A panel structure includes panel members that may be connected together using friction stir welding, Each of the panel members includes a first plate, a second plate substantially in parallel to said first plate, and at least one webbing member connecting the first plate and the second plate. The first plate of the first panel member includes a flange positioned in abutment with a flange located on the first plate of the second panel member. The flange of the first panel member includes a projection and the flange of the second panel member includes a groove. The panel members are connected so that the projection is engaged with the groove to thereby resist relative lateral movement that would separate the panel members.