SPECIFICATION'S FOR STEEL HIGHWAY BRIDGES. 27 
4 feet for plank floors. 
5 feet for strip floors 4 inches or more in thickness and for 
wood blocks on 4-inch plank subfloor. 
6 feet for reinforced concrete floors. 
When the spacing of floor beams exceeds the de- 
nominator given but is less than the axle spacing (14 
feet, inches) each beam shall be proportioned to carry 
the full axle load or loads. 
When the floor beam spacing exceeds the spacing of 
axles the load supported on each floor beam shall be the 
maximum reaction due to the axle loads, assuming the 
flooring between floor beams to act as a simple beam. 
Electric railway wheel loads may be assumed to be uni- 4. Distribution 
formly distributed longitudinally over a length of 3 feet, wheel loadT^^^ 
In the case of ballasted floors, a lateral distribution of 10 
feet for an axle load may be assumed. 
SECTION 5. STRUCTURAL STEEL DESIGN. 
Eifective span. — For the calculation of stresses, span i- Dimensions 
lengths shall be assumed as f oUows : uln!'""" ''''"''" 
Beams and girders, distance between centers of bearings. 
Trusses, distance between centers of end pins or of bearings. 
Floor beams, distance between centers of trusses or girders. 
Stringers, distance between centers of floor beams. 
Effective deptJi. — For the calculation of stresses, effec- 
tive depths shall be assumed as follows : 
Riveted trusses, distance between centers of gravity of the chords. 
Pin-connected trusses, distance between centers of chord pins. 
Plate girders, distance between centers of gravity of the flanges, 
but not to exceed the distance back to back of flange angles. 
Members subject to reversal of stress during the pas- 2. Reversal of 
sage of live load shall be proportioned as follows : Deter- ^*'"^^' 
mine the tensile and the compressive stresses and in- 
crease each by 50 per cent of the smaller; then propor- 
tion the member so that it will be capable of resisting 
each increased stress. The connections shall be propor- 
tioned for the sum of the original stresses. 
No pin-connected member shall be subjected to re- 
versal of stress. 
When the live load and dead load stresses are of op- 
posite sign, only 70 per cent of the dead load stress shall 
be considered as effective in counteracting the live load 
stress. 
Axial and bending. — Members subject to both axial 3. combined 
and bending stresses shall be proportioned so that the^'^^^" 
combined fiber stresses will not exceed the allowable 
axial stress. Members continuous over panel points shall 
be proportioned for the live and dead load bending mo- 
ments computed for a simple beam having a span equal 
to one panel length. 
Stresses due to lateral and longitudinal forces and tem- 
perature. — In proportioning the various parts of the struc- 
ture, provision shall be made for the following stress 
combinations : 
Group A: Dead load; live load; impact; centrifugal force. 
Group B: Lateral force; longitudinal force; temperature. 
