— 174 — 
The jaw plates of the portal struts act also as reinforcing plates, so their thick- 
ness may have to be proportioned for this condition. The pressure on the bearing 
is equal to the greatest working stress on a 1Ä rod, which for an intensity of 5 
tom Table VI. gives as 6.240 + 1.875=8.115 tons. The diameter of the pin was 
determined to be 2^. ,f Consulting Table XV. we find the nocessary width of bearing 
to be J", subtracting from wliicli the web thickness leaves ^ ,r for the thickness of 
the plate. As tho latter will have to resist bending, it will be woll to make the 
thickness +"• The width should vary from 8" to 5", making the average say 7^. 
This width strictly speaking should be calculated, 1311 1 H is liartlly necessary ; for, 
judging by a similar calculation in Chapter XIV., we can conclade that if the pin 
hole be placed as closely as possible to the end of the strut, and if the jaw plates be 
made 8" wide at the pin holes, there will be sufficient material to resist the bending 
produced by the transverse component of the greatest working stress on the portal 
rods. The greatest stress on one channel supposing it to belong to the truss would 
bo 2.1 x 2.488 = 5.12 tons, the intensity being taken from Table YIII. for 3^ 
diameters and one end fixed. The lever arm is 皆 (i + i) oi* i ,r , making tlie mo- 
ment on tho rivets g x 5.12 = 1.92 inch tons, which divided by 0,311 gives 7 as tlie 
number of rivets required. A calculation for bearing would give a smaller num- 
ber. Laying out tho detail to scalo, we find that the required length of the jaw 
plate is about 3’. 
Next come the extension plates. Let us first proportion those for the largest 
post. The thickness for one extension is found by dividing tlie total sectional area 
of the post by the doptli of the channels, or 2M = 1.14 say li ，r 9 which Table XV. 
shows to bo more than su 伍 cient for bearing. This extension plate can bo made tip 
of two plates each ä " thick, the inner one extending two or three inches below the 
upper edge of the stay plate, and the outer one as low as requisite. 
The stress carried by tlio rivets is ^ x 87.122 = 18.0 tons and the lever arm is 
i (办 + 0.88) = 0.45", making the moment 18.6 x 0.45 = 8.87 inch tons. Using J" 
rivets we divide by 0.494 and find the number required to be 17. Only one half of 
those rivets which pass through both thicknesses of plate and the channel web are to 
be counted in making up the 17; and the countersunk rivets passing through only 
the two thicknesses of* plate are not to bo counted at all. 
By laying out the connection to scale we see that the inner plate should extend 
about 10" below the ends of tlie channels, and that we can use five rows of rivets 
with a 8" pitch, permitting of tlie passage of 18 rivets, lialf of which being deducted 
from 17 leaves 10 or 11 rivets to pass through the outer plate anti the web alone, 
making tho former extend 18" below the top of the post channels. The total length 
of the outer plate is, therefore, 28", and that of the inner plate 20." Similar calcu- 
lations for the extension plates of the other posts will give tho dimensions recorded 
in the u Bill of Iron.” 
Tlie thickness of the shoe plate is given in Chapter VI. as V, and its other di- 
mensions have been determined to be 25" x 30". 
The thickness of tlie roller plate is also 1", its width about 25 + 2 x 3 = 31 、 
and its length 80-j-2x3-f2 = 38". 
