STRUCTURAL TIMBER 63 



supporting plank may be reduced to 2J in., in which case the 

 latter shall be laid diagonally with J-in. openings. 



All floor planks shall be laid with the heart side down. 



280. Joists (Highway Bridges). The minimum size of 

 joists shall be 3"X12". They shall be notched over their bear- 

 ings at least in., bringing the top surfaces to exact level. Where 

 they rest on top of the floor beams the intermediate joists shall 

 lap by each other over the full width of same, with ^ in. open 

 space for circulation of air, whereas the outside joists shall have 

 their outer faces flush from end to end of span. 



If the floor plank be continuous each joist may be assumed 

 to carry only two-thirds () of the concentrated load. 



The maximum spacing for wooden joists shall be 2 ft. 6 in. 



281. Wheel Guards (Highway Bridges). There shall be a 

 wheel-guard on each side of the roadway having a minimum 

 size of 4"X6", laid on flat, and blocked up on shims 2"X6" 

 and 12 in. long. The shims shall have a maximum spacing of 5 

 ft. from center to center. The wheel guards shall be spliced with 

 half-and-half joints 6 in. long over a shim and shall be fastened 

 to the joist beneath with a three-quarter (f) in. bolt passing 

 through the center of each shim. 



282. Handrailing (Highway Bridges). The posts shall be 

 6"X4". spaced not more than 5 ft. apart, and shall be firmly 

 fastened. There shall be two lines of railing of 2 // X6 / ' plank, 

 the upper line of which shall be placed on flat and the lower 

 one on edge. 



283. Cross Ties (Railroad Bridges). The cross ties shall be 

 of such size as to give the requisite resistance to bending, under 

 the assumption that the maximum load is distributed equally 

 over three ties and that the impact equals 100 per cent. 



Every tie shall be notched down not less than in. and be 

 brought to a full and even bearing upon the stringers, and every 

 third tie shall be secured thereto by a J-in. hook bolt. 



The minimum sizes of cross ties shall be: 



(a) For electric railroads 6"X8"X8' (laid on flat); and, 



(b) For steam railroads 8"X8"X10'. 



The ties shall be spaced 14 in. center to center, with a free open- 

 ing of not more than 6 in., or less than 4 in. 



On curves the super-elevation of the outer rail shall be obtained 

 by using beveled ties. 



