Development of Construction Method for Reinforced Concrete Grade Beams with a Large Web Opening

Shimizu Corporation (President: Kazuyuki Inoue), Konoike Construction Co., Ltd. (President: Morihiro Tsutada), The Zenitaka Corporation (President: Hisayoshi Zenitaka), Tokyu Construction Co., Ltd. (President: Tsuneo Iizuka) and Koryo Kenpan Co., Ltd. (President: Shinji Ota) have developed a construction method that enables economical construction of web opening (through holes) in reinforced concrete grade beams. They have recently obtained a performance evaluation certificate of construction technology from the General Building Research Corporation of Japan (GBRC), verifying the reliability of the construction method.

When designing a web opening such as a manhole for inspections (usually 600-750mm in diameter) in reinforced concrete grade beams, according to the standards for Structural Calculation of Reinforced Concrete Structures published by the Architectural Institute of Japan, the beam depth must be normally three times or more the diameter of the web opening. This means, for example, that a web opening in the grade beam with depth of 1,800mm or more requires 600mm in diameter. However, the restrictions on the ratio of beam depth to web opening can be waived as long as it can be proven that the specified strength required for construction is attained by reinforcing the area around the web opening properly, even if the beam depth is less than three times the web opening diameter. The five companies have developed a reinforced concrete grade beam construction method for large beams that can reduce the beam depth to web opening diameter ratio to 2.5 times, while maintaining the strength necessary for construction of through-holes.

This construction method easily attains the required strength by using “the Diaren” opening reinforcement hardware produced by Koryo Kenpan Co., Ltd. , upper and lower reinforcement of the web opening, reinforcement beside web opening, and horizontal reinforcement. In order to investigate the performance, a shear loading test was performed on eight half-scale specimens [750mm(D), 300mm(W), 2,250mm(L)] with a web opening of 300mm in diameter having specified parameters for concrete strength and the quantity of the web opening reinforcement. This test figured out the shear capacity of the web opening. The test results also showed a clear correlation between the web opening diameter and concrete strength and quantity of reinforcement, and the companies succeeded in formulating a design that can attain the necessary strength for a web opening of 300mm in diameter, even at a beam depth of 2.5 times the diameter. Having the necessary strength for construction means that the reinforced concrete grade beam will maintain the specified structural performance during a large earthquake, and will not fail in the web opening. By reducing the reinforced concrete grade beam depth to 2.5 times the web opening diameter based on this design formula, it is possible to reduce the amount of excavation for the grade beams and the quantity of concrete of the beam itself. It turned out that this construction method can consequently reduce costs and shorten the construction period.

The five companies will each propose this reinforced concrete grade beam construction method for a large web openings in the future for hospitals, office buildings, and other low and medium-height buildings with the beam depths of around 1,500mm.