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.
In many cases, even the loading on the girder might not be given. In such case, it would have to be calculated from the general plans showing amount and distribution of floor and wall loads. If the loads had been uniformly distributed, details might have been made by determining the capacity of the girder, as noted below.
The first point to be determined is the size of the bearing plate. The reaction is 65,000 pounds; and, allowing a safe bearing on the stone template of 25 tons per square foot, this requires about 1.30 square feet. A plate 12 by 16 inches, therefore, will be sufficient. Applying the formula given on page 97 of Part II, the required thickness is found to be .26 inch; a steel plate 3/4 inch thick is used here, although 1/2-inch plate might have been used.
The size of the bed-plate having been fixed, the spacing of all the stiffeners is the next thing to determine. The end ones are fixed at 12 inches back to back. As the piers come down on top of the girder, it will be sufficient to use one stiffener in the center of each pier; if the pier was very heavy or over 3 feet, it would be well to use two under each pier. The measurements given in the diagram (Fig. 257), therefore, fix the other stiffeners. It is then necessary to look into the shear on the web to see if stiffeners are required on this account. Referring to the diagram (Fig. 246), it is found that for a 3/8-inch web and 18 inches between edges of flange angles, the allowable shear per square inch of web is 6,800 pounds. The actual shear is
76,000/30 x 3/8 = 6,750 pounds, which is therefore entirely safe without stiffeners, as the shear just one side of the end is 11,000 pounds less.
Looking new at the horizontal rivet spacing, we find, at the end, s =65,000/28 = 2,320 = approximate horizontal shear per inch.
Some engineers use the distance between pitch lines of flange rivets, or, in case of double pitch lines, the mean between the two, instead of using distance between centers of gravity for determining the approximate shear. In this case the result would be: s = 65,000/24.75 = 2,630 pounds.
The bearing value is the least for these rivets, and may be taken at 5,060; the end pitch, therefore, is 5,060/2,630 = 1.92 inches.
It is always better to space a little under the calculated pitch; for this reason 1 1/2 inches was used.
The loads being concentrated, the shear is practically constant from the end to the first stiffener, and the only other point to consider is to space from each stiffener so as to conform to the standard gauge in the stiffener angle, and to keep this where previously fixed, leaving room from the back of angle to drive first rivet. The distance back to back being 5 feet 2 1/2 inches, and the standard gauge in one case 2 inches and in the other 1 3/4 inches, the distance center to center of pitch lines in stiffener is 5 feet 6 1/4 inches. It is well to leave not less than 1 inch, and better 1 1/4 inches, from the back of a stiffener to first rivet so that it can be easily driven; leaving 1 1/4 inches will just allow for 40 spaces at 1 1/2 inches.
The shear just to the right of the first stiffener from the end, is 25,000 pounds; therefore, s = 25,000/24.75 = 1,010 pounds.
The direct shearing force from the pier load is 30,000/24 = 1,250 pounds per inch.
If we assume a pitch of 3 inches, this brings 3,750 pounds on each rivet, and the diagram of stress would be as illustrated in Fig. 252, the resultant stress being about 4,850 pounds. A pitch of 3 inches could therefore have been used and need not have been continued much beyond the pier lines. In order to keep the pitch constant, however, and be somewhat under the required pitch, 2 1/4 inches was used. Similarly, the pitch in center way is made 3 inches, although somewhat larger pitch might have been used.
The actual required pitch through flange plates would be found much less than shown, since there are four lines of rivets instead of two as is commonly the case in girders of this length. In order to simplify the shop work, however, they are detailed the same spacing. It is well to note that in such cases the rivet through flange plate on the gauge line nearest to the vertical leg of flange angle, comes opposite the vertical rivet in flange line farthest from the horizontal leg. This is to give all possible room for riveting, and also because it distributes the rivets more uniformly.
The bottom flange spacing is made the same as top, and differs only in having the rivets through bearing plates countersunk, with open holes for anchor bolts.
Fig. 258 shows the detail of a two-web girder. This girder carries a wall on a street front, and is one of a continuous line of several girders. The right-hand end is at the corner of the building; and the open holes shown are for connection of a girder on the other street front. The girder rested on steel columns, and the arrangement of the line members of the columns determined the spacing and arrangement of the end stiffeners on the girder.
The column section coming under right-hand end is shown by Fig. 259. The stiffeners at the extreme left end are simply for connection to similar stiffeners on the end of the girder coming against this one. The intermediate stiffeners are for support of flange under centers of brick piers.
The bottom plates were made 1 inch larger than the top plates for the purpose of securing the ornamental fascia.
In the calculation of the rivets of a two-web girder, the shear is assumed to be divided equally on the two webs; and therefore each line is calculated as before described, except that the shear used is one-half the total. It should be noted, also, in such cases, that the rivets are in single shear.