Irabu Bridge

IrabuBridge

 

Irabu-jima is located to the northwest of Miyako-jima, about 310 kilometers southwest of Okinawa Island. Although ships provided a regular service between Irabu-jima and Miyako-jima, there were problems unique to remote islands such as cancellations due to bad weather, emergency medical treatment at night and early morning, education, and welfare. Some 41 years after Irabu village first requested a safe connection to Miyako-jima in 1974, the opening ceremony for Irabu Bridge was held on January 31, 2015.

In the subtropical Miyako region, it has a beautiful sight of the wonderfully clear sky and blue sea. However, for a bridge, it is a harsh environment where the scorching sun beats down and the structure is exposed to high-temperature winds containing salt. Also, because typhoons often pass through with strong force, and the wind speed and frequency are very high, the design standard wind speed for the Irabu Bridge main marine navigational route was set to U33 = 82.2 m/s. Nippon Engineering Consultants Co., Ltd. was responsible for the detailed design of the main marine navigational route in a joint venture with Chuo Kensetu Consultants, Co., Ltd., and for the landscape design for the whole bridge, and worked to ensure durability for 100 years even in an environment with such severe salt damage and strong winds.

Bridge type

The main marine navigational route of the bridge was initially planned as a medium-profile steel arch bridge where the arch ribs opened upwards and were not laterally connected at the top. However, because storms would greatly shake the arch ribs, causing fatigue damage by repetitive shaking, and because repair after damage would be difficult, it was found that there was a problem with durability for 100 years, and a review of the bridge type was carried out. Taking wind resistance and salt damage countermeasures into account, the review of the bridge type concluded that a steel deck box girder bridge with an octagonal single box girder section with a small surface area should be used. This design reduced the amount of deposition of flying salt and paint replacement, reducing maintenance and management expenses.

伊良部大橋 横断図

Irabu_Bridge_Side_View

 

Wind-resistant design

上面傾斜角θと平場長L_02

In the wind resistant design of the top inclination angle θ and the flat field length L_02 bridge, six cases with different top surface tilt angle θ and flat field length L of the edge cross section were prepared, attention was paid to vortex induced flexural vibration (limited vibration: reference wind speed 82.2 m /s) and flutter (divergence vibration: checking wind speed 108.5 m / s), and a 2-dimensional model wind tunnel test was carried out at 1/64 scale. As a result, it was found that a cross sectional shape with the top inclination angle θ = 18° and L = 1.399 m (actual length) was the most advantageous cross section in terms of wind resistance stability suppressing vortex induced vibration. Flutter was not generated with any cross section.

For Irabu Bridge, the vehicle protection fence handrail was raised by 25 cm to prevent accidents from cyclists or vehicles falling into the sea because the height from the road surface to the sea was very high. This effect was confirmed through a 2-dimensional model wind tunnel test at 1/20 scale. Vortex  induced vibration did not occur even in the 1/20 scale wind tunnel test which had a higher model precision than the 1/64 scale.

嵩上げされた車両防護柵At the time of installation of the raised vehicle protection fence, a 2-dimensional model wind tunnel test (reference wind speed of 59 m / s) was conducted on the cross section without installing the wheel guard / vehicle protection fence. This showed that flexural vortex excitation far exceeded the allowable amplitude. Therefore, the vortex excitation was suppressed by making the aerodynamic characteristics the same as the cross section at completion (by installing the wheel guard / vehicle protection fence from the time of installation).

Durability design

In order to ensure durability for 100 years in a severe salt damage environment, “aluminum / magnesium alloy (Al 95% – Mg 5%) thermal spraying” which can be expected to give the highest level of corrosion resistance at the present time was adopted for the corrosion prevention on the main marine navigational route of Irabu Bridge. This was the first bridge of this size to use this material, and it was decided to use it as a heavy anticorrosion paint in combination with a C-5 type paint from the painting and corrosion prevention manual because there were concerns that securing stable quality would not be easy. Aluminum / magnesium alloy spraying is a corrosion protection method that protects steel materials by sacrificial corrosion protection, has characteristics of self-wearing resistance and excellent abrasion resistance, and has been proven overseas as a corrosion prevention tool for marine structures such as the North Sea oil fields.

In considering the structure details, protrusions were minimized so as not to cause salt adhesion on the outer surface of the girders. In addition, the box girder was made as a large block in order to reduce sections fitted on-site that require on-site painting as much as possible. Agricultural water pipes, etc. were attached to the bridge at the center of the cross section, and since there were complicated parts at the ends of the girders, to prevent the inflow of salt into the girders, lids was placed on the ends of the girders.

排水装置 路面排水装置_04

Although it is common to install the drainage device such that it penetrates the girders, it was planned to discharge rain water falling on the road directly outside the opening of the steel wheel guard in order to eliminate the need for a drainage device metal fitting that would be difficult to protect from corrosion. A steel gutter whereby dirt would not be noticeable was also considered for discharge, but it was decided that dirt itself would not be conspicuous, and on the contrary, because the merit of being able to wash away the salt adhered to the bridge to some extent by rainwater was larger, a design was chosen whereby water flows from the top surface of main girder overhang to the sides and bottom, and drains to the sea through drains installed near the bearings.

Landscape design

伊良部大橋_景観検討

The girder shape and bridge shape were designed without changing the cross-sectional shape of the main girder general part determined based on wind resistance stability, focusing on the intermediate support portion where the girder height changes. The target main marine navigational route is more than 2 km away from land so the detailed form cannot be seen, but people on ships passing under the bridge can view it. Given this, it was judged that a “modest and regular form” was desirable for the landscape, and the intermediate support part of the main girder adopted an A proposal in which the change in the height of the spar stands out, while the lower part adopted a straight-line stick model which was modeled with straight elements.

Munsell value N 7.5 (light gray) achromatic color was adopted, but since the color of the sea is reflected, the girder color of the main marine navigational route appears bluish.

伊良部大橋の親柱A request was received asking if the main pillar of Irabu Bridge on the Miyako-jima side could feature a motif of the island’s heroes “Hisamatsu-goyushi*,” and a design was created to express the shape of five walls surrounding and protecting the sea.
The main pillar on the Irabu-jima side is a design that expresses a grey-faced buzzard.

* Hisamatsu-goyushi
Five heroes who rowed 170 km over 15 hours by Sabani boat from Miyako-jima to Ishigaki-jima with a do-or-die spirit to convey the arrival of the Russian Baltic fleet to the mainland just before the battle of the Sea of Japan in the Russo-Japanese War.

Superstructure Construction Plan

In order to ensure quality without affecting the coral, it was planned to divide the main girders into three large blocks and erect them with a 3000-t capacity crane ship (Actually, a 4000-t suspended crane ship). In order to drop the large block girder of the center span as the final step, it was necessary to make a gap so that the hanging girder and the installed girder did not collide. The side girder on the Irabu-jima side was preliminarily erected positioned 250 mm towards the side of Irabu-jima, and after installation of the center span, the Irabu-jima girder was returned to its original position after completion. In order to make this possible, a setting beam (girder temporarily received after FC installation) installed in the large block installation was made so that it could be slid. In addition, a moving scaffold for the setting beam (a scaffold for moving to the joint position after installation) was placed and techniques were devised to enable welding, spraying and painting of the joint parts.

3000 t hanging FC large block installation schematic drawing
Setting beam and moving scaffold
Setting beam and moving scaffold of temporarily placed center span block

Installation was done from late March to May when the sea is relatively calm and typhoons do not pass. Although the large blocks were assembled at the port around Miyakojima due to construction conditions, actual construction was done by assembly of large blocks at the production factory and measurements were taken during marine transport to prevent the danger of shaking. The Irabu-jima side span was erected on April 28, 2012, and the Miyako-jima side on May 16, but the center span had to be postponed due to unseasonable weather, and construction was completed in April of 2013, the following year.

伊良部大橋_架設写真_01

Consideration from actual bridge measurements and test results

Wind observation results, loading tests, etc. were made on the bridge, and the results were reported, showing that the numerical values set in the design were appropriate.

Within these results, we will focus on the structural attenuation (logarithmic decay rate).

Since the structural attenuation of the actual bridge is unknown at the design stage, estimation formulas in road bridge wind-resistance design manuals that were developed through investigating existing bridges are used. Irabu Bridge has a value of 0.026 when calculated with the estimation formula (0.35 / √L, L: maximum span length 180 m) for “rubber bearings” for the classification of “Girder Bridge.” There was a concern that the value was lower than that indicated by the estimation formula because the bridge had no bolted joints on the outer surface of the girders, giving it a unique girder cross section shape. For that reason, the structural attenuation was set as low as 0.02 and studied. Since the load test result this time was 0.029 on average, for wind-resistance stability, the force to stop shaking is larger than the design estimate, ensuring a margin of safety. In addition, it turned out that the estimation formulas in the manuals could be used even for Irabu Bridge.

The completion of the bridge in this manner is the result of the principal, Okinawa Prefecture, which established a large number of committees for the bridge, and involved many people in design and construction. We are truly honored that we could be involved in this project over around 10 years with responsibility for the detailed design of the main marine navigational route and the landscape design for the entire bridge, and we would like to continue to support the bridge together with the local residents in the future.

 

Bridge Name Irabu Bridge (Ship passage section)
Location Kugai, Hirara, Miyako City, Okinawa Prefecture – Ikemazoe, Irabu
Principal Okinawa Prefecture Miyako Civil Engineering Office
Bridge Length 420m
Span Length 119 +180 +119m
Structure Type 3-Span Continuous Steel Plate Floor Box Girder Bridge
Construction Period Detailed design: February 2009 to March 2010.