54 BULLETIN 852, U. S. DEPARTMENT OE 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 from a settling reservoir or velocities in the 
pipe are so high that a clean-scoured condition ^ill 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 ravelin gs 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 ^ill 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 cf 
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 conve}^ance of 60 second-feet 
