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.
As previously stated, beam connections to girders and columns are generally made after standard forms for the different size beams. From an inspection of these standard connections it will be seen that 3, 4, 5, and 6-in. beams and channels all have the same number of rivets; 7, 8, 9, and 10-in. sections have the same number; and of the larger beams the different weight beams of a given size have the same number, whether the lightest or heaviest section is used. It is evident that these beams which are of different capacities, would not require the same number of rivets, if the number was calculated for the exact load of each case. It would not be economical, either from the standpoint of time or money, to detail in this way, however, and therefore these standard forms are always used unless peculiar conditions made it impossible to frame with these size angles, or unless because of peculiar conditions of loading, these connections would not be sufficiently strong.
These standard connections are proportioned for uniformly distributed loads with spans commonly used for the different size beams. When beams are used on short spans and loaded to their full capacity, it would be necessary to design special connections with the required number of rivets; the same is true where a concentrated load comes on a beam very near to one connection. The tables on pages 42 and 43 of the Cambria Handbook give the minimum spans of the different size beams and channels for which these standard connections can be used when the beams are loaded uniformly to their full capacity, based on 10,000 lb. per sq. in. for shear and 20,000 lb. for bearing. For cases of concentrated loading near the ends, no general rule can be given. For all cases of loading on spans shorter than those given by the table, the draftsman should calculate the load on the connections and determine the number of rivets required.
Connection angles are always riveted to beams centrally as regards the depth of web unless conditions make it necessary to raise or lower them. Such conditions arise when certain beams of different depths frame opposite to each other to the same girder. There are standard conditions concerning many of these cases and these are shown in Figs. 135 to 139, Part II. Such connections should be made by changing the position of the angles rather than the spacing of the holes in the angles if possible, so that the standard framings can be used.
Where beams frame on opposite sides of the same girder, but the center lines of the two beams do not lie on the same straight line special size angles and rivet spacing is required. If the distance between the center lines of the beams is less than 8 1/2 in., as shown in Fig. 194 the one line of rivet holes must be common to both beams. The minimum distance between rivets of beams framed to the same side of a girder for which standard connection angles can be used is shown in Fig. 137. In cases where beams are spaced closer than this, a single angle with the required number of rivets is used in the outside of each web; or where there is sufficient depth of girder a shelf angle below the beams can be used. In this case stiffeners fitted to the outstanding leg of the shelf angle should be used, as under deflection the beam will bear near the outer edge of angle and without the stiffeners would tend to break off this leg. The full number of rivets required to carry the load should be put in the stiffeners and shelf, even if angles on the web of the beam are used to hold it laterally. It is not good design to rely on the combined action of two sets of connections, such as a shelf connection described above, and a web connection, to carry a load. In such a case the deflection of the beam would bring the bearing on the shelf, and this connection would take the whole load; or if the shelf was not stiffened to resist bending under the load, this would throw the load on the web connections. "Wherever a shelf with stiffeners is used it should contain enough rivets for the full load.
Where beams frame to deep girders or to columns, even if the connection is made by angles on the web of the beams, it is customary to put a shelf angle under the beam. The student should not confuse this construction with the one just described. The object of such a shelf is to facilitate erection and not to support the beam after the web connection is made. Where such an angle is used, therefore, no stiffeners should be used under the beam, as these would prevent the web connection from performing the work for which it was designed. The draftsman must see that the connection angles are not placed so as to interfere with the fillet of the beam or of the girder. This consideration arises where the connection is raised or lowered on the beam, or where the beam does not frame flush with the girder, or where a small beam frames flush with a large one, as for instance a 5-in. beam to a 24-in. beam. Fig. 36, Part I, gives rules for determining the distance from outside of the flange to the commencement of the fillet. These distances are given also in the Cambria Handbook. It is possible to encroach a little on the fillet but generally not more than 1/8 in.
The standard form of connections of beams to columns is by a shelf angle with the stiffeners under it, with the required number of rivets, and with a cap angle over the top. The beam is riveted both to the cap and the shelf angles. Generally there are four rivets in each flange - sometimes only two in each flange are used. The shelf angle is usually a 6 X 6 X 1/2-in. angle and the cap angle a 6 X 6 X 7/16-in. angle where four rivets in the flange are used; if only two rivets are used the outstanding leg would be 4 inches instead of 6 inches. The size of stiffener angles varies with the size necessary to conform to the rivet pitch of the column, and to keep the outstanding leg of the stiffener the required distance from the finished line of the column. As stated previously, the deflection of the beam tends to throw the load near the outer edge of the angle and therefore the stiffener should come as near this edge as is practicable. Another point to be considered in choosing the size of stiffeners is to bring the outstanding leg as near as practicable under the center of the beam as this is the portion of the shelf loaded by the beam. It is not always practicable to do this, however, and sometimes two stiffeners are used coming a short distance each side of the center of the beam.