Suspension Bridge Design

Date - Current

A new 30m span bridge was required for a new trail being constructed on the West Coast of the South Island. GRIT was engaged to provide technical support within a design-build framework to ensure the bridge was designed safely, with constructability and cost-savings in mind.

Background

The construction of a new trail over the river required a 30m span bridge for pedestrians and cyclists. The bridge construction company was responsible for the procurement and set up design, and construction of this bridge and engaged GRIT as a technical specialist partner to work collaboratively to help them provide the most practical and fit for purpose solution.

The Challenge

The rural location of the bridge meant that access for construction would be very difficult and reducing concrete volume was high on the priority list due to the construction cost and time associated. The design also needed to address how to achieve the required design life of the bridge in a coastal zone prone to high levels of corrosion.

GRIT’s Approach

Given the construction constraints and location, we opted to design the structure as a suspended cable suspension bridge to achieve the 30m span unsupported and avoid any interference with the underlying embankments or riverbed.

We completed a thorough design analysis to establish maximum cable tensions forces and design wind loads. This enabled us to reduce the materials required as far as reasonably practical by utilising the hard limestone rock properties on either side of the river crossing.

We also completed a durability analysis on the bridge to ensure the required design life of the structure was addressed in the design, and that some components could be replaced relatively easily if required after ongoing inspection/maintenance.

The Outcome

We collaborated closely alongside the constructor to understand the likely construction constraints and provide a practical, constructible, cost-effective, and safe solution that could be installed by skilled rope access personnel. We also went to the extent of investigating available cable and rigging products in NZ to ensure a smooth procurement process for the project.

Key Highlights

Tailored for constructability – The suspended cable 30m span suspension bridge was designed to suit construction constraints and be able to be installed by skilled rope access personnel.
Sustainability – The design minimised the impact on the environment as it avoided interfering with the underlying embankment and riverbed.
Cost savings – Significant cost and time savings were achieved for the project due to the reduction of materials and concrete volume required for construction.

Added value – Our investigation of available cable and rigging products in NZ ensured a smooth procurement process for the project.
Durability – The design addressed the required design life of the bridge in an area prone to high levels of corrosions and mitigated likely maintenance issues.