We were super thrilled and honored on Tuesday night (2nd August 2022) for the Water of Leith Cable-stayed Footbridge designed by DC Structures Studio to be recognized as the WINNER at the Bridge NZ Awards 2022 for the highly prestigious “Bridge Design Excellence” Award.
We were humbled by the fact that the Water of Leith Bridge was described as the “unanimous” choice by the team of 10 highly respected and accomplished judges. The judges made the following comments which really caught the true intention of our bridge which really excited us:
“It’s become a gateway for the local community and a positive legacy for the people and city of Dunedin…. The overarching bridge form and architecture combined with sustainability and the approach to recycled materials in the deck and the use of New Zealand sourced engineering timber [to replace 30t of steel] has produced a striking structural form and is an example for now and into the future. An excellent example of a walking and cycling bridge for Aotearoa New Zealand.”
[NZ Bridge Awards 2022 judges]
The awards were hosted by Brightstar and endorsed by Waka Kotahi (NZTA).
We’d like to thank Anton Kivell of Hoffcon for accepting the award on our behalf (I was unable to attend because of Covid). Anton did a sterling job being Dan Crocker for the night! I’m sure after a few beers he even toyed with putting on a mock British/Westcountry accent for the full effect!
We would also like to thank Eli Maynard (Geosolve, Geotech), Liam Edwards (WSP, Peer Review), Walter Raikes (Edifice Contracts, Constructor), and HML Steel (Fabricator) for helping make this bridge an award winner. Thanks also to the Dunedin City Council and the GHD & Bonisch client team for having faith in our brave new ideas from the start.
Key features of our submission:
- First bridge in NZ to use a 93% recycled deck.
- Our streamlined cable stayed bridge form, in combination with widespread use of FSC accredited glulaminated timber and recycled decking planks, saved 30 tonnes of steel (50%) when compared to the original Specimen Design. This is a substantial reduction in the carbon footprint of the project and underpins the overall design direction.
- Lighting design is both discrete and elegant whilst adopting industry leading “Dark Skies” design ethos to reduce impact on the wider environment.
- First bridge in NZ to use stainless steel mesh as a primary balustrade infill system.
- First bridge in NZ to be modelled (3D) and drawn (2D) using Trimble Sketchup + Trimble Layout software respectively. This substantially reduces design related overheads whilst still creating a fully 2-way integrated design process between 3D and 2D.
- Uses complex buckling analyses, historical aerodynamic wind records, and well considered liquefaction philosophies to optimize design for tangible material and cost savings for our client.
Full submission content is below so everyone can read more about some of the novel sustainability aspects. Hopefully this might inspire other designers out there to push the boundaries! Please get in touch if you want to find out more.
(full submission below…..)
Background to the water of Leith Cable-stayed Bridge
The Water of Leith Bridge is a 45m span x 3.5m wide cable stayed footbridge with a 22m high steel mast. The bridge is adjacent to the Forsyth Barr Stadium and crosses the Leith south of the adjacent railway. The bridge was selected by Dunedin City Council following a successful Design and Construct (D&C) tendering process. The bridge was accepted as an alternative to the steel truss bridge specimen design originally contained in the D&C tender documents. Design work started in January 2018 and it was opened to the public in December 2018 (12 months). The total cost was $1.4 million which includes all design, review, and construction monitoring.
Noting that this project was delivered as a Design and Construct project, it stands apart from the industry perception that D&C projects result in “functional no thrills design”. The solution put forward during the tender (and meticulously delivered to construction) threw out the clunky steel truss bridge specimen design and promoted the use of a streamlined, elegant, and environmentally responsible cable stayed bridge. The bridge has become a local landmark and forms an integral part to encouraging uptake of greener transport solutions and use of Dunedin’s new cycleway networks. The bridge also sits proudly alongside the Forsyth Barr Stadium ready to welcome guests from all over NZ during large sporting and musical events.
Bridge Innovation: New ideas in design
Revolutionary use of cost-effective but fully holistic 3D-2D software
We believe this is the first large-scale bridge in NZ to be modelled in 3D using Trimble Sketchup™ with the subsequent 2D drawings produced using the Trimble Layout™ software. This is a full 2-way integrated software system with the 3D model remaining the primary element for change and never becoming redundant (which is often the case as the 2D drawings progress).
Why is this important? The advancement of 3D modelling in recent decades for the design of bridges has seen a steady reduction in onsite clashes and mismatches that are not otherwise obvious when drawn in 2D planes. Unfortunately, these 3D platforms are often expensive and timely to adopt which limits their take-up by smaller firms not wanting the high overheads. Trimble Sketchup and the associated Trimble Layout are significantly cheaper (USD$240 per year) and easier to adopt than their traditional alternatives such as Revit, Solidworks, Microstation, etc.
The benefit in demonstrating the power and suitability of Sketchup for complex bridges such as Water of Leith, is that it highlights opportunities for smaller scale firms to master, utilize, and incorporate 3D modelling into their everyday working practices for the benefit of their clients.
Innovative bridge balustrade infill systems
We believe this is the first instance of stainless-steel mesh infill systems being used as the primary fall restraint system for bridge balustrades in NZ. There are past examples of the stainless-steel mesh systems being used for anti-throw screens, but none to our knowledge where it is used as the primary balustrade infill system. Stainless steel wire infill systems are a fantastic option for lightweight footbridges that are used internationally as both an economic and durable solution. Unfortunately, the NZ Building Code Section F4 (Safety from Falling) has always been the limiting factor with numerous councils flagging the traditional horizontal wire infill systems as being deemed non-compliant by failing the non-climbable requirement.
Using a 35mm maximum horizontal opening means the system is deemed fully NZ Building Code compliant. By teaming this system with glulam balustrade posts we have also successfully navigated the common issue of isolating the stainless-steel system from the mild steel balustrade posts that would otherwise be adopted.
The use of the stainless-steel infill mesh was cost effective, durable, quick to install, and overall provides a lightweight architectural appearance that contrasts well with the timber post and stainless-steel rail.
Optimized bridge design to save material and cost
- Design of cable stayed bridges is a highly sophisticated branch of bridge engineering. To understand and optimize the behavior of the cable stayed bridge, a 3D finite element model was created that ran all loads (except modal) as non-linear load-cases. Design was performed to the New Zealand Transport Agency Bridge Manual v3.
- A key cost-saving design approach was to model the movement of the dead-man as the surrounding soil liquefies. The movement of the dead-man during liquefaction increases as the passive soil effects are lost and the piles become the primary (but more flexible) restraint. This movement was assessed and correlated to resultant bridge sag effects. The bridge was proven to remain stable without collapse, and although this sagging is undesirable, bridge camber can be reinstated with subsequent cable restressing. This philosophy of considering the overall consequences of liquefaction, rather than just preventing liquefaction, saved unnecessary, and significant, investment in ground improvements. A great outcome for our client.
- The resultant buckling shape recorded during our buckling analysis is not the buckling shape of a typical cantilever (effective length 2H) and is instead consistent with the effective length of a mast pinned at both ends (effective length = H). Using a sophisticated buckling analysis in this way to quantify the mode shapes was an excellent way to justify a longer buckling length and significantly reduce the quantum of steel. Our findings were confirmed by our peer reviewer (WSP-Opus) and supported by respected text on the matter. This approach helped reduce mast diameter and thus streamlined the overall aesthetic of the bridge.
- Wind vibration / aerodynamic effects on the bridge are assessed using BD 49/01 “Design Rules for Aerodynamic Effects on Bridges” (Highways Agency UK, 2001). Our bridge was compliant with the majority of rules but showed potential vulnerability to torsional divergence. BD49 is an empirical code applicable to a vast array of different bridge types, material, and forms and thus has unwanted conservatism built into it. To prevent the need for detailed wind tunnel testing (timely and costly) our designers found, referenced, and correlated the Water of Leith cross section to previous test data as recorded in “TRL Report 530[1]” (a document referenced by BD49). By correlating our bridge to this historical test data, we were successfully able to prevent the need for wind tunnel testing and increase our critical wind speed for torsional convergence to acceptable levels.
Structural efficiency & sustainability of bridge materials
- The Water of Leith Bridge is a good example where carefully considered bridge design has led to sustainable and environmentally conscious outcomes. When the project was released for tender in 2017 it was initially conceived as a 60 tonne steel superstructure (steel truss, with steel deck, & steel balustrade system). As part of the early design process, we set about considering alternative bridge forms and materials. The chosen option was a cable-stayed footbridge that combines steel + engineered timber + recycled decking planks into an elegant crossing. By considering a more efficient bridge form and supplementing steel where possible with engineered timber and recycled decking, the total superstructure + mast steelwork was reduced by 30 tonnes.
- All of the glulaminated timber used for joists and balustrades are from Forestry Stewardship Council (FSC) accredited sources. FSC certification is internationally recognised as the most rigorous environmental and social standard for responsible forest management.
- Our design uses decking manufactured from 93% recycled materials (HDPE plastics and bamboo). The product is highly durable with a non-slip surfacing integrated into the top surface. It has incredible sustainable credentials when compared to concrete, steel, and/or FRP equivalents. A key benefit of using the recycled decking (compared to concrete or steel decking options) comes from its lightweight properties and proven performance in marine environments. We believe this is the first time a 93% recycled product has been used for bridge decking in NZ[2]. In using this product, we worked tirelessly with the manufacturers’ technical team to confirm load, durability, and slip performance consistent with the Building Code. This investment of effort and time has yielded a truly unique and responsible outcome to bridge decking systems which we believe is a first in NZ. Weighing only 0.27 KPa this is significantly lighter than alternative concrete systems and has lower ongoing maintenance than the conventional steel + epoxy anti-slip topping systems.
- By eliminating 30 tonnes of steelwork (50%), and using NZ sourced FSC glulam timber in combination with the recycled decking, we were thus able to significantly reduce the impact on the global environment caused by the production and transportation (generally from China) of energy intensive steelwork.
- Lighting design was added to our scope during the project. When designing the architectural and functional lighting systems, “Dark Skies” best practice was adopted. The Dark Skies movement helps “champion best lighting practices which illuminate our surrounds with care and respect whilst safeguarding human, ecological and environmental health”. Traditionally, landmarks have been lit using ground mounted floodlights pointing upwards. The benefit of this approach is the simple underground wiring and the overall ability to light large areas. Unfortunately, this approach is poor for the surrounding environment as these upward facing wide angle projectors spill large amounts of light to the environment thus adding to the unwanted “city glow”. When lighting the Water of Leith Bridge mast, the projectors were all top mounted pointing downwards. The projectors nominated were 6-degree narrow beam angle low energy LED projectors with a carefully selected 3000 colour temperature. By modelling these lighting fixtures within the overall 3D Sketchup model (using a lighting add-on app), we were able to optimize fixture placement and performance. All wiring is concealed in steel conduit that passes through the air-tight pressure tested mast. The outcome to the bridge lighting design process achieves multiple benefits: 1) it is discrete during the daytime, 2) elegant and safe at night, and 3) light spill is minimized and “dark skies” best practice is achieved.
Bridge Constructability
- Our cable stayed bridge is optimized by using 18 sets of high strength spiral strand cables. Stressing of cable stayed bridges is highly complex and can often result in retrospective “tweaking” of cables to get the desired geometry which is frustrating, timely, and costly. Using our non-linear analysis model we were able to create a cable-by-cable stressing sequence that documented both the cable forces and the bridge geometry expected at each stage. To keep things simple, the constructor knew at every stage what force the next cable needed to be stressed to. Following each stressing a simple survey was taken from a laser level running the length of the bridge (fixed for the duration of the stressing). Offset measurements (from the laser) were recorded at each cross beam and communicated back to the designer to check and correlate real-world behavior back to the analysis model.
- This meticulous and methodical approach to the design documentation, and on-site communication/collaboration with constructors, prevented the need for any retrospective (costly and timely) stressing. All cables on the project (all 18!) were only ever stressed once and the bridge has a beautiful and symmetrical curve. This is considered a major success. We believe this level of stressing coordination and documentation far exceeds previous industry practices and should be considered a benchmark for designing and constructing future cable stayed bridges built in NZ as it saves both time and money for clients.
Urban Design of Footbridge
By using traditional bridge building materials such as concrete and steel and combining them effectively with the soft modernity and sustainability offered by engineered timber and recycled composite plastic decking, we have tried to create a truly unique bridge which is economical, durable, and architecturally striking.
[1] A. F. Daly (TRL Limited) & B. W. Smith (Flint and Neil Partnership), “Wind tunnel tests on plate girder bridges” (TRL530), 2002.
[2] To the authors knowledge
