The form of section to be given to this important member of a roof-truss, especially of the classes (a) and (b) above alluded to, will largely influence the details of connections and the general type of construction, and will always be found to demand careful consideration.

In timber roofs this member takes the form of a simple rectangular beam, as the functions of this tie are usually as much to resist transverse stress due to the weight of ceiling rafters, or possibly of a floor, as to resist in tension the spreading effort of the rafters.

In composite roofs of timber and iron combined the practice has usually been to employ wrought-iron tie-rods of circular section.

In wrought-iron roofing for moderate spans, and even up to spans of very considerable dimensions, the general practice for many years was in favour of the round rod or circular section.

This form admitted of a nice adjustment of cross-sectional area to theoretical requirements, and so far represented an economical construction, while the appearance was light, and the nature of the material offered no special difficulties or risks in the manufacture of eyes and jaws or in the welding processes which usually accompany those details, provided only that the smith-work was properly and soundly done.

In many roofs of large span heavy circular rods were used with screwed ends and coupled connections, with a view probably of avoiding the risks of defective smith-work in such large jaws or eyes as the size of section demanded.

Recent events have, however, thrown some light upon the general policy of providing a single member only to act as the main tie in roofs of large span, and it may be doubted whether, in view of possible hidden flaws arising from defective smith-work, a duplication of this important member in a large roof-truss is not desirable.

Upon the introduction of steel into roof construction some attempts were made to utilize the material possessing a high tensile resistance, by employing it in the main tie, while constructing the remainder of the truss in wrought iron. In the early days of steel construction a harder grade of steel, with higher carbon content, was in vogue, and difficulties manifested themselves in the smithing of jaws and eyes. In some cases the difficulty was met by the use of a ferrule of soft wrought iron being welded on the steel rod, and the whole smithed out to the desired form; while in other directions steel tie-rods with plain screwed ends, involving no smith-work or welding, were used, the connections being of a special character, consisting of coupling boxes designed to give the required connections at junctions with intermediate braces, etc.

With the more general use of mild steel of a lower carbon content, and more amenable to smith-work, a new set of conditions has arisen, and the present-day practice is in favour of the use of mild steel throughout the entire truss, with such occasional use of cast iron in special details, such as lanterns, skylights, and guttering, as is desirable. Some designers, however, prefer, in the case of round-rod ties, to use wrought iron, on the ground of greater security in the welds, reserving mild steel for the remainder of the truss, thus reversing the procedure of an earlier date; but the tendency as a rule has been to abandon the circular section, and to adopt for moderate spans the flat-bar tie with riveted connections.

This form of tie is less economical than the circular section, inasmuch as a loss of one rivet-hole in the cross-section is involved; the appearance, although not objectionable, is not so light as the round rod, but, on the other hand, the method of construction is cheap, involving no smith-work, while the riveted connections are as a rule of simple type. If the slight increase of weight in the flat-bar type is set against the greater economy in manufacture, it is probable that the difference of cost in the two types, flat-bar section and circular section, is not very appreciable.

In roofs of large span, the ordinary suspension-bridge link with swelled eyes and pin connection has been employed with good results both in wrought iron and steel.

In this case all the precautions necessary, both as to methods of manufacture and in the design of the proper shape of head and dimensions of pin connection, are as applicable as in the case of suspension-bridge design, or in the lower chords of trussed girders, with eye-bar tension members.

Hitherto we have regarded the main tie as subject to direct tension only, and this assumption is probably correct for all roofs of large span and of considerable dead load in proportion to any inequality or obliquity of loading or wind pressure which may come upon them. Certain cases may, however, arise, especially in roofs of small span in very exposed situations, where it is expedient to stiffen the main tie as against any small element of compression which may arise from an extreme horizontal component of wind pressure, or where the roof principal performs the function of a strut or tie between the heads of lofty columns, acting as a gauge-keeper between the parallel rails of a traveller gantry, or transmitting a proportion of wind pressure in a lofty building from one side to the other. Again, such a stiffening of the main tie may be desirable in the case of roof principals spanning the interval between lattice roof girders possessing but little transverse stiffness, and where a certain amount of stiffness in the tie-rod is desirable on general grounds.

Considerations of this kind will occasionally lead to the adoption of angle or tee, or other stiffened section, for the main tie, although, of course, the economy of section as for a purely tension member is lost, owing to the practical difficulties in connection with end connections, which lead to an inequality of tensile stress over the entire cross-section.

It is unnecessary to remark that where a tie is subjected to transverse stress, as from supported loads, the weight of a ceiling or floor, or the like, then the form of section must be one specially adapted to meet these conditions.

Some examples of the use of the circular section of tie-rod, with the details appertaining thereto, are given in Figs. 290 to 295.

The treatment of flat-bar ties in mild steel is indicated in Figs. 280 to 289, and of the stiffened form of tie in certain cases in Figs. 270 to 279.