SPECIFICATIONS. 57 



31. Loads on Timber Piles. The maximum load carried by a pile shall not exceed 40,000 

 lb., or 600 lb. per sq. in. of its average cross-section. The allowable load on piles driven with a 



drop hammer shall be determined by the formula P = - - . Where P = safe load on pile 



in tons; W = weight of hammer in tons; h = free fall of hammer in ft.; s = average penetration 

 for the last six blows of the hammer in in. Where a steam hammer is used, tV ' 8 to be used in 

 place of unity in the denominator of the right hand member of the formula. 



Piles shall have a penetration of not less than 10 ft. in hard material, such as gravel, and not 

 less than 15 ft. in loam or soft material. 



PROPORTION OF PARTS. 



32. Allowable Stresses. In proportioning the different parts of the structure the maximum 

 stresses due to the combinations of the dead and wind load; dead and snow load; or dead, minimum 

 snow and wind load are to be provided for. Concentrated loads where they occur must be pro- 

 vided for. 



33. Tensile Stress. Allowable Unit Tensile Stresses for Structural Steel. For direct dead, 

 snow and wind loads. 



Lb. per Sq. In. 



Shapes, main members, net section 16,000 



Bars 16,000 



Bottom flanges of rolled beams 16,000 



Shapes, laterals, net section 20,000 



Iron rods for laterals 20,000 



Plate girder webs, shear on net section 10,000 



Shapes liable to sudden loading as when used for crane girders 10,000 



Expansion rollers per lineal inch 600 X d 



where d diameter of roller in inches. 



Laterals shall be designed for the maximum stresses due to 5,000 pounds initial tension and 

 the maximum stress due to wind. 



34. Compressive Stress. Allowable Unit Compressive Stress for Structural Steel. For 

 direct dead, snow and wind loads 



S = 16,000 70 - 



where S = allowable unit stress in lb. per sq. in; 



/ = length of member in inches c. to c. of end connections; 

 r = least radius of gyration of the member in inches. 



35. Plate Girders. Top flanges of plate girders shall have the same gross area as the tension 

 flanges. 



36. Shear in webs of plate girders shall not exceed 10,000 lb. per sq. in. of net section. 



^ 37. Alternate Stress. Members and connections subject to alternate stresses shall be 

 designed to take each kind of stress. 



38. Combined Stress. Members subject to combined direct and bending stresses shall be 

 proportioned according to the following formula: 



^S= P - + - 

 re 5 = stress in lb. per sq. in. in extreme fiber; 

 P = direct load in lb.; 

 A = area of member in sq. in.; 

 Af = bending moment in in-lb.; 



yi = distance from neutral axis to extreme fiber in inches; 

 / = moment of inertia of member; 



/ = length member, or distance from point of zero moment to end of member in inches; 

 E = modulus of elasticity = 30,000,000. lb. per sq. in. 



When combined direct and flexural stress due to wind is considered, 50 per cent may be 

 added to the above allowable tensile and Compressive stresses. 



39 Stress Due to Weight of Member. Where the stress due to the weight of the member or 

 due to an eccentric load exceeds the allowable stress for direct loads by more than 10 per cent, the 

 section shall be increased until the total stress does not exceed the above allowable stress for 

 t loads by more than 10 per cent. 



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