GIRDERS AND TRUSSES 135 



Roof Loads 



For information as to proper roof loads and the effect of wind 

 the student is referred to pages 305-307, 1913 edition " Carnegie 

 Pocket Companion." This will also be dealt with in the chapter 

 on " Graphic Statics." Usually city ordinances specify that a 

 roof shall be capable of carrying 40 Ibs. per square foot of hori- 

 zontal surface, hi addition to its own weight, this allowing for 

 wind, snow, live load, and roofing. Some cities require only 25 Ibs. 

 and others 30 Ibs. For a steeply pitched roof 25 Ibs. is proper, but 

 for a very flat roof the designing load should not be less than 50 

 Ibs. per square foot. Each joint in a frame carries a load, P, 

 equal to the truss spacing times the panel length multiplied by 

 the load per square foot. 



The Signs Used for Stresses 



The author mentioned that the positive (+) sign indicates 

 compression and the negative (-) sign indicates tension. This 

 is the way he was taught, and thirty years ago this use of the 

 signs was common with American and British writers. There 

 was a certain mnemonic aid in using the signs thus, for compres- 

 sion thickens a body and tension makes it thinner, so the " minus 

 sign " expressed the idea of thinness. In drawings the pieces 

 in compression were indicated by heavy lines and the pieces in 

 tension by light lines. 



Continental European writers used the signs in a directly 

 opposite sense, for strict mathematical analysis in which careful 

 attention must be paid to the signs of quantities resulted in bring- 

 ing compression out at the end with a negative sign and tension 

 with a positive sign. The well-trained mathematician needs no 

 aid from mnemonics. The result of late years has been to unsettle 

 American and British authors, and a reader of modern books must 

 be careful to ascertain just how the signs are used by the author. 

 It is to be hoped that at some not distant day all writers will 

 agree upon a definite use of the signs, but for the purposes of the 

 present work the author believes the mnemonic value, as given 

 above, ia too great to be neglected. 



