Onewa Footbridge - Convention Management

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.
Contacts:
Tel: 00 64 9 3009000
Fax: 00 64 9 9009300
E-mail: dhirendra.singh@beca.com 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]
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Onepoto Footbridge, Singh
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,
Figure 1: Initial Concept
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Onepoto Footbridge, Singh
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.
Figure 2: The Wave
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.
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Onepoto Footbridge, Singh
•
3.0
Deck vibration checks in accordance with BS5400.
DEVELOPMENT OF DESIGN
Architectural Modelling
The architect used 3-dimensional software to develop and design the complex nonuniform '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 selfsupporting shutter formwork to limit impact on the protected tree.
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Onepoto Footbridge, Singh
Figure 3: A 3-Dimensional Architectural Model of Onepoto Footbridge
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Onepoto Footbridge, Singh Figure 4: Plan
Figure 5: Cross Section
Glue Laminated Waves
The material selected for the architectural ‘wave’ cladding to the bridge was gluelaminated 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
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Onepoto Footbridge, Singh
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.
Figure 6: Look-Out
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.
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Onepoto Footbridge, Singh
NORMAL WAVE BRACKET BREAKING WAVE BRACKET
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.
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Onepoto Footbridge, Singh
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.
Figure 7: Handrails
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.
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Onepoto Footbridge, Singh
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.
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Onepoto Footbridge, Singh
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.
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Onepoto Footbridge, Singh
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/Releases2007/April/Good-design-focus-on-new-cycle-pedestrian-path.html, April 2007.
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Onepoto Footbridge, Singh