BEAMS.

377

Fig. 615.

CONNECTION OF CHOSS-DE&UU WITHGIRDERS—SCALE, J INCH TO THE FOOT.

bolt-holes are made in these, by which the cross-beam d is bolted to the flange.By this method the whole of the cross-beams and girders are united together,and the “ girder will be prevented from ‘ canting,’ as the strain has a tendencyto bring the top flange inwards, and to force the top flange outwards, thus pro-ducing a lateral strain upon the girder, which would in a great measure beresisted by the bolts in the cross-beams. The best security for girders loadedin this way will, however, be good broad flanges, cast upon the top and bottomsides, in order to resist the lateral thrust of the cross-beams.”

1390. When the cross-beam or binding-joist is of cast-iron, and of the samesection as the girder, Mr Fairbaim recommends an arrangement to be adopted,illustrated in fig. 615, a narrow shelf a being cast on the side of the flange,from one end of the girder to the other (not merely,it is to be noted, at the places where the cross-beamsoccur) ; “ this would give a sufficient bearing forthe cross-beam by forming the ends, as shown in thefigure at d ; and the bolt-holes c should perforatethe vortical rib as near as possible to the neutralaxis of the girder.” The cross-beam is shown at b.

1391. With reference to this plan of perforatingthe girder, and also of making perforations for pass-ing through the tie-bolts, often used in trussingcast-iron girders, Mr Fairbaim says, that in “ cast-iron girders,” even where the hole, or rather themetal immediately surrounding the aperture, isgreatly strengthened, “ such a process, if not fatal,would be, to say the least of it, exceedingly in-jurious. I have,” he continues, “ decided objections to anything like perfora-tions in cast-iron girders; and it is even with some reluctance I would havea bolt-hole through the neutral axis, unless thickened so as to compensate forthe part taken out; besides, it is exceedingly objectionable to cut off theconnection between the two resisting flanges of a girder, or to damage in anyother way a casting of this description. There is nothing I should be moretenacious about than the cutting or boring of any part of a well-proportionedgirder ; and I believe there is nothing so dangerous in the hands of personsunacquainted with the laws which govern the strength of these importantstructures.'”

1392. Mr Fairbaim remedies the evil arising from the use of perforatedgirders by the plan illustrated in fig. 616. It is of essen-tial importance that the bottom flange shall not be per-forated, the cross-beam being supported only by hook-bolts. The other method is to lay the cross-beams atonce upon the girders.

1393. Where tie-rods are used to truss the girders andprevent their lateral movement, it is important that theyshould be placed in the proper position. This practicallyis at the soffit of the arch, in which position the tie-rodswall perforate the “ neutral axis ” of the beam. Themaximum point of tension of the tie-rods is at the bottomof the flange; but this, although the safest, is not themost convenient position in practice, as the tie-rod wouldthen stretch across the chord of the arc. Tie-rods should never be placedabove the arch near the top of the girder. For cases where the weight to

SUSPENSION OF CROSS-BEAMS

PROM GIRDERS-SCALE,

£ INCH TO THE FOOT.