28 BULLETIN 1369, U. S. DEPARTMENT OF AGRICULTURE. 
Members subject to the stresses of Group A in com- 
bination with the stresses of Group B, either direct or 
fiexural or both, shall be designed for any of the follow- 
ing combinations, at unit stresses 25 per cent greater 
than those specified, but the resulting sections shall be 
not less than would be required if the stresses of Group 
A were considered alone. 
1. The combined stresses of Group B in combination with 
dead load only. 
2. The combined stresses of Group A in combination with 
50 per cent of the combined stresses of Group B. 
3. The combined stresses of Group A in combination with 
temperature only. 
stte^^?^^^^^^ Members and their details shall be proportioned to 
reduce secondary stresses to a minimum. In simple 
trusses without subdivided panels the secondary stresses 
due to deformation in any member whose width meas- 
ured in the plane of flexure is less than one-tenth of its 
length need not be considered. "When this ratio is ex- 
ceeded, or where subdivided panels are used, the second- 
ary stresses shall be computed. 
In members designed for secondary stresses in combi- 
nation with other stresses the specified allowable unit 
stresses may be increased 30 per cent, but the sections 
shall be not less than required for primary stresses. 
5. Allowance For the Calculation of stress reversal or counterstresses, 
the specified live loads, either uniform or concentrated, 
shall be increased 100 per cent and for this loading con- 
dition the specified unit stresses shall be increased not 
more than 50 per cent. The resulting sections shall be 
not less than would have been requii'ed had the allow- 
ance for overload not been considered. 
f). Compression The gross area of the compression flanges of beams and 
and^gfrders^^^™^ plate girdcrs shall be not less than the gross area of the 
tension flanges. 
The laterally unsupported length of the compression 
flanges of beams and girders shaU not exceed 40 times 
the flange width. When the unsupported length of 
flange exceeds 12 times the flange width, the compres- 
sive stress in pounds per square inch shall not exceed 
19,000-250 -g-- (Maximum value, 16,000 pounds.) 
where 
L = length, in inches, of unsupported flange, be- 
tween lateral connections or knee braces. 
h = flange width in inches. 
7. Proportion- RoUed beams shall be proportioned by the moments 
ing rolled beams. ^£ inertia of their sections. Proper allowance shall be 
made for anv reduction in strengtli due to rivet holes in 
the tension flange or to any reduction in allowable stress 
due to the length of unsupported compression flange. 
