Art. 40 



METHODS OF APPLICATION. 



45 



The terms "horizontal" and "vertical" are used because 

 the loads which act on structures are usually weights which act 

 vertically, and because it is usually convenient to resolve inclined 

 forces into horizontal and vertical components, but it is under- 

 stood that the terms may be applied to any forces at right angles 

 to each other. 



That equations 5 and 6 make R = is shown in the force 



polygon, Fig. 29. 



Forces (or components) or 

 moments acting in opposite direc- 

 tions to have opposite signs in any 

 member of the above equations. 

 It is burdensome to use an adopted 

 convention with regard to signs, 

 and it is usually convenient to 

 place the known quantities on one 

 side of the equation and the unknoicns, 

 whose directions may not be 



Fig. 29. 



on the other side. To illustrate the rule, the simple case shown in 

 Fig. 17 gives 



Knowing the line of action of a force but not its direction, 

 it may be entered in the equation of equilibrium with a -{- sign, 

 upon the assumption that it acts in the corresponding direction; 

 if the solution makes a quantity of it, it acts in the opposite 

 direction from that assumed. 



40. flethods of Application of the Equations of Equilibrium. 

 Having written the equations of equilibrium, it becomes a simple 

 matter to calculate stresses. But the beginner will find his great- 

 est difficulty in writing these equations. To do this successfully 

 requires the strictest adherence to some method. The following 

 is an outline of. a method which is easily remembered, and it is 

 very essential to remember to apply it : 



1. Make a good sketch (preferably to scale) giving all the 

 elementary dimensions of the structure, and completely locating 

 all loads by means of proper dimension lines not confused with 

 anything else. A poor sketch is usually misleading, and the time 

 used in making it, worse than wasted. 



2. Take time to consider the problem in all its bearings. 

 Time spent in deciding upon a method of attack is always well 



