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necessary members are omitted, and hence comparatively few counter-ties are used. In the Fig. only two are shown-one each side of the centre. The number of counter-ties depends upon the relation of the moving load to that of the weight of the bridge (see articles 107 and 108 of Wood's Treatise on Bridges and Roofs).

The lower chord is sometimes made of links of iron (Fig. 168), which pass over cast-iron blocks under the vertical

Fig 168.-One of the links of the lower chord.

posts (Fig. 169). The lower chord may be, and at the pres

Fig 169.-A joint in the lower chord of a Whipple Truss.

ent day often is, made of eye-bars (Fig. 170). The proper

Fig. 170.-One end of an eye-bar used in tension members of bridges and roofs.

form and dimensions of the eyes and the proper size of the pins has been the subject of considerable experiment.

At first it was supposed that the total section on both sides of the eye should equal half the section of the pin, but experiments quickly showed that when made in this proportion the eyes would tear out before the shearing strength of the pin was reached. According to some experiments made by Sir Charles Fox, he concluded that it was best to make the bearing surface between the pin and concave surface of the eye about equal to the least section of the link; or, in other words, the diameter of the pin should equal about two-thirds of the diameter of the link.

This rule, however, is not rigidly adhered to by our most eminent bridge builders. Each has a rule of his own. Some make the eye thicker than the link; others make them somewhat pear-shaped by adding material back of the eye (Fig. 171); while still others make them of the form shown in Fig.

172.

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But in all cases the total section of the material through the eye is made to exceed that through the bar, and the section of the pin also exceeds that of the bar.

Fig. 172-Another form of eye-bar.

634. Modifications of Whipple's Truss. Different bridge builders have modified the details of Whipple's Truss, so as to suit their convenience or fancy, or to make them conform with modern practice. It is useless to attempt to give all these modifications. They have, however, given rise to certain names of bridges, such as the Murphy-Whipple bridge, Linville bridge, Jones's bridge, etc., etc.

635. Linville Bridge. This bridge, the details of which (Figs. 173 and 174) have been very thoroughly and carefully worked out, has a wide reputation.

The improvements consist in employing tubular forms of wrought iron for members used to resist compressive strains, and weldless eye-bars to resist tensile strains, by this means economizing material and reducing the dead weight of the structures. In the accompanying details of the chords, struts, and ties, and the floor system and lateral connections, some of the leading principles of the Linville truss are illustrated.

The upper chords A, are composed of channel ([) bars and I beams, to which are riveted top plates, and sometimes bottom plates, forming a tubular compressive member of great strength. When the lower plate is used, elliptical holes are cut out in order to admit of painting the interior. The chords are generally made in sections, one panel in length. The connection between the suspension ties and upper chords are effected by means of angle blocks, through which pass the suspension ties, with enlarged screw threads and nuts for adjustment, or by means of pins passing through the chords, and through loops or eyes on the suspension ties.

The struts B are circular or polygonal tubes (Fig. 174a), composed of two or more rolled bars united by rivets through flanges, or by transverse tie-bolts passing through the struts between the flanges. The struts are generally swelled and opened to allow the interior to be repainted in order to prevent their rapid destruction by oxydation.

The lower chords are made by upsetting the enlarged eye ends, by compressing them when highly heated into moulds or dies. They are afterwards forged and rewelded under a

hammer.

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Figs. 173, 174-Details of Linville's truss. Fig. 173 is a cross section, and Fig. 174 a right section of a portion of the truss

AA, upper chord, composed of channel bars ([) and I sections. B, the post. (See Fig. 174 A.) CC, the lower chord. D D, the lower end of a main tie; and H H, the upper end of & main tie.

E is a counter-tie.

II, suspenders for supporting cross-ties,
K, horizontal diagonal tie.

G G, bases of the posts or struts.
J, cross horizontal diagonal tie.

B

Fig. 174 A-Cross section of one of the forms of post used in a Linville truss.

These weldless chords and tubular posts have, in many cases, superseded older forms. The lower chords CC disposed at each side of the suspension ties D, and counter-tie E, and between ribs in the bases G of the posts or struts, are effectually combined with the struts and ties by means of a connecting-pin. The tendency to bend the connecting-pin is obviated by this distribution of the strains.

The pin can fail only by shearing.

From the connecting-pins depend loops or suspenders, II, which support the rolled cross-girders F, that sustain the trackstringers and track. The upper lateral struts of wrought or cast iron are secured at the connecting-pins, the ties being attached to an eye-plate, or in a jaw-nut secured to the connecting-pins.

The lateral ties J are adjusted by means of sleeve-nuts with right and left hand-screws.

The lower laterals K K are attached to the cross girders, and adjusted in a similar manner.

The bases and capitals of the posts are made either of wrought or cast iron.

To secure greater efficiency in the struts by dispensing with the round bearing, and at the same time retain the pin

Fig. 175-An Arched Truss after the general plan of Whipple's. The lower chords or tierods pass through the ends of the arch, and are secured by nuts on the ends of the rods.

connection between the chords and ties, the lower chords are brought compactly together between and outside of the suspension-ties and suspenders, and a bearing provided on the upper edges of the chords for the lower ends of the posts. The upper ends also have a flat bearing.

636. Arched Truss. Fig. 175 shows the general form of a Whipple Arched Truss. The upper chord is composed of hollow tubes, made in sections of about a panel length. 637. Bollman's Truss. The general outline of Bollman's Truss is shown in Fig. 176.

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Fig 176-Bollman's Truss. A D, D E, etc., are sections of the uppper chord-cast iron and usually hollow. D C, E F, etc., are hollow cast-iron posts. A C, C B; A F, F B, etc., are tension rods; D F, C E, etc., are pane! rods,

One of the leading features of this bridge is, the load at each post or joint is carried directly to the supports at the ends by means of a pair of tension (or suspension) rods. Thus a load at E is supported by the post E F, and is thence supported by the rods AF and F B. The panel rods D F, EC, E G, etc., serve to keep the upper chord in place, and in case of an undue strain upon, or failure of, one of the long suspension-rods, may transmit the strains to the other members of the truss.

The suspension rods being of unequal length will be unequally elongated or contracted by the same strain, or by changes in the temperature. In order to prevent severe cross strains upon the posts due to these causes, the suspensionrods are connected to the lower ends of the posts by means of a link which is a few inches in length, and which permits of a small lateral movement at the ends of the rods without any corresponding movement of the posts. The suspensionrods are made of flat iron, and pass through the ends of the upper chord where they are secured by means of pins which pass through the ends of the chords.

If the roadway passes above the upper chord, it is called a deck bridge, and the lower chord may be dispensed with.

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