JOINTS AND CONNECTIONS 155 



All bolts should have a driving fit. This may be obtained by 

 boring all holes from one side with a bit the right size to assure 

 a driving fit. If any hole is large then use the next size larger 

 bolt. In determining the working loads on bolts the washers played 

 no part, so on bolts used for this type of fish-plate joint, standard 

 washers may be used, drawn tight enough to insure a good bear- 

 ing. " In general the fewer bolts there are to place, the less will 

 be the cost for labor, and the more certain will be the combined 

 action. Against these considerations must be weighed the amount 

 of metal in the bolts and the availability of the chosen size. Stock 

 bolts are, of course, cheaper than special sizes." (Dewell.) 



In Fig. 92 is shown a shear-pin joint for tension members. This 

 is very reliable for thoroughly seasoned timber but should not be 

 used in green timber. Any shrinkage of the timber will allow a 

 slip in the joint. Hardwood pins may be used, but metal is better 

 and square bars or round are equally serviceable. Iron pipe is 

 generally used. The pieces are cut and fitted together, then spiked 

 to hold them in position during the fitting of the bolts. After the 

 bolts are driven and the washers drawn tight, holes are bored and 

 the shear pins driven. The drawing shows the shear pins vertical, 

 but they may just as well be horizontal, if the designer fears 

 vertical pins may fall out. 



Assume the diameter of the shear pins to be 1J ins. and the 

 fish-plates to be 3 ins. X 8 ins. The net section of the two plates 

 will be 4.5 x 8 = 36 sq. ins. The unit tensile stress in the plates 



45 000 

 will be =5 = 1250 Ibs. per square inch, which leaves the platea 



.)!) 



safe, as the allowable stress is 1500 Ibs. per square inch. 



The number of pins required is fixed by the bearing area on the 



A c* rwi 

 end of the fibers in the holes, = n _. Tonn - 6.25. Use 8 



U.fO X O X L4(J(J 



pins, for there must be an even number. 

 Total shearing area required - ' - 375 sq. ins. 



375 

 Spacing of pins for shear - ^ ^ - 5.85 ins. Adding the thick- 



o X o 



ness of the pins, 5.85 + 1.5 - 7.35 ins. Make it 8 ins. center to 

 center. 



Subtract from the thickness of the fish-plate one-half the thick- 

 ness of the pins, which leaves 3 - 0.75 - 2.25 ins. for the uncut 



