140 REINFORCED CONCRETE CONSTRUCTION 



which represents the proportion of load carried by the reinforce- 

 ment parallel to the shorter axis. The proportion of the total 

 load carried by the shorter system for various ratios of length 

 (/) to breadth (6) is approximately as follows: 



When the length of a floor panel is large compared to its breadth, 

 the longitudinal reinforcement (that is, reinforcement parallel 

 with the length) is of little value in carrying loads, but a small 

 amount is nevertheless generally desirable in preventing 

 shrinkage and temperature cracks and in binding the entire 

 structure together. It is more important for wide beam spacing 

 than when the beams are closely spaced. The amount of steel 

 to use is usually selected somewhat arbitrarily, and 1/4-in. or 

 3/8-in. rods spaced 18 to 24 in. apart is common practice. The 

 top of the slab over a girder should be reinforced transversely, 

 not only for stiffening the girder, but also to provide for the 

 negative bending moment produced with the bending of the slab 

 at right angles to the direction of the principal slab steel. 



66. Distribution of Slab Load to Cross-beams. If a floor slab 

 is reinforced in one direction only, the load will practically all be 

 transmitted to the beams at right angles to the direction of the 

 reinforcing rods. A small part, however, will be transferred 

 directly to the girders at the sides of the panels, but this may 

 well be neglected in the calculations for cross-beams. In fact, 

 even with reinforcement in two directions, the load should be 

 assumed as all transferred to the cross-beams unless the panel is 

 nearly square. 



If panels, nearly square, are reinforced in both directions, 

 the loads carried to the cross-beams and girders will not be uni- 

 formly distributed over the length of such beams and girders, 

 but may be assumed to vary in accordance with the ordinates 

 of a triangle. This assumption is surely on the safe side in re- 

 gard to moment, if the area of the triangle is made equal to that 

 part of the total load on the panel which is transmitted to the 

 beam in question as determined by the table of the preceding 



