Art. 44. 



RULES TO BE OBSERVED. 



51 



The advantages and disadvantages of the two methods for 

 finding reactions and the four methods for finding stresses, to- 

 gether with those resulting from the use of the different equations 

 of equilibrium, will be brought out in what follows. 



44. Rules to be Observed in the Use of the Graphic Hethods. 

 In order to measure forces, it is necessary to use a scale. For this 

 purpose a decimal scale (an engineer's scale) is necessary one 

 divided into tenths, twentieths, thirtieths, fortieths, fiftieths and 

 sixtieths of an inch. It is not necessary to know how many pounds 

 an inch represents, but the scales for distances and forces should 

 be given on the drawing as a "scale of 10, " a "scale of 40, " etc. 

 By placing the given scale upon a line representing a known 

 quantity, it is at once apparent how an unknown quantity should 

 be read. 



The scaling of stresses should not be complicated by requir- 

 ing scaled distances to be multiplied or divided by 2, 3 or 4, ex- 

 cept that, when convenient, such a scale may be chosen as will 

 require scaled stresses to be multiplied or divided by two. Stresses 

 should be scaled as closely as may be, estimating between divi- 

 sions. Good instruments should be used. Triangles and straight 

 edges should be tested before using. Soft paper should be 

 avoided. 



Truss and stress diagrams should not be drawn on separate 

 sheets, and for all ordinary problems the sheet need not be larger 

 than 8"xlO", and the diagrams should not be allowed to run off 

 the sheet. A little preliminary sketching will serve to indicate 

 where a diagram should be started. 



Prick points made with a fine needle point should locate 

 all intersections. 



A well sharpened hard pencil should be used for drawing all 

 lines and these should not be ' ' inked in, ' ' unless the intersections 

 are protected by small circles made with a bow pen. 



Truss diagrams should be laid out accurately to as large a 

 scale as the sheet will permit, and great care should be taken in 

 transferring the direction of any member to the stress diagram. 



Stress diagrams (force polygons) should be drawn to as 

 small a scale as is consistent with the required degree of accuracy 

 in reading stresses. If stress and truss diagrams are both large, it 

 will be found difficult to transfer directions from one to the other. 



If inks of different colors are used, say red for the lower 

 chord, black for the upper ch^rd, and blue for the web, the dia- 

 grams will be much clearer than when made with a single color. 



