290 



EXPERIMENT STATION EECORD. 



[Vol. 35 



eral summary of concrete principles; aggregates; principles of proportioning; 

 mixing concrete; placing concrete; protection after placing; cold weather 

 concreting ; recommended mixtures ; forms ; concreting tools ; foundations ; 

 principles of reinforcing; materials for reinforcing; walls and fences; posts; 

 rubble concrete; tanks and troughs; cisterns; form removal; hog wallow; 

 manure pit; repairs of leaks in tanks, etc.; hotbeds; roofs for small build- 

 ings; pavements, feeding floors, and walks; steps; and well curbs and plat- 

 forms. 



Tests of three larg'e-sized reinforced-concrete slabs under concentrated 

 loading-, A. T. Goldbeck and E. B. Smith {U. S. Dept. Agr., Jour. Agr. Re- 

 search, 6 (1916), No. 6, pp. 205-2S4, pi. 1, figs. 2S).— Tests of three large rein- 

 forced-concrete slabs to determine their effective widths under concentrated 

 loading, as defined by McCormick (E. S. R., 33, p. 487), and "to demonstrate 

 the application of the theory of narrow rectangular beams to the design of wide 

 slabs supported at two ends and subjected to concentrated loads," are reported 

 All three slabs were 32 ft. wide, of 16 ft. span, and reinforced but not trans- 

 versely, made of machine-mixed 1:2:4 concrete. A complete description of the 

 slabs is given in the following table : 



Description of reinforced-concrete slabs used in tests. 



The data of results are graphically reported. 



It was found that " with an increase in load the effective width seems t» ia- 

 crease slightly. Values for effective width were obtained from the ste^ de- 

 formations as well as from the concrete deformations, but . . . the concrete 

 deformations gave the most conservative widths. ... As the thickness in- 

 creases the effective width decreases, varying from 109 per cent of the span 

 length for a 6-in. slab to 75 per cent of the span for a 10.5-in. slab. The least 

 value for effective width shown by these tests is roughly, then, about 0.7 of 

 the span length. ... It would seem that under extremely heavy loads, requir- 

 ing 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." 



The effective widths of the spans tested are given in the following table : 



Effective widths of reinforced-concrete slabs, 16-ft. span by 



center loading. 



ft. loide, foi' 



Center load. 



Pounii. 

 15,000 

 20,000 

 2.5,000 

 32,500 

 35,000 



Safe load... 



Slab (lOH in. effective thick- 

 ness). 



11.6 ft. =72.3 per cent of span. 

 11. 5 ft. = 71.9 per cent of span. 

 12.1 ft.= 75.7 per cent of span. 



12.1 ft. —75.7 per cent of span. 



Slab (8Vg in. effective thick- 

 ness). 



11.4 ft.= 71.6 per cent of span. 

 13.0 ft.= 81.2 per cent of span. 

 12.9 ft. =81.1 percent of span. 



14.5 ft. =90.7 per cent of span. 



12.9 ft. =81.1 per cent of span. 



Slab (6 in. effective thick- 

 ness). 



12.7 ft. = 79. 5 per cent of span. 

 17.5 ft. = 109.3 per cent of span. 



17.5 ft. =109.3 per cent of span. 



