An adjustable furniture attachment clamp for attachment to the underside of a furniture item with two spaced apart arm fasteners, the clamp comprising a clamp body having oppositely disposed laterally extending forked arms, each arm having a fastener slot having a width greater than the shanks of the arm fasteners, the fastener slots including inner ends spaced apart a distance less than the arm fasteners, such that lateral and forward-to-back play is provided for adjustment of the clamp relative to the furniture item, the clamp also having a downwardly facing beam pocket including a beam capture surface for holding the right upper beam flange of a support I-beam and a cam having a binding face for pressing against the lower portion of the left upper beam flange when a cam fastener is tightened for attaching the clamp body to the I-beam.