232 



Journal of Agricultural Research 



Vol. VI, No. 6 



Z lOO 



depending on the manner of considering the zero readings. The more 

 conservative values are obtained by basing the calculations on the "new 

 zero" readings, as was done in the case of slabs 930 and 934. Note that 

 with an increase in load, the effective width seems to increase slightly. 

 Values for effective width were obtained from the steel deformations, as 

 well as from the concrete deformations, but it was found that the 



concrete deformations 

 gave the most conserv- 

 ative widths, and these 

 were therefore plotted. 

 In figure 28 the effect 

 of variation in thick- 

 ness of slab on effective 

 width may be seen. 

 Note that as the thick- 

 ness increases, the ef- 

 fective width decreases, 

 varying from 109 per 

 cent of the span length 

 for a 6-inch slab to 75 

 per cent of the span 

 for a loj^-inch slab. 

 The least value for ef- 

 fective width shown by 

 these tests is roughly, 

 then, about 0.7 of the 

 span length. Judging from the curve of variation, it would seem that 

 under extremely heavy loads, requiring very thick slabs, the effective 

 width might be decreased as low, possibly, as 0.6 of the span length. 

 However, 0.7 of the span will always be safe, and in general is a sufficiently 

 conservative figure to use. 



Table III. — Effective widths of reinforced-concrete. slabs, 16-foot span by 32 feet zvide, 



for center loading 



EFFCCTIV-E TMICKNCSS . IN MSCMES 



Fig. 28. — Curve showing effective width versus thickness. 



Center load. 



Pounds. 

 15,000. . . 



20,000 

 25,000. 



32.500 ■ 

 35,000. 



Safe load . . 



Slab 83 s (ioJ4 inches effect- 

 ive thickness). 



ii.6feet=72.3 percent 



of span. 

 11.5 feet=7i.9 per cent 



of span. 

 12. 1 feet=75.7 percent 



of span. 



12. 1 feet=75.7 percent 

 of span. 



Slab 930 (85-^ inches effect- 

 ive thickness). 



II. 4 feet=7i.6 per 



cent of span. 

 13.0 feet=8i.2 per cent 



of span. 

 12.9 feet=8i . I per cent 



of span. 



i4.5feet=9o.7 percent 

 of span. 



Slab 934 (6 inches effective 

 thickness). 



12.7 feet=79.5 percent 



of span. 

 17.5 feet=io9.3 P^*" 



cent of span. 



i2.9feet=8i.i per cent 17.5 feet=io9.3 per 

 of span. cent of span. 



