PURPOSE: To reduce a tensile strength acting on cooling pipes by a method wherein the trunks of water chambers are secured to the end plates of the trunks of a main frame by reinforcing ribs and both end plates are connected by reinforcing pipes at the inside of the main frame to where the reinforcing ribs for the end plates of the trunks of the main frame are secured.

CONSTITUTION: Distributed load W generated by a hydraulic pressure p in the water chamber is not transmitted to the cooling pipes 2 substantially and is transmitted to the reinforcing ribs 14a, 14b, 14c, 14d. The end plates 4a, 4b for the trunks of the main frame are connected by the reinforcing pipes or rods 15a, 15b between the end plates 4a, 4b at the positions inside the main frame to where the reinforcing ribs are secured, therefore, the distributed load W generated at the fore and aft sections of both of the water chambers 8a, 8b are received through the reinforcing pipes or rods 15a, 15b. Accordingly, the distributed load W is not transmitted to the trunk of the main frame substantially and, therefore, the deformation of the trunk 1 of the main frame will not be generated.