Abstract: A clamp device for clamping a boom hinge rail 12 of gantry crane in-situ comprises a cap portion 14 slidably mounted on a base portion 13 and secured together by a Locking bolt 15, the cap portion 14 comprising a body 18 and a nose 19 which extends laterally outwardly from the body 18 along a nose axis. The clamp portion 13 and the body 18 of the cap portion 14 comprise complementary opposed arcuate mating surfaces 21, 22 which permit rotational movement to the cap portion 14 relative to the base portion 13 about an axis P which extends perpendicular to the 10 nose axis. The base and cap portions 13, 14 of the clamp always engage each other regardless of the position of the nose of the clamp, so that forces applied to the nose 19 of the clamp 10 by the rail 12 are directly applied to the base portion 13 of the clamp 10, thereby providing a clamp which is less prone to failure.