138 STEEL HIGHWAY BRIDGES. CHAP. III. 



Each tower shall have a sufficient base, longitudinally to be stable when standing alone, 

 without other support than its anchorage. Tower spans for high trestles shall not be less than 

 30 ft. 



12. Proposals. Contractors in submitting proposals shall furnish complete stress sheets, 

 general plans of the proposed structures, and such detail drawings as will clearly show the dimen- 

 sions of all the parts, modes of construction and sectional areas. 



13. Drawings. Upon the acceptance and the execution of the contract, all working drawings 

 required by the engineer shall be furnished free of cost (168). 



14. Approval of Plans. No work shall be commenced or materials ordered until the working 

 drawings have been approved by the engineer in writing. 



FLOOR SYSTEM. 



15. Floorbeams. All floorbeams shall be rolled or riveted steel girders, rigidly connected 

 to the trusses at the panel points, or may be placed on the top of deck bridges at panel points. 

 Floorbeams shall preferably be square to the trusses or girders. 



16. Joists and Stringers. All joists and stringers of bridges of classes A, B, EI, E 2 and E 3 

 shall be of steel. Joists for classes C, Di and D 2 may be either of wood or steel as specified. 

 Steel joists shall be securely fastened to the cross floorbeams, and steel stringers shall preferably 

 be riveted to the webs of floorbeams by means of connection angles at least -fa in. thick. 



17. End Spacers for Stringers. Where end floorbeams cannot be used, stringers resting on 

 masonry shall have cross-frames at their ends. These frames shall be riveted to girder or truss 

 shoe where practicable. 



1 8. Wooden Joists. Wooden floor joists shall be spaced not more than 2| ft. centers, and 

 shall lap by each other so as to have a full bearing on the floorbeams, and shall be separated | in. 

 for free circulation of air. Their width shall not be less than 3 in., or one-fourth the depth in 

 width. The proportion of the concentrated live load carried by one joist shall be taken equal to 

 the spacing of the joists in feet divided by four feet. No impact shall be considered in the design 

 of wooden joists, planks or ties. Oak, longleaf yellow pine and Oregon fir shall be designed for a 

 safe bending of 1,500 Ib. per sq. in., bearing across the fiber of 400 Ib. per sq. in., and shearing along 

 the grain of 140 Ib. per sq. in. Outside joists shall be designed for the same live loads as the inter- 

 mediate joists. 



19. Steel Joists. Steel I-beams when used as joists shall have a depth of not less than one- 

 thirtieth of the span, and one-twentieth of the span when used as track stringers. The proportion 

 of the concentrated live load carried by one joist shall be taken equal to the spacing of the joists 

 in feet divided by four feet when timber flooring is used, and divided by six feet when a reinforced 

 concrete or other rigid floor is used. Outside joists shall be designed for the same live loads as the 

 intermediate joists. 



20. Floor Plank. For single thickness the roadway planks shall not be less than 3 in. thick 

 nor less than one-eighth of the distance between centers of joists, and shall be laid transversely with 

 y in. openings and securely spiked to each joist. All plank shall be laid with heart side down. 

 When an additional wearing surface is required it shall be i^ in. thick, and the lower planks of a 

 minimum thickness of 3 in. shall be laid diagonally with in. openings. 



21. Footwalk plank shall be not less than 2 in. thick nor more than 6 in. wide, spaced with 

 5 in. openings. 



All plank shall be laid with heart side down, shall have full and even bearing on and be firmly 

 attached to the joists. 



22. Wheel Guards. Wheel guards of a cross-section of not less than 6 in. by 4 in. shall be 

 provided on each side of the roadway. They shall be spliced with half-and-half joints with 6 in. 

 lap, and shall be bolted to the stringers or joist with f in. bolts, spaced not to exceed 5 ft. apart. 



23. Solid Floor. For bridges of classes A and B a solid floor, consisting of wooden blocks, 

 brick, stone, asphalt, etc., on a concrete bed is recommended. For this case the floor shall con- 

 sist of buckle plates or corrugated sections or reinforced concrete slabs, and a waterproof 

 concrete (bitumen or cement) bed not less than 3 in. thick for the roadway and 2 in. thick for the 

 footwalk, over the highest point to be covered, not counting rivet or bolt heads. The floor shall 

 be laid with a slope of at least one inch in 10 ft. 



Reinforced Concrete Floor. See specifications for reinforced concrete floor on page 112 h, 

 and distribution of loads on page 112 f. 



24. Buckle plates shall not be less than -& in. thick for the roadway and j in. thick for the 

 footwalk. The crown of the plates shall not be less than 2 in. 



25. For solid floor the curb holding the paving and acting as a wheel guard on each side of 

 the roadway shall be of stone or steel projecting about 6 in. above the finished paving at the gutter. 

 The curb shall be so arranged that it can be removed and replaced when worn or injured. There 

 shall also be a metal edging strip on each side of the footwalk to protect and hold the paving in 

 place. 



