1918] 



RURAL ENGINEERING. 



491 



the assumed eccentricity of the load; and (3) supply the deficiency by design- 

 ing the curb of the parapet to provide a resisting moment equal to that of a 

 slab of width equal to the loss in effective width due to eccentricity, maliing 

 allowance for the greater stiffness of the section under the parapet. Thus, sup- 

 pose a slab of l&-ft. span and 20-ft. width is to be designed to carry a con- 

 centrated load of 20,000 lbs. applied at a point 4 ft. from one edge, then 



Total width 20 



=—=1.25 



Span 16 



from the table for central concentrated loading, the effective width--=0.69X16 

 ft. =11.04 ft. Consider the load of 20,000 lbs. to be carried by a width of 11.04 

 ft., u.se the ordinary formulas for rectangular-beam design and determine the 

 effective depth of the slab and the area of steel required. Now, by the relation 

 indicated above, determine the effective width with the load in the critical 

 position 4 ft. from one edge, then 

 11.04 



(be)= ^4=5.52+4=9.52 ft. 



2 

 the difference between the values of he and be is 11.04 — 9.52=1.52 ft. Therefore 

 the curb of the parapet should be so designed that it will have a resisting moment 

 equal to that of a width of 1.52 ft. of the slab, maliing allowance for the 

 greater stiffness of the parapet section. In constructing slabs designed in this 

 manner . . . the curb of the parapet must be added before the concrete of 

 the slab has taken initial set." 



How the surface of a road affects tractive effort {Engin. News-Rec, 19 

 (1911), No. 8, pp. 361-369, figs. 5).— Traction tests under California conditions 

 are reported with a standard farm wagon equipped with steel axles of equal 

 length and 38- and 46-in. wheels, all wheels having 4-in. tires. The gross load 

 was 3 tons, and the speed was kept close to 2.4 miles per hour. Typical results 

 are summarized in the following table: 



Tractive resistance for various road surfaces. 



Kind of road. 



Condition of road. 



Tractive resistance. 



Total. Per ton. 



Concrete (unsurfaced) 



Concrete (unsurfaced) (load apparently accelerated 



when test was started). 

 Concrete (|-inch surface, asphaltic oil and screenings), 



Do 



Macadam (water- bound) 



Topeka on concrete 



Gravel 



Oil macadam (drawn with motor truck at 2h miles i>er 



hour). 

 Oil macadam (drawn with motor truck at 5 miles per 



hour). 



Gravel 



Topeka on plank 



Earth road 



Topeka on plank 



Earth road 



Earth 



Gravel , 



Smooth, excellent. 

 do 



-do. 

 .do. 

 .do. 

 -do. 



Compact, good condition 



G ood, new 



.do. 



Packed, in good condition... 

 Good condition, soft, wagon 



left marks. 

 Firm, IJ in. fine loose dust... 

 Good condition, but soft . . . . 



Dust f to 2 in 



Mud, stiff, firm underneath, 

 Loose, not packed 



Lbs. 



83.0 

 90.0 



147.6 

 155.0 

 193.0 

 205.5 

 225.0 

 234.5 



244.0 



247.0 

 2G5.0 



276.0 

 278.0 

 298.0 

 654.0 

 789.0 



Lbs. 

 27.6 

 30.0 



49.2 

 51.6 

 64.3 

 68.5 

 75.0 

 78.2 



81.3 



82.3 



88.3 



92.0 

 92.6 

 99.3 

 21S.0 

 203.0 



"The resistance encountered on oiled surfaces was considerably more than on 

 concrete. . . . The base supporting oiled surfaces affected the amount of trac- 

 tive effort required. For example, tests ... in which a concrete base was in- 



