156 PRACTICAL STRUCTURAL DESIGN 



portion, the tension acting through and considered as concentrated 

 in the center, which is thus 3 - 1.125 = 1.875 ins. from the shearing 

 joint between the main piece and the fish-plates. Add to this 

 one-half the projection of the shear-pin into the main member 

 = 1.875 + 0.75 = 2.625 ins., which is the moment arm for the 

 couple acting in the joint. The moment = 2.625 x 22,500 = 59,063 

 in. lb., the action tending to raise one end of the plate from its 

 seat. This is resisted by tension in the bolts. 



Assuming the bolts to be set halfway between the shear pins 

 the length of the moment (or lever) arm from the edge of the 

 hole to the center of the bolt = 4 - 0.75 = 3.25 ins. The stress 



59 063 

 in the bolts = '. = 18,200 Ibs. Four bolts will be used, as 



o.ZO 



18 200 

 shown, and the stress = - - = 4550 Ibs. per bolt. Use |-in. 



bolts, the net area of which, at the root of the threads, = 0.42 sq. in., 



4550 

 which causes a stress = -^ ~ = 10,830 Ibs. per square inch, which 



U.4Z 



will be ah 1 right for a wrought iron bolt. 



For developing the bolts plate washers may be designed. The 

 tension on each bolt = 4550 Ibs. and the area for each washer 



4550 



= 13 sq. ins. Make each washer 3f x 3| ins. The student 



can compute the thickness as an exercise by the formula on page 

 151. Standard round washers of equal area may of course be used. 

 The area of the chord will now be checked. Vertically there 

 will be a hole 1^ diameter (half on each side), which subtracts 

 12 sq. ins. Horizontally there will be two f-in. holes, with an 

 area of 11.375 sq. ins., which, added to the area of the vertical 

 holes, = 11.375 + 12 = 23.375 sq. ins. The gross area of the 

 chord is 64 sq. ins. and the net area = 40.625 sq. ins. The 



fiber stress = . ' oc = 1100 Ibs. per square inch. The chord has 



plenty of area for the maximum tension. 



In Fig. 93 is illustrated an old type known as a tabled fish- 

 plate splice. It may be considered to be reasonably effective when 

 the entire stress can be taken by not more than two tables on 

 either side of the chord joint. There is comparatively little sec- 

 ondary tension in the bolts, therefore they can act in their most 

 efficient manner. Washers of generous size must be provided in 



