Fig. 154 shows an effective section consisting of four channels connected at the external corners by four angles.

We now approach a group of sections in which the rolled joist is the principal feature, used to a very large extent in ordinary building construction, and which combine a considerable amount of stiffness with simplicity and ease of construction, and economy in riveting.

Fig. 154.

Fig. 155.

Fig. 156.

Fig. 157.

Fig. 153 is a form of section composed of four channels as shown.

Fig. 155 is the simple rolled joist, in which no riveting is required except in end connections when used as a plain column or strut. The principal defect in this section is the inequality of the radii of gyration round the axes severally square to the web and flanges. The relative values of these radii will be found for various sections in the table of the mechanical elements of rolled joists (p. 97), and in columns or struts exposed to lateral shock the liability to flexure in a plane square to the web must be borne in mind. This defect has apparently been recognized by some manufacturers, who have produced a section of rolled joist of exceptional width in the flange. This weakness is also to some extent corrected in the next development of this form of column shown in Fig. 156, where plates (one or more in thickness) are riveted to the flanges of the rolled joist, whereby the moment of inertia round the axis parallel to the web is increased. This section is very useful, and is largely used in columns for general building purposes.

Fig. 158.

Fig. 157 shows a pair of rolled joists, connected by plates as shown, or by a system of lattice bars in the same planes. A practical example of this type on a large scale will be referred to in detail hereafter.

Fig. 158 shows a strong column for heavy loads, composed of three rolled joists connected by plates as shown, or by latticing.

Fig. 159 shows a combination of three rolled joists which is the prototype in miniature of the more elaborate section shown in Fig. 167.

Fig. 160 shows the same combination, with the addition of external flange plates, which may be replaced either by latticing or flat stiffening plates at intervals.

Fig. 159.

Fig. 160.

Fig. 161 gives a column composed of one rolled joist and two channels, a simpler form of the type shown in Fig. 165.

It not unfrequently happens that a built-up section of plates and angles, though more expensive, offers greater facilities to the designer in certain details of connections or in arrangement of cross-section than a simple rolled section of similar type of outline, and so we frequently find the built-up section consisting of a plate and four angles, shown in Fig. 162. This section, similarly to that shown in Fig. 156, may be further elaborated by the addition of flange plates riveted to the angles.

Fig. 161.

Fig. 162.

Fig. 163.

Fig. 163 gives a box section of great strength and stiffness, frequently used in columns carrying heavy loads. The same amount of metal disposed as shown in Fig. 164 will yield a greater uniformity in the value of the radii of gyration about different axes, but the riveting is more difficult to get at, and must be dealt with in a manner similar to that adopted in ships' masts, sheer legs, derricks, etc.

Fig. 164.

Fig. 165.

Fig. 165 gives a valuable section, of good appearance, and great stiffness in all planes, composed by the addition of channels riveted to the section shown in Fig. 162.