SPECIFICATIONS. 209 



ii. Wind Force. Viaduct towers shall be designed for a force of 50 Ib. per sq. ft. on one 



and onr-li.il! times tin- vertical projection of the structure unloaded; or 30 Ib. per sq. ft. on the 

 .X.HIH Miriuce plus 400 Ib. per linear ft. of structure applied 7 ft. above the rail for assumed wind 

 I. 'in MM train when the structure is cither fully loaded or loaded on either track with empty cars 

 " to weigh 1,200 Ib. per linear ft., whichever gives the larger strain. 



1111 s 1111 % 



S <a ^ "^ ^ <^ ^ ^ ^<^^<i Tram Load **> K> 



" ^ ^ ^ S&SS ^ ^J5^5^ 4000 It. per Ft. (~\C\ 



OOOO ^oon^-n OOOO noon v; ;/;;;/;//;, \J\J 



Special Loading 



12. Longitudinal Force. Viaduct towers and similar structures shall be designed for a 

 longitudinal force of 20 per cent of the live load applied at the top of the rail. 



13. Structures located on curves shall be designed for the centrifugal force of the live load 

 applied at the top of the high rail. The centrifugal force shall be considered as live load and be 

 derived from the speed in miles per hour given by the expression 60 2\D, where "D" = degree 

 of curve. 



III. UNIT STRESSES AND PROPORTION OF PARTS. 



14. Unit Stresses. All parts of structures shall be so proportioned that the sum of the maxi- 

 mum stresses produced by the foregoing loads shall not exceed the following amounts in pounds 



sq. in., except as modified in paragraphs 22 to 25: 



15. Tension. Axial tension on net section 16,000 



16. Compression. Axial compression on gross section of columns 16,000 70 



irith a maximum of 14,000 



here "/" is the length of the member in inches, and "r" is the least radius of 

 gyration in inches. 



ct compression on steel castings 16,000 



17. Bending. Bending: on extreme fibers of rolled shapes, built sections, 



rders and steel castings; net section 16,000 



extreme fibers of pins 24,000 



. 18. Shearing. Shearing: shop driven rivets and pins 12,000 



eld driven rivets and turned bolts 10,000 



plate girder webs; gross section 10,000 



19. Bearing. Bearing: shop driven rivets and pins 24,000 



field driven rivets and turned bolts 20,000 



expansion rollers; per linear inch 6oo<f 



where "d" is the diameter of the roller in inches. 



on masonry 600 



^20. Limiting Length of Members. The lengths of main compression members shall not 

 eed 100 times their least radius of gyration, and those for wind and sway bracing 120 times 

 ir least radius of gyration. 

 21. The lengths of riveted tension members in horizontal or inclined positions shall not 

 exceed 200 times their radius of gyration about the horizontal axis. The horizontal projection 

 of the unsupported portion of the member is to be considered as the effective length. 



22. Alternate Stresses. Members subject to alternate stresses of tension and compression 

 shall be proportioned for the stresses giving the largest section. If the alternate stresses occur 

 in succession during the passage of one train, as in stiff counters, each stress shall be increased by 

 50 per cent of the smaller. The connections shall in all cases be proportioned for the sum of the 

 stresses. 



23. Wherever the live and dead load stresses are of opposite character, only two-thirds of the 

 load stresses shall be considered as effective in counteracting the live load stress. 



24. Combined Stresses. Members subject to both axial and bending stresses shall be pro- 

 portioned so that the combined fiber stresses will not exceed the allowed axial stress. 



25. For stresses produced by longitudinal and lateral or wind forces combined with those 

 from live and dead loads and centrifugal force, the unit stress may be increased 25 per cent over 



15 



