142 REINFORCED CONCRETE CONSTRUCTION 



and its live load, for an area whose length is the length of the 

 girder and whose width is the average length of the beams run- 

 ning from each side into the girder. The sum of these loads 

 divided by the length of the girder gives a uniformly distributed 

 load for which the ordinary formula for bending moment may be 

 used. 



Thus, in Fig. 72, instead of computing the moment on the 

 girder as the sum of the moments produced by loads of the tri- 

 angles abc plus the concentrated loads from the beams at c plus 

 the weight of the girder, the entire live and dead load of the area 

 dddd may be considered as uniformly distributed over the girder 

 in the length aa. 



a 



An exception occurs, however, in the case where two beams 

 run into a girder at the one-third points. Here the maximum 

 moment obtained by the uniformly distributed method gives 

 slightly too conservative results, and may be reduced by 10 per 

 cent. Moments in a girder other than the maximum must be 

 computed for individual conditions. 



58. Arrangement of Beams and Girders. The spacing of 

 columns and girders will be determined largely by architectural 

 considerations and consequently the best spacing of cross-beams 

 will vary for different cases. The designer must decide whether 

 to omit all cross-beams, to insert them only at columns so as to 

 form a square or nearly square panel, or to space them at much 

 closer intervals using two or more to a girder panel, as shown in 

 Fig. 66. For economy, slabs with square or nearly square panels 

 should have both transverse and longitudinal reinforcement. 



For the sake of lateral stiffness, it is generally desirable to 

 place cross-beams at columns. In some cases, however, where 

 such stiffness is not needed, the cross-beams may be entirely 



