Enter the length of a path around the outside edge of the tube layout. This can be calculated by counting the number of tubes on the outside of the layout and multiplying that number by the tube pitch. When a tubesheet might be controlled by shear stress, the program requires the perimeter and area of the tubesheet for the shear calculation. An error message displays when these values are required but not given. The result will be conservative if you overestimate the area and underestimate the perimeter. This input is only needed for TEMA tubesheets.