DESIGNING. 167 



the ring is less than the required theoretical thickness de- 

 manded by the formula for the assumed conditions. If the 

 parallelogram projects beyond the diagonal, the plate has 

 greater thickness and strength than is theoretically necessary 

 to resist the hydrostatic pressure at that point, the projecting 

 area representing the excess of thickness and weight of the 

 plate metal, and to that extent increasing the cost of the 

 structure ; in the same way the area included in the section 

 between the diagonal and the vertical line when the latter is 

 within the diagonal represents the proportion of insecurity. 

 Obviously, the nearer the vertical projected line, intersecting 

 with the horizontal, approaches the diagonal, the more nearly 

 are the theoretical conditions of thickness of plate to applied 

 pressure complied with ; hence, in graphic design, the decrease 

 in thickness of plate, corresponding with reduced pressures, 

 should be shown as rising like steps along the diagonal, the 

 foot of each rise just touching the diagonal line, and the three 

 intersecting lines forming triangles whose area represents the 

 excess of strength and plate metal beyond the theoretical re- 

 quirements. This will be clearly understood by a slight study 

 of the strain-sheet on page 164. 



Joint Efficiency. It is also customary to indicate upon the 

 strain-sheet, graphically, the joint efficiency, or the percentage 

 of strength of the joint as compared with the strength of the 

 plate, showing by vertical dotted lines in each section the 

 ratio of strength which the specified character of the joint 

 bears to the strength of the solid plate. 



In the formula for determining the thickness of the plate 

 to resist safely the applied pressures, it was assumed that -J- of 

 the strength of the plate would be lost by punching and rivet- 

 ing; hence the line indicating the relative efficiency of the 

 joint, or the " rivet-efficiency line," should be drawn to repre- 

 sent 66.6 per cent, of the theoretical strength of the plate as 

 indicated by its thickness as measured on the scale or base-line 



