In the first article in this series, Stephen Cummins and Andrew O'Connell outline the unique design and the inherent engineering challenges of the Nelson Street Cycleway and explain how it works together with the Canada Street Bridge. The Canada Street Bridge has a unique structure. The design philosophy that was adopted endeavoured to find a subtle balance between providing something simple, sculptural and elegant while also highlighting the complex engineering. For this reason, the inverted slender triangular shape for the deck and trapezoidal-like shapes for the piers were selected as providing the most advantageous solution. All bridges in New Zealand are designed in accordance with the criteria defined in the New Zealand Transport Agency Bridge Manual and this refers to both New Zealand and Australian codes, where applicable. These codes are limited in their scope and, as such, refer designers to the old BS 5400:3:2000 for the design of steel orthotropic box girders. The piers and substructure were designed in accordance with NZ design standards. In addition, the vertical pedestrian induced vibrations checks had to be carried out in accordance with BD37/01, but the horizontal checks had to comply with the requirements of NA to BS EN 1991-2. BD 49/01 had to be utilised for assessing the effects of aerodynamic vibration. An elastic design philosophy was adopted for the entire bridge. This approach was adopted because Auckland has a low level of seismicity and ductile design philosophy would not have yielded any design efficiencies. Also, as outlined above, because the structural design of the bridge had involved two independent design codes, an elastic design approach ensured there was no conflict in the design philosophy between the super and sub structure.

Challenging articulation of Canada Street Bridge

[caption id="attachment_32561" align="alignright" width="300"]bridge-lift-ghd CLICK TO ENLARGE: Bridge life (Image: GHD)[/caption] The articulation of Canada Street Bridge on the existing Off-Ramp proved to be quite challenging, as it had to provide a balance between a number of design features, such as:
  • Provide lateral restraint against wind- and pedestrian-induced vibrations. Due to the lack of transverse stiffness of Pier No.1, this proved to be a key design consideration for the bearing;
  • Facilitate temperature movement of the bridge without inducing excessive lateral forces into the Off-Ramp;
  • Minimising the out-of-phase seismic displacements of the Off-Ramp and the new bridge; and
  • An efficient means of delivering the support reaction to the Off-Ramp below.
After of number of design iterations, it was found that a single guided pot bearing with the direction of free movement parallel to centreline of the main span of SH1 yielded the most satisfactory compromise to challenges outlined above. To help preserve, protect or indeed avoid a redesign of the architecturally striking Pier No.1, the existing roadside barriers were extended and upgraded to provide the maximum level of protection against possible collision loading. Due to the unique geometry of Canada Street Bridge, the limited design guidance within BS 5400:3 did not cover the triangular nature of the deck for the effects of distortion and out-of-plane bending. Therefore, specific analyses techniques had to be developed to satisfactorily account these affects. To observe the behaviour and establish the order of magnitude of distortional stresses (transverse distortional bending and distortional warping stresses), a large finite-element-analysis shell model was built for the large sections of the bridge. This was then broken up in segments (local models) corresponding to the locations of the internal diaphragms. The global forces corresponding to each segment were then applied to in order to examine in detail the magnitude of the effects resulting from the complex relationship between the vertical bending, St Venant torsion and torsional warping.

Steel, triangles and curves

[caption id="attachment_32563" align="alignright" width="300"]282c5771_v2-courtesy-of-tammy-peters-photography CLICK TO ENLARGE: Canada Street Bridge (Image: Tammy Peters Photography)[/caption] As the Canada Street Bridge spans were to be constructed by being lifted into place over the busiest section of roadway in NZ, steel was chosen as the primarily structural material as it is substantially lighter than an equivalent concrete type span. In addition, as the new bridge had to land on top of an existing bridge, the use of steel was substantially more advantageous. As steel is the primary structural material, the section of the bridge spanning directly over SH1 can be lifted as one section. This had the advantage of minimising the amount of time and therefore de-risking the amount work that is to undertaken overhead State Highway 1. The triangular shape of the deck and diamond shape of the piers naturally amplify a clean sculptural form while simultaneously providing natural structural efficiency. This is exemplified in the shape of the deck with its inherent torsional stiffness. As a whole, the bridge’s peculiar yet simple geometry subtly expresses to the observer a slender, elegant yet effortless structural form. As the Canada Street Bridge is a cycle path, the bridge had to be curved in plan in a number of locations to facilitate a smooth change of direction while still avoiding on-site obstacles such as buildings and encroaching over SH1. The alignment was designed in accordance with the AUSROADS Cycleway Guidelines. The final bridge arrangement consists of seven spans in total, two haunched spans, five horizontal curves ranging from 24m to 36m, two vertical curves and a varying longitudinal gradient. The longest span is 39m spanning over SH1. This resulted in the final bridge scheme representing an optimised balance of form and function. The curves were a result of working within the tight site contraints and using AUSTROADS Cycleway Guidelines. Some key design parameters used were:
  • Min radius 24m
  • Width 3.5m
  • Camber 2%
  • Max gradient 4.4%
  • Barrier 1.4 m high

Why so long?

[caption id="attachment_32564" align="alignright" width="300"]282c5652-pano-edit-courtesy-of-tammy-peters-photography CLICK TO ENLARGE: Canada Street Bridge (Image: Tammy peters Photography)[/caption] On the Off-Ramp side, the Canada Street Bridge had to align itself with Off-Ramp in order to provide a seamless transition for the cyclists. In order to facilitate this, the bridge had to be curved as it spanned over the motorway underneath. On the other side of the motorway the bridge had to traverse an embankment with services, contaminated land and a 100-year overland flow path. In addition, the bridge had to weave its way between the narrow space available between the adjacent buildings while still avoiding encroaching over the motorway before it connecting to Canada Street. The Canada Street Bridge may look curved out of flamboyance, but in reality it simply has to be this way to make it all fit. Below is a time-lapse video of Canada Street Bridge fabrication: Authors: Stephen Cummins MSc CEng MIEI is a principal engineer with AECOM. He is a chartered engineer (2011) with 10 years’ experience in project management and design of transportation projects in Ireland, the UK and New Zealand. This includes involvement in PPP, design & build, employer’s design and re-measurable contracts. He has experience managing a range of transportation projects from the design phase, through procurement, construction and into maintenance. Andrew O’Connell is a chartered engineer with Engineers Ireland and has 10 years’ experience in industry with experience in both civil and structural engineering in Ireland, Poland, the UK and New Zealand. O’Connell spent 3.5 years working in Ireland undertaking flood studies and a range of other civil works. He also spent one year in Poland involved in building structures, as well 1.5 years in the UK and three years in New Zealand involved in bridge design. O’Connell graduated from UCD with a BE in Civil Engineering in 2006 and an MSc with distinction in Structural Engineering from the University of Surrey in 2014.