the general drawings, Fig. 18. The gage lines of angles are placed on the center line of the truss. When an angle has two gage lines, the one nearest the back is used. The center of gravity of the top chords and end-posts are placed on the center line of the truss. The size of the gusset plates is usually determined by the space required for the rivets necessary to connect the members to the 13.20x14 900=196 600 12.16 x 12 230=148 800. Lo 5.39x16000-8 5.50x16000-88000 27 4.78 x 16 000-76500 Lo (6) JOINT Lo plate. Except in extreme cases, the size of the gusset plates required by the rivets will be sufficient for strength. Rivets should be symmetrically placed in order that the stress may be uniformly distributed. 17. Joint U1.-A diagram showing the stresses in members fastened to the gusset plate at U1, is given in (a) Fig. 16. The gusset plate will be shop riveted to U1U, and field riveted to all other members. Bearing controls the number of rivets in LU1. The number of field rivets required will be 13.20 X 14,900 .29 X .75 X 20,000 = 45 rivets, or 23 on each side. The number of shop rivets in U1U, at joint U1 is determined by bearing, and is 2 The number of field rivets in U1L2 is determined by single shear, and is The number of field rivets in U1L1 is determined by single shear, and is 18. Joint L.-Cast iron shoes will be used at the fixed end and cast iron rockers at the expansion end. The details of the shoes and rockers are shown in Fig. 17. The pin at Lo should be made as large as the channels of Lo U1 will permit, even though a smaller pin would safely carry the stresses. A 3-inch pin will be used if the following investigation shows it to have sufficient strength. For the detail used here the forces acting on the pin are all vertical and have a magnitude equal to one-half the maximum pedestal reaction. This maximum will occur with the bridge fully loaded and will equal one-half the sum of the dead load, live load and impact joint loads, multiplied by the number of panels. (16,700 + 10,000 + 2,430)7 = 102,000 lb. The arrangement at the joint is shown in Fig. 16 and Fig. 18. gusset plate is determined by the bearing area required, and is The minimum thickness of the = - 0.29 The thickness of the web is 0.29 in., so the required thickness of the gusset plate is 0.61 0.32 in. As far as bearing is concerned, ain. plate would be sufficient, but a 4-in. plate will be used on account of the large connection at this joint in order to give more rigidity. The maximum bending moment is and the maximum shear is V d = 51,000 X 1.79 = 91,300 in.-lb. 51,000 lb. The diameter required by bending moment is = (32) 1 = 2.17 ( 4 ) * = 2.17 (21,300)* = 3.39 in. The diameter required by shear is = The 3 in. pin is satisfactory, so will be used. For bending moments on pins, see Table 27, Appendix III. A diagram showing the stresses in the members fastened to the gusset plates at Lo is given in (b), Fig. 16. The end-post will be shop riveted to the gusset plate, and the lower chord and floorbeam will be field riveted. The number of shop rivets required in L。U1 at joint L。 is determined by bearing on the web of the channels, and is The number of field rivets in LoL1 at joint Lo is determined by single shear, and is The number of field rivets required between the connection angles of the floorbeam and the gusset plate is = The section area of the gusset plate required to carry the stress in LoL is equal to the net area of the member or 4.78 sq. in. With two gusset plates each in. thick, the effective width must be 4.78/(2 X 1) 4.78 in. net. It is evident from the detail drawings that sufficient area has been provided. The effective section through the pin hole is more than 25 per cent of the net section of the member, and the net area behind the pin is at least 75 per cent of the area through the pin hole ($75). The number of field rivets required for lateral L。L1 is 1.00 X 20,000 The other joints are designed in a manner similar to the method above described. The intermediate floor beams require The number of rivets at joint U2 between the upper chord and the gusset plate must be sufficient to transfer to the gusset plate the maximum difference in stress between members U1U and UU1⁄2 This occurs when U2L has its maximum stress. The difference is 10/7P tan 0+ impact + difference in dead load stress = 10/7 X 10,000 tan 0 + 0.243 X 10/7 X 10,000 tan + 13,400 11,400 + 2,800 + 13,400 lb. 27,600 lb. = = The number of shop rivets required will be determined by bearing, and is In order to make a rigid connection a greater number will be used. 19. End Bearings.-Rocker or roller bearings are required on spans of 70 ft. or more, must be used for this bridge. 20. Design of Cast Iron Rockers.-The type of rocker shown in Fig. 17 will be used. T maximum pedestal reaction is 102,000 lb. so the area of each masonry plate must be 102,000 = 170 sq. in., where 600 lb. per sq. in is the allowable bearing stress on concrete masonry. plate 12 in. x 2 in. x 24 in. will be used. The length of the rocker will be taken as 24 in., se bearing stress between the rocker and the plate is p = 102,000/24 allowable bearing stress is 300d pounds per lineal in., or 300 X 18 = = 4,250 lb. per lin. in. 5,400 lb. per lineal inch |