This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Assume effective depth between center of gravity of flanges, 28 inches; then approximate horizontal shear per linear inch = 60,000/28 =
2. Given the same web as above, with an end reaction of 95,000 pounds, determine pitch at end.
Here 95,000/28 = 3,400 = Horizontal shear per linear inch; and
5,060/3,400= 1.49 or 11/2 inches.
This makes it necessary to use an angle deep enough to give two lines of rivets either a 5-inch or a 6-inch leg. If the pitch between rivet lines is 2 1/4 inches, and horizontally between rivets 1 1/2 inches, then the actual distance between rivets is about 2 11/16 inches, which is more than three times the diameter of the rivet. Where the top flange of a girder is loaded directly, as by a heavy wall, it becomes necessary to calculate the rivets for direct shear as well as horizontal shear. The combined stress on the rivet must not exceed its value, and therefore a spacing somewhat less than that determined for horizontal shear above must be used. This can best be illustrated by a problem.
3. Given a girder having a web-plate 36 inches by § inch, with an end reaction of 75,000 pounds, and loaded directly on top flange with 3,000 pounds per foot of girder, 75,000/34 = 2,206 = horizontal shear per inch. Assume a pitch of 2 1/4 inches; then 2,206 X 2.25 = 4,963 = Horizontal stress on rivet;
3,000/12 = 250 = Direct vertical shearing force per inch, and
250 X 2.25 = 560 = Direct vertical load on rivet. These forces act on the rivet as indicated by Fig. 252. The resultant, therefore, is the square root of the sum of the squares of these two forces, and equals 4,994. As the value of the rivet is 5,060, this is about the nearest even pitch which could be used for these combined stresses.
The maximum straight distance between rivets which can be used is 6 inches, or sixteen times the thickness of the thinnest metal riveted. For a flange having 5/16-inch angles, therefore, 5 inches would be the maximum pitch; or, if a 1/4-inch cover-plate were used, 4 inches would be the maximum in rivets through these cover-plates.
Vertical spacing of rivets in stiffeners does not generally require calculation. For end stiffeners there should be at least sufficient to take up all the end shear. In other stiffeners the pitch is generally made 2 1/2 or 3 inches.
Flange Splices. In long girders it becomes necessary sometimes to splice the flange angle and cover-plates. Sometimes, for purposes of shipment or erection, the girder has to be made in two or more parts and spliced.
In splicing the angles, the full capacity of the angles should be provided in the splice, regardless of whether the splice is at a point of maximum flange stress or not; it preferably should not be so located. Angles are used on either side of the flange angles, with the corner rounded to fit accurately the fillet of the flange angle, and having the same gross or net area as these angles.
Fig. 253 shows the splice of the top flange of a plate girder. Note that the angles are spliced by cover-angles and also by cover-plates. In flange splices, provision should be made as far as possible to splice each leg of the angles directly and with sufficient rivets to provide for the proportional part of the stress carried by this leg. If cover-plates form part of the section of the flange, these should, if possible, be spliced at a point where one of the cover-plates is not required for sectional area, and then this cover-plate should be carried far enough beyond the splice to provide rivets sufficient for the stress in the plate. If the plates are of different areas, an additional, short cover-plate over the splice would be required, to make up the required area.
When the shop work has to be very exact and reliable, a planed joint is sometimes used to take a portion of the stress by direct compression between the abutting ends. In such cases the cover anglea should be used, but may be of slightly less area. It is preferable, when possible, however, to have the flange fully spliced without relying on the planed joint. The number of rivets should be sufficient to provide for the full capacity of the flange angles without exceeding the value of a rivet. If one portion of the splice is hand-riveted, the values must be determined accordingly. Rivets are in double shear or bearing on the angles.
Web Splices. If the girder has been designed without considering that the web carries part of the flange stress, then the web splice need have only sufficient rivets to provide for the shear. If the web were considered as helping to carry the stress due to bending moment, then the splice would have to have sufficient rivets to resist this portion of the bending moment carried by the web. In such a case, if two lines of rivets each side of the splice are used, and these rivets are spaced 2 1/2 or 3 inches center to center, they will be sufficient to provide for the shear and the bending moment also. In general it is better to use such a splice as illustrated in Fig. 254, whether the intention is to provide for bending moment or not.
The splice plates should have a net area equal to or a little greater than the net area of the web. If possible, the splice should be located at a point where the flanges are not fully stressed, so that they can help to splice the web.