Steel Plates are generally riveted, with the best quality of steel rivets. slightly smaller in diameter, and closer in pitch, than for wrought-iron plates. Steel rivets should be made from bars of a very mild quality, otherwise the heads are liable to fly off from jars. Plates both hot and cold, are tested on a true surface-plate, the radius of the corner over which they are bent being inch, the distance from the edge of the plate to the part bent, is from 3 to 6 inches. When plates are tested hot, they are heated to an orange colour; the plates are bent down to the required angle by hammering.

Tests for Rivets.-They should be made from bars of the toughest quality, and admit of being bent double, without fracture, when cold. The heads should admit of being hammered down to inch in thickness without fracturing the edges, when hot.

Test for Wrought-Iron Bridge-Plates.-A piece of plate is cut 2 inches wide and inch thick, of sufficient length to have 7 inches under tension, the plates being rejected if the extension of the test-piece is greater than inch under a test of 18 tons, inch under 21 tons, inch under 23 tons, inch under 24 tons. All bar iron to stand a tensile strain of 25 tons, per square inch of section, before fracture.

Diminution of Tenacity of Iron Boiler-plates at high temperatures, the mean maximum tenacity being at 550° F. 65,000 lbs. per square inch. From the experiments of the Franklin Institute.

Effects of Re-heating and Rolling Iron, from the experiments of Mr. Clay.

Puddled Bar. Tenacity in lbs. per square inch

43.904

The same iron, 5 times piled, re-heated, and rolled. Tenacity in 61,824

lbs. per square inch

The same iron, 11 times piled, re-heated, and rolled. Tenacity 43,904

in lbs. per square inch.

Steel Plates and Bars used in place of wrought iron, to be of equal strength, may in a general way be made 20 per cent. thinner than wrought iron plates and bars.

ROPES AND CHAINS.

The Breaking Strain of hemp-ropes, is I ton, for each lb. weight per fathom.

The breaking strain of iron-wire ropes is 2 tons, for each lb. weight per fathom.

The breaking strain of steel-wire ropes is 3 tons, for each lb. weight per fathom.

Table 91.-SIZE, WEIGHT AND Strength of STEEL- AND IRON-WIRE ROPES AND HEMP-ROPES.

II

3g

3 g

3

39

3

18

16

14

13

I

4 4 4 3 3 3 3 3 3 2 2 2 2 2 2 2 2 ----

110101001100

98 776 5 4 4 4 333

:

2

12

32

II

888

30

28

Hemp-Ropes.-Tarred ropes are weaker than white ropes, hot-spun

tarred ropes are stronger than cold-spun, but are not so pliable.

Wet-Ropes. When a rope is wet, it expands in diameter, and contracts in length, owing to the fibres being drawn in by this increase of diameter. Hemp-Fibres are about a yard in length, the tensile strength of hempfibres is 6,400 lbs. per square inch of sectional area.

Table 92.-SIZE, WEIGHT, AND STRENGTH OF STEEL, IRON AND HEMP FLAT ROPES.

Table 93.-WEIGHT, WORKING LOAD, PROOF STRAIN AND BREAKING

Standard Proportions of the links of chains in terms of the diameter of the iron from which they are made:

Stud-link = 6 diameters extreme length, and 3'6 diameters extreme width.

Close-link 5 diameters extreme length, and 35 diameters extreme width.

Open-link = 6 diameters extreme length, and 3.5 diameters extreme width.

Middle-link = 5.5 diameters extreme length, and 3.5 diameters extreme

width.

End-link of each, 15 fathom length of chain, 6.5 diameters extreme length, and 4'1 diameters extreme width.

Strength of Chains and Ropes.-To find the breaking strain in tons of short-link chains, square the number of eighths of an inch in the diameter of the iron from which the link is made, and multiply by 375.

To find the breaking strain in tons of stud-link chains, square the number of eighths of an inch in the diameter of the iron from which the link is made, and multiply by '44.

To find the breaking strain in tons of ropes of hemp, and of iron and steel wire :

For hemp ropes, square the circumference in inches and multiply by 25. For iron-wire ropes, square the circumference in inches and multiply by

5.

For steel-wire ropes, square the circumference in inches and multiply by

2.5.

The working or safe load for ropes is from one-sixth to one-seventh of the breaking strain, for round hemp ropes, and for round iron-wire ropes: one-eighth for flat hemp and for flat iron-wire ropes: one-sixth for round steel wire and one-seventh for flat steel-wire ropes.

GIRDERS.

Girders and Beams.-To find the breaking weight in tons, of solid beams of wood or iron, square or rectangular, with both ends supported, and loaded in the middle:-Rule: Multiply the square of the depth in inches by the breadth in inches, and divide the product by the length in feet between the supports; the result will be the breaking weight in tons of a cast-iron beam. For wrought-iron, multiply the said result by 1'5; for oak, multiply by 25; and for pine or fir, multiply by 2.

Wood Girders with wrought-iron flitch-plates.-To find the breaking weights in cwts., when loaded in the middle, with both ends supported.— Rule for fir: Multiply five times the square of the depth in inches, by the breadth in inches, including the iron flitch plate, and divide the product

by the length in feet. For oak, use 6 as a multiplier instead of 5. The thickness of the iron flitch-plate should be one-tenth that of the wood, for which thickness the above rule applies.

Solid-rolled wrought-iron joists and girders, Fig. 148. To find the breaking weight in tons when loaded in the middle, with both ends supported. Rule: Add one-fourth the area of the web in inches, calculated on the full depth of joist, to the area of the bottom flange in inches;

I

Fig. 148.-Rolled-Joist.

multiply that sum by the depth in inches; multiply the product by 6.6, and divide the result by the length in feet between the supports.

Box-girders are about 10 per cent. less in strength than solid rolled joists or girders, of equal depth and weight.

Single-web girders are about 20 per cent. less in strength than solid rolled joists or girders, of equal depth and weight.

T girders are about 40 per cent. less in strength than solid rolled joists or girders, of equal depth and weight.

Riveted joists are about 50 per cent. less in strength than solid rolled joists or girders, of equal depth and weight.

The Deflection of solid-rolled joists is about 50 per cent. less than that of riveted joists, of equal depth and weight.

A girder fixed at one end only, and loaded at the other end, will support only one-fourth the load that a girder of the same length will bear, when supported at both ends and loaded in the middle.

A girder will support only one-half the load at the middle, that it will if distributed over its length.

Factor of safety for girders. The safe dead load for wrought-iron girders is generally the breaking weight, and for cast-iron girders the breaking weight; for moving loads the factor of safety should be double that used for dead loads.

Solid Round Beams and girders. To find the breaking weight, in tons, of a solid round beam, with both ends supported, and loaded in the middle-Cube the diameter in inches, and divide by the length in feet between the supports; the result will be the breaking weight in tons of a wrought iron round beam; for cast-iron multiply the said result by '66; for oak multiply by 17; for fir or pine multiply by 13.