NSCC2009
Stonecutters Bridge, Hong Kong: Design and construction of the
composite upper tower in stainless steel
T. Vejrum1, D.W. Bergman2 & N. Yeung3
1
COWI Consulting Engineers A/S, Lyngby, Denmark
2
Buckland & Taylor Ltd., COWI Group Member, North Vancouver BC, Canada
3
Ove Arup & Partners Hong Kong Ltd., Kowloon Tong, Hong Kong SAR
ABSTRACT: Stonecutters Bridge in Hong Kong – spanning across Rambler Channel at the
entrance to Kwai Chung Container Port and opening for traffic at the end of 2009 – is currently the second longest cable-stayed bridge in the world. Several unique design issues were
incorporated into the detailed design. Special attention had to be paid to durability because of
the harsh marine and industrial environment in order to meet the required 120 years design
life. One of the main design features of Stonecutters Bridge is the mono-column towers which
rise up to 298m tall and support the 1596m long bridge with a 1018m main span. The towers
are formed of concrete to +175m, are of composite construction with an outer steel skin to
+293m and are topped by a lighting feature to +298m. Access for maintenance of the outer
steel skin of the upper tower was identified as being of particular difficulty leading to high
maintenance cost if corrosion protection was based upon a traditional system of coating. Instead stainless steel was adopted to provide a low maintenance solution. Selection of steel
grade, design considerations and details of the stainless skin are discussed. The stainless steel
skin segments are assembled by bolted compression only splices to minimize site welding.
The total quantity of stainless steel used for the outer steel skins amounts to 1,600t which is
unprecedented for this type of application.
1 INTRODUCTION
An international design competition in 2000 selected a reference scheme (RS) for the proposed new
Stonecutters Bridge which featured dramatic 298m tall mono-column towers supporting the cablestayed twin box girder deck spanning 1018m over Rambler Channel. Stonecutters Bridge then became the first cable-stayed bridge with a main span over 1km for which detailed design has been
completed. Particular challenges included designing for typhoon winds at the exposed bridge site
and taking account of the severe restrictions on construction operations imposed by the busy harbour.
The 1596m long cable-stayed bridge has a steel main span of 1018m, with concrete back spans each
side of 79.75m, 70m, 70m and 69.25m. The circular tapered mono-column towers stand on the
bridge centre line between the two longitudinal box girders of the twin girder deck. The towers are
formed of concrete to +175m, are of composite construction with an outer stainless steel skin to
+293m and are topped by a lighting feature to +298m. Stay cables are in two planes arranged in a
modified fan layout and attached to the outside edges of the deck girders. The deck girders are connected with cross girders spaced at 18m in the main span, coinciding with the stay anchorage spacing, and 20m in the back spans where the stays anchorages are spaced at 10m. The concrete back
span decks are monolithic with the piers.
This paper describes the design of the Stonecutters towers with special emphasis on the composite
upper towers. Details of the design of the superstructure can be found in (Falbe-Hansen et al. 2004)
and (Vejrum et al. 2006).
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Figure 1. Photo of Stonecutters Bridge in the final stage of construction (February 2009).
2 REFERENCE SCHEME TOWER
The reference scheme (RS) tower chosen in the competition proposed the use of a hollow concrete
section for the lower 167m and an all steel section for the upper 115m stay anchorage zone. At the
start of final detailed design of the towers in 2001, a review of the technical viability of the RS
tower was carried out, (Falbe-Hansen et al. 2006). The review considered structural performance,
constructability and durability of the RS towers and included wind tunnel testing and analysis of the
arrangement.
The review indicated that the response of the proposed circular RS tower to vortex shedding was
unacceptably high. In service peak amplitudes of the RS tower were estimated at 0.45m at the top of
the tower. The response was mainly harmonic and the 1st lateral frequency of the RS tower coincided with the natural frequencies of the longer cable stays raising the possibility of linear resonance causing excitation of the stays.
The review concluded that an all concrete tower would have significantly less response to vortex
shedding due to increased mass and damping. In addition, the natural frequency of the all concrete
tower would not coincide with that of any of the longer stays, thereby reducing the risk of parametric excitation of the stays. Constructability and schedule concerns with the extent of site welding
and the heavy lifts required for the RS all steel upper tower were also identified in the review.
As a result, a concrete tower with a composite stainless steel skin in the upper anchorage zone was
selected for final detailed design. The modified scheme respected the intent of the original RS
scheme while addressing performance and constructability issues identified in the review.
3 FINAL TOWER DESIGN
The general arrangement of the final tower is shown in Figure 2. The lower tower from the pile cap
to level +77.75mPD is formed by a transversely elongated circular hollow cross-section with 2m
thick walls. The diameter of the circular end sections varies from 18m at the base to 14m just under
the deck at elevation +77.75mPD. From level +77.75mPD to level +175mPD the tower section is
formed by a circular hollow cross-section which varies in diameter from 14m down to 10.9m. The
tower wall thickness in this section reduces from 2.0m down to 1.4m. An internal concrete diaphragm is provided at the deck level to distribute forces delivered to the tower from the deck.
The upper section of the tower which contains the stay anchorages is a circular concrete cross section, which tapers from 10.9m in diameter at elevation +175mPD to 7.16m at elevation 293mPD.
The tower wall thickness in the upper tower varies from 1.4m to 0.82m at the top.
The exterior surface of the upper tower from elevation 175mPD to 293mPD is a 20mm thick composite stainless steel skin. The upper 25 sets of stay cables are anchored to an interior composite
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steel anchor box as shown in Section 2 of Figure 2. The lower three sets of stay cables are too steep
to make anchorage into anchor boxes practically feasible. Instead the cable forces are transferred directly to concrete corbels on the inside face of the tower wall as shown in Section 3 of Figure 2.
The top of the tower above the anchorages contains space for mechanical and electrical services and
provisions for a future tuned mass damper if necessary. The light feature at the top 5m of the tower
is a circular glassed facade which tops the tower off to elevation +298mPD. The glassed facade
contains aesthetic lighting as proposed in the RS scheme and also houses a retractable derrick crane
which can be used to access the outside surfaces of the tower.
Figure 2. Final tower design: General arrangement.
4 DESIGN OF THE LOWER TOWER
The vertical reinforcement ratio in the lower tower varies from 1.4% at the base to 3.6% at the connection to the upper tower. The lower tower reinforcement is arranged with multiple layers near the
outer face and a single layer near the inner face. For durability, S50 stainless steel bars are used in
the outmost layer to level +175mPD. T50 or T40 plain bars are used in the remaining outer layers.
More details of the design of the lower tower including foundation design as well as global analysis
of the towers can be found in (Bergman et al. 2006).
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5 DESIGN OF THE UPPER TOWER
At an early stage during detailed design access for maintenance of the outer steel skin of the upper
tower was identified as being of particular difficulty leading to high maintenance cost if corrosion
protection was based upon a traditional system of coating. Consequently, a study was undertaken to
identify a suitable grade of stainless steel for the skins, determine an appropriate surface finish and
test fabricate a prototype section of the skin to confirm detailing, including joint design, attachments
to the reverse face of the skin, geometry and overall dimensions.
5.1 Stainless steel skin
5.1.1 Material grade
The key to long-term durability of stainless steel is the selection of an appropriate material type and
grade for the anticipated environment. The service environment corresponds to a polluted coastal
condition and the tower skin is exposed to airborne contaminants including chlorides from the marine environment as well as industrial pollutants. Furthermore, the design of the tower skins requires
that the steel used have a yield strength in the order of 450N/mm², which limits the choice of
stainless steel grades. Under these conditions austenitic and duplex (austenitic/ferritic) grades of
stainless steel were considered for the tower skin.
Conventional austenitic steels such as grades 1.43xx and 1.44xx commonly used in building construction are characterised by relatively good corrosion resistance and moderate mechanical properties. The corrosion resistance of these steels is provided by the alloying element chromium; improved resistance is provided by the addition of 2 to 3% of molybdenum. The addition of nickel
ensures a fully austenitic microstructure and the required mechanical properties are obtained.
Although these basic grades of steel are corrosion resistant in many naturally occurring environments some staining and pitting can occur in marine and polluted environments. The incidence of
this type of corrosion can be difficult to predict and where these steels are used in these conditions a
high quality of surface finish is often required to ensure good performance.
The mechanical properties of base grade austenitic steels are somewhat less than those of typical
structural grades of carbon steel. The properties can be improved by the addition of small quantities
of nitrogen however these grades are not readily available in significant quantities and are considerably more expensive than other grades of material.
Duplex steels have a combined microstructure of austenite and ferrite, the alloy content and heat
treatment of these steels is carefully controlled to provide an optimum microstructure of approximately 50% ferrite and 50% austenite. The control of this balance is important in ensuring the correct balance of corrosion resistance and mechanical properties. It is equally important that subsequent forming and fabrication processes, particularly those involving heat, do not disturb this
balance.
The alloying elements used in duplex steels are similar to those used in austenitic steels but the contents vary. Duplex stainless steels are characterised by improved corrosion resistance in naturally
occurring environments, compared to austenitic stainless steels, particularly with respect to localised corrosion such as pitting and crevice corrosion. Duplex steels can therefore be used more confidently in a wider range of environments than austenitic steels and the quality of surface finish is
less critical to the overall long-term performance. This greater tolerance to quality of surface finish
is important when considering the use of hot rolled plate materials where the achievement of high
quality polished finishes maybe difficult on all types of stainless steel.
The mechanical properties, in terms of yield and ultimate strength, of duplex steels are higher than
those of austenitic steels and conventional hot-rolled structural steels. Typically the 0.2% proof
stress is comparable to the yield strength of a quench and tempered carbon steel.
Composition and mechanical properties for common austenitic steels and for duplex steel 1.4462
(318) are listed in Table 1.
Table 1. Chemical composition and mechanical properties of selected stainless steels, hot rolled plate.
C
Cr min
Ni min
Mo min
AISI grade
Proof
UTS
EN10088
grade
(% by
(% by
(% by
(% by
stress
mass)
mass)
mass)
mass)
(N/mm²)
(N/mm²)
1.4301
≤0.07
17.5
8
0
304
210
520 to 720
1.4436
≤0.05
16.5
10.5
2.5
316
220
530 to 730
1.4462
≤0.03
21
4.5
2.5
2205/318
460
640 to 840
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Based on the considerations described above, duplex steel grade 1.4462 was selected.
Duplex steel grades have been used in fabrication a number of pedestrian bridges. Previous application is consequently in a much smaller scale (in the order of 100t - 500t) than the required quantity
for the towers of Stonecutters Bridge (1,600t).
5.1.2 Surface finish
As discussed above, duplex stainless steel is less susceptible to pitting and crevice corrosion and
therefore surface finish is less critical to long term performance. This characteristic permitted a
wider range of possibilities in the surface finish which could be used to achieve the desired final
visual effect for the skin. The goals for the surface finish of the skin were:
− low reflectance comparable to that of a ground finish
− no clearly defined directional texture as it would be difficult to match panels
− minimal surface roughness to limit retention of pollutants
For a welded fabrication the issue of surface finish is further complicated by the need to blend, or
dress, the welds to match the surrounding plate as the surface of the weld area has a different hardness to the parent materials and therefore responds differently to finishing processes. The higher the
quality of finish and the more directional the finish texture the more difficult this dressing becomes.
This tends to favour shot peening, as the finish is none-directional and thus more easily blended to
the surrounding plate. Thus a shot peening process was selected to achieve the desired surface finish
and a selection of test panels were prepared to confirm feasible processes and the resulting finish.
A skin thickness of 20mm was selected to ensure that welds for the shear connectors and stiffeners
on the inside of the skin would not be reflected to the outside surface of the skin and a prototype
fabrication confirmed this result, see Figure 3.
The final specifications called for the stainless plate to be supplied from the steel mill to the fabricator with a polished 1K finish which is a uni-directional satin finish as specified by EN10088. For
the prototype, a two stage process was then used to achieve the final finish. The surface was first
blasted with aluminium oxide and then shot peened with a glass bead media to achieve a uniform
surface roughness in the range of 1 to 1.25 microns. The resulting finish can be seen in Figure 3.
Prior to actual production further investigations have been made to establish a simplified one stage
process to achieve the same finish. Through a series of trials the fabricator has determined the appropriate mixture and grade of aluminium oxide and glass bead to be used at the final stage of the
skin fabrication process.
Figure 3. Left: Stainless steel skin prototype. Right: Prototype - stainless steel skin surface finish.
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5.1.3 Prototype
The prototype was fabricated in Sheffield, UK. The form was selected to represent all details envisaged in the design including welded joints, bolted site joints, attachments to the reverse face, cone
forming, geometry, overall dimensions and fit of completed parts. In elevation each part of the prototype is about half the height of a typical section and in plan a quarter of the circumference.
The conical shape was achieved by cold forming. It was observed that some slow relaxation took
place over a number of days. This appears to be an inherent characteristic of the plate material that
can be overcome by the plate being formed to a tighter radius to allow for this relaxation.
In general the prototype fabrication was achieved without unforeseen difficulty and used conventional welding processes, procedures and techniques. As anticipated the skin-to-flange-plate, i.e. the
horizontal skin splice, proved to be the most problematic area and the resulting bolted connection
between sections initially demonstrated inadequate fit. The prototype fabrication allowed a number
of alternatives to be discussed between the designer and the fabricator and in this way a suitable detail was developed and included in the final design, see Figure 4, right.
5.2 Structural design
The structural design is carried out to BS5400. The typical upper tower section is shown in Figure
4. The stainless steel skin and the internal steel anchor box section are designed to act compositely
with the reinforced concrete section to resist global demands. Composite action is achieved by
means of shear studs which are designed to transfer short-term and long-term loads between the
concrete and the skin or anchor box. Shear stud spacing is designed to ensure yield of the skin or
box in compression before local buckling. The upper tower is primarily subject to compressive
loading. The horizontal skin and anchor box splices are detailed such that intimate contact between
faying surfaces can be achieved without extensive preparation of bearing surfaces or site welding.
The typical skin splice is shown in Figure 4, right. The anchor box splices are similar in concept.
The splices are designed to transfer full compressive yield strength of the skin and anchor box. Tensile SLS stresses, where present at all in the splices, are low and are transferred by the bolts in prying. Global tensile stresses at ULS are resisted by concrete wall reinforcing, and the tensile capacity
of the anchor box and skin splices is assumed to be negligible. Global shear and torsion in the upper
tower section are primarily resisted by horizontal reinforcement. The stainless steel skin is also utilized to the extent possible to resist shear and torsion above level +220mPD where global bending
effects are of such magnitude that the skin is not fully utilized in resisting global vertical bending
moments and axial loads. The skin here is utilized for shear and torsion in a similar manner to horizontal layers of reinforcing so no shear is assumed to be transferred across the bolted skin splices.
Figure 4. Left and centre: Typical arrangement in upper tower. Right: Typical skin splice.
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The vertical splices in the two halves of the stainless steel skin are designed to resist the local horizontal effects in the upper tower section. For the ULS effects, it is assumed that the bolts may slip
and resist the applied loads in bearing. For SLS, the bolted connection is designed to remain free of
slip under the governing loads using a friction factor of 0.2 which was confirmed in testing.
The anchor box side wall plates span the circular tower section between the box flanges and have
openings which create discrete tension ties across the tower section. The side wall ties act to resolve
opposing components of horizontal cable stay force in the tower. Approximately 90% of the balanced or average horizontal stay force at a level is resolved by the ties. The remaining 10% is carried around through the concrete walls. The unbalanced horizontal stay force at a level is delivered
to the concrete section as a shear at each end of the anchor box.
The global design of the upper tower section is governed by wind buffeting. Transverse bending
demands govern from level +175mPD to the mid-height of the anchorage zone and a combination
of longitudinal and transverse bending demands govern from mid-height of the anchorage zone to
the top. Reinforcing ratios in the upper tower vary from 3.6% at +175mPD to 1.2% at the top, excluding the contributions of the composite stainless steel skin and anchor box.
6 CONSTRUCTION
The stainless steel was supplied from Sweden to the tower steelwork fabrication workshop located
in Zhongshan at the Pearl River a few hours by boat from the bridge site in Hong Kong.
The geometry of the upper tower is determined by the geometry of the steel skin and anchor box.
Both anchor boxes and stainless steel skins are assembled on site by bolting. For the anchor boxes
shim plates allow for some adjustment of the vertical position. However, for the stainless steel skin
sections there was no possibility for on-site adjustments of the vertical position since it was not acceptable to use shim plates between the segments of the stainless steel skin for esthetical and durability reasons. Therefore, an important element of the quality control was the trial assembly in the
workshop. Furthermore, anchor boxes and stainless steel skins are connected by the guide pipes
which penetrate both. The relative geometry of the skin and anchor box is therefore important and
had to be controlled carefully. In order to accomplish this, the anchor box and stainless skin sections
were trial assembled where three consecutive segments were stacked at one time to check verticality
and gap around the erection joint. One segment from each assembly was then carried forward to the
next assembly in order to monitor and control the overall and relative geometry of the skin and anchor box.
Figure 5. Left: Assembly of stainless skin section. Right: Trial assembly of three sets of stainless steel skins.
The plastic protection shall prevent cross-contamination from contact with carbon steel in the workshop.
Examples of the tolerance requirements specified for the tower steelwork are given in the following:
− Stainless steel skin diameter: +/-10mm.
− Edge gap between the horizontal bolted splice plates in the stainless steel skin and anchor box:
not more than 1mm anywhere along the splice and not more than 0.25mm over 600mm length in
any 1000mm length of splice.
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− Alignment of slotted holes for the splice in the guide pipe: within 1mm of centred in any direction during trial assembly.
− Drift and cumulative drift of the plan centreline of the trial assembled skin and anchor box sections: not greater than 1:1500.
− Drift of the plan centreline of the trial assembled skin and anchor box sections over a single erection section: not greater than 5mm.
− Cumulative elevation of any point in the trial assembled stainless steel skin and anchor box sections: within 25mm of theoretical.
These tolerances were generally met during fabrication and installation on site. The installation of a
stainless steel skin and an anchor box on site is shown in Figure 6.
Figure 6. Left: Installation of a stainless steel skin. Right: Installation of an anchor box.
7 SUMMARY
The 298m tall mono-column towers are a dramatic feature of the new Stonecutters Bridge in Hong
Kong. The final design of the towers respects the aesthetic features and form of the reference
scheme selected in the International Design Competition while taking best advantage of material selection and advanced design methods to satisfy requirements for strength, serviceability, aerodynamic performance, constructability and durability. The total quantity of duplex steel for the
stainless steel skins, which was supplied from Sweden, amounts to 1,600t. The use of stainless steel
in this quantity is unprecedented for civil works of this type and scale.
8 ACKNOWLEDGEMENTS
This paper is submitted with the permission of the Highways Department, the Government of the
Hong Kong Special Administrative Region.
REFERENCES
Bergman, D., Ibrahim, H., Radojevic, D. Cuperlovic, N., Thompson, P. & Cheung, J. (2006), "Detailed Design of Stonecutters Bridge Towers", Proceedings of International Conference on Bridge Engineering –
Challenges in the 21st Century, Hong Kong.
Falbe-Hansen, K., Vejrum, T. & Carter, M. (2004), “Stonecutters Bridge – Design of the Steel Superstructure”, Proceedings of Steelbridge 2004, Millau.
Falbe-Hansen, K., Hauge, L. & Wong, C. (2006), “Stonecutters Bridge - International Design Competition
and Reference Scheme Reviewing”, Proceedings of International Conference on Bridge Engineering – Challenges in the 21st Century, Hong Kong.
Vejrum, T., Carter, M. & Kite, S. (2006), “Detailed Design of Stonecutters Bridge Superstructure”, Proceedings of International Conference on Bridge Engineering – Challenges in the 21st Century, Hong Kong.
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