Strains To Which, Various Parts Of Building Structures Are Subjected -Modes Of Estimating Pressures Or Strains On Do Dimensions of various parts of Building Structures.
The subjects briefly detailed in the heading to this chapter are of the greatest importance to those engaged in the practical work of building construction, and the more complete the knowledge of the principles upon which the practice is founded, the more accurate will be the work which they design, and the greater the reliance which may be placed upon it, as regards its capability to resist the weights or pressures to which its various parts may be subjected; in other words, the stability and the economy with which it may be erected. This latter point, although often overlooked, will be obvious on very slight consideration; for if parts are made heavier, or of larger dimensions than the necessities, so to say, of any particular point demands, then there is just so much extra material used which might have been saved; while, on the other hand, if these be made lighter or of smaller dimensions, with a view to securing the economy which alone an accurate knowledge of principles can give, then the safety of the structure is endangered, and this attempt at economising material may be carried so far that the structure cannot possibly be safe, except in the most favourable circumstances, and will, when these are unfavourable, which a variety of causes may bring about at any moment and in the most unexpected of ways, result in the entire destruction of the building, in whole or in part, thus causing heavy loss of time, labour, and material; or what may be worse, of life, a very likely occurrence, considering the uses which building structures are as a rule put to. To a large extent in daily practice, certain empirical rules or standards of dimensions, so to call them, are in use; and while these rules, in a wide variety of instances, are found to be wonderfully successful, this arises mainly from the circumstance that they yield results in excess, rarely in deficiency, of material employed; still it is obvious that, if so, it brings about the waste of it already noticed; while it may be said with equal truth that it also involves the risk of instability and consequent damage, also above noticed; and this from the variety of circumstances connected not merely with the nature of the materials employed, but from the circumstances in which they may be placed, either as regards position, construction, or one or other of the many disturbing influences to which in all cases buildings are subjected in greater or less degree. What these are the student will have learned, with more or less fulness of detail, as space permitted of, in the various chapters and paragraphs which have been given in preceding parts of the two volumes which make up the present course. These are so varied in character, and are often so dependent upon circumstances over which the ablest designer and most careful constructor may be said to have not the slightest control; as, for example, unknown defects or flaws in materials, or from the treacherous character of some, to wit, cast-iron; so that it may be said, as has indeed not seldom been so, that the constructor who trusts to the empirical rules or standards, or that vague system described so graphically by the term " rule of thumb/' is just as likely to be successful in his work, as regards its economy and safety, as he who brings to his work all the resources of the highest theoretical knowledge, as well as those derived from sound practical skill and the most careful and extended observation. But that this is not so, and that the fallacy lurking in the statement or belief is easily enough exposed, needs not to be here further gone into; it is sufficient to exemplify this by stating that the constructor trusting to the "rule of thumb" is treading upon ground, so to say, of the nature of which he really knows nothing, however much he may conjecture, as conjecture or guess he must, while he has at the same time all the elements of unknown sources of danger we have alluded to, to contend with; while he who brings to his work true principles knows what he is dealing with, and has only the chances of those unknown elements to combat, and which his superior knowledge best fits him to meet; in short, in no branch of technical work is the truth of the saying, " Knowledge is power," more strikingly employed, than in that of building construction, using this term in its widest acceptation.
The whole subject, the importance of a knowledge of which to the constructor we have thus but in the briefest manner glanced at, is one which embraces so many considerations, and the points and calculations connected with it are so diverse in character, as well as extensive in point of numbers, that to do justice to it a volume as large as the present might be written upon it without exhausting all the details. It will be obvious therefore that, within the limits of the confined space now at our disposal, we can do little more than glance at some of its leading points, and this even in the briefest fashion; and in order to the economization of space, as well as for the purposes of reference, we deem that the best way will be to give what that space admits of rather in the form of brief sentences, or what might be called notes, than in that of more formal descriptive matter or elaborate disquisitions.
The title of Strains of Materials has been chosen for the sake of brevity, although, strictly speaking, the subject involves or carries with it other departments; thus, it may be said to be divided into three great classes : " Materials used in Construction - their Characters and Peculiarities;" second, " The nature of the Strains, and the different kinds of them, to which the Materials are subjected;" and third, "Their Relative Value or Strengths, which enables them to bear certain Weights, or to resist certain Pressures or Strains to which they may be subjected." Polio wing these, however, and as their natural result, come what may be called two other classes: first, the methods in use by which the direction and value of the pressures or strains are ascertained; and, lastly, the various formulae upon which the rules used in practice, to determine the dimensions of the parts of materials designed to resist these pressures or meet these strains, with the maximum of efficiency and the minimum amount of weight of material.