Fig. 62 shows a common form of pillar foundation (Hennebique system), being very similar to the grillage foundation shown in Fig. 184, Volume IV.; and as in that case the size of steel joists was arrived at by considering them as cantilevers, so here the necessary thickness of concrete and size of rods are calculated by considering the portions on either side of the central load as cantilevers, with an evenly distributed load acting upwards produced by the reaction of the earth. A plate is embedded in the footing to better distribute the load brought by the pillar reinforcing rods. Stirrups are used here, as may be seen in the illustration, while Fig. 63 shows the use of the Kahn bar with its diagonally sloping wings.

Armoured Or Reinforced Concrete For Various Uses P 107

Fig. 64.

Where it is necessary to construct a concrete raft over the whole site, armoured concrete undoubtedly forms the most practical and economical method of doing this. Fig. 64 shows plan and sections of a portion of such a raft, which may be regarded as a similar construction to that of floors, but in a reversed position. At the centres of spans between supports the tensile stresses occur at the upper side of the beams and slabs, and the reinforcement is consequently placed in this position, while under the pillars the reinforcement is placed near the bottom surface.

spaced with a pitch of i to 1/10 of diameter of the winding.

Only concrete within the hooping may be considered as resisting compression; that outside the hooping acting merely as a protection. Besides the greatly increased resistance offered by this method of construction, it has the further advantage that any signs of distress in the material are at once made evident by the breaking away of the outer protecting skin of concrete, which takes place long before the ultimate resistance is reached.

Armoured Or Reinforced Concrete For Various Uses P 108

Fig. 60.

The wire ties in the pillars shown in Figs. 36, 68, etc., besides holding the reinforcement in position while the concrete is being filled in, act to some extent as do the hoopings mentioned above, while the nearer they are placed together the greater resistance will the resultant material have.

Armoured Or Reinforced Concrete For Various Uses P 109

Fig. 61.

Walls

The thickness of a wall, together with the extent of its reinforcement, must largely be a matter of practical consideration, for the thrusts that it may be called upon to meet as a general rule cannot be even approximately arrived at.

Fig. 61 shows the method of reinforcing a wall according to the Hennebique system, the wall being constructed as a slab capable of resisting thrust from either side.

Foundations

Concrete always finds an important place in foundation work, and the nature of armoured concrete makes it particularly suitable for use in this position, while the moulds, which usually form a considerable portion of the cost of armoured concrete, are here reduced to a minimum.

Foundations 110

Fig. 62.

Foundations 111

Fig. 63.

Armoured concrete may be economically used for all

Armoured or Reinforced Concrete for Various Uses 43 the methods of forming foundations mentioned in reference to steel frame buildings in Chapter XVI (Building Stone. Sandstones). Part II. Volume IV., while there is greater certainty of the thorough protection of steelwork than in the case of the grillage of steel beams.

Fig. 62 shows a common form of pillar foundation (Hennebique system), being very similar to the grillage foundation shown in Fig. 184, Volume IV.; and as in that case the size of steel joists was arrived at by considering them as cantilevers, so here the necessary thickness of concrete and size of rods are calculated by considering the portions on either side of the central load as cantilevers, with an evenly distributed load acting upwards produced by the reaction of the earth. A plate is embedded in the footing to better distribute the load brought by the pillar reinforcing rods. Stirrups arc used here, as may be seen in the illustration, while Fig. 63 shows the use of the Kahn bar with its diagonally sloping wings.

Foundations 112

Fig. 64.

Where it is necessary to construct a concrete raft over the whole site, armoured concrete undoubtedly forms the most practical and economical method of doing this. Fig. 64 shows plan and sections of a portion of such a raft, which may be regarded as a similar construction to that of floors, but in a reversed position. At the centres of spans between supports the tensile stresses occur at the upper side of the beams and slabs, and the reinforcement is consequently placed in this position, while under the pillars the reinforcement is placed near the bottom surface.

Piles

Where the use of piles is necessary, those formed of concrete are highly to be recommended for a structure which is intended to be permanent, as they are not only practically proof against damp, but they form one with the structure above.

Fig. 65 gives a section of the Coignet pile, in which a spiral rod about 1/4 inch in diameter and having a pitch of 3 or 4 inches encircles the longitudinal rods. To prevent unwinding and to add to the rigidity of the framework, another spiral wire of very long pitch runs in the contrary direction to the first. The two flat surfaces shown in the section are intended for guiding purposes during driving operations. With the exception of the increased pitch of the winding, this construction is very similar to the hooped pillars considered with reference to Fig. 60. Square piles are also made on this system.

Piles 113

Fig. 65.

Fig. 66 shows the reinforcement of a hollow pile on the Hennebique system before the concrete has been filled in.

Piles 114

Fig. 66.