Abstract: First and second elongated members are slidable along each other to form a beam of variable length to span the top surface of a structure on which a worker is positioned. Clamping surfaces extend from the ends of the beam to engage the sides of the structure. A lever pivotally mounted on the first elongated member is connected through a link to the second elongated member to shorten the beam and urge the clamping surfaces against the structure. A worker's safety line may be attached to the device such that the lever is locked relative to the first elongated member to secure the device to the structure whenever the safety line is attached.