216 PRACTICAL STRUCTURAL DESIGN 



chord are added and one-half the total gives the reaction for each 

 end. The reactions are laid off as shown, overlapping on the 

 load line. The cambered tie rods are transferred to meet these 

 points and extended to intersect with the rafter lines. The 

 horizontal part of the tie rod is drawn from the middle point of 

 the load line. The diagram is not hard to construct if care is 

 taken. 



The drawing for a truss with a horizontal tie rod, or horizontal 

 lower chord, is similar. First the loads on the top chord are set 

 off on the vertical load line. Then the loads on the lower chord 

 are added, the reactions obtained and set off and horizontal lines 

 drawn from the points marking the amount of the end reactions. 



The Fink truss is economical because the struts are short and 

 most of the members are in tension. Partial loading cannot 

 cause maximum stresses in the members as it will in other com- 

 mon forms of trusses. It is a difficult roof to frame when the 

 slope is slight, so it should be used only for pitches exceeding 27 

 degrees. The form shown in Figs. 130 and 131 is the most simple 

 one, a common form of the Fink truss being illustrated in Fig. 

 132 and Fig. 133. 



In Building Age for May, 1916, Mr. Harry B. Wrigley, Allen- 

 town, Pa., presented the following method for dealing with the 

 web members of a Fink truss, without substitution or change. So 

 far as the author knows, this method has never before been pre- 

 sented in print, so Mr. Wrigley may be justified in claiming it to 

 be original with himself. 



In the graphical analysis of forces in a Fink, or Belgian, truss, 

 a difficulty is encountered at joint 5, support ing the load CD, 

 Fig. 132; for after determining the forces in the members meet- 

 ing at joints 1, 2, and 3 there remain three unknown forces at 

 joint 5, namely, in members DP, PO, and ON. A similar diffi- 

 culty is met with at joint 4, where there are three unknown 

 forces, namely, in members NO, OR, and RK. 



It is a well-known fact that in order to construct a polygon of 

 forces in equilibrium, acting in the same plane, through the same 

 point, all conditions but two must be known. 



In Fig. 132 (a) is shown the truss, with the force diagram at 

 (6). To illustrate the new method consider the left half of the 

 truss as shown at (c) and lay off the load line abcdek, and reactions 

 AT' and r'a, in the usual manner; then construct the stress diagram 



