This useful section is shown in Fig. 15. It is largely used in the frames of ship and caisson work, having a considerable moment of inertia for its weight, as compared with angles or tees, with ample width of flange for riveted connections.

The web is rolled with parallel sides, the flanges having a taper and being connected to the web by curves at the internal angles. In the British standard section the flanges have no taper, but are of uniform thickness.

Increase of weight beyond the minimum section is obtained by thickening the web, the width of flange being slightly increased.

The section is frequently rolled with a uniform thickness of web and flange, the latter being tapered as above described, and the quoted thickness being the mean between that of the root and of the point of the flange. The flanges of the British standard section have a thickness in excess of that of the web.

Occasionally the flanges are rolled of unequal width; this is a convenience where additional width is required for heavy riveting, and in those cases where the lesser width of flange is sufficient for the riveted attachments, then the increased width of the other flange yields a larger moment of inertia. The thicknesses given in the following table are approximately those to which the various sections are rolled as a minimum; for other thicknesses than those given, the moment of inertia may be taken for approximate calculations as proportional to the sectional area or weight for each section.

The table on page 104 gives the dimensions, sectional area, weight per foot lineal, moments of inertia, radii of gyration, etc., for Zed angles.

Further reference will be made to the use of Zed angles in the practical design of columns or struts.

In the preceding pages the sections which have been described and of which the tables of the principal mechanical elements have been given, viz. angles, equal and unequal-legged, tees, bulb tees or deck beams, rolled joists, channels, and Zed angles, are those which may be called the elementary or standard sections, which in combination with plates and bars are most ordinarily employed in riveted constructional steelwork. It is not possible to consider in detail the very numerous forms of rolled sections, other than those above mentioned, employed for special purposes. These include, for example, the varied sections of railway bars (bull-headed, bridge, and flat-footed), fish plates, tramway rails, guard rails, sections of trough flooring (usually formed in hydraulic presses), quadrant sections for pile-work, half-round, segmental, or cope steels, sash-bars, and fancy and other sections.

Fig. 15.

Table No. 29. The Principal Mechanical Elements Of Zed Angles

(See Fig. 15.)

The mechanical elements of square, round, hexagonal, and octagonal bars have not been given in the tables, as these can be easily obtained by the usual arithmetical processes.

With respect to the use of plates and bars, it is sufficient to point out that the dimensions to which these can now be rolled are amply sufficient to meet all legitimate demands of the designer of constructional steelwork. Various makers have their own standard maximum dimensions to which plates, sheets, or flats can be rolled, and it is customary to assign a limit of superficial area for each thickness of plate, which is not exceeded without entering into special arrangements. Thus for a ⅜-inch plate, the limit of area is given by one authority as 135 square feet, the maximum length of plate being 42 feet, and the maximum width 7 feet 6 inches, it being understood that maximum length and maximum width are not rolled together, but that, given the length, the width is such as not to exceed the limit of area, or vice versa. Again, for a plate ⅞ inch thick, a limit of 250 square feet is given, • the maxima of length and width being 56 feet and 10 feet respectively.

With respect to flats, usually so called when the width does not exceed 12 inches to 15 inches, the available length obtainable without joint will usually be found to meet all practical requirements, as other considerations, such as the maximum length permissible for transport or shipment, very frequently rule the case.