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NSCC2009
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 cable-stayed
twin box girder deck spanning 1018m over Rambler Channel.
Stonecutters Bridge then be-came 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 har-bour. 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 con-nected
with cross girders spaced at 18m in the main span, coinciding with
the stay anchorage spac-ing, 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).
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 cur-rently 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. In-stead 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.
Stonecutters Bridge, Hong Kong: Design and construction of the
composite upper tower in stainless steel
T. Vejrum1, D.W. Bergman
2 & N. Yeung
3
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
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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 1
st lateral frequency of the RS tower coin-
cided with the natural frequencies of the longer cable stays
raising the possibility of linear reso-nance 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 paramet-ric 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 dia-phragm 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 sec-tion, 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 com-posite
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 di-rectly 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 con-nection 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
ma-rine 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/mm, 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 con-struction
are characterised by relatively good corrosion resistance and
moderate mechanical proper-ties. The corrosion resistance of these
steels is provided by the alloying element chromium; im-proved
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 environ-ments 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 consid-erably 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
approxi-mately 50% ferrite and 50% austenite. The control of this
balance is important in ensuring the cor-rect balance of corrosion
resistance and mechanical properties. It is equally important that
subse-quent 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 con-tents vary. Duplex stainless steels are
characterised by improved corrosion resistance in naturally
occurring environments, compared to austenitic stainless steels,
particularly with respect to local-ised corrosion such as pitting
and crevice corrosion. Duplex steels can therefore be used more
con-fidently 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. EN10088
grade C
(% by mass)
Cr min (% by mass)
Ni min (% by mass)
Mo min (% by mass)
AISI grade Proof stress
(N/mm)
UTS
(N/mm) 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 applica-tion
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 hard-ness 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 fabrica-tor 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 ap-propriate 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 envis-aged 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 pro-totype 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 conven-tional 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 de-tail 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. Ten-sile SLS stresses, where present at all in the splices,
are low and are transferred by the bolts in pry-ing. 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 util-ized 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 hori-zontal
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 hori-zontal 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 bal-anced or average
horizontal stay force at a level is resolved by the ties. The
remaining 10% is car-ried 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, ex-cluding 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
ac-ceptable to use shim plates between the segments of the
stainless steel skin for esthetical and dura-bility 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 an-chor 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 direc-tion during trial assembly.
Drift and cumulative drift of the plan centreline of the trial
assembled skin and anchor box sec-tions: not greater than
1:1500.
Drift of the plan centreline of the trial assembled skin and
anchor box sections over a single erec-tion section: not greater
than 5mm.
Cumulative elevation of any point in the trial assembled
stainless steel skin and anchor box sec-tions: 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 se-lection and advanced design
methods to satisfy requirements for strength, serviceability,
aerody-namic 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 De-sign of
Stonecutters Bridge Towers", Proceedings of International
Conference on Bridge Engineering Challenges in the 21
st Century, Hong Kong.
Falbe-Hansen, K., Vejrum, T. & Carter, M. (2004),
Stonecutters Bridge Design of the Steel Superstruc-ture,
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 Chal-lenges in the
21
st Century, Hong Kong.
Vejrum, T., Carter, M. & Kite, S. (2006), Detailed Design of
Stonecutters Bridge Superstructure, Proceed-ings of International
Conference on Bridge Engineering Challenges in the 21
st Century, Hong Kong.
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