This section is from the book "Notes On Construction In Mild Steel", by Henry Fidler. Also available from Amazon: Notes On Construction In Mild Steel.
The angle-steel, or, to use the older nomenclature, the angle-iron, is perhaps the most commonly in use among all those sections of material which go to make up riveted work. It may be "equal-legged," as in Fig. 7, or "unequal-legged," as in Fig. 8.
The equal-legged angle in its ordinary form has a rectangular outline with a square corner on the outside, the interior faces being sometimes slightly tapered with a connecting round in the inner corner, and the edges rounded off to a quadrant of small radius.
These tapers and the radii of the roundings are not quite the same in all section books, varying with the shape of the rolls of the respective makers, although proposals for the adoption of a uniform standard in this as in other sections have not been wanting.
These proposals have now assumed a definite form in this country, by the issue of the "British Standard Sections," compiled under the direction of the Engineering Standards Committee in 1904, and to these sections the attention of the student is directed; and in the work entitled "Properties of British Standard Sections" will be found the standard sizes, thicknesses, slopes of taper, and radii of connecting curves, together with tables of the mechanical elements of the standard sections.
The unequal-legged angle presents the same general characteristics, while its name speaks for itself. Both the equal- and unequal-legged section is, in the British standard section, of uniform thickness, without taper.
Variations from these forms are found in the acute-angled angle and the obtuse-angled angle, used where oblique connections of riveted work have to be made. The use of acute-angled angles is attended sometimes with the difficulty of getting the rivets into the acute angle, which must be borne in mind in these cases, it being sometimes necessary, if the angle of connection is very acute, to use a bent plate of sufficient dimensions in lieu of an angle.
Both equal- and unequal-legged angles are also rolled with a round back, as in Fig. 9. They are most commonly used to effect the splice in the main angles of plate or lattice girders, an example of which is given in Fig. 101, the round back of the connecting angle fitting into the interior rounding of the main angles to be spliced. This method of splicing the main angles of a riveted plate girder is the one most commonly adopted, and leads to the consideration of the net sectional area of the angles to be spliced, the corresponding thickness of the angle "cover," which must necessarily be greater than that of the main angles, and as a consequence the spaces left for the rivets, their heads, and the amount of metal left outside them.
When, however, angles are used for ties or struts, either singly or in pairs, as in roof trusses, it becomes a simple matter to splice each leg of the angle with a flat of suitable width and thickness.
Another variation in the equal- or unequal-legged angle is that in which the legs are rolled of equal thickness without taper, and the edges and corners, both internal and external, are square and sharp. This, however, is usually considered a special section, and not frequently adopted in ordinary riveted work.
A section of angle-iron used frequently in the frames of ship or caisson work, as beams subject to transverse stress, is the so-called bulb-angle, shown in Fig. 10. This angle is usually unequal-legged, the object of the bulb being to increase the moment of inertia of the beam in the plane of its greatest depth as a beam, while it also serves the purpose of thickening and rounding the edge of the angle where exposed to passing traffic, etc.
The bulb may be rolled on one side of the longest limb as shown, or on the opposite side. The sectional area of the bulb varies slightly with different makers, but is standardized in the British standard section.
According to the usual practice, the vertical limb or web with the bulb is made parallel, the taper being given to the other limb of the angle. In the British standard section of bulb-angle, both limbs are of uniform thickness, there being no taper. If the section be increased beyond a certain minimum thickness the dimensions of the sides are increased proportionately, while the bulb retains the same projection from the face of the web.
The uses of angle-steels are multifarious. In addition to their primary function of connecting members of a structure in planes at right angles to one another, such as the web and flanges of a plate girder, or in oblique connections, they are also found used to a great extent either as beams, struts, or ties. As beams we find them in purlins to roofs, as secondary beams in a variety of structures, in the framing to sides of corrugated iron sheds, and the like. As struts they are employed in the members of lattice girders, in the compression members of roof principals, and as secondary bracing where some lateral stiffness is required in a number of cases. As ties they appear more or less successfully in the tension members of light trusses, but their effective use in tension is somewhat qualified by the necessity of securing them in many cases by one leg only. This remark also applies under similar conditions to their use as struts. In this latter capacity they will be further referred to in the chapter on columns.
The selection of the dimensions and scantlings of angles will be determined by a variety of considerations depending on the use to which they are put. As connecting members simply we shall find their dimensions ruled to a large extent by the size of rivets employed, the bearing stress allowed, and so on; as beams, by their moment of inertia; as struts, by their least radius of gyration. But in most cases considerations of rivet spacing in connections, etc., will be ruling factors in the design; and the young draughtsman will in this, as in so many other cases, be wise in drawing all doubtful details full size, or to a large scale, before he finally determines his section.