Notes on Iron Arches. 83 



Let us now suppose the lower flange to be removed, thus 

 reducing the amount of material employed, in the case of 

 wrought iron, by nearly one-half, and we shall find the 

 upper or curved flange alone to be fully competent to endure 

 the load, provided that the supports or abutments be so 

 constructed as to resist the horizontal as well as the vertical 

 resolved parts of the. compression at the ends of the remain- 

 ing flange. 



We have now gradually transformed our structure from 

 an ordinary parallel girder with two flanges and a web into an 

 iron arch, and in so doing we have reduced the amount of 

 material theoretically requisite by almost exactly one-half. 

 From this it follows that as far as material is concerned an 

 arch is a far more economical means of supporting an unva- 

 rying load than a girder whenever a good abutment is 

 available capable of resisting a horizontal thrust as well as 

 a vertical pressure. 



In working this form of bridge out in practice we are, 

 however, met by certain difficulties, in order to overcome 

 which we are obliged to relinquish a part of the economic 

 advantage which theory indicates. 



1. The arch will be exposed to variations of temperature, 

 which may amount to as much as 100° Fahrenheit in a 

 Victorian climate, and which will cause considerable varia- 

 tions of dimension through alternate expansion and contrac- 

 tion of the metal. These changes of dimension, though 

 perfectly h aimless in the case of girders free to elongate 

 horizontally, may lead to very serious if not dangerous 

 results in the case of arches placed between immovable 

 abutments; and it is imperatively necessary to take such 

 precautions as shall prevent injury to the structure under 

 extreme variations of temperature. 



The most thorough method of meeting this requirement 

 is to divide the arch rib into two parts at the crown, and 

 connect these two parts together, and the ends of the arch 

 to the abutment by joints possessing the character and 

 performing the functions of hinges (see Fig. 5). The arch as 

 thus modified will rise slightly when the temperature 

 increiases, and fall slightly when the temperature diminishes, 

 and the change of temperature will be powerless to produce 

 any sensible variation in the stress to which the material is 

 subject. 



Sometimes the arch rib is made with hinges at the ends 

 only, and the elasticity or spring of the iron itself is 



