COMPLEX STRESS DISTRIBUTIONS IN ENGINEERING MATERIALS. 371 
Graduated Stress Specifications. 
In one is given a series of stresses, carefully graduated according to the length and 
character of the bridge member having regard to the relation of fz + f. to fa, and the 
expected effect of fatigue combined with the relative values of f, and /,. 
. The late Sir Benjamin Baker’s well-known specification for steel main-line bridges 
is of this type and has been extensively used in the design of a very large number of 
bridges and other structures in this country and abroad. This specification was 
prepared in 1892 and published later.! 
In another form of graduated stress specification, the graduation is done according 
to the range of stress. The following are examples of formule which have been 
used by two British Railways : 
Maximum permissible stress in member or part, 
Minimum load or B.M. 
aero? (1 + 2 x Maximum load or Sm) gee bee 
HA (1 4 Minimum load or B.M. ) t r 
cerca pon ee 2 Xx Maximum load or B.M./ °07* Per 84: 17. 
These graduated stresses are obviously extensions of the simple stress limiting 
rule originally made by the Board of Trade. The type of specification may be 
called the graduated stress specification. It is perfectly definite and easy to apply 
and has many advantages over the alternative form. 
Standard Stress and Impact Specifications. 
The second type may be called the single or standard stress specification. In 
it the-load or moment to be borne by any particular member is increased by a factor 
so that that larger load represents the increased or more damaging effect of the actual 
load travelling at speed and of any other influence. The pazticular member is then. 
designed to carry this increased load on the basis of a standard maximum stress. 
This type of specification is of American origin, and there are many variations of it. 
One of the first formule for getting this increase of load to be added appeared 
1 The graduated stresses are as follows : 
TENSILE STRESSES, 
For Main Girders, Cross Girders and Rail Bearers of plate construction : 
Tons per sq. in. 
Under 20 feet span . - ; : : ‘ - 43 
20 feet and under 25 feet . ; : ‘ ; : 43 
25 ,, Pi 30 .cjonn ‘ : : ‘ : 5 
30 ,, i 50) 45! « 3 : : 3 , 5} 
50 feet and above . - é ; 4 2 54 
For Truss or Lattice Girders : 
80 feet and under 160 feet span— 
Bottom chords F : : I ; ; 5 
Diagonals . 5 . : 
160 feet and under 200 feet span—- 
Bottom chords { : 2 : 5 
Diagonals . ; 5 ; : c ; . 44 to 5% 
200 feet to 400 feet— 
Bottom chords . : : ¢ . . en Gutons 
Diagonals . : 2 - : ‘ é - 44 t07 
For wind bracing, all spans F : : i - : 84 
For floor suspenders cf 24 
Norr.—The 4} tons stress on the diagonals will apply to those at the central por- 
tion of the span, and to the counterbracing at the same point. The higher stresses will 
apply to those at the end portions of the span where the variations of stress are not so 
great. Intermediate diagonals will be subject to stresses lying between the two limits. 
‘The object in all cases of the varying working stresses is to guard against the relatively 
destructive action of suddenly applied loads on lightly loaded girders or members 
of girders, having reference both to the effects of impact and to the inferior ultimate 
resistance of material subject to considerable and frequent variations in stress. 
Special regard must be had to these conditions in the general design of the bridgework, 
and in the form and proportion of gussets and other minor details. 
