Mr. Edward Godfrey, in his volume on "Con crete," speaking in regard to the quality of the steel used in reinforcing, touches upon some vital and debatable points on which there has been much controversy, when he says: "Steel for reinforced concrete should preferably be open-hearth steel, though Bessemer steel may safely be used for rods and for plates and shapes that are punched, if the punched holes are reamed.
"The ultimate strength of the steel is not a matter of much importance, neither is the elastic limit, except as these properties indicate uniformity in the product. It should be a good grade of soft steel. It is of more im portance that it stand the bend test of soft steel than that its ultimate strength and elastic limit be high. The reason why high elastic limit and high ultimate strength are not essential characteristics of steel embedded in con crete, is the simple fact that these qualities cannot be made use of in proper design of reinforced concrete. This is directly contrary to a great amount of trade literature and some technical literature. Commercial soft steel is almost universally of an ultimate tensile strength of from 50,000 to 60,000 lbs. per sq. in., and a strength at elastic limit of 30,000 to 40,000 lbs. per sq. in. This latter is about three times the safe value that ought to be allowed on the steel, because above this value cracks begin to appear, and there can be no justifi cation for a design that anticipates cracked beams and slabs. There is therefore ample margin of safety in any good soft steel.
"If high steels showed smaller elongations for a given unit-stress (for stress within the elastic limit) than soft steels, there would be some justification for their use, as they would then not stretch out as much under a given stress, and the concrete would be less liable to be cracked. But the modulus of elasticity is one prop erty that is practically constant for all grades of steel. Even soft wrought iron has a modulus of elasticity almost as great as the hardest steel. The simplest conception of the modulus of elasticity, designated as E, is a unit stress that would stretch a piece of steel out to double its original length, at the rate at which it stretches within the elastic limit. The modulus of elasticity of steel is about 30,000,000; if, then, a piece of steel is stressed to 10,000 lbs. per sq. in., it will stretch one-three-thou
sandth of its length. Beyond the elastic limit, different grades of steel exhibit different characteristics. Soft steels stretch out more before failure, while high steels and soft steels that have been rolled or drawn cold or twisted cold, break without much stretch or reduction of area at point of fracture. This lack of stretch beyond the elastic limit is held out as a benefit in trade litera ture. It is a positive detriment. If failure occurs in steel that will not stretch, it will be sudden and without warning; whereas, if the steel stretches out, it will allow a beam or slab to sag before failure. Besides giving warning of failure, the sagging will in many cases reduce the stress in the steel very materially. The author has seen tests of slabs reinforced with soft steel that sagged enormously and could not be broken.
"The reason why it is important that steel shall stand the cold-bend test is because rods are very often curved and bent in construction. This bending should be done cold; for, if the steel is heated, its internal structure is changed, and annealing would be necessary to restore it. Soft steel of ordinary manufacture will, in general, stand more punishment than harder grades of steel. The threading of rods and punching of plates or shapes are less liable to cause incipient cracks or hardened metal in soft steel than in high steel. These are also processes to which the embedded steel may be subjected.
"Special steel, while it has a high sound, does not possess any needful characteristics, as an element in reinforced concrete, that are not possessed by the cheap commercial article. This is true because of the limita tions of the concrete. Good soft steel is not a special steel, but is the commonest product of the steel furnace. It is important, however, that it be good and that it be soft—that is, not a high-carbon steel.
"There should be a wide margin of safety in the amount that a steel rod will bend. A piece of steel of high ultimate strength may stand a bend of 100 degrees, and fail if bent 105 degrees. It is clear that this steel would not be fit to use where it is bent at an angle anywhere near approaching this.