Silicon Steel.
It is widely known that the Sydney Harbour Bridge is made from Silicon Steel. Such a material is not available, at least by that name, in modern times so the question must be asked as to what exactly is Silicon Steel?
In 1934 four very comprehensive papers, (they total 322 pages with many sheets of gate-fold plans as well.), were given before the British Institution of Civil Engineers and in the discussion and correspondence which ensued Ralph Freeman disputed a description of the material given by Dr Steinman, as a “high-alloy” steel. Dr D B Steinman was a partner in the New York consulting engineering firm, Robinson and Steinman who had been retained by the English Electric Company of Australia to prepare a design for a tender for the bridge. Freeman said that this description of silicon-steel might convey a wrong impression.
That steel was rather a superior quality of ordinary mild steel. It was produced for Sydney Harbour bridge at a cost very little above that of mild steel, and was practically as easy to work with in the shops. (ICE 1934 p459)
In one of Freeman's papers in the set of four, (he wrote another jointly with Ennis), he gave a detailed description of the material.
Quality of Steel .- One of the most important decisions before the detailed preparation of a design could proceed was the quality of steel to be used. Three qualities of steel required consideration:-
(a) Carbon-steel:- ordinary mild steel of British Standard type. 27.7 to 32.2 tons per square inch tensile breaking strength.
(b) Silicon-steel :– a mild steel with similar physical characteristics, and having a breaking strength of 35.6 to 42.3 tons per square inch.
(c) A nickel or other alloy-steel with appreciably higher tensile strength.
The effect of using these various qualities of steel was studied, and the conclusion was reached that the reduction in the weight of steel in the bridge, if an alloy-steel were used, would not be sufficient to effect the extra cost of the raw material and manufacture.
A high-tensile mild steel, having a tensile strength about 30 per cent. above that of ordinary British Standard mild steel used for structural steelwork, could be obtained without difficulty in the large quantities required, provided the design were made without introducing an undue multiplicity of sections ; it was not considered advisable, however, to attempt the manufacture of a steel with similar characteristics but with a higher tensile strength. The cost of the raw material was estimated to be about 25 per cent. above that of mild steel, and the additional difficulty of working would add only a small sum to the cost of manufacture.
It was obvious that these extra costs, amounting to only about 5 per cent. of the total cost per ton, would be more than offset in the arch span by the reduction in weight and erection costs subsequent upon using steel with a strength 30 per cent. greater, as the weight of the arch structure itself accounts for a large part of the total stresses. It was therefore decided to use high-tensile steel for the whole of the [main arch] trusses, the cross-girder flanges [in the deck of the arch span], and the principal lateral bracings [between the two arch trusses].
The steel to be used was left to the decision of the contractors, and complied with the following physical tests and analyses : –
As a general rule members of the bridge were made wholly of silicon-steel, or wholly of carbon-steel, but the cross girders [in the main deck] were built with webs of carbon-steel and flanges of silicon-steel. Carbon-steel was generally used for those members in which the calculated thickness of silicon-steel would not have provided sufficiently substantial sections.
Although the approach-spans are structures of considerable size, each span weighing about 1,000 tons, it was found that it would not pay to use silicon-steel for the main girders, and carbon-steel was used. A sufficiently extensive demand for steel would enable it to be supplied at a price enabling the cost of ordinary bridges and structural steelwork to be much reduced. To obtain the full benefit of possible economies it would be necessary the reduce the minimum thicknesses commonly specified for structural work, and to find a rivet-steel that would be proportionally stronger, or to modify the somewhat low stresses now allowed in rivets.
Bradfield in his paper in this set also discusses the specifications for the steel used :-
Steels.– All steel in approach-spans and in the stringers and decking of the main span is basic open-hearth structural carbon-steel. Rivets throughout the structure are of carbon-steel. Main-span truss members, [i.e. the arch], hangers, lower laterals and flanges of the main-span cross-girders are of structural silicon-steel. Cast steel was used for the bearings. The specified properties of the various steels are given in Table III.
Carbon-steel specimens cut longitudinally or transversely, up to 1 inch in thickness, had to bend cold 180 degrees round a rod of diameter equal to the thickness of the specimen. Above 1 inch in thickness the diameter of the rod was increased gradually until at 3 inches thickness the diameter of the bend was 6¼ inches.
Rivet-steel had to bend 180 degrees flat, hot or cold, and a piece of rivet-steel 2 inches long had to flatten to a thickness of ¼ inch when hot and laid longitudinally under the hammer.
Silicon-steel had to bend cold 180 degrees round rods of varying diameter. For longitudinal bend tests the diameter of rod increased from ⅜ inch or specimens ⅜ inch thick, to 7 inches for specimens 2½ inches thick. For transverse specimens the diameter of bend increased from 9/16 inch for ⅜-inch material to 10½ inches for 2½-inch material.
For cast steel, specimens had to bend cold 120 degrees round a rod twice the thickness of the test piece.
