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.
Fig. 140, a simple flat bar, is met with as a strut or compression member in the webs of multiple lattice girders. It is obviously weak in the direction of its least radius of gyration, and is prevented from failure in that direction by the frequent intersection of the tension diagonals and the use of stiffening vertical members. Outside the particular application mentioned, it is not frequently used as a compression member, being obviously less adapted for that purpose than other and stiffer sections.
Fig. 141 is an example of the use of flat bars in pairs, frequently used as compression members in the diagonals of roof principals of small span, say up to 30 or 40 feet or thereabouts. The bars are usually pin-connected at the ends, and swelled apart at the centre by cast-iron distance pieces as shown. It is a convenient form of strut for light loads, but is apt to fail by weakness at the ends, the bars buckling near the pin connections.
Fig. 142 is a simple angle, either equal or unequal legged section. This section is in common use in the compression diagonals of small lattice girders and as a single angle in the rafters of small and light roof principals and their diagonals.
The double angle shown in Fig. 143 is a variation of the same type used for the same purposes, where the loads are heavier. The angles are occasionally riveted close together, back to back, and thus form practically a riveted tee. The mode of attachment of the single or double angle as a compression member is nearly always by one leg only. Under these conditions the distribution of stress is conceivably very unequal over the whole cross-section, and it is to be regretted that while numerous experiments have been carried out upon the ultimate strength of angles in compression, so little has been done to elucidate under actual practical working conditions the ultimate strength of angles connected in the usual way, and with the direction of the compressing forces out of centre with the centre of figure of the section.
A further elaboration of the use of angles is given in Fig. 144, which is a somewhat special section, occasionally used in the compression diagonals of roof trusses of large span. The four angles are brought near together at the ends and swelled out at the centre by cast-iron distance pieces.
A more simple example of the use of four angles is given in Fig. 145, also used as a compression member of large roof trusses, and consisting of four angles connected by internal stiffening plates at intervals as shown, or by light lattice bracing in the same planes. Such a strut is frequently pin ended, but may also be riveted and "fixed" ended. This strut is an example on a small scale of the type shown in Fig. 164, which is intended for large columns with heavy loading.
Fig. 146 will be at once recognized as the compression or top flange of ordinary plate or lattice girder construction of moderate spans. The increase of sectional area required is usually obtained by increasing the thickness or number of the plates.
Fig. 147, showing a simple tee section, is in common use as the rafter or compression member of roof trusses up to about 40-feet span, and in their compression diagonals. Its place is sometimes taken by the double angles shown in Fig. 143. It is also frequently used as the upper flange of small lattice girders or trussed purlins in roof work.
The double tee (Fig. 148) is used in the compression diagonals of large roof trusses, especially in those types of "crescent"-shaped principals shown in Fig. 345, where the stresses in the diagonals are not great. In such cases the double tees are brought near together at the ends and swelled out in the centre with cast-iron distance pieces, and are not so liable to the local weaknesses that may occur with the use of double flats, as in Fig. 141.
Fig. 149 is the simple channel, frequently used alone, but perhaps more commonly in combination, either in pairs, as in Figs. 150, 152, or as in Fig. 161.
Fig. 150 gives a pair of channels connected by lattice bars on both sides, and forming an open section used in the compression rafters of roofs of considerable span, say from 70 to 100 feet, as ordinary columns, or in compression members in large triangular girders.
Fig. 151 is the same combination of channels, but with a solid plate connection on the one side and open latticing on the other, used for similar purposes to those above mentioned, but where heavier stresses have to be provided for.
Fig. 152 shows a section of column frequently used for heavy loads in buildings, warehouses, dock sheds, and the like, consisting of two channels and two solid plate sides forming a closed cell.