Riveted, or, as they are more usually called, plate girders may be constructed of sizes far exceeding those of the largest rolled beams.
The best depth for these girders is about 1/12 the span.
The simplest form of plate girder consists of angle irons (ff), riveted to a vertical plate (w), as shown in Fig. 258, the former being the flanges 1 and the latter the web of the resulting girder. The rivets are generally at a pitch (or distance between centres) of from 3 to 5 inches - most frequently 4 inches.
Fig. 258. Section.
Fig. 259. Elevation.
Some particulars regarding riveting are given at page 83. In some cases the rivets of the lower or tension flange of the girder are pitched at wider intervals than those in the upper or compression flange.
Fig. 260. Section
Fig. 261. Elevation.
In larger girders separate plates p p may be used for the flanges, and are fixed to the web by angle irons riveted as shown in Figs. 260, 261.
1 Called also "booms" in large girders.
When the web is deep, or of slight thickness, it has a tendency to buckle sideways, and requires support.
Extra stiffeners are also placed under points where heavy loads are expected.
The stiffeners may either be bent outwards at an angle as shown, or they may be cranked or joggled, that is bent close round, over the angle irons of the girder (which latter is a more expensive arrangement), or they may be kept out by means of distance pieces placed under them so as to clear the angle irons. This last arrangement is simple, but adds unnecessary weight to the girder.
In girders to carry great loads several plates are required in each flange, and when the flange-plates are wide, gussets or vertical plates are added; but such heavy girders are not likely to be required in any ordinary building.
"There are in existence plate-girder bridges of almost all possible dimensions, and some of the largest are objects of universal admiration; yet it may be broadly stated that the plate girder, if made beyond a span of 50 feet, loses those advantages which, up to that span, its simplicity affords as against the lightness of other systems." l
Box Girders are made up of plates united by angle irons and rivets into a hollow rectangular box section, as shown in Fig. 262.
Box girders should be large enough to admit a man or boy, so that they can be painted periodically on the inside to prevent corrosion.
When the girder is necessarily too small to allow of this, the plates are made of extra thickness to allow for corrosion; and sometimes the interior of the girder is filled with concrete to protect the iron from the action of the air and to prevent oxidation.
Comparison of Plate and Box Girders2 - The comparative advantages of plate and box girders are summed up by Sir W. Fairbairn as follows: - "On comparing the strengths
Fig. 262. Section of Box. Girder, of these separate beams, weight for weight, it will be found that the box beam is as 1: 93, or nearly as 100: 90.
1 Works in Iron, by Ewing Matheson.
2 Sir W. Fairbairn, On the Application of Cast and Wrought Iron to Building Purposes.
"This difference in the resisting power of the two beams does not arise from any difference or excess in the quantity of material in either structure, but from the better sectional form of the box beam. The box beam, it will be observed, contains a larger exterior sectional area, and is consequently stiffer and better calculated to resist lateral strain, in which direction the plate form generally yields before its other resisting powers of tension and compression can be brought fully into action.
"Taking this beam, however, in a position similar to that in which it is used for supporting the arches of fireproof buildings, or the roadway of a bridge, when its vertical position is maintained, its strength is very nearly equal to that of the box beam.
"But while the plate beam, in the position thus described, is nearly equal, if not in some respects superior, to the box beam, it is of more simple construction, less expensive, and more durable, from the circumstance that the vertical plate is thicker than the side-plates of the box beam, and is consequently better calculated to resist those atmospheric changes, which in this climate have so great an influence upon the durability of the metals.
"Besides it admits of easy access to all its parts for purposes of cleaning, painting, etc."