362 STEEL HEAD FRAMES AND COAL TIPPLES. CHAP. X. 



207. Breaking Load. The head frame shall be designed for a load in one or all of the hoisting 

 ropes equal to the breaking stress of the hoisting rope as given in the manufacturer's catalog. 



208. Machinery Loads. The stresses due to machinery, crushers, tipple equipment, etc., 

 shall be considered the same as the stresses due the working or live load. 



209. Wind Loads. Where the head frame or tipple is enclosed the wind load shall be assumed 

 as 30 Ib. per sq. ft. of exposed surface acting horizontally. Where the framework is open the 

 wind load shall be taken as 50 Ib. per sq. ft. acting on the projection of the members of the head 

 frame or tipple. In calculating the stresses due to wind, the wind loads may be assumed as 

 applied at the joints of the structure. Where one side of the structure is open so that a deep cup 

 or pocket is formed the wind load shall be taken as not less than 60 Ib. per sq. ft. on the projection 

 of the cup-like surface. 



210. Snow Loads. Snow loads shall be taken the same as for steel frame buildings. 



ALLOWABLE UNIT STRESSES. 



211. Steel head frames, coal tipples, coal washers and breakers, and similar structures shall 

 be designed for the following allowable stresses. 



212. Dead Load Stresses. The allowable unit stresses for dead loads shall be the same as 

 for steel frame buildings given in "Specifications for Steel Frame Buildings." Snow loads shall 

 be considered as dead loads. 



213. Working Load Stresses. The allowable unit stresses for working loads shall be one-half 

 the allowable unit stresses for dead load stresses as given in "Specifications for Steel Frame 

 Buildings." 



214. Bins. Bins shall be designed for two thirds the allowable unit stresses for dead load 

 stresses as given in " Specifications for Steel Frame Buildings." 



215. Breaking Load Stresses. The allowable unit stresses for the maximum stresses due 

 to breaking one or all the hoisting ropes shall be equal to the allowable unit stresses for dead load 

 stresses, plus 50 per cent, equal to three times the allowable unit stresses for working loads. The 

 breaking loads and working loads for any shaft compartment or machine need not be assumed 

 as acting together. 



216. Machinery Load Stresses. The allowable unit stresses for the maximum stresses due 

 to machinery and moving loads shall be the same as the allowable unit stresses for working loads, 

 equal to one half the allowable unit stresses for dead load stresses. 



217. Wind Load Stresses. The allowable unit stresses when the wind load stress is com- 

 bined with the dead load stress plus twice the working load and machinery load stresses shall not 

 exceed the allowable unit stresses for dead loads by more than 25 per cent. If the sum of the 

 wind load unit stress, the dead load unit stress, and twice the working load and machinery load 

 unit stresses exceed the allowable unit stress for dead loads by more than 25 per cent the area of 

 the section shall be increased to reduce the actual stresses to within the prescribed limit. Wind 

 load stresses need not be combined with breaking load stresses. 



218. Reversal of Stress. Members subject to a reversal of stress due. to a combination of 

 dead load stresses and working load stresses shall be designed to take both tension and com- 

 pression, each stress being increased by one half the smaller of the two stresses. Members subject 

 to a reversal of stress due to wind stress combined with dead load stresses and working load 

 stresses, or breaking load stresses combined with dead load stresses shall be designed to carry 

 both stresses. 



EQUIPMENT. 



219. Skips and Cages. Skips and cages shall be made of structural steel, as shown on the 

 detail drawings. They shall be provided with guide shoes and safety devices. For inclined 

 shafts the wheels shall have phosphor bronze bushings. 



220. Safety Detaching Hooks. All skips and c|ges shall be provided with effective detaching 

 hooks. The case shall be designed to take the stress due to a loaded cage or skip dropping a 

 vertical distance of two feet. 



221. Bin Gates. Unless otherwise specified all bin gates shall be of the undercut ^ type. 

 All gates shall be equipped with operating mechanism so that they can be opened in service by 

 one man. 



222. Screens. Fixed screens shall be made of bars as shown on the drawings and shall be 

 supported so that the bars will not be permanently deflected under the load. The screen bars 

 shall be placed at an angle so that they will screen the ore or coal without choking up. 



223. Shaking screens shall be carried on rollers and be driven by eccentric connecting bars. 

 They shall be placed at proper slopes, and shall be provided with all necessary gates. Unless 

 otherwise specified the screens shall be made of structural steel. 



224. Rotary screens shall be made of structural and machinery steel, and shall perform the 

 work required by the specifications. 



