This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol5", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
It is clear that if stirrups be used in conjunction with horizontal members, to resist the curved disposition of tensile stress, some rigid connection is necessary between the two. This connection is generally supplied by the "adhesion" of the concrete to the metal, and under ordinary stresses there is little doubt that this is generally sufficient to meet the case; but if they are rigidly connected the beam will be doubly secure in this respect. This condition is met by the " Kahn trussed bar" (Fig. 31). This bar is rolled with projecting wings on either side, which wings are sheared and bent up to form the inclined reinforcement. The condition is likewise met to some extent in the Henne-bique system by the sloping up of the reinforcement to pass over the supports; and most frequently by the Wells system, to be explained immediately.
As before stated, the various systems all endeavour to meet the same theoretical requirements in the most simple and practical manner. Many firms secure patentable systems by the employment of special forms of reinforcing bars. For instance, the Ransome system, much used in America, employs a reinforcement consisting of a square rod twisted throughout its length, the object being to prevent its slipping in the concrete, while at the same time the elastic limit of the metal is considerable increased by the process. Fig. 32 shows another special bar by the Patent Indented Steel Bar Company Ltd. Other systems again employ special arrangements of bars of ordinary section.
A system which has been much used in England is that of M. Hennebique (Fig. 23), the special feature in this being the hoop-iron stirrup (Fig. 27). Fig. 33 shows the general arrangement of rods and stirrups in a beam. It is seen that the reinforcement consists of two rods vertically over one another, and that where they pass over the supports the upper rods are bent up to the upper surface. Fig. 34 shows a detail at a point where a beam passes over a pillar. The join in the rods is formed by simply overlapping them, their ends being slightly split; and in order to connect them more intimately with the concrete small stirrups are placed about them at these points. The beam being in the form of a tee, the necessary area of concrete to resist compression is provided in the horizontal arm, while a comparatively small area exists in the lower portion of the beam. Over the supports, however, where the stresses are reversed, the concrete to resist compression is reduced to that in the lower portion of the beam. It is assisted, however, by the lower reinforcing rods, which are carried straight through, and also to some extent by the splayed portion at the top of the pillar.
A detail section through beam and slab is shown in Fig. 35. Small stirrups are seen to be used in the slabs as well as in the beams. Where either slabs or beams are freely supported the rods are not bent up as shown in these illustrations, but are carried straight along the lower surface.
The reinforcement of pillars may be seen in Fig. 34, while a detail showing the wire ties which bind the vertical rods may be seen in Fig. 36. The splayed portion at the head of the column is further reinforced by horizontal rods (Fig. 34).
In this system the principal reinforcement is carried along the whole length of the lower surface of the beams, while smaller bars are introduced near the upper surface in order that, with the addition of stirrups, the tensile and compressional portions of the beam may be connected in a practical and mechanical manner (see Fig. 37). These upper or secondary bars also assist in resisting the compressive stress and improve the resistance of the concrete itself as mentioned elsewhere. All intersections between bars and stirrups are secured with annealed wire, thus producing a framework with a certain amount of rigidity, which, when formed, can be lifted up and fixed in position in the moulds, and the concrete can then be filled in and packed about it.
Slabs of large span are constructed in the same manner as are beams, as in Fig. 38, while thinner slabs, 3 or 4 inches thick, supported by ribs only a few feet apart, are formed as in Fig. 39. Annealed wire binding is used at every other intersection of the rods of slabs.
Figs. 40 and 41 show the general adaptation of the Kahn trussed bar, a detail of which was shown in Fig. 31. The sizes of bars used are given in the following table -
Section of Diamond.
Width across Wings.
Thickness of Wings.
Weight per Fool Run.
The diagonals are ordinarily placed opposite one another, and in deep girders they thus come rather far apart To avoid this they may be staggered, being placed alternately on either side of the bar. In this way diagonals 24 inches long may be obtained 12 inches apart. Where a beam or slab passes over its support, reversed bars are used with their diagonals sloping downwards, as may be seen in Figs. 40 and 61. In deep girders the straight reinforcement along the bottom may be supplemented by a shorter bar sloping upwards from the centre, as shown in the top illustration of Fig. 41.