Esquema de la intervención en el Congreso de los Diputados

Sumitomo Mitsui Construction, Tokyo, Japan

ABSTRACT: Some examples of multi-span extradosed bridges in Japan are introduced together with their structural features. Furthermore, a new solution to extend the span length of extradosed bridges to around 500 m is proposed, using a new technology called the butterfly web. The structural behavior of a multi-span long extradosed bridge, including the results of wind tunnel tests, is discussed.

1 INTRODUCTION

A large number of extradosed bridges have been constructed around the world since the theory was presented by Mathivat in 1988 (Mathivat 1988), and the Odawara Blueway Bridge was constructed in 1994. Japan is the country with the largest number of bridges constructed (Kasuga 2006). The most distinguishing structural feature of extradosed bridges is that the designer can freely choose the stiffness balance between stay cables and main girder. This principle has been adopted in the Japanese standards (JPCI 2000), providing a continuum spanning the design of girder and cable- stayed bridges. Depending on the stiffness balance between stay cables and main girder, the design can be performed with a safety factor of 1.67 for the stay cables (limiting cable tension to 0.6fpu,) in

spite of being a cable supported structure. This is the value used for internal or external cables. The factor of safety can also be varied linearly, allowing a limit value for cable tension ranging between 0.4fpu, the value specified for cable-stayed bridges, and 0.6fpu. The span length of extradosed

bridges built to date has been in the range from 100 to 250 m. Even for a composite bridge using steel girders, 275 m is the maximum span length that has been achieved so far.

Innovation in cable-stayed bridges and extradosed bridges consists of finding ways to make the main girders lighter. Recently, there has been remarkable progress in composite structures that permit lighter main girders, including the successful construction of many corrugated steel web bridges and composite truss bridges. Further progress is now being made in Japan with the construction of bridges using a new form of composite structure, the butterfly web (Ashizuka et al. 2012). Butterfly web structures use panels of steel or fiber reinforced concrete as an alternative to a double warren truss, and make it possible to achieve a similar level of lightness to corrugated steel web bridges (Fig. 1). Furthermore, it is a highly durable member since there are no reinforcing bars placed in the 15 cm thick butterfly web as the tensile side is reinforced with pretensioning steel. There are now a number of extradosed bridges constructed with main girders that use these sorts of composite structures.

Furthermore, a method to further extend the applicable span length of extradosed bridges up to the range of 500 m is presented. To achieve this, the butterfly web is used. With the use of butterfly webs, the total weight practically stays the same even if the depth of the main girder is increased. This means that the main girder stiffness can be considerably increased. Hence, the main tower can be shorter than cable-stayed bridge towers at the 500 m span length range (Kasuga et al. 2015). Here, an actual long-span cable-stayed bridge is compared to the case in which it is designed as an extradosed bridge. One issue that has to be taken into account with such a bridge is its wind-resistant stability. Consequently, the effect of the butterfly web, which has openings in

Figure 1. Butterfly web bridge (Takubogawa Bridge).

Figure 2. Ikuchi Bridge Figure 3. Nhat Tan Bridge.

the webs of a deep girder, was verified through wind tunnel testing. The results of the study of the long-span extradosed bridge are shown below.

2 LONG SPAN CABLE SUPPORTED BRIDGES 2.1 Composite Girder Construction Techniques

Composite bridges are structures that combine steel and concrete, selecting the material to suit the characteristics required for each part of the bridge. In Japan, where there are frequent earthquakes, reducing girder weight brings a range of benefits, and this has been a driving force behind the remarkable pace of development over the past twenty years. Reducing the weight of the super- structure is the key to innovation in long span bridges supported by cables. The reason is that this approach enables stiffness to be increased without increasing girder weight.

The hybrid girder is a well-used approach for composite bridges. In Japan, long span cable supported structures with hybrid girders are used, for example, in the Ikuchi Bridge (Fig. 2) and in the Nhat Tan Bridge (Fig. 3).

The corrugated steel web bridge dates back to 1965 when Shimada proposed that corrugated steel plate may be used in the web of main girders (Shimada 1965). However, it was a long time before this marvelous concept was realized in an actual bridge. That was in 1984, when a composite bridge using this approach was completed in France, far away from the birthplace of the idea. By replacing the concrete web with corrugated steel plate, a corrugated steel web bridge enables the weight of the main girder to be reduced by around 10 to 15%. An example of corrugated steel

Figure 4. Himi Bridge. Figure 5. SBS Link Way Bridge.

Figure 6. Fudo Ohashi Bridge Figure 7. New concept of composite truss bridge.

webs used in cable supported structures includes the Himi Bridge (Fig. 4) (Maeda et al. 2002). The Himi Bridge, an extradosed bridge, is particularly interesting as it was the world’s first use of a corrugated steel web in a cable supported structure.

One form of composite truss bridge that permits substantial weight reduction is the space truss structure. A cable-stayed structure using the space truss can achieve weights that are around 40% less than conventional concrete girders (Muller 1990). The only actual bridge using this structure is a very small pedestrian bridge in Singapore (Fig. 5), but in Japan, there is one example of the extradosed bridge utilizing composite truss techniques but replacing just the web with a steel pipe truss, as can be seen inFigure 6. In addition, as a means of reducing axial forces acting on the truss nodes, it is possible to use a composite structure combining a concrete box girder with a space truss as shown inFigure 7.Table 1shows that selecting an appropriate mix of concrete and steel trusses in combination with a cable supported structure enables the maximum axial force acting on the trusses to be controlled to within 1500–2000 kN. The concrete-steel composition is a parameter to be considered when designing an optimum structure.

2.2 Tay Cable Design

Japan was also the location for the world’s first extradosed bridge, the Odawara Blueway Bridge constructed in 1994, which marked the debut of a structural form that could cover span lengths for which cable-stayed bridges are uneconomic. This represented a major innovation in thinking about the design of stays. Cable-stayed bridges and external-cable structures could be conceived as points on the same continuum when determining the maximum allowable stress for stays. Taking this approach enables the use of stays with allowable stress in a continuous range between 0.4fpuand

0.6fpu. Moreover, the allowable stress can be determined according to the range of live load stresses

Table 1. Maximum axial force in various composite truss structures.

in the cables. Instead of the conventional approach of stipulating a single value of allowable stress for the whole structure, the structure could be designed with different values for each stays. This design method was formalized in Japan’s specifications for highway bridges, enabling engineers to produce more rational designs for stays (Fig. 8).

2.3 Ibi River Bridge – An example of multi-span extradosed bridge

The Ibi River Bridge is a five-span continuous extradosed bridge which was completed in 2001 (Fig. 9). The longest span length is 271.5 m long and was built with precast segments of up to 400 tons (Fig. 10). It is a hybrid structure made with 100 m of steel girders at the span center. The two-point bearing supports on top of the piers serve to increase the overall stiffness of the bridge, while the shape of the main towers provides additional stiffness in the longitudinal direction of the bridge. Since it is an extradosed bridge, the stiffness of the main girder is high. However, the maximum stay cable stress variation due to live load is 112 N/mm2_{because it is multi-span. A limit}

value of 0.6fpuis used for the stays since prefabricated cables (DINA) were used.

One characteristic feature of an extradosed bridge in comparison with a cable-stayed bridge is its ultimate limit state behavior (Kutusina et al. 2002). Since the increase in stay cable tension due

Figure 8. Allowable stress versus stress change owing to live loads.

Figure 9. Ibi River Bridge. Figure 10. 400 tons precast segment.

to the ultimate load starts from 0.6fpu, the stay cables yield before the ultimate limit state is reached

(Fig. 11). In contrast, the stay cables of a cable-stayed bridge starts from 0.4fpuand do not yield.

In an extradosed bridge, the construction error distribution at the time of introduction of the stay cable forces produces a small error distribution in the bridge because it is the stay that cables yield, which may lead to a smaller factor of safety at ultimate load like external tendons (Woelfel 1990). Extradosed bridges inherently have higher main girder stiffness than cable-stayed bridges, which makes it possible to raise their overall stiffness even with multiple spans. This suggests potential for exploring the possibility of ranges beyond the commonly stated applicable span length of 250 m for extradosed bridges.

3 BUTTERFLY WEB BRIDGE 3.1 Outline of Butterfly Web

The butterfly web (Kasuga et al 2010) is a new structure with butterfly-shaped web members having the following characteristics.

(1) The web is configured with butterfly-shaped panels placed independently and not joined con- tinuously. The shape limits the orientation of compression and tension in the panel due to shear forces, meaning that the structure is similar to a double warren truss (Fig. 12).

Figure 11. Behavior of extradosed cables up to ULS.

(2) The butterfly web uses 80 MPa steel fibre reinforced concrete, and has prestressing steel oriented in the direction that tensile forces act (Fig. 13), limiting the occurrence of cracks. It does not use steel reinforcements, relying instead on steel fibers and prestressing to achieve the required strength.

(3) Transmission of shear forces between the butterfly web and deck slabs is achieved by the joint between the slab concrete and dowels embedded in the panel.

Many corrugated steel web bridges and steel truss web bridges have been built. These bridges had rational structures and excellent structural characteristics, but at the same time, they required complex machining of steel members, on-site welding, or other special skills for fabrication or construction. In contrast, as the butterfly web is a precast product, all that is needed to construct a girder is to combine the web with the slabs on site. The prestressing steel oriented in the same direction as the tensile forces in the web is pre-tensioned at the factory, so there is no need to work on the butterfly web at the construction site. The potential weight reduction of the main girder is similar to that of a corrugated steel web bridge, achieving about a 10% to 15% reduction compared to a conventional box girder section. Consequently, the length of segments that can be constructed using a form traveler can be 50% longer because of light weight of the girder.

A butterfly web bridge, which uses butterfly-shaped panels instead of a double warren truss, is a new structure that has both the corrugated steel web bridge’s advantage of being able to simplify the joints with the concrete slabs, and the truss bridge’s advantage of not needing on-site work to make joints between the butterfly-shaped panels that carry the shear forces.

3.2 Innovative Bridge Project

One of these solutions for long span extradosed bridges is being used in an innovative project cur- rently under construction. The Mukogawa Bridge, shown inFigure 14andFigure 15is an extradosed bridge using butterfly web technology. This is a 5-span continuous rigid frame bridge with a span length of 100 m. The tallest piers are 81.2 m, and they were designed for rapid construction. The cross-section incorporates four butterfly webs, and the extradosed cables are located in the center of the cross-section. The main girder is constructed by free cantilevering, with individual segments having a length of 6.0m and incorporating two butterfly web panels parallel to the longitudinal direction. After setting panels, the concrete deck is cast in place. The reduction in superstructure weight achieved enables a substantial reduction in pier thickness and the size of foundations. The cables for the Mukogawa Bridge use ultra high-strength strands (Kido & Hoshino 2010) that are 30% stronger than regular strands.

Figure 12. Behavior of butterfly web bridge. Figure 13. Butterfly web panel.

Figure 14. Mukogawa Bridge.

Figure 15. General view of Mukogawa Bridge.

Table 2. Comparison of material quantities.

CSB EDB Concrete Girder m3 _{48900 43700}

Pylon m3 _{18500 14400}

Rebar ton 15960 13410 Prestressing steel ton 259 261 Stay cables ton 6030 6300

4 STRUCTURAL BEHAVIOR OF MULTIPLE SPAN EXTRA DOSED BRIDGE

The models used for this comparative study are a cable-stayed bridge and an extradosed bridge composed of five continuous spans with a central span of 500 m (Fig. 16). In order to raise the overall stiffness of the multi-span cable supported structure, overlapping cables are distributed at the middle of the span, and the stiffness of the bridge piers and main towers is increased. Moreover, highly stiff butterfly webs with a 6.0 m depth are used for the extradosed bridge. Using these two structures, the main girder stresses at the serviceability limit state and stay cable stress variations, as well as other variables such as the behavior during earthquakes, were compared, and the structural feasibility of the proposed continuous long span extradosed bridge is verified.

4.1 Service limit state

With regard to the limit stresses, concrete stress at dead load is 0.4f ’ck= 18 N/mm2 on the

compression side and the full prestress on the tension side. At design load, concrete stress is 0.6f ’ck= 27 N/mm2on the compression side and −0.5f ’2/3ck for 50% live load on the tension side;

for 100% live load, cracking is allowed, with the amount of reinforcement set such that the rein- forcement tensile stress is 0.6fsy or less. The bending moment and stress diagrams of the main

girder at 50% live load are shown inFigs. 17to20. From these results, it was confirmed that the structure is sufficiently sound even with the extradosed bridge configuration.

Figure 21shows a comparison of the stress variations in stay cables due to live loads. In the JPCI code of Japan, up to 0.6fpu, the limit value for stay cable tension, is allowed for road bridges as long

as the stay cable stress variation is 70 N/mm2_{or less. However, it was confirmed that almost all of}

the cable stress variation exceed 70 N/mm2_{except some cables of the side span because multi-span}

structures are very flexible.

Table 2shows a comparison of the material quantities for the two configurations. The amount of concrete for the extradosed bridge using butterfly webs is lighter, despite having a deeper girder. Moreover, the material required for the pylons was reduced. However, there is no big difference of the weight of stay cables in the multi-span structures.

4.2 Seismic resistance

In verifying dynamic behavior under seismic loading, it is interesting to examine the differences between cable-stayed bridges with their tall main towers and flexible girders, and extradosed bridges with short main towers and stiff main girders. Seismic response analysis was performed using the wave of Japanese seismic specifications (Fig. 22) for soft soil sites. The natural periods for each of these bridge types are shown inTable 3, and the maximum bending moments of girders and pylons are shown inFigs. 23and24.The response value of the extradosed bridge girder is larger than that of the cable-stayed bridge, but the bending moment of pylons has no large difference. For long span bridges, adoption of an extradosed bridge with a light main girder instead of a cable-stayed bridge was found to be advantageous in regions prone to earthquakes.

Figure 16. 500 m five-span bridge.

Table 3. Natural period.

1st 2nd 3rd CSB 7.92 5.14 4.17 EBD 8.00 5.38 3.99

Figure 17. Girder bending moment (CSB). Figure 18. Girder bending moment (EDB).

Figure 19. Girder bending stress (CSB). Figure 20. Girder bending stress (EDB).

Figure 21. Stay cable stress variations due to live load.

Figure 22. Wave of Japanese seismic specifications.

Figure 23. Girder bending moment. Figure 24. Pylon bending moment.

Figure 25. Girder configurations of butterfly web.

Figure 26. Girder configurations of conventional web.

5 WIND TUNNEL TEST

When bridge structures are stretched to very long spans, one concern is the more noticeable aerodynamic vibration as the entire structural system becomes more flexible. Therefore, for this chapter, we conducted wind tunnel tests to examine the wind-resistant stability of the main girder in the 500 m span extradosed bridge described in the previous chapter. Note however that a wider bridge deck was used as the main girder section, as shown inFigure 25. Furthermore, in order to study the effects on wind-resistant stability of the web openings in the butterfly web structure, a comparative study was conducted through wind tunnel testing on another model with the main girder’s butterfly web structure openings covered (Fig. 26).

5.1 Test outline

Modal analysis was performed for the bridge to set the test conditions. The results are shown in Table 4, and the wind tunnel test specifications are provided inTable 5. To tailor to the test facility, a scale of 1/100 was used. A logarithmic decrement of about 0.03 is usually used for cable-stayed and

Figure 27. Test outline and setup.

Table 4. Results of eigenvalue analysis.

Vibration mode Natural frequency Girder, heaving 1st mode 0.139 Hz Girder, torsion 1st mode 0.682 Hz

other bridges to take the damping ratio under aerodynamic vibration into account, but the damping ratio in this test was minimized as much as possible, so that the aerodynamic properties induced by the shape of the butterfly web structure itself can be compared to those of the conventional box girder section. Wind-resistant stability for the actual bridge was determined from excitation forces (damping factor) for the various vibrations exhibited. Wind tunnel testing was performed on two-dimensional rigid body spring models. The tests are outlined inFigure 27.

5.2 Test Results (1) Heaving vibration

For the closed section box girder studied for comparison, the single-degree-of-freedom heaving vibration test results are given inFigure 28(smooth flow, angle of attack + 3 deg). The figure shows the test results when the model was excited with a 5 m/sec full scale wind velocity and then left to freely vibrate. It also shows the results when the model was left to freely vibrate without adding excitation (initial amplitude of vibration is 0), which confirms the presence of limited amplitude vibration that is apparently vortex-induced vibration, with a double amplitude of about 600 mm for the actual bridge. However, for subsequent wind velocities, there were no vibrations for the tests without added excitation. For the tests with added excitation, unsteady amplitude increases as the wind velocity increases. Although peak amplitudes were large for full-scale wind velocities over 65 m/sec in particular, these were not considered as divergent vibrations such as galloping as these large vibrations did not occur in a stable form.

Table 5. Wind tunnel test specifications.

Item Actual bridge Test mode