Rivet Spacing

Enough rivets must he placed in the end stiffeners of girders to transmit the shear to the web. For example, 100,000 lb. is the reaction on a girder constructed as in Fig. 22; the web is 3/8 in. thick, the rivets 7/8 in. diam. The allowable unit bearing value being 15,000 lb. (by Table XXIX, page 131), the ordinary bearing of a 3/8" plate on a 7/8" rivet is 4,920 lb.; adding \ because the plate is web bearing, gives 6,560 lb. Since the double shearing value of the rivet is greater than the web-bearing value of the plate, 6,560 lb. must be taken as the resistance of one rivet. Then the number of rivets required in the two pairs of angles is 100.000 / 6,560 = 15.2, say 16, or 8 rivets in each pair. Rivets in intermediate stiffeners are usually spaced the same as in end stiffeners, their spacing not being calculated. No rivets in stiffeners should be spaced more than 6 in. apart, or more than 16 times the thickness of the angle leg.

As the shear increases from the point of greatest bending towards the supports, the number of rivets placed in the vertical legs of the flange angles, to resist their tendency to slide on the web, must also increase as the supports are approached. The horizontal stress, per inch of length, which is transmitted from the web to the flange at any point, is equal to the maximum shear at any point divided by the depth of the girder in inches.

For example, in the girder shown in Fig. 28, the shear at a is R1 or 100,000 lb.; at b, it is 100,000 lb. - (5,000 lb. X 4) = 80,000 lb., at c, 60,000 lb., etc. The horizontal stress per inch of length at a is 100,000 / (4 X12) = 2,083lb.; at b, 80,000 / 48 = 1,667 lb.; at c, 60,000 / 48 = 1,250 lb.; at d, 833 lb., etc. Using 7/8" rivets, the safe bearing value of each is 6,560 lb. At a, the stress being 2,083 lb. per inch of run. the rivets should be placed 6,560 / 2,083 = 3.14 in. center to center; at 6, 6,560 / 1,667 = 3.93 in.; at c, 6,560 / 1,250 = 5.24 in.; at d, 6,560 / 833 = 7.87 in. In practice, rivets are spaced alike in both flanges; and as 6 in. is the greatest allowable pitch in a compression member, further calculation is needless. The rivet spacing from a to b is, then, 3 1/4 in.; b to c, 4 in.; c to d, 5 1/4 in.; beyond d, as the theoretical pitch is more than 6 in., the 6-in. pitch should be used.

Design Of Riveted Girders 265

Fig. 28.

At the ends of each flange plate, sufficient rivets must be used, spaced 2 1/2 to 3,in. on centers, to transmit the allowable stress on the net section of the plate to the adjacent members. The remaining rivets should be spaced the greatest allowable pitch for a compression member, namely, 16 times the thickness of the thinnest outside plate - provided the pitch does not exceed 6 in. For example, in a 3/8" X12" flange plate, after deducting the area of two 1" holes (7/8" rivets + 1/8"),the net sectional area is 3 3/4 in. Assuming the safe unit fiber stress at 15,000 lb., 3.75 in. X 15,000 = 56,250 lb., strength of plate. The safe value of each rivet in this case will be taken at 5.410 lb.; then the number required in each end of the plate is 56,250 / 5.410 = 10, say 5 each side of the web, and they should be spaced from 2 1/2 to 3 in. on centers. In splicing the lower flange angles, which are in tension, enough rivets should be placed therein to equal in resistance that of the net section of the angles. In splicing the top angles, since they are in compression, only enough rivets need be used to securely hold them in abutting position.