1 Proceedings Institute of Civil Engineers, vol. xlix. 2 Pole. 3 From Experiments of Committee of Civil Engineers.
Such loads are seldom met with in buildings, except perhaps in the effect of wind upon roofs; but they are of frequent occurrence in railway bridges and other engineering structures.
With regard to the effect of live loads, it will be sufficient to say that such loads have a greater effect than if they were gradually applied as dead loads.
In practice, the effect of a live load is generally taken as equal to twice that of the same load considered as dead.
Live and moving loads frequently produce stresses (upon any member of a structure) which vary considerably in intensity from time to time - e.g. a bar in a bridge may be subject to a stress of 3 tons per inch of section when a light train is passing, and 5 tons per inch when a heavy train is passing.
It has been shown by Wohler that in either case the intensity of stress that the bar can bear is much lower than what it can bear when the stress is of the same kind (either tension or compression) throughout, and also of the same intensity.
To put it in another way, the stresses produced are much more trying to the bar than a stress which is unvarying in kind (being either compression constantly or tension constantly), and which is also unvarying in amount.
It has already been pointed out that repeated loads do not tend to cause rupture so long as they are kept below the limit of elasticity of the material.
The effect of such loads, or of vibration, has been commonly supposed to be dangerous, and eventually to cause fracture by changing the internal structure of the iron from a fibrous to a crystalline structure. There is still considerable difference of opinion on the subject.
Dr. Percy, who has carefully considered the cases bearing upon this question, says : -
"The question will naturally suggest itself whether gentle vibration - the result of very frequently repeated light blows, or of vibration without impact, caused by jarring grinding action - as in an axle working in badly lubricated bearings, or of straining and torsion in shafts, etc., very much less intense than would be produced by heavy hammering - would tend to incline permanent disaggregation of the crystals of iron, and consequent tenderness. . . .
"Opinions are divided upon it, and I am not acquainted with any precise experimental data to justify any very positive conclusion on the subject. . . .
"Another point remains to be considered, namely, whether vibration, caused by impact or otherwise, may induce a crystalline arrangement which did not previously exist, or was only imperfectly developed. I have not met with any evidence to justify an answer in the affirmative." 1
The effect of extreme cold upon the strength of iron and steel is another open question.
It has already been pointed out (see p. 270) that in some castings, the bulkier parts, being the last to cool, are left in a state of tension.
Now, if such castings are exposed to cold, the parts already in a state of tension may endeavour to contract still farther, and rupture may ensue.
With regard to the effect of cold upon wrought iron and steel many experiments have been made, but they afford up to the present time very conflicting data.
The discrepancies between the results obtained seem to have been caused in some measure by differences in the composition of the materials experimented upon, the presence of phosphorus especially having a marked influence.
1 Percy's Metallurgy.
Iron tyres, chairs, and other parts of a railway which are made of iron or steel, break more frequently during frosty weather than at other times. This however, has been accounted for by pointing out that the hardness and rigidity of the ground during such weather causes the shocks to have much greater effect upon the permanent way.
As a rule practical men incline to the opinion that frost and extreme cold have a weakening effect upon iron and steel, and render them specially liable to be broken by a sudden shock or concussion.
Thus it is the custom to pass the chains used for lifting heavy weights through the fire on frosty days; and there is no doubt that while the question is unsettled it is safe to take some precaution of this kind.