TKUS8ED BEAMS, ETC. 



Qi:l 



In some parts ui Australia the beams may be ruuiid trees,, 

 grown not far i'lom the bridge site and rolled into position. The site 

 may l)e ditttcult oi' access — one to which the conveyance of a pile 

 engnie might be a costly matter; and to cnt up timber on the spot, 

 in order to construct a pile engine, would involve nearly as much 

 labour as building a completely braced girder. Even if it be urged 

 that, when selecting pieces for a braced girder, heart and sap-wood 

 may be avoided, yet, if timber is abundant, as it sometimes is, a 

 certain excess of material may be permitted in the beam to be 

 trussed, which will practically amount to ignoring the sap-wood in 

 ci.nsidering the strength of the beam, if any sap-wood is left on it. 



Want of headi'oom may, of course, preclude the use of any 

 structure in which the trussing is below the deck. If the roadway 

 be wide, as comjjared with the length of span, and if an intermediate 

 girder in the middle of the roadway is objected to, then the cross 

 beams nmst be deep. It may be that the combined depth of stringers 

 and cioss-beams, in such a ease, w(nil(l l>e quite as great as the 

 depth of suitable trussed beams, placed longitudinally, but close 

 together. The latter may be placed so close together under the 

 deck that nothing but the planking need be placed on top of them. 

 Shallowness, of course, involves severer stresses and greater weight 

 of material, both of steel and timber, .and tlie cost of this has to 

 be compared with the saving- of labour claimed for the trussed beam. 



The details, in the case of a trussed beam, aie of -in exceedingly 

 simple character. Two tension rods to one beam would be prefer- 

 al)le to one rod if the boring of the holes for the latter woidd be 

 attended with difficulty. Little need be said about the joint at the 

 end of the beam, which can easily be designed. The joints at the 

 feet of the posts need pins, or other firm attachments, to jjrevent 

 slipping, if the posts are vertical, as in Figs. 2a and 3a. Other- 

 wise, if the posts are caused to bisect the angles betAveen adjacent 

 sections of tension rods, the rods need onty pass underneath the 

 posts, being prevented from slipping sideways. If unusual vibration 

 is expected, then special precautions nmst be taken to render this 

 joint secure. Wooden posts, with tension rods under them, would 

 be apt to s])lit along- the grain. Some shoe Avould then be necessary, 

 or, in the case of a bridge, where several trussed beams are placed 

 side by side, a piece of timber might be introduced, as at j in Figs. 

 1, 2, 3, and 3a, transversely to the bridge, passing under the feet of 

 the similarly placed posts. These would hold the trussed beams 

 together, and, where wind bracing is necessaiy, these transverse 

 timbers could lie connected together by diagonal tie rods or timlier 

 struts. 



A simple joint for connecting tlie posts to the beams is shown 

 at H on Figs. 2 and ■), where a small straining piece is introduced. 

 This is preferable to notching the main beam, which would weaken 

 it. Moreover, if the structure is to be a temporary one, it is 

 desirable to avoid cutting the beam. Seeing that tlie posts always 

 endiu'e equal stresses, the bolts, which attach the sti-aining piece to 

 the main beam, are not called upon to take any part of the thrust 

 in the posts. 



