geometry transforming from an octagon base to a square form at the top. Of which the octagonal base was retained. The octagonal transformation is carried out by tapering four of the eight sides while maintaining the other four at a constant dimension. This approach lends itself to an efficient climbing formwork arrange- ment because out of the eight jumping vertical formwork components, only four change dimensions at each jump. A design decision was made to taper the faces which are orthogonal to the bridges primary axes. Tapering faces at 45 degrees to the primary axes would have resulted in a “square geometry” that is incompatible with stay cable geometry: the fan of cables will intersect with the cor- ner of the tower meaning that some of the anchorages will pass through the section’s corner. Keeping the diagonal faces constant resulted in a “diamond geometry”, simultaneously resolving the geometric conflict between cable stays and section corners and creating a unique and instantly recognizable tower form.
Figure 2: Gerald Desmond Bridge main span bridge tower sectional evolution
Figure 3: Development of the tower section
Reducing the need for maintenance Arup’s monopole tower and fused viscous damper solution provides a non-invasive post-seismic remediation plan where the bridge deck is repositioned with jacks, and broken fuses are replaced without the need to alter the bridge substructure. Towers and end bents are simplified to be less congested with fewer items to inspect and maintain, while means of access are provided to conveniently access each viscous damper for inspection or fuse replacement without the need for hoists or manlifts.
Figure 4: Evolution of tower cross section from tower base (left) to top of tower (right)
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october 2020
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