bridges 



DESIGN OF RAILWAY BRIDGES. 175 



is slightly higher than for riveted truss bridges, the total cost erected of the structural 

 strrl in the- pin-connected bridge is less than the steel in the riveted bridge. (2) The pin-con- 

 mvii-d t russ bridge can be erected in less time at a very much less cost than the riveted truss bridge. 

 (3) The secondary stresses in the pin-connected truss bridge are smaller than in the riveted truss 

 Initial' and the structure is more efficient. (4) With the present ballasted floors the vibration 

 and impact stresses are no greater in a pin-connected truss bridge than in a riveted truss bridge. 

 i\< ted tension members are difficult to design and are expensive of material and labor. Eye- 

 arc ideal tension members in which the material is used efficiently. For the above reasons 

 author predicts that the pin-connected bridge for spans of 200 ft. and over will regain its 



as a standard type of railroad bridge. 



The Pratt truss with parallel chords is used for pin-connected spans up to about 250 ft., 

 iile riveted truss spans are made with Pratt or Warren trusses; double and triple intersection 

 isses are also used for riveted trusses. For long span bridges the subdivided Pratt truss with 

 inclined chords (Petit truss) is generally used. The width center to center of trusses should not 

 be less than one-twentieth of the span, and preferably not less than one-eighteenth. The height 

 the center should be from one-fifth to one-seventh of the span; the Municipal Bridge at St. 

 iris has a center height of one-sixth of the span. The height at the ends should be only sufficient 

 an effective portal. The most economical inclination of diagonals is very nearly 40 degrees, 

 that in a Petit truss the panel length should be about 0.42 times the height. For the most 

 momical web system the panels should vary in length as the depth varies, but this increases 

 ic weight of the floor and also increases the shop cost and cost of erection, so that constant panel 

 :ngths are commonly used. One railroad specification requires that panel lengths shall not 

 exceed 35 feet. For truss bridges of the Pratt type with two stringers and an open timber floor 



E-~ present practice is to use a panel length of 22} to 27^ ft., with 25 ft. as an average. Increasing 

 length of the panels increases the weight of the floor system, and decreases the weight of the 

 sses. The economical panel lengths for bridges with ballasted floor is less than for bridges with 

 open timber floor. Riveted truss bridges with triple-intersection web members, Fig. 41, are 

 made with very short panels. 



With the increase in the size of the sections in a bridge great care must be taken in detailing 

 to use details that will develop the full strength of the members. Increased details increase the 

 shop cost and for this reason there is a tendency for bridge companies to cut down details and to 

 change details so as to simplify shop work even at the expense of added weight in order to obtain 

 a low pound price. For this reason detail drawings, not necessarily shop drawings, should always 

 be made by the designing engineer. The author has in mind a case where to change the details 

 a plate girder so that multiple punches might be used required the addition of details equal to 

 per cent of the weight of the span and the addition of 25 per cent to the number of field rivets, 

 :h no increase in efficiency. It is needless to say the change was not made. 



An empirical rule for calculating the economical depth of plate girder spans is to make the 

 area of the flanges equal to the area of the webs. The actual depths of plate girders are commonly 

 slightly less than the depth given by the above rule. The minimum thickness of f inch for plate 

 girder webs should be used only for stringers with short spans, and the thickness of the web 

 lould be increased as the span and depth of the girder increases. For the depths and spacing of 

 .te girders designed undor Common Standard Specifications 1006, see Table I. 



DETAILS OF RAILWAY BRIDGES. It is very important that the details of railway 

 idges be worked out with great care. A few standard details will be briefly described. 



Sections for Chords and Posts. Chord sections are shown in (a) to (i) in Fig. 22. Sections 

 and (b) are used for light chords and (c), (d) and (e) for heavy chords. Sections (a) and (d) are 

 also made by turning the angles in, as in section (i). Sections (f) to (i) are used for chord sections, 

 for intermediate posts and for columns. Sections (n) and (p) to (t) are used for column sections. 

 Chord sections, posts and columns with diaphragms or webs at right angles to each other as in 



I to (e), (n), and (p) to (t) give much better results under actual service than laced sections as 

 (f) to (i) and (o). Sections (j) to (m) and (o) are used for struts and braces. 



