Development of extradosed structures in the bridge construction

Transcription

1 Development of extradosed structures in the bridge construction S keda, Yokohama National University, Japan A Kasuga, Sumitomo Construction Co. Ltd, Japan 25th Conference on OUR WORLD N CONCRETE & STRUCTURES: August 2000, Singapore Article Online d: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete nstitute All Rights reserved for C Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of C Premier PTE LTD Visit Our Website for more information

2 25 th Conference on Our World in Concrete & Structures: August 2000, Singapore Development of extradosed structures in the bridge construction S keda, Yokohama National University, Japan A Kasuga, Sumitomo Construction Co. Ltd, Japan Abstract Extradosed bridges are similar to cable-stayed bridges in that stay cables are used for strengthening. Mter introducing the concept of extradosed bridges by J. Mathivat, this new type of bridges have been constructed in Japan. This paper shows some examples and the difference between extradosed and cable-stayed bridges in a structural aspect. Keywords: extradosed bridges, cable-stayed bridges, 1. ntroduction Extradosed bridges, a concept introduced by J. Mathivat in 1988, have two main characteristics [1]. One is the allowable stress of stay cables which is the same as that of internal cables. The other is the saddle at the top of the tower. n the Japan Highway Bridge Specifications, the allowable stress of the cables is provided for stay cables of cable-stayed bridges (O.4,/;,u : hu represents the strength of the cables) and ordinary cables (0.6hu). However, the problem of how much allowable stress should be used for stay cables of extradosed bridges is still controversial because these cables are considered external cables arranged outside the box girder. Basically, extradosed bridges should be distinguished from cable-stayed bridges in their structural aspects. However, the distinction between them must not be inconsistent with the idea of the specification, because more than 100 concrete cable-stayed bridges have been designed in Japan. The purpose of this paper is to outline some extradosed bridges in Japan and clarify the structural distinction between cable-stayed and extradosed bridges using the Japanese examples. 2. The Significance of Extradosed Bridges The structures which are characteristic of cable-stayed bridges and conventional box girder types had not yet been constructed at the time the concept of extradosed bridge was introduced. According to Mathivat, the optimum girder height is 1/30 to 1/35 of the main span length and the tower height is 1/15. n particular, the tower height is half of the optimum value of cable-stayed bridges. The significance of extradosed bridges lies in their aqility to associate cable-stayed bridges with conventional box girder bridges. Extradosed bridges are an intermediate type between girder bridges and cable-stayed bridges as can be seen from the quantities of materials used, shown in Fig. 1 and 2 where the relationships between maximum span length and average thickness of main girder concrete (concrete volumelbridge area), and between equivalent span length and prestressing steel quantities are plotted for previously constructed bridges. t can be seen that the quantities of all materials in extradosed bridges lie mid-way between girder bridges and cable-stayed bridges. Extradosed bridges may be constructed similarly to girder bridges, with no cable force adjustment of stay cables during construction and simultaneous stressing of stay cables and of cables within girders. Construction is also easier than for cables-stayed bridges, as the height of the main tower is only around half that of a cable-stayed bridge of similar size. n the same way, the concept of 85

3 partially prestressing associates prestressed concrete with reinforced concrete. Therefore, though the optimum tower and girder height should be determined primarily by economic considerations, the optimum values can vary depending on the design conditions. n other words, cable-stayed, extradosed and externally prestressed bridges can be treated as the same category of the structure reinforced by stay cables. The difference among them is which member carries the live load more, stay cables or girders. This is the same idea as the category of arch bridges including deck-stiffened arch types and stiff arch types. However, one problem arises here. n the specification, allowable stress of prestressing steel for stay cables is different from that of internal and external prestressing steel. Therefore, stay cables should be distinguished from ordinary cables by some criteria. By using the concept of extradosed bridges, designers can select the distribution ratio of stay cables against the live load freely. n the external cables, this ratio is almost zero and stay cables carry almost 100% in the cablestayed bridges. This means that the structural variety and degree of freedom can be expanded by selecting the load distribution ratio of stay cables positively. 2.5 ~ ;;-... 0) ) >- 0 :: 1.S J:: c- O) c 0.5 0) bo.." >- 0) > -< 0 :: 50 o -g 0) t- Box Girder ~ridge 0- Tsukuhara ~\ Co l~ :r~- Fig ~ - -~ Kan i sawa Odawara Cab e-stayed Br i dge Span Length VS Average Depth 150r o 50 Fig.2 Box Girder Bridge 0- Tsukuhara Cable-Stayed -- Bridge Co..J>-fr t ~ Co _--- b. -,." Co ~-- Kanisawa Odawara Span Length (m) Span Length VS Tendon Weight 3. Extradosed Bridges in Japan Major extradosed highway bridge projects are shown in Table 1. There is another bridge for the Nagano bullet train and other extradosed bridges are in the process of design. 3.1 The Odawara Blueway Bridge The Odawara Blueway Bridge is the first extradosed bridge (Fig. 3,4). An allowable stay cable stress of O.6/pu has been adopted in this bridge. Stay cables are anchored outside the saddle at the top of the tower in order to satisfy the requirement of not allowing them to slip, which would create a difference in cable force in the right and left sides. Moreover, high damping rubber dampers were installed at the bottom of each stay cable to suppress rain induced vibrations. The validity of strength of the saddle was first confirmed by testing on a full-scale model. Then the flexural fatigue test of the stay cables and the performance test of the dampers were carried out. Construction is the same as that of cable-stayed bridges in that the free cantilevering method is used. Cable force adjustment during and after construction is not required, since all forces on stay cables decrease with the progress of creep and shrink-age, just as when prestressing steel is placed inside concrete girders. The anchorages for the stay cables are identical to those for the prestressing tendons in the girders as no cable force adjustment is needed and the stress change due to the live load is low. The highly damped rubber dampers are used to absorb stay cable vibrations. This type of damper is installed between the pipe and the stay cables, so that the dampers can be hidden inside the cable cover. This arrangement also has advantages from the point of view of the aesthetics of the bridge. 86

4 Bridge Name Odawara Blueway Bridge Structural Type 3-Span Rigid Frame Table 1 The Major Projects Bridge Maximum Width Tower Girder Length Span Height Height (m) (m) (m) (m) (m) Tsukuhara Bridge 3-Span Rigid Frame Kanisawa Bridge Okuyama Bridge Shikari Bridge bi River Bridge 3-Span Continuous 3-Span Continuous 5-Span Continuous 6-Span Composite (Sidewalk) Kiso River Bridge 5-Span Composite Miyakoda River Bridge 2-Span Rigid Frame Mitanigawa Daini Bridge 2-Span Rigid Frame HimiBridge 3-Span Rigid Frame Haneji Bridge 2-Span Rigid Frame Sajiki Bridge 3-Span Continuous HoduBridge 6-Span Rigid Frame Hukaura Bridge 5-Span Rigid Frame Yukizawa Bridge 3-Span Continuous Yubikubo Bridge 2-Span Rigid Frame Siukawa Bridge 5-Span Continuous Fig.3 '-""' Odawara Blueway Bridge Fig.4 Odawara Blueway Bridge 87

5 3.2 The Tsukuhara Bridge The Tsukuhara Bridge is a three-span frame structure with a main span 180 m long (Fig.5). This clear span was needed to avoid disturbance of the lake it crosses, which is used for drinking water. n order to reduce the overturning moment of the piers, a concrete counterweight has been placed inside the box girder of each side span. The ratio of the center span to the tower or girder height is the same as on the Odawara Bridge. The Tsukuhara Bridge is constructed with single-cell box girders instead of the conventional two-cell girders. The efficiency of erection was enhanced by using form travellers, and by reducing the dead weight of the girders, resulting in a slab span of 9 m. Twelve external tendons, comprising 19 strands 15.2 mm in diameter, are placed inside the box girder at the center span to resist positive bending moments. The internal tendons, which are mainly used for cantilevering, are 12 strands of 12.7 mm diameter. Moreover, a single 28.6 mm strand "after bond" system is used for transverse prestressing, which means that epoxy resin inserted between the strands and the polyethylene sheathing hardens after tensioning. Cement grouting is not necessary in this system. Piers and pylons are V-shaped and Stay cable forces can flow straight from top to bottom. Cross beams between the pylons are not necessary because the pylon is shorter than for a comparable cable-stayed bridge. Saddles are used at the top of the pylons to simplify the installation of reinforcement. The stay cables of the Tsukuhara Bridge form a replaceable external cable system. Each stay cable consists of 27 strands 15.2 mm in diameter, protected against corrosion with a double layer of polyethylene. As the maximum stress change in the stay cables due to live load is 3.7 kgf/mm 2, 0.6 J;,u (fpu is the strength of the stay cables) is adopted as an allowable stress of stay cables. Therefore, there is no need for anchorages with high fatigue strength, such as those used in cable-stayed bridges. The stay cables of extradosed bridges are functionally comparable to external post-tensioning cables. As they are located on the outside of the girders, adequate provision has to be made for the damping of wind vibration. Vibrating stay cables not only are unsettling to bridge users, but vibration may lead to actual breakage of the cables. The dampers installed in the Tsukuhara Bridge use the high damping capacity of rubber for base isolation. This type of damper is easy to maintain and its performance is relatively immune to temperature variations. The dampers are designed to achieve more than a 0.03 logarithmic decrement, which is necessary to suppress windrain-induced vibration. The magnitude of the January 1995 earthquake in the Kobe area was greater than that of the Great Kanto Earthquake in 1923, the previous benchmark for seismic design in Japan. A new set of provisional specifications for seismic design has subsequently been drawn up based on the 1995 quake. The Tsukuhara Bridge was designed according to these new specifications because detail design started just after the 1995 earthquake. n order to simulate the behavior of the bridge during earthquakes, nonlinear seismic response analyses allowing for concrete cracking and yielding of reinforcement were also carried out in both the longitudinal and transverse directions. The girders are constructed by the free cantilevering method, using form travellers (Fig.6). An extra-large traveller with a maximum block length of 7 m was used for the critical P pier, in order to shorten the construction schedule. t was around twice the size of a standard form traveller. 7@700J =49000 Fig.5 Tsukuhara Bridge 88

6 Fig.6 Tsukuhara Bridge 3.3 The Kanisawa Bridge The Kanisawa Bridge (Fig. 7,8) is the same size as the Tsukuhara Bridge. Because this bridge is located on the river, the girder is of the continuous beam type. Accordingly, the stress change of stay cables due to live load is 10 kglmm 2 and O.4/pu was used as the allowable stress of the stay cables. Because this bridge is an access point to the new airport, the tower height is limited. f the extradosed concept had not been introduced, the cable-stayed structure type could not have been constructed. This is a good example proving the expansion of structural variety and degree of freedom. Fig.7 Kanisawa Bridge Fig.8 Kanisawa Bridge 3.4 The Thi and Kiso River Bridges The bi River and Kiso River Bridges are "twin bridges" and represent the world's flst example of the composite 89

7 extradosed bridges located at the mouth of the two rivers. An interchange is going to be constructed in the area between the two rivers. The bi River and Kiso River bridges can be considered as the more advanced version of the extradosed bridges. They would have been designed as cable-stayed bridges if the conventional design standard were to be applied. Combination of the composite structure of the extradosed bridge led to a good result from the viewpoints of economy and aesthetics. These technologies, the extradosed bridges and the composite structures, have been considered as the most promising keynote technologies toward the 21st century. When combined together, they can surely widen the new area of application. The superstructures of these two bridges are 33 meter wide carrying 6 lanes carriage ways on the monolithic deck slab supported by the rubber bearings. The Fig.9 shows the general view. As for the substructures, so called steel pipe sheet pile foundation was adopted the reach the bearing stratum which exists at more than 40 meter depth aiming at a good cost saving and shortening of the construction period. t can be said that one of the most noticeable features of this bridge is the first application of the composite extradosed structure in the world. A lot of efforts have been made to reduce the dead weight of the superstructure. The central part of each span is composed of 100 m long steel girders, whereas the remaining part of span is made of concrete segments. Even at the part of the concrete segment, weight reduction was achieved through prestressing by extradosed cables. The concrete portions of the spans are composed of numbers of prefabricated segments. The following three reasons made it possible to adopt the short line match cast method. Transportation of heavy segments to the site was possible using barges on the sea. The field fabrication yard of 80,000 m 2 area was available in 10 km distance. The high strength concrete material (60 MPa strength) was to be used which can reduce the weight of segments. The maximum sizes of the segments were set at 7 meters in height, 33 meters in width, 5 meters in length and 400 tf in terms of weight. This weight is as heavy as five times of the concrete segments fabricated in Japan so far. Prestressing of the concrete segmental girders were introduced by extradosed cables, external and internal tendons. The cross sectional configuration of these bridges is three-cell box girders as shown in the Fig.lO and extradosed cables are installed in the center transversally. n order to reinforce the slabs and webs to give necessary transverse rigidity, rib members are attached along the upper slabs and the outer webs. Pylons are set up in the transversal center also, which makes the transverse formation fairly flat as shown in the Fig.ll. This is also designed as to reflect the image of a sail of a ship, which as a result contributed highly to enhance the transverse ductility against earthquakes. n parallel with the detailed design, a number of tests were carried out to verify the design reliability and check the newly used materials of concrete. Particular attention was paid to clarify the materials properties of concrete by fabricating two actual size models of segment in advance to check the following points. The 60MPa concrete material has not been used in the expressway project in Japan, and materials properties were not well known. The segmental sections were so large-sized as nobody experienced and structurally complicated unusually. As is shown in the Fig. 12, four fabrication lines are set up in the prefabrication yard for 375 segments. Two lines share one crane to carry the 400 tf segments in the yards as is shown in the Fig. 13. One 400 tf capacity crane is set up to carry and mount the segments on to the barge (Fig. 14). First of all, the pier segments comprising three segments are fabricated. The segments are placed on the pier top after installation of the rubber bearings (Fig. 15) by a floating crane. Then concrete girders transported by barges are lifted up by the erection nose (Fig. 16). The approach spans are cantilevered out from the pier abutment side toward the main piers supported by the temporary stay cables. Finally the 100 m long and 2000 tf heavy steel girders are lifted up from the tip of concrete portion. 90

8 Kiso River Bridge bi River Bridge Fig.9 bi and Kiso River Bridges ~ / i i i i 1\ i i i ~l Fig.lO Cross Section Fig.ll Pylon Fig.12 Prefabrication Yard Fig.13 Yard Crane (400t1) 91

9 Fig.14 Shipping Crane (400tt) Fig.lS Erection of Pier Head Segments Fig.16 bi River Bridge 4. The Structural Difference with Cable-stayed Bridges The stress change due to live load is the most important element of the safety factor of stay cables. The stiffness ratio between stay cables and girders greatly affects stress change. n this paper, the index f3 is introduced in order to express the distribution on stay cables and girders of vertical uniform load on the main span. f3 is derived by Load carried by stay cables f3 = X 100 (%) (1) Total vertical load Also f3 represents the stiffness ratio between stay cables and girders and is easy to calculate. Fig. 17 shows the relationship between f3 and the maximum stress change of stay cables due to live load (design vehicular load) using actual bridge data. White dots, triangles and squares represent extradosed bridges (EDBs) and black dots show cable-stayed bridges (CSBs). Cable-stayed bridges include various structural types. The maximum span length is from 100m to 260m and the number of spans ranges from one to three. According to Fig. 10, f3 = 30 % or L1 (J = S kg/mm 2 could form the boundary between cable-stayed and extradosed bridges. Therefore, the two types of allowable stress defmed in Fig. 18 can be proposed in design without contradicting the safety factor of stay cables for existing cable-stayed bridges. 92

10 15- A LJ. Olruyama 0 0 : 0 0''',," : 0"/,, " " /~ -.,,,," - """ o - / )'i""j!.h A SingleSpan(CSB) Odaw~ --l-ara 0 2-8pan(CSB),,/: 3-Span(CSB) 00"/ 2'0 4'0 6'0 '-,--8'0--"&100 Distribution Ratio of Virtica1 Load f3 (%) Fig.17 f3 VS Maximum Stress Change due to Live Load <l <l G)!:l tzl G) ~ u G) ~ ~.sa < fpa Load carried by stay cables /3 = X 100 (%) Total vertical load 0.6fpu ---~ TypeA L O.4fpu TypeB {3(%) Distribution Ratio of Vertical Load Fig.18 dea of Allowable Cable Stress 5. Conclusion Extradosed bridges, which have been brought up successfully in Japan, have to be clarified the difference from the ordinary cable-stayed bridges both in terms of structural characteristics and design specifications. We understand that this burden falls on the Japanese engineers firstly and at present the Japan Prestressed Concrete Engineering Association has undertaken the creation of design specifications for these types of bridges. We expect that the establishment of these specifications coupled with the successful completion of extradosed bridges will further contribute to the advancement of the extradosed bridges. References [1] J. Mathivat : "Recent Development in Prestressed Concrete Bridges", FP Notes, 1988/2 93