A mat that includes first and second side beams, a core support structure located between and connected to the side beams, with the core support structure including longitudinal members and having a height that is less than that of the side beams; one or more elongated members attached to the upper portion of the core support structure to form with the beams the top surface of the mat, and one or more elongated members attached to the lower portion of the support structure to form with the beams the bottom surface of the mat. Preferably, a plurality of joining rods attach the side beams to the support structure, with the joining rods passing through the sides of the side beams and the sides of the longitudinal members. Also, steel plates can be provided between the beams and longitudinal members to increase the strength and ruggedness of the mat.