A clamp device includes a clamping plate having a first side portion that is sized and shaped for reception into a channel defined by a bottom channel portion of one of the rails of the cable tray. The clamping plate has a second side portion configured for fastening to a cable tray support such that the bottom channel portion of the rail is clamped between the clamping plate and the cable tray support. The use of the clamp device increases the load-bearing capacity of the cable tray assembly over an identical cable tray that does not include the clamping plate.