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 21 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 com pression between the abutting ends. in such cases the cover anew 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 bear ing 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 21 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 pro vide 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 arc not fully stressed, so that they can help to splice the web. • 1. As an illustration of the use of the exact formula for pitch of rivets, the following problem will be worked out: Take the girder given in the problem illustrating the cutting-off of flange plate. This girder 40 feet long has a web-plate 36 inches by inch; and section of flange at end consists of two angles 6 x 6 x inch. At point 10 feet from end section, are two angles 6 x 6 x inch, and 2 plates 14 x inch.
Determine first the pitch of rivets at end where the shear is 60,000 pounds. The formula is: The first step is to determine position of center of gravity of flange. As there are no cover-plates, this is taken directly from "Cambria" and is 1.64 inches.
The web is 36 inches; but in all girders where flange plates are used, the depth back to back of angles is or inch more than the depth of web, in order to allow for any variation in the depth of plate. In this case it will be taken as 36+ inches back to back of angle.
To determine the pitch at point 10 feet from end, we have to calculate the neutral axis of the flange as follows: Angles 2 X 4.36 X 2.39 = 20.9 10.5 X .3S = 4.0 24.9 24.9 ÷ 19.22 = 1.3 inches from back of cover-plate to neutral The maximum pitch is as stated 6 inches. At this point the actual pitch would be made somewhat less—say, 5 inches.