STEEL HIGHWAY BRIDGES. 



CHAP. III. 



under each plank length. When the latter method is used the floor planks are fastened to the 

 intermediate joists by bending spikes, driven through the floor plank, around the upper flanges of 

 the joist. For specifications for plank floors, see the author's "General Specifications for Steel 

 Highway Bridges." 



The thickness of plank for different loadings and spans calculated for the allowable stresses 

 required by the author's specifications are given in Table V. 



Laminated Timber Floor. Highway bridge floors are sometimes made by placing 2 in. by 

 4 in., 2 in. by 6 in., or 3 in. by 8 in. timbers on edge and spiking them together. A waterproof 

 wearing surface is placed on top of the laminated base.. The safe spans for a laminated timber 

 floor may be taken the same as for planks 12 inches wide. 



The Oregon Highway Commission uses laminated wood floors made of 3 in. by 8 in. timbers 

 placed on edge and spiked together at intervals of not less than 18 in. " The timbers shall prefer- 

 ably be long enough to extend the full width of the roadway, and in no case shall more than two 

 lengths be used in the width of roadway. Every fifth timber shall project in. above the inter- 

 vening four pieces, to furnish a grip for the waterproof wearing surface." 



A laminated floor made of 2 in. by 4 in. pine timbers placed on edge and spiked together was 

 used for reflooring 23d Street Bridge, Denver, Colorado. The laminated timber base is covered 

 with an asphalt paving i^ inches thick. 



TABLE V. 

 THICKNESS OF 12-iNCH FLOOR PLANK. 



For 8-inch plank add 23 per cent to the thickness of plank. 

 Thickness in Inches, Actual Size, No Impact. 



Creosoted Timber Floor. Creosoted timber may be used as a sub-floor for a creosoted timber 

 block wearing surface, for a bituminous wearing surface, or may carry a gravel or earth fill, or may 

 have no wearing surface. 



Specifications for Creosoted Timber. Timber used for all creosoted floor timbers except 

 blocks shall be first-class oak, long-leaf yellow pine or Oregon fir. It shall be cut from live trees and 

 Shall be straight grained, free from shakes, large or loose knots, decayed wood, worm holes or other 

 defects that will impair its strength or durability. It shall be sawed straight and true and shall 

 be full size. All timber shall be impregnated with at least 12 lb. of creosote oil per cubic foot of 

 timber. The creosote oil shall be a pure coal-tar product free from any adulteration. It shall be 

 free from any tar or any petroleum oil or petroleum residue. The specific gravity at 100 F. shall 

 be at least 1.03, but not more than 1.07. The creosote oil shall comply with the specifications of 

 the American Railway Engineering Association for creosote oil. The timber shall be impregnated 

 with creosote oil by the full cell process. The details of the treatment shall comply with the 

 specifications of the American Railway Engineering Association for the treatment of ties with 

 creosote oil. 



