Onewa Footbridge - Convention Management · Beca was engaged by North Shore City Council to design...

Onewa Footbridge Dhirendra Singh, Structural Engineer, Beca Infrastructure Ltd

Dhirendra Singh was the principal structural engineer for the design of the Onewa Footbridge. Dhirendra graduated from the University of Auckland in 2004. He worked as a graduate engineer with Opus International Consultants before working as a site engineer with Fletcher Construction supervising the sub-structure construction for the Rewa Bridge, Fiji. Dhirendra has been with Beca since 2006 and is currently a structural engineer working on the Newmarket Viaduct Project.

This paper is an entrant for the Young Authors Award.

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1 Onepoto Footbridge, Singh

Onewa Footbridge, Auckland Dhirendra Singh, Structural Engineer, Beca Infrastructure Ltd

SYNOPSIS

This project was an architectural footbridge and pathway for North Shore City Council.

The footbridge has been translated from a virtual architectural concept into reality. This was achieved through attention to detail and innovative use of construction materials. It provided synergy between the functional requirements of the pathway, environment and architectural design.

Some of the engineering challenges faced when designing this architectural bridge included construction in a sensitive coastal environment with native trees and tighter construction tolerances and finishes not normally associated with bridges. In addition, solutions had to be found to address issues of durability and graffiti removal.

This paper gives an insight into the evolution of the design for this architectural bridge and incorporates views on managing stakeholder expectations. Some of the design challenges faced and the construction issues are also discussed.

1.0 INTRODUCTION

In December 2004, North Shore City Council, in partnership with the New Zealand Transport Agency (formally Transit New Zealand) identified a need for an alternative pedestrian-cyclist route over the Onepoto Stream. The existing footpath on the Onewa Road Bridge had to be removed as part of the bridge widening project. This was necessary to incorporate the new Onewa Road Transit Lane.

Beca was engaged by North Shore City Council to design a new footbridge and pathway to serve as an entrance to Northcote for pedestrians and cyclists. The route linked a major local arterial, Onewa Road to the Onepoto Basin. The decision to relocate the pedestrian access to a new bridge off a major arterial route emphasised the client’s commitment to safety for pedestrians, cyclists and traffic.

This paper will summarise the background, concepts and design development of an architectural footbridge. The key aspects include working in an environmentally sensitive coastal marine area and the choice of construction materials and details for this bridge. The paper also demonstrates the value that can be added when engineers and architects deliver infrastructure projects collaboratively.

2.0 BACKGROUND

Landscape

The project is located within Onepoto Basin, Northcote, North Shore City, Auckland, New Zealand. The Onepoto Stream is a tidal stream that runs into a basin. The basin is a heavily modified volcanic crater that includes a marine reserve, mangroves, areas of native bush, sports fields and historically protected timber abutments of an old road bridge. The mangrove perimeter of this basin is an important food source for local marine and bird life. [1]


Figure 1: Initial Concept

The hillside surrounding the basin was initially covered with species of gum and pohutukawa trees, as well as some weed species and grasses. The growth was such that the basin was not visible from the heavily trafficked Onewa Road. This project included clearing and replanting the hillside with a structured coverage of native tree and grass species with view shafts of the basin. A small number of existing pohutukawa trees at the bridge abutments were retained.

North Shore City Council Brief

The council’s objective was to provide a prominent entrance to Northcote for both pedestrians and cyclists by connecting Onewa Road to Sylvan Avenue and the Onepoto Reserve. It was envisaged that the new pathway would maintain the current ecosystem and preserve the historic timber abutments. The footbridge was to be an elegant structure that created a safe environment, maximised visual links and enhanced the mangrove and estuarine experience.

Architect’s Vision

The architect’s vision in designing this bridge was to recreate a journey across the Onepoto stream. A journey sensitive to the area’s history and environment. The architect wanted to achieve this with timber incorporated into the bridge form. The natural weathering of this material would allow the bridge to slowly integrate into the surrounding landscape.

This vision was developed into an architectural form simulating a breaking wave. The journey and final form were open to the user’s imagination. In the architects words, “a carcass of a fish, slowly deteriorating in the waves. Over time flesh eroded slowly to reveal it’s pure structure”. [2]

Concept Development

A number of routes were investigated to maximise the mangrove experience, protect native Pohutukawa trees and preserve the old historic timber bridge abutments. The final location of the bridge was based on maximising the view shafts while incorporating three protected pohutukawa trees near the abutments.

From a safety perspective, the pathway was split to separate the cyclists and pedestrians because of the steep longitudinal gradient on the proposed route.

The major engineering challenge faced was incorporating the wave concept in the bridge form.

Initial concepts were based around replicating the wave in plan by manipulating the bridge deck width as shown in Figure 1.

This conceptual form was later developed in three-dimensions,


through the bridge elevation and cross section. The result a convex shaped timber surround encompassing the bridge simulating a breaking wave as shown in Figure 2.

Geological Profile

The geotechnical investigation indicated weak underlying ground conditions. This consisted of a deep layer of marine deposit underlain by a weaker alluvium strata founded on weathered Waitemata group sandstone (on average 18m below existing ground level). The conventional solution for these conditions was large diameter bored concrete piles.

Span Arrangement

A two span concrete superstructure was selected based on cost. The resource consent requirements dictated the central pier position to be located outside the Coastal Marine Area, resulting in spans of 9m and 27m. The resource consent sought to minimise the effects of construction on the local mud snails, burrowing crabs, horn shells and mangroves [1].

Bridge Design Criteria

The principal design criteria for the footbridge were:

• Maximum live load of 5.0kPa in accordance with Transit New Zealand Bridge Manual (TNZBM);

• Wind, seismic and light vehicle loadings derived from AS/NZS 1170;

• Other loads such as temperature, creep and shrinkage applied in accordance with the TNZBM.

• All TNZBM load combinations were used, with the exception of the ultimate limit state (ULS) 2A combination of wind and live loads. The AS/NZS 1170 load combination was the preferred, which allowed 40% of live load (2kPa) in combination with wind loads.

Figure 2: The Wave


• Deck vibration checks in accordance with BS5400.

3.0 DEVELOPMENT OF DESIGN

Architectural Modelling

The architect used 3-dimensional software to develop and design the complex non-uniform 'wave' form of the bridge. The model outputs as shown in Figure 2 and 3 were used to accurately portray the final design solution to the client.

An appreciation of the non-standard bridge construction materials was required on an ongoing basis throughout the modelling process. This required the design team understanding how materials such as curved glue-laminated timber, steel fixings, colour oxides and other architectural finishes could be integrated into the bridge structure. From a bridge engineer’s point of view, some of these detailing tasks are generally finalised towards the latter part of the design process. In this case, all details needed to be finalised in the early stages as inputs to the modelling exercise.

Final Bridge Form

The bridge width was set as 3200mm, with a 100mm central divider for separate pedestrian and cycle lanes.

The superstructure comprised of single 1200mm deep pre-cast concrete super-tee girders, simply supported in a two span arrangement as shown in Figure 4. The cast in-situ reinforced concrete deck had a minimum thickness of 200mm to support the architectural cladding, steel brackets and handrails.

The abutments and pier headstocks were supported on single 1000mm diameter reinforced concrete bored piles, socketed into competent Waitemata group sandstone. The piles had to be grooved within the rock socket to develop part of the required axial resistance by skin friction. Approach slabs were detailed on the abutments to mitigate differential settlement between the footpath and deck surface.

The architectural concept essentially comprised a complex wave of independent curved glulam units. The unit heights varied from 1000mm to 3300mm above the deck surface, with a 100mm clear space between each unit.

An outrigger seating detail on rubber bearing strips provided the torsional restraints for the overturning wind load effects.

A Special Pier

The pier headstock shape provided clearance to the deeper underlying roots of one of the protected Pohutukawa trees. The profile, as shown in Figure 5, tapered into the pile versus a more common rectangular profile, to minimise impact on the tree roots.

The construction methodology for this pier was based on hand excavation and self-supporting shutter formwork to limit impact on the protected tree.


Figure 3: A 3-Dimensional Architectural Model of Onepoto Footbridge


Figure 4: Plan

Figure 5: Cross Section

Glue Laminated Waves

The material selected for the architectural ‘wave’ cladding to the bridge was glue-laminated timber (Glulam). It was selected in preference to other materials such as fibreglass and plastics, because it is a natural product that blends into the surrounding landscape.

The initial concepts, panel thickness, specifications, and construction tolerances were developed with the glulam manufacturers. The ‘wave’ profile required the


Figure 6: Look-Out

glulam units to be designed with varying lengths. There were two wave types; a normal wave with a single curve and a breaking wave with a compound/‘S’ curve. For ease of manufacture and cost efficiency, the units were detailed with a single 2100mm radius curve of varying length, 90mm nominal thickness and 400mm width.

The detail design drawings were produced from the architect’s 3D “wave” model, with start and termination angles specified for each individual unit.

A number of glulam wave members were notched on the southern side of the bridge. This created windows for the appreciation of the old timber bridge abutments and the Auckland City skyline (Figure 6).

The marine environment required the glulam to be specified H3.2 treated in accordance with NZS 3602 to meet the NZ Building Code design life requirement of 50 years.

The units were designed in accordance with NZS 3603. Material properties were based on the manufacturer’s specifications.

The finish requirements for the glulam units were the most important aspect to the bridge design, and an appearance grade A was specified. This required all surface voids to be plugged or filled to match the existing natural finish. Each unit was then inspected and reviewed prior to installation.

Steel Brackets

The glulam wave panels were fixed to two different types of curved steel brackets, as shown in Figure 7. For normal waves the bracket form extending above the deck surface and for the breaking waves the brackets form extended below the deck surface.

Both bracket forms were similar in that a single 20mm thick plate replicated the curve and width of glulam wave. Two 16mm vertical plates connected this curved plate onto a 20mm anchorage plate. The anchorage plate protruded beyond the deck surface and provided formwork to the deck pour. All the steel plate connections were detailed as structural shop welds.

The bracket anchorage system consisted of four 20mm diameter deformed bars fully developed into the concrete deck and welded onto the anchorage plate. The thin concrete deck and cyclic wind loading limited the use of proprietary post-drilled or cast in-situ anchorages.


Figure 7: Steel Brackets

Corrosion protection for the brackets comprised of hot dipped galvanising with an epoxy urethane finish coat. The specified period to first maintenance was 25 years with a minimum galvanising coating weight of not less than 600 grams per square meter.

A waterproofing membrane was detailed between the steel bracket and glulam wave panel to prevent trapped moisture that could have an adverse effect on both the timber and steel material. A 5mm thick bitumen compressible membrane was used and sealant was applied to the top and side edges to prevent water ingress.

Timber Kickboard

A 90mm high timber kickboard was installed as a cover on the vertical plates of the steel brackets for the length of the bridge. This maximised the view shafts of the windows by providing a 200mm wide viewing platform on which pedestrians can step onto to sneak a view outside the windows (as shown in Figure 6).

Separating a Shared Use Pathway

A clear separation between the pedestrian and cycleway was achieved with 100mm white pebble tiles laid as a median. This is further emphasised with different marking symbols and concrete surface finishes for each user along the length of the bridge and pathway. The cyclist lane had an exposed aggregate finish while the pedestrian side had a broom finish towards the deck edge. To enhance the pathway divide and vision at night, special bi-directional blue road lighting (Smart Studs) were laid at 2m centres in between the tiles.

BREAKING WAVE BRACKET

NORMAL WAVE BRACKET


Figure 7: Handrails

Handrails

Two different types of handrail details were used on the bridge to meet the requirements of the different user types.

The cyclist lane had a 1200mm high cycle friendly handrail that met Austroads Part 14 requirements. The posts of the handrails were welded onto the glulam support brackets. A special weave mesh was used in between steel posts to maximise the natural light ambience while maintaining the safety requirements.

For the pedestrian lane, the glulam wave units provided an unconventional yet conforming handrail solution. The glulam units were detailed in accordance with the NZ Building Code requirements, with no openings greater than 130mm sphere below a 1000mm level. As a result, the glulam units were designed for handrail loads and persons hanging off or jumping on the glulam waves.

Anti-graffiti Treatment

The choice of timber for the architectural cladding provided a number of challenges with respect to durability and future maintenance. Timber structures are prone to vandalism such as carving, burning and tagging within the urban environment. The only proper way to mitigate this is by limiting the use of timber in the high-risk areas.

Painting timber is the usual practice for anti-graffiti protection. In this case, painting was not considered acceptable as the architect wanted to emphasise the natural look of the timber. While a number of different anti-graffiti measures were investigated, most had a relatively limited period to first maintenance, high costs associated with proprietary products for graffiti removal and a limited product history in the New Zealand market.

The chosen system was a proprietary duplex system with some product history in the Australian market. This system consists of a base “Silaskin” applied to the glulam units, with “Tag Off” as the sacrificial cleaning product. The system requires that graffiti be removed within 24 hours.

For concrete elements, a conventional sacrificial anti-graffiti treatment was specified, with the exception of the underside of the super tee girder. These were painted black to provide a camouflaged shadow look for the delete glulam units.

Black Finish

The concrete bridge deck including the footpaths beyond were coloured grey with a black oxide. This was to provide a visual impact to the pathway and complement the black finish to the underside of the super tee girder.


Whilst the use of oxides is common in the concrete industry, especially in footpaths, its use on bridges in New Zealand is rare. The key concern about its use for structural components was the effect of oxides on performance.

The oxide pigments bind into the cement mix semi-permanently during production, penetrating colour throughout the concrete matrix. In a paper by Chusid et al.(1999) it is stated that pigments are virtually inert and do not compromise the strength of concrete; provided dosage rates are limited to 10% by weight of cement and as specified by the manufacturer. [3]

The durability of the colour system was the prime concern for the architect. The use of high quality pigment provides resistance to fading due to sunlight and chemical reactions during the concrete curing process. It is important to choose an effective curing process in hot weather conditions, as poor curing increases the susceptibility of the concrete and colour particles to erosion and removal. While carbon pigments do not fade, some grades of oxides can slowly leach out of concrete that is not sealed. [3] The sealant specified on the bridge and pathway was Sikafloor-ProSeal 22.

Figure 8: “The Onepoto Pathway”

4.0 MANAGING STAKEHOLDER EXPECTATIONS

From the project outset, the use of architectural images and models helped in managing stakeholder expectations. Presentations to stakeholders focused on each element of design from cost, environment, landscape, lighting, safety, and performance. Of particular importance was the focus on the impacts of construction tolerances, finish types, and weathering.

It was important to communicate to stakeholders that timber elements were prone to warping and twisting over time. This meant that wave form could change over time. The stakeholders were also informed that in future the glulam units would likely weather to a silvery grey.


5.0 CONSTRUCTION

Architectural Finishes

A high standard of finish and quality of work was required to build the architectural wave concept into reality. The architect’s 3D images were used to emphasize the importance of these to the construction team.

To manage the quality control of colour, texture and finishes, the specification requested test samples and prototypes be constructed to the required finish for the majority of the architectural details.

Installation of Glulam Waves and Brackets.

It was necessary to work closely with the contractor to achieve the architect’s wave profile. The profile had to be set out in three directional geometry; achieving the specified tolerances was hampered by the limited ability to provide tolerance as the brackets were cast in to the deck pour.

To achieve tight tolerances and overall quality control, the glulam manufacturer made a two piece timber model of the bracket and glulam to ensure a tight fit on site in terms of curvature. The bracket model was used to manufacture the glulam units and the glulam model used to fabricate the steel brackets.

The hog of the pre-stressed beams also affected the set out of the brackets on site. It was imperative that assessment of the hog and measurements on site were accurate to within a couple of millimetres.

Normal survey techniques were used to set out the positions of each bracket. This included the use of a string line attached to the top of the brackets at each end of the bridge to align the brackets. A 2-3mm error at bracket level resulted in a 50mm error at the top of the glulam units. Levelling nuts positioned in the front and back of the anchorage bar length were used to adjust the final vertical position of the steel brackets.

6.0 CONCLUSION

The construction of the Onewa Footbridge and Pathway was completed in March 2008. The client’s expectations were met through the engineers, architect and contractors working collaboratively to deliver this elegant structure. The sculptured wave design blended into the surrounding landscape and opened up views over the Onepoto Basin. [5]

The most important aspects to the design of this footbridge were the choice of materials, finishes, and details for the prominent wave form. It was critical to work closely with the architect from the beginning of this project to achieve a great outcome.

This footbridge has since won the following awards:

• New Zealand Institute of Architects 2008 Auckland Architecture Award - Urban Design Category;

• New Zealand Wood Timber Design Awards - Outdoor Infrastructure Category.


Figure 9: The “Wave”

ACKNOWLEDGMENTS

The permission granted by North Shore City Council to publish the information in this paper is gratefully acknowledged.

Shwan Alhashimi, of Beca Architects was the architectural designer and compiled the 3-D image (Figure 3) used in this report.

A special thanks to Kevin Crawford, Ted Polley, Stuart Tucker, Paul Noble, Vincent Lobendahn and the rest of the Beca team for the help throughout this project.

REFERENCES

[1] Larcombe, M. Onewa Road Western Approach: Assessment of the Effects in the Coastal Marine Area, Application for Resource Consent, December 2006. Appendix B

[2] Alhashimi S. Onepoto Footbridge Design Report, Beca, 2008.

[3] Chusid, M and Paris, N. Color in Concrete: Beauty and Durability, Concrete International Magazine, American Concrete Institute, January 1999. Pg 60- 3,

[4] Press Release. Good design focus on new cycle/pedestrian path and bridge, www.northshorecity.govt.nz/your_council/News_Releases/Releases- 2007/April/Good-design-focus-on-new-cycle-pedestrian-path.html, April 2007.

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