196 PRACTICAL STRUCTURAL DESIGN 



and Reinforced Concrete. By John C. Ostrup. Sold by the U. P. C. 

 Book Company, Inc., New York, for $1.00 



General Specifications for Structural Work of Buildings. By 

 C. C. Schneider. Sold by the publisher of this book for 75 cts. 



Specifications and Tables for Steel Framed Structures. Pre- 

 pared by the American Bridge Company. Distributed by the 

 New York and Chicago offices free of charge. 



Building Code recommended by The National Board of Fire 

 Underwriters. Address the officers of the Board, 76 Williams 

 Street, New York, N. Y. Similar information is to be had in 

 the building ordinances of all large cities. Small details, however, 

 of the sizes of members and spacing of rivets, etc., can only be 

 had in specifications similar to the first three mentioned. 



A number of standard specifications for structural work are 

 sold and a list can be obtained from any large publishing and 

 bookselling concern. 



Secondary Stresses in Framed Structures 



Secondary stresses in framed structures are due, primarily, 

 to faulty details. In the general design of a framed structure 

 it is assumed that all forces meet at a common point, which is 

 the intersection of the axes through the centers of gravity of the 

 members forming the joint. In the case of a pin connected truss, 

 with the pin clean and the joint in first-class condition, this 

 assumption is very nearly met. In the case of rivetted joints 

 the direction of each member is rigidly fixed and when the struc- 

 ture deflects under load all members are placed in double curva- 

 ture. This condition of secondary stress is accentuated by faulty 

 joints, the resulting stresses with carefully studied joints being 

 often negligible. With faulty joints a structure may fail because 

 of secondary stresses. 



The effect of faulty design can best be shown by an example 

 and Fig. 115 shows a joint in the top chord of a Warren truss. 

 Taking A as the center of moments the total bending moment, 

 due to eccentricity, is 35,600 x 7.5 = 267,000 in. Ibs. This mo- 

 ment is apportioned among the four members meeting at the 

 joint in accordance with their relative rigidities, which is found 

 by dividing their Moment of Inertia (to be found in the steel 

 handbooks) by one-half the length of the member, all in inches. 



To understand this question of relative rigidity assume that 



