The best Yorkshire ironmakers roll plates for boilers 20 per cent. larger in area than given in the above table.

The following sizes have been selected from the Dalzell Steel and Iron Works list :

Extras are charged for thicknesses of iron and steel plates under inch, and for lengths over 25 feet; for widths over 4 feet 6 inches in iron and over 6 feet in steel; for weights over 10 cwt. in iron and 20 cwt. in steel; and for areas over 60 square feet.

Land T bars are rolled in a great variety of sections. For ordinary sections the lengths may be 30 feet without extra charge; generally the length and depth united should not exceed 9 inches, or extras will be charged. Dalzell Steel Company, however, allow 11 inches for angles and 10 for T's without extras.

The following are probably the largest sections obtainable in iron and steel angles : 10′′ × 33" from " to 3" in thickness. In steel, 10" x 4" and 11" x 4" from " to 3" may be obtained. T's may be obtained in steel 10" in the stalk by " thick, and 7" in the table by 3" thick. Channel bars are rolled to a maximum size of 15" x 4" x 1", and rolled girders to a maximum size of 20" x 8", at mean weight of 100 lb. per foot, but extras are charged when the sum of the depth and width exceeds 16 inches.

Flat bars are rolled up to a maximum width of 12 inches, and may be obtained from of an inch in breadth and of an inch in thickness, from 1" x 1" to 6" x 1" without extra charge.

Round and square iron and steel bars may be obtained from g" to about 3" without extra charge; the maximum sizes are 10" diameter or 10" square. The relative cost of the various rolled

Sizes of Plates and Bars-Joints and Connections.

201

sections may be approximately expressed, for the purpose of estimating the relative cost of varicus designs, as follows:

Economy may be effected by making use of angle and flat bars in preference to plates wherever they can be used without a corresponding disadvantage, and by reducing the riveting as

desirable to incur the extra charges for longer bars and plates, rather than to increase the number of joints, as the total cost is reduced.

Unequal-sided angles should be avoided unless used in large quantities, and generally the fewer the sections used in a particular work the better. It is most desirable to reduce the amount of smith's work, such as cranking, bending, joggling, as much as possible, as the iron is injured in the process and the cost of the work increased. Welding should never be resorted to in girder-work if it can possibly be avoided.

Bolts, Nuts, Union Screws.-Bolts and nuts, suspension and tie-rods, are specified by English engineers to have Whitworth threads, but American engineers generally use the Seller thread (see Figs. 250, 251, and 252).

Whitworth threads are rounded at the tops and bottoms,

while Seller's threads are flat at top and bottom, and consequently they can be cut with one tool.

Strength of Bolts and Screwed Rods.-Let P denote the tensile stress on the rod or bolt; f, the safe intensity of working stress; a, the area at the bottom of the thread given in the following table for Whitworth threads; then

f should not be taken more than 4 tons per square inch for a steady load, since the stress is not uniformly distributed in consequence of the abrupt change of section from the full area of the bolt to the area at the bottom of the thread.

Unwin's "Machine Design," part i. p. 145.

In bridge-work the safe working stress must be determined from the range of stress, as explained in Chapter I. If the ratio of the minimum to the maximum stress is 1, then ƒ may be taken at 3 tons per square inch. When the bolt is screwed up torsional stress will be developed, which, if the maximum load producing tension is also on the bolt, will necessitate a further reduction in the working stress. Professor Unwin has shown1 that this stress requires the diameter of the bolt to be increased about 15 per cent., and he gives the following formulæ for machine bolts where the range of stress varies from zero to a maximum :

P=2400d2 to 3000d2

where Pthe total load in pounds, and d = the full diameter of the bolt in inches.

In suspension bolts for timber truss bridges, and in cable suspension bridges, also in the tension rods in iron roofs and wind-bracing, the rods should be upset before the thread is cut, so that the diameter at the bottom of the thread is never less than the full diameter of the bolt. Under these circumstances the effect of unequal distribution of stress may be neglected. The torsion may also be neglected, since it is not likely to be The screwing up, applied when the full load is on the rod.

however, will in most cases put initial tension upon the rod, the exact amount of which is difficult to determine. In roofs and wind-bracing this initial stress may be neglected in fixing the intensity of working stress, since the maximum load is rarely applied. It will generally

have to be considered in addition to the range of stress due to dead and live loads.

Fig. 253 shows a common bolt and nut; Fig. 254, a rag bolt which is sometimes used to secure foundationplates to the masonry or concrete bed upon which it rests. A taper hole is cut

in the masonry, and the tail of the bolt is square, with jagged ends; the space between the bolt and the hole is filled with molten lead.

Unwin's "Machine Design," part i. p. 150.

Union Screw Coupling.-Figs. 255 and 256 represent two union-screw couplings; the threads cut upon the upset ends of the rods are right and left handed, so that when the coupling is turned the rods are caused to approach each other until they are tight. In Fig. 255 the coupling is turned by inserting a bar in the centre hole, and in Fig. 256 the middle of the coupling is forged hexagonal, so that it may be turned by means of a

FIG. 255.

b

2/2
C

FIG. 256.

screw-key. The outside diameter of the screwed portion is equal to the diameter of the rod + 2 x depth of thread, which in Whitworth threads is of the pitch; hence, if d1 = the outside diameter, and d = the diameter of the rod

d=d+ pitch

Table 44 will give us at once the outside diameter if we look out the diameter which has the diameter at the bottom of the thread equal to, or not less than, the diameter of the rod; thus if the rod is 1 inch in diameter, we find that 14 will give 1.067 at the bottom of the thread.

The depth of the thread in the coupling should be made equal to the outside diameter of the thread, to ensure that the threads will not strip, and the total length of the coupling will be twice the outside diameter of the thread plus the portion, C, left for adjustment. There only remains, therefore, the outside diameter of the coupling, a, to be determined, so that the sectional area through the hole in Fig. 255 may be equal to that of the rod. Let d2 the inside diameter of the coupling, d the outside diameter; let the diameter of the hole be of an inch; then d2 will equal the outside diameter of the screwed

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