2376

tails throughout the design and planning of the crossing.

The rounded corners and reduced bolt connections of the main towers' columns and beams, as well as the configuration and surface finish of the anchorages, resulted from aesthetic considerations. The graceful configuration of the bridge is illuminated at night by white, green and coral pink lighting. The lighting pattern varies with the seasons and times of day. For its illu-

mination design, the Rainbow Bridge was awarded the Paul Waterbury Award of Distinction for Outdoor Lighting by the Illumination Engineering Society of North America.

Owner/Engineers: Metropolitan Expressway Public Corp. (MEPC), Tokyo

Construction duration: 6.5 years Service Date: August, 1993

Esko Jarvenpaa, Technical Dir. Pekka Pulkkinen, Civil Eng. Suunnittelukortes AEK Ltd, Oulu, Finland

Reiner Saul, Managing Dir. Leonhardt, Andra and Partner, Stuttgart, Germany

Introduction

In 1988 the Finnish Roads Administration organised a design competition for a technically advanced bridge which would adapt well to the lake landscape near the city of Heinola. Heinola is one of the most beautiful cities in Finland, about 130 km north of Helsinki on the route to a lake area where many vacation cottages are located.

The winner of the competition, now known as the "Star of Heinola", is a single-pylon cable-stayed bridge (Figs. 33, 34). It is the largest bridge in Finland, with a total length of 924 m. The

main span is 165 m and the width of the deck varies from 22 to 30 m.

Fig. 33

Design Competition

The competition was limited to four Finnish bridge engineering companies. The design brief was very demanding. Among the most important parameters were the site, the difficult foundation conditions, the curvature of the roadway and the need to maintain freeflowing marine traffic, including a busy logging channel. Maximum water depth at the site is 24 m, and the bridge is situated in a natural lake landscape, near the town.

Ten proposals were delivered to the competition jury. Five were traditional composite girder designs; one was a

forcing bars are placed in the uppersection of the piles.

The cross section of the deck is a traditional composite structure of two open girders. The girders are 3.2 m high I- beams (Fig. 33). There are crossbeams at 7 m centres. On the crossbeams there is a 1 m high secondary longitudinal girder. In the north part of the bridge, where the deck is 30 A wide, there are two secondary girders.

The cable crossbeams are inclined to the cable direction. Between the main girders, the cross section of the cable crossbeam is an open I-beam. The girders are stiffened with steel bars. Outside the main girders, the cable crossbeam is a composite box girder.

In the competition proposal, the pylon was a steel structure. In the general design, the costs of steel, composite and concrete pylon were compared. The concrete pylon proved to be the most economical.

The pylon is formed of two T-shaped hollow concrete towers which are joined together by two crossbeams. There is an elevator inside one tower and stairs inside the other. The horizontal geometry of the road has a curvature of R = 4000 m. This caused large horizontal forces on the towers. Therefore, the pylon was not placed centrally to the deck cross section, but with an eccentricity of 0.6 m. Due to this placement, the horizontal forces, and hence, the number of steel piles decreased considerably.

The bridge has 24 factory-fabricated parallel wire cables. The cables consist of 7 mm steel wires (253-403/cable) injected with grease inside a high den-

sity polyethylene pipe that is covered with white Tedlar tape. The pylon and cables are illuminated at night.

The fixed bearing of the bridge is at the southern abutment. It is a hinged structure, where horizontal forces are transferred large steel bars.

Construction

The piling and the concreting of the foundation slabs and columns took 14 months. The joint welding of the steel girder sections and the launching were completed in one year. The largest concrete works, such as towers and deck slabs, were placed in summer, because the temperature in winter can be as low as -25 °C.

At the beginning of the construction period, the design for the pile foundation was changed to a caisson structure. The caissons were built in a dammed trough on the north side of the bridge. The caissons were floated into position and the piling work was done through access holes in the caisson slab. Finally, the piles and the inner part of the caisson were cast in dry conditions. Except for the slab of supports T3 and T9, all casting work on the substructure was done in dry conditions. The casting of pile slabs succeeded well and there were no faults typical of underwater concrete casting. The steel girders of the superstructure were launched from both abutments and joined by welding between supports T5 and T6. Three auxiliary piers had to be built for the cable:stayed spans during launching. The top of the girder was lifted on a pontoon when

the spans exceeded 70 m. The launching bearings were vertically adjustable because the girder was very stiff. The longest beam sections were 42 m long and the maximum weight was almost 80 t.

The bridge was originally designed using steel grade Fe 355 E, but the steel contractor and steel manufacturer proposed to change the steel grade of the support sections of the girder to a new thermomechanically manufactured S 420 ML steel. Using this material, the contractor slightly reduced the weight of the girder and benefited from the superior welding properties of the new steel grade. The total amount of steel in the superstructure is about 50501.

The towers of the pylon were cast using a climbing formwork, with the height of one casting section being 4.15 m. The longitudinal reinforcing bars were jointed with screw sleeves.

The crossbeams between the towers and the cable anchor areas were pre: stressed. The surface of the concrete tower had to meet stringent quality requirements. A textile covering was placed on the formwork surface to enhance the strength of the concrete surface and to reduce air blisters. A green concrete paint was applied to the surface of the pylon.

Construction of the pylon was complicated because it has three inclined walls. Therefore, the scaffolding had to be adjusted at every stage. The reinforcing of the cable anchor area was difficult because of restricted space. The temporary support structure of the

upper crossbeam was also difficult to install and there were problems to stabilise it. Nevertheless, the pylon was completed on schedule and all casting work was completed before winter.

The deck slab of the superstructure was cast using two movable scaffolding units. The longest casting section was 24 m. The slab was prestressed transversely in two equal stages.

The stay cables were installed from the bottom upwards. Cable stressing was accomplished using four jacks at the level of the deck slab. The cables were stressed in pairs. The total weight of stay cables is 214 t. They were tested against fatigue and ultimate load, the test cable being the largest cable of the bridge. In the fatigue test, where a 5 m long test Cable was loaded 2 million times with a stress variation of 193 MPa, only 9 wires broke. After this test the same cable was stressed up to the breaking load, 24.6 MN.

Client:

Mikkeli Road District

Design:

Suunnittelukortes AEK Ltd, with Leonhardt, Andra and Partner

Main contractor: WT Corporation Ltd

Steel contractor: PPTH Steel Ltd

Construction duration: January 1991 to November 1993

Initial, seismic analyses were performed in accordance with the local seismic design criteria. These analyses provided controlling conditions for design of the piers and foundations.

A more refined analysis using earthquake response spectrum developed for the vicinity of the site, was used later to verify structural dimensions and reinforcement requirements.

Substructure Design

Each pier consists of a four-walled cellular reinforced concrete shaft, heavily reinforced to form a ductile cellular element. All fixed piers participate in providing the required capacity to resist seismic and horizontal forces. For transverse seismic loads, the piers behave as cantilevers, with significant moments developed at the bottom only. For resisting longitudinal seismic excitation, the main piers behave as members in a rigid frame. The creep, shrinkage, and temperature deformations of the superstructure are adequately accommodated in the monolithic piers by the flexibility of the foundations and the pier shafts.

The basic cross section of the main piers continues through the depth of the box girder cross section to provide diaphragms capable of developing the plastic moment of the pier into the girder. These diaphragms also provide a convenient location for anchorage of the longitudinal prestressing tendons in the superstructure.

The abutment is designed as a conventional cantilever wall which supports the bridge girder on sliding bearings. The footing blocks supporting the piers

were designed for the static dead and live impact loads and the seismic loads such that the movements at the base of the piers are within allowable limits and no plastic hinges develop below the ground surface.

Foundations consist of 2.0 m diameter drilled, cast-in-place, reinforced concrete piles, supporting reinforced concrete footing blocks. The piles, up to 14 m in length, act as friction piles with some limited end-bearing support.

Analysis and Modelling

Constructability will largely determine the success of a project. Therefore, a thirty-eight step stage-by-stage analysis of the bridge was performed to validate the time-dependent behaviour of the construction sequence. The analysis included:

-development of structural computer models for construction procedure

-evaluation of stresses and deflections at critical construction stages

-evaluation of stresses and deflections over the service life of the bridge. The bridge was modelled using a commercially available general-purpose structural analysis program which is capable of static and dynamic analysis of finite element models for a wide range of structure types.

All important structural features of the bridge were modelled, including the piers, the box girder, and the deltashaped piers. The girder was modelled as a line of frame elements representing the entire cross section, oriented along the centroidal axis of the prototype girder. The piers were modelled

similarly, including semi-rigid elements in the pier-girder intersection. The intersection zone between the delta-shaped piers and the girder was modelled by a separate finite element

model in order to capture the stress distribution between the diaphragms and the webs in this area.

rib-through-arch and a concrete stayed girder bridge.

The box-rib-through-arch was selected for the steel alternative based on an indepth study of three steel bridge types: trussed-through-arch, box-rib-through- arch, and cable-stayed girder. The twisting alignment required for the mountainous terrain made it difficult to get a tangent alignment long enough to accommodate minimum-length back spans for the cable-stayed and trussed arch bridges.

The arch span was the most economical. It adapted readily to the curved alignment of the approach structures and had superior aerodynamic properties. The welded steel arch was the overwhelming favorite of most bidders. It was bid at 30% less than the concrete alternative.

Arch Rib Design

Existing terrain and future lake levels were important factors in establishing the arch rib geometry. The arch springline is at Elevation 649.5 m, well below the 200-year flood elevation of 663 m. To keep the steel arch above water, the lower 158 m of the arch rib is made of concrete. The resulting composite steel-concrete structure consists of the following sections: 21.6 m of concrete rib, 323 m of steel rib, and 21.6 m of concrete rib, for a total arch rib length of 366.4 m and an arch span of 329 m.

The steel and concrete portions were made continuous by anchoring a base plate at each of the steel-concrete con-

nections. This required 32 MN of prestress force at each base plate.

The geometry of the entire arch rib optimizes the use of material through the length of the rib. The depth of the steel box section varies from 2.44 m at the crown, where compressive forces are the smallest, to 4.27 m at the base, where compressive forces are the greatest. The shape of the arch rib was also established to minimize the dead load bending stress in the arch. The resultant shape is defined by both second order and fourth order curves, making the entire arch appear smooth and continuous.

The steel arch rib section is made up of two 1.2 m-wide flanges with two web plates that vary from 2.44 m at the crown to 4.27 m at the base plate. The webs of the box section are stiffened by two T-sections welded to each web. The stiffeners are continuous and participate fully in carrying the various loads. Sixteen longitudinal fillet welds, over 13.4 m long, were required to connect the flanges and the stiffeners to the webs.

Stability against global buckling and transmission of wind loads for the arch rib, which is 15.2 m center-to-center, was achieved through the use of a K- bracing system fabricated from structural tubing and welded box sections. The selection of the K-bracing was based on both first-cost and life cycle costs. The K-bracing required 901 less steel and reduced wind deflections to half as much as those caused by a comparable Vierendeel system. The closed tubular sections were selected to minimized corrosion due to entrap-