ON THE DESIGN OF GIRDER BRIDGES. 483 



The admissible stress to be less than when subject to tension or com- 

 pression alone, and may be determined from the formula 



,, 7/11 Max. B'\ 



in which h' is the admissible stress in a bar subject to alternate 

 stresses of which Max. B' is the numerically lesser, and Max. B the 

 numerically greater, and 60 the admissible stress in the material for ^=0. 

 This expression is derived from that of Dr. Weyrauch by substituting for 

 the coefficient derived from the primitive strength of the material by an 

 arbitrary factor of safety, the value of h^, determined by the preceding 

 formula with respect to the ductility of the material. 



Table of Coefficients of Safety. 



1 . For vibration, shock, and other dynamic effects. For wrought iron 

 or steel ; minimum 1"33 for structures over 25 feet span, for 20 feet span 

 1-42, for 15 feet span 1-60, for 10 feet span 1'75. 



2. For unequal distribution of stress and secondary stresses. Mini- 

 mum for wrought iron 1-20, for steel 140. Additional for bracings 

 generally : — In pin-jointed structures 1'05 ; in riveted structures where the 

 breadth of bars is less than ji^th length of bay or depth of girder, depth 

 of girder is greater than ^th span, and bars are not joined at cross- 

 ings I'lO; in riveted structures otherwise 1-15. Additional for steel 

 plate girders 1"10. 



3. For ambiguity of stress or failure of continuity. Minimum I'OO. 

 For ambiguous systems of bracing 1'33, for continuous girders generally 

 1-16. 



4. For errors in design and workmanship. Minimum, 1'03. Addi- 

 tional for punched holes : — In iron plate girders 1-05, in iron framed 

 ■structures 1'15, in steel plate girders 1"15, in steel framed structures I 'SO. 



5. For irregularities in section and rusting generally 1'03. 

 (Product of minima coefficients for iron 1-70, for steel 1'98.) 



The specified coefficients of safety are not intended to include pro- 

 "vision for increase of stress due to an obvious want of symmetry in the 

 attachments or section of members ; bending stress due to their weight, 

 or liability of struts to buckling : these and other calculable additions to 

 be made to the stresses estimated from external loading. 



In the case of solid beams or plate girders, the admissible stress to 

 represent the extreme fibre stress ; accepting the ordinary theory of 

 bending. 



Experimental determination of resistance to flexure is recommended 

 in the case of solid beams of unusual section. 



For solid round pins the extreme fibre stress may exceed the specified 

 stress by 33 per cent, in iron and soft steel, and 20 per cent, in hard steel. 



Shearing stress in general to be taken as iths the admissible tensile 

 stress in the same material, but when of different materials the shearing 

 .strength of rivets and pins to be based upon the strength of the materials 

 of which they are made. 



Coefficients applicable to members joined to be applied to joints. 



Pressure on bearing area not to exceed 1*5 times the admissible 

 iensile strength of the weaker material, whether of rivet, or pin, or that 

 in which the hole occurs. 



ii2 



