— 142 — 
^ 11.25 X ^ 3^ X 49 X 7 X 2 w , 
ili = ij = 1 h) inch tons, nearly. 
The difference between 115 and 82.5, oi. 32.5 inch tons, is greater than tlic re^istiug- 
moment of tlie material lost by the pin liolo: so the dimensions assumed aro ample. 
A similar calculation is necessary at the portal rod connection to portal struts. 
It. is evident tliafc tlie pin holes just treated should be placed as near tho ends of t-ho 
strut as circumstances will permii, iji order to reduce the bending moments on 
the jaws. 
It is not customary to calculate tlie thickness of a beam liangev plate, but to 
make it from an inch to an inch and an eighth: it can, though, untlev certain as- 
sumptions be calculated. If the load on a plate be considered uniformly distributed 
over the portion between the beam-liangei* holes, and if the flange oi tlie beam bo 
supposed to take up no bending- stress, the plate may be considered as a beam sup- 
ported at tlie ends, and uniformly loaded. For instance tako the case of a twentv- 
foot panel of a single track bridge ； the reaction at each end of the beam is about. 
18.5 tons. 
Suppose the centres of the beam hunger holes to be situated on the comers of 几 
4J" x 6" rectangle, the latter dimension being transverse to the bridge, and tliafc 
the sides of the plate are 8" and 9"， tlion the bending moment is 
M = 1 - W/. = i x 18.5 x 4.5 - 10.4 inch tons. 
The resisting moment is where R = 4 tons, I - moment of inertia = i l){ ^ - 
3 d 3 and d t = Equating the moments, substituting and solving, gives d = ： l.H 
inches. • 
Bat as the flanges do assist m resisting the bending, and as some of the weight 
comes upon the plate outside of the beam hangers, it is safe enough to take the 
thickness as low as an inch and an eiglitli. 
Lacing, or, as it is often improperly termed, single latticing, is about tlie most 
common detail foi* keeping pairs of channel bars in line: nevertheless, it must he 
inferior to latticing, especial^ when the lattice bars are riveted together at their 
intersection. By inspecting Tables XXII. and XXIII. it will be seen that a system 
of lacing-bars with one rivet at each end of a bar requires much larger stay plates at 
the ends than does a corresponding system oFlatticiug or double-riveted lacing. 
Tlie actual sizes of lattice or laciug bars for any strut can be detenuined only 
by experiment: it is thought that those given in Tables XX. ami XXI. are so 
strong, that the struts on which they aro employed would break in the channels 
rather than in the bars, aud yet not so heavy as to cause much ud necessary wsa of 
material. It will be seen also in these tables, that the requisite dimensions of 
latticing aud lacing bars depend not only upon the sizes of the channels winch they 
connect, but also upon the distance apart of these channels : this ia due to tlie fact 
that the bars are subject to compression as well as to tension. The lengths and 
weights of latticing and lacing bars can be found from Table XIX. It must not be 
forgotten that these lengths are to be used for estimate onhj ； as they were obtainocl 
from a diagram, and not checked by calculation. 
