88 Notes on Iron Arches. 



It was in this way that my attention was first directed to 

 this subject, and it is in compliance with a request made by 

 him that I bring the subject before you to-night. 



Figs. 8 and 9 respectively show a half-elevation and half- 

 cross-section of the bridge to a scale of eight feet to one inch. 

 The span, as before stated, is 100 feet in the clear, and the 

 rise of the soffit of the arch twelve feet. The section of 

 each flange of each of the two arched ribs is about twenty- 

 four square inches at the crown, and increases slightly to 

 the springing ; and the web varies from J inch thick at the 

 crown to J inch at the springing. The arches are con- 

 tinuous at the crown, but are probably capable of a very 

 slight hinge action at the springing. Assuming them to be 

 hinged at the springing, the following results have been 

 obtained by calculation : — 



1. Maximum compression of the metal, bridge half-loaded 

 with load of 84 lbs. per square foot, at a temperature 40° 

 below that at which it was erected =7180 lbs. per square inch. 



2. When the load extends over the whole span the cross- 

 bending stress vanishes at a temperature of about 16° Fahren- 

 heit above that at which it was erected. 



3. With a load extending half-way across, as in Fig. 6, 

 the minimum stress occurs at a temperature 13° Fahrenheit 

 above that at which the bridge was erected. 



4. Ordinary plate girders to carry the same load would 

 have contained from 30 to 40 per cent, more material than 

 the iron arches. 



The spandrils and roadway are constructed of timber 

 as shown, and possess no doubt some stiffness and power 

 of resisting the effect of irregular loads. In the previous 

 calculations, however, no account was taken of this fact, it 

 being considered unwise to rely upon two such different 

 materials as wood and iron acting to any considerable degree 

 in concert. The arch was therefore made strong enough to 

 endure all irregular stresses without assistance from the 

 spandrils. 



In Fig. 10 a detailed section of one arched rib is given, and 

 a portion of the lateral bracing connecting the two ribs 

 together at intervals is shown. 



