Bearing Strength

The resistance of wrought iron to indentation by bolts or rivets varies, of course, according to the quality of the iron.

For most ordinary work the safe statical pressure per square inch of bearing surface may be taken at 5 tons,1 but in chain-riveted joints it may be taken at 71/2 tons.2

Mr. Stoney takes it at 11/2 times the safe tensile stress, or 71/2 tons for all structures.

Steel

The factor of safety applied to steel structures should depend (cceteris paribus) on the nature of the steel and its temper.

Thus a very hard steel, with high tensile strength and slight ductility, should be worked at a smaller proportion of its breaking stress than a mild and soft steel.

Working Tensile Stress

Mr. Stoney recommends a working stress of 8 tons per square inch for mild steel plates, being about 1/4 of their ultimate tensile resistance (see Table III p. 322).

Opinion Of Committee Appointed By The Board Of Trade

The use of steel in railway bridges and other structures is not at present provided for by the Board of Trade regulations, and hence the working stress per square inch to which it may be submitted is not officially laid down.

This has prevented the extensive use of steel for other structures in this country, and consequently no conclusions can be drawn from actual practice.

A committee have, however, recently reported to the Board of Trade on this subject.

The composition8 of this committee renders their opinion of the greatest value to engineers generally.

They base their recommendations on an analysis of the experiments on steel made by a committee of civil engineers in 1868-70 (see p. 320).

The following extracts from their report will give the conclusions at which they arrive :-

"As regards the ordinary steel of commerce, there appears to be no difficulty in obtaining the usual amount of tensile strength, varying from 29 to 35 tons per inch. A point requiring equal attention is the toughness or malleability." . . .

"We assume that with steel, as with iron, the engineer will take care that, as well as the required strength, he secures a proper amount of ductility. . . .

"The steel employed should be cast steel, or steel made by some process of fusion, subsequently rolled or hammered, and that it should he of a quality possessing considerable toughness and ductility." . . .

1 Latham On Wrought Iron Bridges. 2 Unwin's Wrought Iron Bridges and Roofs.

3 Sir John Hawkshaw, C.E., f.r.s.; Colonel W. Yolland, E.E., f.r.s.; W. H. Barlow, Esq., C.E., F.K.S.

"The greatest load which can be brought upon the bridge or structure, added to the weight of the superstructure, should not produce a greater strain in any part than 61/2 tons per square inch."

From other parts of their report it appears that they consider that the working stress upon steel should bear the same proportion to its ultimate strength that the working stress upon iron does to its ultimate strength.

Thus, taking the ultimate strength of iron at 20 tons per inch, and the working stress allowed by the Board of Trade for bridges at 20/5 = 5 tons, they infer that the ultimate strength of steel may very safely be taken at 26 tons per inch, and the working stress applied to it at 26/4 = 61/2 tons.

Working Stress In Compression

With regard to the working stress in compression Mr. Stoney says : -

"The crushing strength of steel is so high that 12 or even 15 tons per square inch is perhaps a safe compressive strain. When the material is not permitted to deflect, but when in the form of a solid pillar, the strength of mild steel seems to be only 1 3/4 times that of wrought iron.

"Experiments are, however, still wanted to determine this, and until such are made it will scarcely be safe to adopt for steel pillars a higher load than 50 per cent above that which a similar section of wrought iron would safely carry."1

Bearing And Shearing Stress

In default of experiments, the working stress for bearing and shearing may be taken at the same proportion with regard to the ultimate stress as in the case of iron.