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.
Fig. 67 shows the construction of a pile on the Williams system, it being reinforced with a rolled steel joist, the web of which is cut away and the flanges bent in to form the point of the pile. Around the joist, at intervals of about 12 inches, rings of 3/16 inch wire are placed, while the flat steel bars a are added when increased bending resistance is necessary in the direction at right angles to the web of the joist.
Concrete piles are pitched and driven as are ordinary timber piles, and the fact that this is possible forms abundant proof of the resistance of the material to shock. In driving the piles the head is protected by a steel cap containing a pad of sawdust, the upper side of which is fitted with a hard wood dolly about 3 feet long; this arrangement being used to bring the blow evenly upon the head of the pile and to deaden the shock.
When the pile has been driven the concrete about its head is cut away, leaving the reinforcement projecting. The latter is then embedded in the concrete superstructure, or the pile may be lengthened in the same manner as it was originally constructed. Fig. 68 shows a pile foundation to a pillar, in which the intimate connection between the pile and the spread foundation may be seen.
Another variety of pile, the " Simplex Concrete Pile," is shown in Fig. 69. In constructing this an iron tube is first driven into the ground to the required depth, being closed in at its lower end by an " Alligator Point," the driving being done by the ordinary pile driver. After this form has been driven it is filled with concrete to a height of about 3 feet above its bottom end. The form is then pulled up 2 feet, the jaws of the alligator point opening wide and allowing the concrete to pass through. The concrete is then rammed with a 600 lbs. drop hammer. The process is thus repeated until the pile is complete. The first diagram in Fig. 69 shows the process of withdrawing the metal form and ramming the concrete; the second shows the pile completed, and also the bucket used for discharging the concrete; while the third diagram shows a wharf pile, the upper portion being reinforced to meet lateral Armoured or Reinforced Concrete for Various Uses 45 pressure and shocks, the reinforcement in the form of a cage being lowered into the tube while it is being filled with concrete, and the tube being left in position for so much of its length as would be surrounded by water or friable earth.
The advantages of this system are that the piles are made on the spot to the exact length that is found necessary; that the sides are rough, and offer great frictional resistance between pile and earth, while the shape of the pile is also conducive to great carrying power.
These form a particularly striking instance of the economy that is attainable with this form of construction. In the case of masonry, very heavy massive walls are necessary to resist the overturning effect of the earth pressure, and at the same time to avoid the presence of tensile stresses. In armoured concrete, ledges may be formed at the back of the wall, and the earth vertically above these, which they support, adds to the weight of the wall, and greatly assist its resistance to overturning; while, at the same time, a toe can be conveniently formed at the face of the wall which will bring the pressure more evenly upon the foundation-bed. Fig. 70 shows such a retaining wall reinforced with Kahn trussed bars. It will be observed that the face of the wall acts in the same manner as a floor slab, the reinforcing bars being placed nearer together at the foot where the pressure is greatest. The top half of the wall forms a simple cantilever of tee section, the " counter forts " or webs being in tension and the wall slab in compression.
Fig. 71 shows another example of this form of construction, in conjunction with concrete piles, for a quay wall on the Hennebique system. The lightness of the construction is very noticeable.
Arches of armoured concrete have the considerable advantage over those of masonry that the line of pressure need not necessarily pass through the middle third of the depth of the arch ring, for the tensile stresses, set up by the departure from this rule, are taken by the reinforcements, and the depth of the ring may consequently be much reduced.
A double reinforcement, such as is shown in Fig. 72, is generally adopted, but if the only continuous reinforcements be placed near the lower surface, further reinforcements should be used at the upper side of the arch, enbedded in the abutments, and extending inwards for not less than a quarter of the span.
The exact position at which tensile stresses may be induced cannot be definitely ascertained, and depends upon shape of arch and distribution of loading. Thus the line of pressure may be disposed as shown by the thick dotted line in Fig. 73, producing tension at the points marked T. It is therefore advisable to use a double reinforcement throughout. Stirrups or similar members are also advisable in order to bind the layers of concrete together.
In the case of a three-hinged arch - that is, one with hinges at springings and crown - the line of pressure must pass through these three points, and can be located fairly accurately. Also, the arch is independent of the effects of temperature and of the uneven settlement of the abutments. It will seldom happen in the case of an arch without hinges that the line of pressure will pass through the axis of the arch at these three points, and the disposition of stress and the effect of temperature or unequal settlement must be uncertain.
It will probably not be far from the truth if it be assumed that the line of pressure passes through the axis of the arch at its crown, and that the horizontal thrust for an evenly distributed load = H = wl2/8R where w = total load per foot-run (dead + live).
l = span of arch in feet.
R = rise of arch in feet. The depth of ring may then be proportioned to resist this load by the pillar formulae given at the commence-
Armoured or Reinforced Concrete for Various Uses 47 ment of this Chapter. The strength of the arch at any other point may be calculated as follows -
T = thrust at point under consideration. (This may be split up into two thrusts parallel and normal to the axis of the arch. The latter will produce shear only, and may be neglected.)
Let P = component of T parallel to axis, and e = the eccentricity, or distance between pressure curve and axis of arch. The effect of P will be to produce a total stress P distributed throughout the section + the stresses produced by bending moment P . e.
Let C1 equal the compressive stress produced by the direct compression of P.
Then c1 = P.
bd+ 10(ac + ar)
Then, if c = 400 lbs. per square inch = maximum allowable stress on concrete, the maximum stress per inch available to resist the BM = c2 = c - c1.
Now, applying the beam formulae (near the end of last Chapter), and having found the value of h from the formula there given -
Mr = 2/3bhc2(d-2/5h)+a2c2rh-g/h(d-g)
The latter value will check the ring's resistance to bending, which must not be less than Pe. The stress in the tensile reinforcement
=f= 2bh1c + 3accr(h-g) -rc1
3ath a = angle between axis of arch at point under consideration, and the horizontal.
The greatest bending moment will, as a rule, be in the proximity of 1/4 span, where the thrust may be taken as roughly w2l2/16R(2w1 +w2) sec. a. and BM = w2l2/64 where w1 = dead load per foot. w2 = live load per foot. / = length of span in feet. R = rise of span in feet.
By the use of reinforcements, bending moments may likewise be allowed in the abutments, producing proportions which at first sight might appear to be much too slight to resist the thrust of the arch. This is seen in Fig. 74, in which an evenly distributed load on the arch will produce tensile stresses in the reinforcements T shown in full lines. The arch may, in fact, be considered as springing from ground level, instead of from the apparent springing level, while the line of pressure becomes somewhat as shown in a heavy dotted line.
Fig. 75 shows the pleasing effect and lightness of this construction when used for floors (Hennebique system), the span being 46 feet.