The limiting or working stresses that can be safely applied in practice to cast iron, wrought iron, and steel respectively, depend not only upon the quality and characteristics of the material, but upon the nature of the load which causes the stresses, and in many cases also upon the form of the member or structure under stress.

These points, and many others which bear upon the question, cannot here be entered upon without anticipating the information to be given in Part IV., where the subject will be more fully discussed.

Factors Of Safety

It will be sufficient at present to call attention to the following Table, which shows the "factors of safety" 1 recommended by eminent engineers for application in various cases that arise in practice.

Authority.

Nature of Structure.

Nature of Load.

Factor of Safety.

Cast Iron.

B.

Girders ....

Dead.

3-6

S.

Do......

Do.

6

S.

Pillars.....

Do.

6

s.

Water tanks ....

Do.

4

s.

Crane posts or machinery

Live.

8

s.

Pillars subject to vibration

Do.

8

s.

Do. do. transverse shock

Do.

10

Wrought Iron.

S. R.

Girders ....

Dead.

3

S.

Do......

Live.

6

B.

Bridges ....

Mixed.

4 in tension.

U.

Roofs .....

Do.

4

s.

Compression bars subject to shocks

Live.

6

s.

Compression bars not subject to shocks

Dead.

4

Steel.

C.

Bridges ....

Mixed.

4

B. Board of Trade. S. Stoney.

U. Unwin. C. Commissioners.

1 See page 449.

The working stresses are obtained by dividing the known breaking strength of the particular class of material to be used, by the factor of safety applicable to the structure and load for which it is to be used.

The breaking strength is found by experiment, or taken from tables giving the results of experiments on iron or steel of a similar class (see pp. 314 to 325).

The factor of safety is varied according to judgment and experience, or, in the absence of these, may be taken from the Table above.

It is necessary here to state that the working stress should in no case exceed the elastic limit of the material. The reasons for this are given at p. 329.

It will be seen, however, that the elastic limit is generally about 1/2 of the ultimate strength, whereas the working stress is seldom more than 1/4 of the same, so that if the factors of safety are carefully applied there is no danger of passing the elastic limit of any ordinary material.

In the absence of experimental knowledge with regard to the particular material about to be used, the engineer takes care to calculate for a low working stress, so that he may be sure not to overtax the strength of the material.