Mitre gates, a common solution for medium-sized locks
Mitre gates are the most commonly used type of lock gates for small and medium-sized locks. For locks with a width of more than 25m, a different gate type would usually be considered. Mitre gates are one of the oldest types of lock gates and were mainly made of wood. At present, the gates are usually made of steel. In recent years, mitre gates have also been manufactured in composite materials, with mixed succes.
The operation of a mitre gate during its closed position can be schematised as an accumulation of several three-hinged arches. By acting as a three-hinged arch, a significant proportion of the hydraulic load is transmitted to the walls of the lock head by bending instead of by normal force. This makes larger lock widths possible compared to single leave gates (especially when the gates were still made of wood) as normal force can be absorbed more efficiently than bending forces through a cross-section.
The force transfer between the gates and the walls of the lock head in closed position can be realised in different ways. In the past, continuous wooden support beams were used. Nowadays, discrete support points (quoin blocs) are often used which are placed at the level of each gate line (three-hinged arch). Depending on the angle of these quoin blocs with the axis of the gate, one or more supports should be used. If the quoin bloc is directed according to the resultant of the hydraulic load, one support is sufficient. However, in that case, a certain proportion of the other loads (collision, ice load) must then be absorbed by the gate hinges, as the direction of their resultant is different from that of the hydraulic load.
During movement, the loads are absorbed by a pintle and a top hinge. The pintle usually carries the vertical load and the horizontal load, while the top hinge only carries the horizontal load.
The pintle and tope hinge are often designed with a small radial clearance to ensure that the high reaction forces due to the hydraulic head are absorbed merely by the quoin blocs This requires a a carefull installation and calibration of the mitre gates. Inaccurate calibration could lead to unforeseen high reaction forces on the pintle and top hinge when the gate is subjected to a hydraulic head. In smaller locks on inland waterways there is an increasing tendency to design mitre gates without quoin blocs. This means that there is no longer any distinction between the system carrying the forces during movement and during the closed position. This makes the adjustment and installation of the gate much easier and eliminates the risk of unforeseen overloading of the hinges. This is only possible for small locks where the reaction forces at the hinges are somewhat limited.
SBE has an extensive experience with the design of mitre gates and manages to deliver thoughtful designs.