282 CONCRETE DESIGN 



but the slanting webs b, which are in tension, always have 

 steel in them. The upper and lower chords contain each 

 a set of three reinforcing rods, as shown at d and e, respect- 

 ively. The beams may be made in any convenient location 

 and put in place on the job afterwards. When solid rein- 

 forced-concrete columns are used with this system, these 

 columns are cast in place and are provided with brackets 

 to hold the principal girders. These girders are then hoisted 

 on to these brackets. Part of the end of the girder is made 

 solid, as shown at c, in order to re'sist the stresses in shear 

 and direct compression due to the reaction of the support. 



DESIGN OF CONCRETE STRUCTURAL 

 MEMBERS 



PLAIN CONCRETE 



CONCRETE BEAMS 



Method of Design. Stone and plain-concrete beams are 

 designed by exactly the same method as any other kind of 

 beam, except that the weight of the beam itself can hardly 

 ever be neglected. The formula employed is as follows: 



M-'-l-Ss. 



c 



in which M is the bending moment; s, the unit stress pro- 

 duced; /, the moment of inertia; c, the distance from the 

 neutral axis to the most remote fiber; and S, the section 

 modulus. 



b<P 



In a beam of rectangular section, S = , in which b is 



6 



the breadth of the beam and d is its depth of the beam. 



The modulus of rupture for various kinds of stones and 

 other materials is given in the accompanying table. 



With the values in this table, a factor of safety of from 

 10 to 20 is usually employed when problems dealing with the 



