一 123 — 
which for the middle point becomes 
w being the total uniformly distributed live and dead load per lineal foot including 
tho allowance for shock, l the length of the beam and x the distance from the nearer 
e nd to the point considered. 
、 The shearing stress on the web should never exceed 2 A r , wliere A f is the area 
oi 、 the section of the web : except for extremely shallow girders, which are seldom 
permissible in good practice, the shear can never be as great as 2 A\ so tliis considera- 
tioii may be usually omitted. For reasons stated in the last chapter, a multiplicity 
°[ l )ai *ts iu the flanges of beams or girders is to be avoided, so it is well to use no top 
pLtte and but one bottom plate for track stringers, and never more than tliree 
thicknesses of plate for the flange of tlie largest girder. The rules for web stiffening 
aie given iu Cliaptei* YI. The sizes of stiffening angles may vary from 2|" x 24 " 
x to 8" x 8" x 普 " • 
When the panel length does not exceed fifteen feet no bracing frames between 
hade stringers will be needed, but for greater panel lengths there must be one frame 
a t tlie middle of the stringers, and, if tlie latter rest on top of the floor beams, one 
^ eai ea ch end also. A bracing frame can be made out of ono piece of 8" x 8" angle 
yon i)ofc ! ess than thick, by cutting right angled nicks out of one leg, bending tlie 
11011 u ntil the cut edges meet and welding tlie junction. 
1 oi* plate girders consocutive bracing frames slioulcl not be separated by moi’e 
lau seveu feofc, and tliose at the ends slioulcl be made stronger than tlio others. As 
s ^atetl iu Chapter YI, when tlie length of span exceeds twenty-five feet, tlie in ter - 
j^ediate frames are to bo replaced by diagonal braces of angle iron both above and 
e °w, the braces making with each other angles of about sixty degrees. 
• The only essential clifforeuce between a plate girder and a track stringer is in the 
^clined stiffeners at tlie ends ； and those track stringers which rest upon the masonry 
8 0u ld b® Provided with this detail. 
The following table gives with sufficient exactitude for all practical purposes tlio 
unifoniily distributed load per lineal foot for one track stringer or plate girdei. 
= ail y : it includes the live load with the allowance for shock, the constant 
la clc load and tlie weight of tlie girder. » 
bpau 
Unif. Loud. 
öpan 
Unit, ljoad. 
o to 18' 
and 
21' „ 22 ^ 
W « 24 ， 
,, 26 ， 
27 ； „ 2 ^ 
7 9 ' »» ?o ; 
to )0 to K) U 0 (0 
4^v^«-r> Os_>、l 00 00 
•— 、1 vO 一 、1 
00000000 
…… 5 g 
Sr 
ll' and 34' 
げ， ， 3 か 
IV « 
39% 40， and 4 ぴ 
4 八 4 ” ，， 44 ' 
45 ，， 4 6 ' ，， 47 ’ 
48', 4V ,, 5 ぴ 
5 ド to 6c/ 
2 370 pounds 
2300 ” 
221 〇 ,， 
2160 ，, 
2090 m 
2020 ,i 
I95O ” ' 
1 83 o ，， 1 
Tlie value of the greatest bending moment for a floor beam for a single track 
u dge may be dotevmiucd as follows. 
