Kamihirai Flood Gate: a stainless steel solution to global climate change

The water gate under construction.

In recent years, climate change has brought about more frequent and intensifying natural disasters, causing enormous damage to people’s lives. Nippon Steel Stainless Steel Corporation (NSSC) contributes to Japan’s initiatives to enhance national resilience as a stainless steel manufacturer, as exemplified by the Kamihirai Flood Gate, one of Japan’s largest stainless-steel water gates.

Text & images by NSSC

The original carbon steel Kamihirai Flood Gate was built in 1970 to prevent damage from tsunamis. This year, the flood gate system was upgraded with retrofitted stainless steel plates for reinforcement. IHI Construction Service Co., Ltd. was responsible for the construction work.
The Kamihirai Flood Gate is an essential facility of the river levee system that protects the lives of three million people in the watershed areas from flood damage. Since 2015, the Tokyo metropolitan government has been working on seismic retrofitting of the Kamihirai Flood Gate in preparation for a possible Tokyo inland earthquake.

Type of joint Test object symbol Welding position Steel symbol  Plate thickness (mm) Manufacture (Welds tested by IHI)
 Butt welding  TF  Downward SUS323L 36 x 36 NSSC
Fillet welding SF Horizontal SUS323L 36 x 36 NSSC
Fillet welding SV Vertical SUS323L 36 x 36 NSSC
Butt (pipe) TP All positions SUS323L 22 x 22 NSSC

Stainless solution

The Kamihirai Flood Gate is one of Japan’s largest water gate systems. This system consists of water gates with a long span (30m) supported by a pipe Vierendeel truss structure (see box), making it the only such construction in Japan.
The Kamihirai Flood Gate is located in a dense residential area, making it particularly difficult to conduct and coordinate the slide gates’ periodical re-painting and maintenance work. Furthermore, considering the worsening shortage of re-painting and maintenance workers for steel structures, the Tokyo metropolitan government decided to replace the existing carbon-steel slide gates with all stainless steel while meeting the latest seismic resistance standards.
It was apparent that making the slide gates all stainless steel would drastically solve these problems. However, using conventional stainless steel, such as SUS316L, would significantly increase the weight of the slide gates compared with the existing ones, posing a concern about the insufficient strength of the RC structure supporting the water gates. For this reason, replacing the Kamihirai Flood Gate’s existing slide gates with all stainless-steel slide gates was considered difficult early in the planning stage.

Use of duplex stainless steel SUS323L

The retrofitting of the Kamihirai Flood Gate required high-strength, highly corrosion-resistant materials to comply with the latest seismic resistance standards as a heavy structure and avoid a weight increase of the slide gates. Eventually, duplex stainless steel SUS323L was adopted for the entire slide gates to satisfy these requirements. SUS323L is a resource-saving duplex stainless steel containing 23% chromium, 4% nickel, and 0.15% nitrogen. This stainless steel has a pitting resistance equivalent number (PREN) of 27, significantly higher than that of general-purpose stainless steel. While SUS323L has high corrosion resistance equivalent to SUS316L, its strength is considerably higher. In addition, SUS323L has a nickel content one-third that of SUS316L.
In examining the possibility of adopting duplex stainless steel for this application, IHI Construction Service established a penetration welding technique for large diameter pipes (1m), confirming that the welding of duplex stainless steel satisfied the required corrosion resistance, strength, toughness, and appearance.

Format Pipe Vierendeel Structural Steel Roller Gate
Quantity 4 gates
Pure span 30.000m
Height of the gate 11.100m
Design water level Outside water A.P.+7.200m (Design embankment height)
Inside water A.P.+2.100m (Operational water level during typhoon alert)
Door bed height A.P.-3.500m
Watertight method Double-sided three-way rubber watertight
Gate body material Duplex stainless steel (ASTM S323-4)
Opening and closing method 1. Motor, 2. Drum wire rope winch type (With hydraulically controlled self-weight lowering device)
Operation method Machine side, distant and remote
Lifting height 8.900m during operation / 9.700m during maintenance
Opening and closing speed For main and standby motors 1.0m / minute
Self-weight lowering (high speed) 3.3m / minute
Self-weight lowering (low speed) 1.0m / minute

The world’s only stainless steel pipe-Vierendeel structure

The Kamihirai Flood Gate adopted a pipe Vierendeel-type structure using duplex stainless steel for the first time in the world. However, manufacturing that structure was extremely difficult. A pipe Vierendeel structure (also known as a Rahmen structure) can transmit loads efficiently through the entire structure.
However, because stress is concentrated at its rigid nodes, ensuring the welded joint quality is a significant challenge to overcome.
Particularly in establishing a penetration welding technique for large-diameter pipes made of duplex stainless steel, there was a need to devise enabling elemental technologies through a trial-and-error process, such as looking for the optimum welding conditions for duplex stainless steel from scratch. These efforts successfully found the specific welding conditions required to make high-grade weld joints, including optimisation of the weld groove shape, selection of the welding filler material, composition of the shielding gas, thorough control of the welding heat input, and required high dimensional precision down to the millimetre.

Improved safety and convenience

The Kamihirai Flood Gate is located at the confluence of the Ayase River and the Nakagawa River, which flow in the eastern part of Tokyo’s 23 wards. This location is also known as the hub of river traffic connecting Tokyo Bay and the inland areas.
The retrofitting of the Kamihirai Flood Gate also involved a significant change in the slide gate water tightening design from four-side to three-side. This design change eliminated the need for steel members that ensure the upper side water tightness, improving the seismic capacity of the gate frameworks and the safety and convenience of passing ships due to the increased opening of the slide gates. The weight of each slide gate before the retrofitting was 210 tons. After the retrofitting, the weight of each duplex stainless steel slide gate remained at 210 tons despite an increase in surface area of about 22%. In addition, the opening of each slide gate increased due to the enlarged water gates, helping improve the safety of ships passing under them.

Closing message

The achievement of the Kamihirai Flood Gate Project has demonstrated that duplex stainless steel is a material suited for large-scale steel structures and that it has great potential for broader structural steel applications. In addition, duplex stainless steel is now becoming a standard material used to construct river structures in Japan. NSSC feels proud to have contributed to enhancing Japan’s national resilience through this project in cooperation with other concerned parties as one team.

What is the Vierendeel truss?

In the late 19th century, the Belgian engineer Jules Arthur Vierendeel registered his patent for a new type of beam which would come to be known as the Vierendeel truss. The beam is characterised by the frame’s lack of diagonal members, something which would appear to contradict conditions of steadiness and balance. The application of the Vierendeel truss system surged in popularity during the 20th century, with it being used both in civil engineering and architecture.
Elements in Vierendeel trusses are subjected to bending, axial force and shear, unlike conventional trusses with diagonal web members where the members are primarily designed for axial loads. Vierendeel trusses are usually more expensive than conventional trusses and their use is limited to instances where diagonal web members are either obtrusive or undesirable.
Vierendeel trusses are moment resisting. Vertical members near the supports are subject to the highest moments and therefore require larger sections to be used than those at mid-span. Considerable bending moments must be transferred between the verticals and the chords, which can result in expensive stiffened details. Source: Wikipedia

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This Featured Story appeared in Stainless Steel World October 2023 magazine. To read many more articles like these on an (almost) monthly basis, subscribe to our magazine (available in print and digital format) – SUBSCRIPTIONS TO OUR DIGITAL VERSION ARE NOW FREE.

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