— 127 — 
angles, ami let us use a bottom plate 岳 " x 8' By a careful arrangement of the 
tlii’eo rows of rivet holes through the legs of the angles, wo can have no section 
weakened by more than one rivet liolo, nevertheless it will be better to add say four 
tenths of a square inch to A u as thus found, because the holes of the rows in t lie 
vertical and horizontal legs come pretty closely together. A" m.ay therefore be taken 
equal to 2 x 蚤 x 5 + 0.4 = 1.28 square inches, making the area of the bottom 
flange 8.58 + 1.28 = 9.80 square inches. Subtracting therefrom the area of the 
plato or ^ x 8 = 8.5 square inches, leaves G.B6 square inches as tlio area of the two 
angles, which multiplied by ten sixths gives 10.6 pounds as tlio weight per foot of 
each angle. The most suitable section given in tlio table last used is Bi n x i. r, x 
a nil weighs 10.5 pounds per lineal foot, which is near enough for all practical purpo- 
ses * The assumed thickness of half an iucli uaod in determining k ff gives a slight 
e u*or on tho side of safety. Laying out the sections of the flanges to scale, the 
distance between their centres of gravity is found to be a little over 27〃， so that in 
assuming d s 2(5.5" a slight error on the side of safety was committed. 
The difference is so small as not to necessitate a re- calculation. 
We can either let the bottom plate extend over a little more than tlie middle half of 
the stringer or calculate its theoretically correct length as follows. If in tlio equation 
M - 尤） 
We Su ^stitute for iv and l their values and for M the greatest allowable bending 
foment ou the beam at tlie end of tlie bottom plate, wo can solve for x and thus find 
the length of plate by the equation 
V = l — 2 x 
Ihe effective area of tlio lower flange at tlie end of tlie plate ia about 
6 x 1 0.5 
10 
0.7 7 = 5.5 3 
^Wch multiplied by 4 gives 22.12 tons as tlie permissible stress on the bottom 
ai 】 ge, and this multiplied by tlio effective depth 27" = 2.25’ gives 49.77 as the 
peiimssible bending moment. Substituting this gives 
49.97 
1»37 5 /fx 1 0 \ 
— 2—(21* — a ： 3) 
or x 2 —21 x 
9 9.5 4 
1.0 7 5 
7 2.4 nearly 
therefore 
x 
21 
土 V — 7 2.4 
• on8e quently the length of the plate V is 
^crease it to 14 feet. 
71IV 
4.2 3 
21 
いノ 
8.7 = 12.8 feet : but it is better to 
• 0 Bud tlie rivet spacing for the upper flange angles let us divide the beam 
eiigtlig of two feet commencing at both ends and moving towards the middle, 
Ü et us transform the equation 
