US Patent 10030388 Panel having recesses and protrusions

A panel comprises a reinforcing unit including a plurality of first rectangular regions and a plurality of second rectangular regions. The first and second rectangular regions are in rows and columns, the first and second rectangular regions in each row being arranged alternately and the first and second rectangular regions in each column being arranged alternately. Each of the first rectangular regions includes a fully-covering first protrusion protruding in a first direction perpendicular to an imaginary reference plane and having a flat top surface. Each of the second rectangular regions includes a partially-covering second protrusion protruding in a second direction opposite to the first direction and having a flat top surface. A partially-covering reinforcing protrusion extends between opposite sides and protrudes in the first direction and having a flat top surface. The top surface of the reinforcing protrusion forms one face together with the top surface of the first protrusion.