GRAPHIC STATICS 237 



at the joints. Multiply each load by the horizontal distance 

 from the support. Add the products. Divide by the sum of 

 the loads, to obtain the position of the center of gravity of all 

 the loads. Continue the horizontal reaction line (Ri) to an 

 intersection with the vertical dotted line through the center of 

 gravity. Draw the diagonal line (#2) to show the direction of 

 the reaction at the bottom chord support. 



To draw the stress diagram first lay off on a vertical load line 

 the sum of the loads. From the upper end draw a horizontal 

 line and from the lower end draw a diagonal line parallel to the 

 inclined reaction. The point of intersection, j, on the force dia- 

 gram fixes the amount of each reaction. From j drop a vertical 

 load line on which set off each joint load and close the diagram 

 at the bottom. The rest of the diagram is evident. The vertical 

 fe is not stressed but is merely used to carry the weight of the 

 lower chord in the end panel. 



Accuracy in Drawing 



In graphic statics everything depends on the care with which 

 the work is done. The pencils used should be very sharp and 

 the lines as thin as possible. The lines in the force diagram must 

 positively be parallel with the lines on the truss diagram. The 

 work checks when the reciprocal diagrams close and if they do 

 not close the work must be carefully searched for errors. A use- 

 ful check is to determine some stresses analytically by taking 

 moments. The scale should be one that will not require too 

 large a sheet of paper and will allow a reading of one hundred 

 pounds. For roof trusses of usual spans the scale can be twenty 

 thousand pounds per inch. 



The foregoing presentation of the subject of graphic statics 

 covers the subject only so* far as roof trusses are concerned. It 

 may be applied to any braced structure and other applications 

 will be shown in the following chapter. The principles are simple 

 and any student who works faithfully through the examples 

 given should have no hesitancy in attempting to analyze graphi- 

 cally the forces in any braced frame. 



Continuous Beams 



The continuous beam is not used much in steel buildings but 

 is used in all reinforced concrete buildings. All methods for 

 dealing with the continuous beam are based on the assumption 



