by turning or by sliding. Safety against displacement by turning
is called stability of position; safety against displacement by sliding,
stability of friction.

§ 13. Condition of Stability of Position.-If the materials of a structure were infinitely stiff and strong, stability of position at any joint would be insured simply by making the centre of resistance fall within the joint under all possible variations of load. In order to allow for the finite stiffness and strength of materials, the least distance of the centre of resistance inward from the nearest ed e of the joint is made to bear a definite proportion to the depth 0% the joint measured in the same direction, which proportion is fixed, sometimes empirically, sometimes by theoretical deduction from the laws of the strength of materials. That least distance is called by Moseley the modulus of stability. The following are some of the ratios of the modulus of stability to the depth of the joint which occur in practice:-

Retaining walls, as designed by British engineers . I: 8 Retaining walls, as designed by French engineers I . 5 Rectangular piers of bridges and other buildings, and arch-stones ............ I . 3

Rectangular foundations, firm ground . .....v 1 ~ 3 Rectangular foundations, very soft ground .... I: 2 Rectangular foundations, intermediate kinds of ground 1: 3 to I ~-2 Thin, hollow towers (such as furnace chimneys exposed to high winds), square ......... 1, 6

Thin, hollow towers, circular ........ I . 4 Frames of timber or metal, under their ordinary or average distribution of load ...... I: 3

Frames of timber or metal, under the greatest irregularities of load ..............

1 3

ln the case of the towers, the depth of the joint is to be understood to mean the diameter of the tower.

§ 14. Condition of Stability of Friction.-If the resistance to be N exerted at a joint is always perpendicular to the surfaces which abut at and form

that joint, there is'no tendency of the

pieces to be displaced by sliding. If the resistance be oblique, let IK (fig. 89) be the joint, C its centre of resistance, CR a line representing the resistance, CN a

perpendicular to the joint at the centre of resistance. The angle N CR is the obliquity of the resistance. From R draw RP

parallel and RQ perpendicular to the

joint; then, by the principles of statics, the component of the resistance normal

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FIG. 89.

to the joint is-CP=CR

cos PCR;

and the component tangential to the joint is-CQ=CR.sin PCR=CP.tan PCR.

If the joint be provided either with projections and recesses, such as mortises and tenons, or with fastenings, such as pins or bolts, so as to resist displacement by sliding, the question of the utmost amount of the tangential resistance CQ which it is capable of exerting depends on the strength of such projections, recesses, or fastenings; and belongs to the subject of strength, and not to that of stability. In other cases the safety of the joint-against displacement by sliding depends on its power of exerting friction, and that power depends on the law, known by experiment, that the friction between two surfaces bears a constant ratio, depending on the nature of the surfaces, to the force by which they are pressed together. In order that the surfaces which abut at the joint ]K may be pressed together, the resistance required by the conditions of equilibrium CR, must be a thrust and not a pull; and in that case the force by which the surfaces are pressed together is equal and opposite to the normal component CP of the resistance. The condition of stability of friction is that the tangential component CQ of the resistance required shall not exceed the friction due to the, normal component; that is, that CQ 3l> f . CP,

where f denotes the coejicient of friction for the surfaces in question. The angle whose tangent is the coefficient of friction is called the angle of repose, and is expressed symbolically by4>=tan'1f. Now CQ =CP.tan PCR;

consequently the condition of stability of friction is fulfilled .if the angle PCR is not greater than ¢>; that is to say, if the obliquity of the resistance required at the joint does not exceed the angle of repose; and this condition ought to be fulfilled under all possible variations of the load.

lltés chiefly in masonry and earthwork that stability of friction is re ie on.

§ 15. Stability of Friction in Earth.-The grains of a mass of loose earth are to be regarded as so many separate pieces abutting against each other at joints in all possible positions, and depending for their stability on friction. To determine whether a mass of earth is stable at a given point, conceive that point to be traversed by planes in all possible positions, and determine which position gives the greatest obliquity to the total pressure exerted between the portions of the mass which abut against each other at the plane. The condition of stability is that this obliquity shall not exceed the angle of repose of the earth. The consequences of this principle are developed in a paper, “ On the Stability of Loose Earth, ” already cited in § 2.

§ 16. Parallel Projections of Figures.-If any figure be referred to a system of co-ordinates, rectangular or oblique, and if a second figure be constructed by means of a second system of co-ordinates, rectangular or oblique, and either agreeing with or differing from the first system in rectangularity or obliquity, but so related to the co-ordinates of the first figure that for each point in the first figure there shall be a corresponding point in the second figure, the lengths of whose co-ordinates shall bear respectively to the three corresponding co-ordinates of the corresponding point in the first Hgure three ratios which are the same for every pair of corresponding points in the two figures, these corresponding figures are called parallel projections of each other. The properties of parallel projections of most importance to the subject of the present article are the following:-

(1) A parallel projection of a straight line is a straight line. (2) A parallel projection of a plane is a plane. (3) A parallel projection of a straight line or a plane surface divided in a given ratio is a straight line or a plane surface divided in the same ratio.

(4) A parallel projection of a pair of equal and parallel straight lines, or plain surfaces, is a pair of equal and parallel straight lines, or plane surfaces; whence it follows

(5) That a parallel projection of a parallelogram is a parallelogram, and

(6)dThat a parallel projection of a parallelepiped is a parallelepiped

(7) A parallel projection of a pair of solids having a given ratio is a pair of solids having the same ratio. Though not essential for the purposes of the present article, the following consequence will serve to illustrate the 'principle of parallel projections 1- ', »

(8) A parallel projection of a curve, or of a surface of a given algebraical order, is a curve or a surface of the same order. For example, all ellipsoids referred to co-ordinates parallel to any three conjugate diameters are parallel projections of each other and of a sphere referred to rectangular co-ordinates. § 17. Parallel Projections of Systems of Forces.-If a balanced system of forces be represented by a system of lines, then will every parallel projection of that system of lines represent a balanced system of forces.

For the condition of equilibrium of forces not parallel is that they shall be represented in direction and magnitude by the sides and diagonals of certain parallelograms, and of parallel forces that they shall divide certain straight lines in certain ratios; and the parallel projection of a parallelogram is a parallelogram, 'and that of a straight line divided in a given ratio is a straight line divided in the same ratio.

The resultant of a parallel projection of any system of forces is the projection of their resultant; and the centre of gravity of a parallel projection of a solid is the projection of the centre of gravity of the first solid.

§ 18. Principle of the Transformation of Structures.—Here we have the following theorem: If a structure of a given figure have stability of position .lnder a system of forces represented by a given system of lines, then will any structure whose figure is a parallel projection of that of the first structure have stability of position under a system of forces represented by the corresponding projection of the first system of lines.

For in the second structure the weights, external pressures, and resistances will balance each other as in the first structure; the weights of the pieces and all other parallel systems of forces will have the same ratios as in the first structure; and the several centres of resistance will divide the depths of the joints in the same proportions as in the first structure.

If the first structure have stability of friction, the second structure will have stability of friction also, so long as the effect of the projection is not to increase the obliquity of the resistance at any joint beyond the angle of repose.

The lines representing the forces in the second figure show their relative directions and magnitudes. To find their absolute directions and magnitudes, a vertical line is to be drawn in the first figure, of such a length as to represent the weight of a particular portion of the structure. Then will the projection of that line in the projected figure indicate the vertical direction, and represent the weight of the part of the second structure corresponding to the before-mentioned portion of the first structure.

The foregoing “ principle of the transformation of structures " was first announced, though in a somewhat less comprehensive form, to the Royal Society on the 6th of March 1856. It is useful in practice, by enabling the engineer easily to deduce the conditions of equilibrium and stability of structures of complex and unsymmetrical figures from those of structures of simple and symmetrical figures. By its aid, for example, the whole of the properties of