54 BULLETIN 852, IT. S. DEPARTMENT OF AGRICULTURE. 



In general a lower factor of safety may be used for a jointed pipe 

 of precast units than for a pipe of monolithic construction. This is 

 true because a given surface may be more closely anticipated before 

 construction. 



For pipes of classes 1 and 4 (p. 7) the velocity may be assumed to 

 be within 10 per cent of that computed by the new formula, provided 

 the conditions are favorable. 



For pipes of classes 2 and 3 the writer would suggest about the 

 same factors of safety as for wood-stave pipe: 



Five per cent when only a rough, approximation to the actual needs of the pipe is 

 possible; when water enters the pipe horn a settling reservoir or velocities in the 

 pipe are so high that a clean-scoured condition Mill always be present inside the pipe; 

 and also where conditions of operation are such that no penalties are attached to a 

 slight insufficiency of carrying capacity. 



Ten per cent when the above conditions for a very clean pipe are assured, but where 

 penalties are attached to lack of capacity; or where no direct penalties are attached 

 but silted waters and low velocities may permit deposits. 



Fifteen per cent Where rock ravelings may reduce the interior area of the pipe, or 

 when penalties are attached and silted water is likely to cause excess retardation of 

 flow, or where chemical analyses of the water indicate that accretions may be 

 expected. 



The designer maj safely assume that the capacity of concrete "will not change 

 materially unless the pipe is subject to conditions mentioned on pages 50 and 51. 



As a factor for safety to be used in the design of a pipe line, the 

 writer would suggest adding the percentage to the load to be carried 

 rather than a change of coefficients — that is, if 100 second-feet of 

 water is the desired quantity and a factor of 10 per cent is to be used, 

 then design the line for a capacity of 110 second-feet. 



ESTIMATE DIAGRAMS AND TABLE; SOLUTIONS FOR TYPICAL PIPE 



PROBLEMS. 



(1) An inverted siphon of class 3 is required to convey 60 second- 

 feet of water a length of 2,800 feet, the velocity at peak load to be 

 about 5 feet per second. Water has settled in a reservoir before 

 entering the canal. The siphon is to contain no abrupt turns or 

 obstructions. No direct penalty has been attached for lack of 

 capacity. Required, diameter of pipe. 



Allowing a 5 per cent overload as a factor of safety, the rated 

 capacity will be 60+3=63 second-feet. At a velocity of 5 feet per 

 second the area of the water cross section — that is, the inside area of 



the pipe — must be -^- = 12.6 square feet, which is close to the area 

 5 



of a 48-inch pipe. On Plate VII, from the intersection of diameter 

 line 48 and coefficient line 0.345, follow the guide lines to an inter- 

 section with velocity line 5. This intersection is on loss-of-head line 

 1.7 feet per thousand feet of pipe. Thus for a 4-foot pipe 2,800 feet 

 long the friction loss incident to the conveyance of 60 second-feet 



