CHAPTER VI 

 Graphic Statics 



THE student knows that a line can be drawn to represent a 

 force, because forces act through the center of gravity of 

 a body and this is a point. A line is a succession of points, 

 or is the path of travel of a point. A line representing a force 

 indicates by its direction the direction in which the force acts. 

 When drawn to a scale the length may represent the amount of 

 the force in any selected unit, pounds, tons, etc. 



In earlier chapters some simple graphical methods were pre- 

 sented for obtaining bending moments and shear on beams, but 

 to solve more complicated problems, and to make even those 

 shown a little more 

 simple, reciprocal dia- 

 grams must be used. 

 The reciprocal dia- 

 gram method for 

 making computations 

 is known by the name 

 of Graphic Statics. 



In Fig. 123 is shown 

 the Parallelogram of Fig. 123. - Parallelogram of Forces. 



Forces. The line AB shows the pressure of the wind against a 

 roof truss. All forces act normally to the surface pressed, so the 

 wind always blows perpendicularly to the slope of a roof. It may 

 be resolved into a vertical force represented by BD and a hori- 

 zontal force represented by the line BC. 



The vertical force as weight may be added to the weight of 

 the roof and any loads that may come upon it, and the members 

 of the truss be proportioned to carry all the loads. The horizon- 

 tal force may be assumed to act to push the roof off the support 

 and is a measure of the force to be resisted by bolts which tie the 

 truss to the supports. 



All forces may be resolved into components. The force A B 

 in Fig. 123 might be considered as produced by two given forces, 



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