Transportation on Land 283 



Exercise. Suppose a load is at the center of this bridge. By 

 what parts is the load supported? How is it distributed to the sup- 

 porting parts ? What is the work of the steel rods, R R, running from 

 the girder at d and e to the sides of the triangle ? What is the function 

 of the upper part of the triangle? Is this function the same as it 

 would be if the triangle were not divided? 



Pratt truss. Study the distribution and transmission of 

 the weight in the truss bridge called the Pratt Truss, diagram D. 

 Suppose a load is at the center of the bridge, what members 

 support it? Which are compression and which tension mem- 

 bers? Which of the forms of the truss is best adapted to 

 long spans between supports or piers? 



Exercise : Strength and the form of bridge members. The mate- 

 rials needed are several strips of stiff cardboard three inches wide and 

 twelve inches long; a twelve-inch piece of telegraph wire or similar 

 wire ; and a spring balance of several pounds' capacity, or a series of 

 weights. 



Lay a piece of cardboard flat upon two supports representing the 

 end piers. Supports should be high enough to allow use of the bal- 

 ance or weights in exerting stress upon the span. Make record of 

 results in each case. Test by pulling with balance at the center of 

 the cardboard strip. Test a similar piece placed on edge. How are 

 floor joists placed ? Bend a strip into the form of a semi-circle or arch, 

 support ends to prevent their spreading, and test its strength at 

 center. Make a form with the two sides of a triangle, support so as 

 to prevent spreading, and test its strength at apex. Try the wire in 

 similar ways. 



Compare the load supporting capacities of the different forms 

 tried. How much more strength has the triangular form than the 

 semi-circular? than the piece placed on edge? than that placed 

 flat ? In which form does the wire show greatest strength in support- 

 ing the load? What different forms of the supporting members do 

 you find in the bridges in your vicinity? What three kinds of bridges 

 do the forms tested represent? Which form shows the greatest num- 

 ber of advantages ? 



Cantilever bridges. The cantilever is a special form of the 

 truss type of bridge. The most familiar form of cantilever 

 construction is the springs of a wagon or automobile (Fig. 85), 



