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.
A good many designers use only one stiifener under a beam or girder, and as the load to the stiffener comes from the outstanding leg, this brings a moment on the rivets through the other leg of the stiffener. For usual sizes of beams, there is probably ample strength in the rivets to provide for this moment. It is better design, however, to use two stiffeners back to back, with rivets connecting the outstanding legs, as shown in Fig. 217. This avoids the strain due to the moment on the rivets and also distributes the load to the column symmetrically with regard to the axis, instead of entirely on one side. These points are of very great importance where heavy girders or unusually heavy concentrated loads are concerned. Special column connections will be taken up later on.
The connections of beams to double beam girders, involve the consideration of a number of practical points peculiar to each case. These beams are generally bolted together with only a slight space between the flanges, and if the girder rests on a column, the holes must be arranged where they are accessible. In general this would be in the outside flanges unless the end of the girder was exposed so that the inside flanges could be reached.
Where beams frame to such a girder they cannot be riveted unless it is possible to rivet all the lines of such connections to each beam comprising the girder before they are brought together and bolted up. Where there were several lines of such girders it would be difficult to do this for all of them. In many cases, therefore, these connections have to be arranged for bolts to go through both beams of the girder. Where double beam girders frame into another girder the connection can only be made by single-angles on the outside of the webs, unless the beams are spread far enough apart to allow bolts or rivets on the inside to be reached. If the girder carrying the double beams is deep enough a shelf connection can of course be used, and this would be preferable to the single-angle connection.
Connections by angles on only one side of the web, as shown in Fig. 193, should always be avoided if possible, as they are subject to a bending moment on the rivets in the same way noted for single stiffeners. Where such a connection must be made sufficient extra rivets should be used to provide for this moment. The remarks in regard to double beam girders apply also to girders made up of three and four beams. In these cases, however, there must be room for connection angles on the inner beams, and if the connection cannot be made when the beams are bolted together, it must be arranged so that these beams can be erected before the outside ones. In such an arrangement it is obvious that the standard form of cast iron plate separators could not be used very readily unless rods were used through the separators instead of bolts.
Where different sizes of beams frame opposite to the same girder it is necessary to change the position of the framing angles on the beams in order to use standard connections in each case. These changes in position are generally made to conform to standard practice, which is illustrated in Part II and which in general is as follows: In all cases except where one of the beams is a 7-in. beam, the first hole is 3 1/4 in. from the flanges which are flush with each other, and standard angles are used. Where one of the beams is a 7-in. beam and the other is either a 6,8,9,10 or 12-in. beam the first hole is 2 1/2 in. from the flush flanges; for a 12-in. beam the first hole is 2 3/4 in.
Fig. 190 shows the Carnegie code of conventional signs for rivets. It is important to follow the code in use by the particular shop for which the drawings are intended, as only by the use of such signs can elaborate notes be avoided.
Illustrations of Details. Fig. 195 shows a detail of a punched beam. Note that there should always be a single overall measurement on the sketch. Groups of holes, as for instance holes for connections of other beams, as shown in the top flange and the web, are located by fixing the center of the group. The reason f®r this is that the beam on which is the framing connecting to the holes is located by its center, and therefore it is important to locate this exactly. If the holes are symmetrical with regard to the center it is not necessary to dimension each hole from the center, but simply to give the distance between them, corresponding with the distance in the outstanding legs of the connection angles on the beam framing to this one.
In the case of a channel it is the back of web rather than the center which is always located. For the holes for connections of a channel, therefore, the position of the back of channel is fixed, and then each hole in the group forming the connection must be located with respect to the back of channel as the group is not central with regard to the back. For an example of this see Fig. 196. It always adds to the clearness to put near each group of holes forming a connection for other beams the size of beam or channel connecting to it. Holes at ends of beams for connecting to columns or for anchors are generally spaced by an independent set of measurements from the end of the beam.
The student should note that beams cut by the mill without special directions being given are subject to a variation in length of | in. under or over the length specified. If the beam rests on walls such variation is unimportant. If, however, it frames between columns and has holes connecting to the columns, such variation could not be allowed. For this reason measurements of such beams should always be marked "exact" or else at end of the sketch should be printed "column end 1/4 in. clearance". With such instructions, or similar directions in other cases to indicate how the beam rests with respect to other work, the mill will take the necessary precautions. In the case of framed beams, for instance, such notes are not necessary, as it is self-evident that no variation at these ends can be allowed. Fig. 196 shows a beam framed into another beam, the relations of the top and bottom flanges being such as to avoid coping. Note here that it is necessary to give an end view to show the spacing of holes in the outstanding legs of connection angles. Note also the specification as regards these angles. If the connection is standard and is placed centrally with the beam, always say "standard connection". In such cases if the shop is familiar with the standards referred to, an end view is not always necessary. If the connection is not placed centrally with the beam, or if the spacing of the holes in the legs varies any from the standard it is customary to write "standard connection, except as noted ".