476 



ERECTION OF STRUCTURAL STEEL. 



CHAP. XIV. 



of flood, it may be necessary to use spread footings which are anchored in place. Where it is 

 practicable to obtain piles of sufficient length they may be used for the full height of the falsework. 

 The timber used in building falsework should be sound, strong, free from defects that will affect 

 its strength or interfere with its use. Since the structure is temporary, durability is not an 

 important element in selecting timber for falsework unless it is to be used several times. 



For examples of timber trestles, see Chapter VII. 



Plans of typical four-legged falsework as used by the American Bridge Company are shown 

 in Fig. 25. When trains are to be carried and 2-8 in. X 16 in. stringers are used under each rail, 

 bents must not be spaced over 18 ft. centers for the falsework as shown. 



Zd'o'itotti-ayeler^rComtant^^ 



r6M--i^ 



'yim^m m>- 



'*w\ wrwm*' 



Var.^ (to ^ TrussesJVa_riab[e^ '^Jfar.\ 

 ;, This line oF stringers tobeused^ 

 '<? trusses are erected first--* '' ; 

 Y/ff-^X; 4x8'" -) ,'8'il6"i i ' \; ,'-^'8x1 



*- Dotted lines denote sill to be used wnen necessary 

 The average maximum length of leg nottoexceed30-0. 

 8x16 stringers are to be ordered either ?6-0 or3?-0 

 to suit conditions. 



This type of false work is designed Porheavy slnqle 

 track spans when trains are not carried and for ' 

 sinqle track spans up to 250 when trains are carried. 



' Doffed fines denote ail/ to be used when necessary 



FIG. 25. 



OffiEWYORK 



Piles. Timber piles may be driven with a drop hammer, Fig. 26, or with a steam hammer. 

 A spool roller pile driver with a drop hammer is shown in Fig. 26. The hammer is raised to the 

 top of the leads by the hoisting engine; the hammer is then permitted to fall on the top of the 

 pile, dragging the hoisting rope down with it. The force of the blow of the hammer depends 

 upon the weight of the hammer, the height of free fall, and the resistance of the hammer in the 

 leads. By catching the hammer as it descends the operator can cushion the blow so that the safe 

 bearing power of a pile as calculated from the penetration may be very misleading. 



Details of a pile driver are given in Fig. 27. 



