GENERAL PRINCIPLES OF REINFORCED CONCRETE DESIGN The use of some form of steel reinforcement has already been shown to be a necessity. The stresses in a member subjected to bending have been explained, and attention called to the un equal strength of plain concrete in tension and in compression. It remains for us now to find out where to locate the steel reinforcement, and how much is needed for a given load upon a member.
In the case of a concrete beam or girder, the horizontal steel reinforcing rods should be lo cated as near the bottom of the beam as possible, still allowing sufficient thickness of concrete un derneath to protect them in case of fire or ex posure to liquids, gases, or other agents tending to cause corrosion. This is, as has already been stated, on account of the stresses in the bent beam being divided on each side of an imaginary line located near the center of the beam section (for rectangular beams) and called the neutral axis. The stresses above this line are compres sive stresses, and are taken care of by the strength of the concrete itself; while those below the line are tensile, and, on account of the weak ness of concrete in tension, are taken care of by the steel.
This reinforcing steel may be in the form of rods or bars, unit-frames, or structural shapes, for girders, beams, and long spans of construc tion work, or for short spans bearing heavy loads; but for floors, roofs, etc., of short spans and light loads, the sheet fabrics can often be used to advantage.
At the part in the span where the greatest bending action occurs, the depth of the rein forcement below the top surface of a beam or girder varies with designers from to of the depth of the beam section. This allows a good protection against fire in ordinary sizes of beam section, and also allows for plenty of con crete around the metal to resist shearing action along the rods or to prevent the slipping or pull ing-out of the rods.
Godfrey, in his work on "Concrete," recom mends that the spacing of rods or bars in beams, when the diameter is of the span, should be four diameters for square rods, and three di ameters for round rods. If the rods are of smaller diameter than of the span, the spacing may be closer. The distance from cen ter of outside rod to side of beam should be one-half the spacing.
The web of a beam or girder is often pro tected at the ends against diagonal tension cracks, by bending up the end thirds of the rods and bringing their ends nearly to the top surface of the beam. These ends are then fitted with
anchors which will hold them firmly in the con crete. This construction has been found to strengthen the web of the beam greatly, and to cause it even to approach the condition of an arch in resisting loads. Professor Talbot found, as a result of many tests, that the loads carried by beams with all the reinforcing bars bent up but not anchored did not differ much from those in cases where the bars were all straight. The failure was slower in the case of the bent bars, and warning of approaching failure was given. He found, however, that if part of the bars were left straight and alternated with the bent ones, the web was considerably strengthened.
These tests also showed that the use of U-shaped stirrups generally strengthened the webs of beams, but the amount of additional strength depended largely upon the quality of the concrete. The stirrups did not exert any considerable strength, however, until a diagonal crack had formed in the beam.
Mr. Ransome's rule for placing the stirrups in a beam is to place the first one at a distance from the end equal to the depth of the beam; the second, a distance of its depth beyond the first; the third, a distance of the depth beyond the second; and the fourth, a distance equal to the depth of the beam beyond the third.
The main point in putting in this steel rein forcement is to get just enough steel below the neutral axis to balance the strength of the con crete above it. We do not wish to use too large sizes of rods in light beams, as the result would be a crushing of the concrete on the top side of the beam, or a shearing along the rods, when the beam was bent. Professor Talbot has shown from his experiments that steel rods whose combined area amounted to from one to one and one-half per cent of the area of the beam section above a line drawn through the center of the rods (the half-holes above this line being fig ured as a part of the beam section) would allow the beam to fail by tension in the rods. For percentages of steel higher than one and one half, there is a liability to failure by crushing of the concrete.