It is only proposed, in this chapter (which is a preliminary to Chapter XXV (Calculation Of Strains. Computing Strains)., where the subject is more fully discussed), to deal with the above subject as far as the syllabus of the advanced course requires the student to go in that stage; and, as a necessary preliminary, an explanation will first be given of the general terms in use.
Different kinds of loads cause different stresses on structures, producing different strains, which the principle of equilibrium demands shall be met by reactions of corresponding power, to resist those actions successfully. The stress is the power of the load.
Thus, one man may push another against a wall with such force (equivalent to a load) as to cause a compressive stress, acting in a manner likely to crush him, unless he can call forth a reaction on his own part to resist the strain which that stress or action of pushing puts on his strength, and to prevent his being crushed against the wall.
Or, a team of men in a tug of war, by pulling, cause a tensile stress, which produces a strain on the other team, who must have the power to resist the strain imposed on them by such stress, or else they will be drawn over the line.
Or, again, the blades of a pair of scissors meeting, with a piece of material between them, cause a shearing stress, or action tending to shear or cut that material in two, unless it has the nature or power to set in a reaction, tending to resist or counterbalance the strain put upon it by such action or stress.
Having shown the student the application of the general terms, an explanation of each one individually, with the various forms it takes, will be given in detail.
A load is the weight or power of the external forces acting on a certain structure, together with the weight of the structure itself; and it may be either a live or a dead load, the whole of which is either concentrated at any one point, or at any number of points on the structure, or it is distributed equally over the whole, or any part, of the same.
A live load is a moving force applied suddenly and intermittently, accompanied by shocks and vibrations; such as a train going over a bridge, a sudden gust of wind on a roof, or a weight falling suddenly on a beam or floor; while A dead load consists of a force or weight of a continuous nature, applied gradually and remaining; such as solid independent and unsup-porting walls on breastsummers, materials stored on warehouse floors, etc.
It has been proved that live loads produce twice as great strains as dead loads of equal weight or force; therefore structures will carry twice as much dead load as live load.
A breaking load is that which is so great as to produce fracture to the structure, while a safe or working load is that which a structure will carry with safety and without any apparent risk. A safe or working load, causing a stress, is generally one-fifth of that of the breaking load. The ratio between the two, as one and five in this instance, is called the Factor of safety.
Stresses are the actions or forces which cause the strain, and are of six different kinds, including: -
"Compressive," "tensile," "shearing," "transverse," "bearing," and "torsional" stresses, the latter of which will not be dealt with, as it is not met with in buildings.
Compressive stress is that which crushes a thing, causing a strain of compression, especially present in columns and posts of buildings, while the struts in roof trusses are generally subject to it.
Tensile stress has a tendency to pull out or tear a material asunder, and is met with in all ties and suspenders, in roof trusses, and other framed structures. A simple method to distinguish between compressive and tensile stresses is to consider whether a piece of string or rope, of a flexible nature, could be substituted for the member without detriment to the structure, neglecting, of course, the amount of the stress. If rope or string would answer the same purpose, it may be regarded as certain that the member is in tension; and if not, it may be taken for granted that, in ordinary roof-structures, the member is in compression.
Shearing is that stress which shears or cuts the body, and is especially applicable to riveted joints, where one plate slips along the other, cutting, or shearing in two the rivets which connect them. The web of ordinary girders is subject to shearing, inasmuch as the top flange being in compression and the bottom in tension, or vice versd, the two stresses have a tendency to draw together like the two blades of a pair of scissors Transverse stresses are met with in the bearing members of a structure, such as lintels, joists, rafters, and breastsummers, on which the stress has a tendency to bend the member and break it across, if it has not sufficient power in itself to resist the strain put upon it by the action or stress caused by the load.
Bearing stress may be said to be the opposite of shearing, as the action on the plates, caused by the successful resistance of the rivets to being cut in two, results in the rivets cutting into the plates compressing or stretching them (when of insufficient strength), so that the rivet holes are enlarged.
Shearing affects rivets, and bearing the plates, in riveted joints.