28 Concrete Silos 



duce the amount of frozen silage to the minimum, is the 

 efficient type to build. 



(6) Strength. Experiments have shown, as noted 

 in another chapter of this book, that the bursting pres- 

 sure on a silo wall is 11 pounds per square foot for each 

 foot of depth of silage; that is, in a silo built to a 

 height of 20 feet, the bursting pressure at the bottom 

 of the silo is 220 pounds per square foot. In addition 

 to taking care of this inside pressure, the silo must 

 also be sufficiently strong to withstand storms of vari- 

 ous kinds. Winds of high velocity and much force are 

 not uncommon and tare especially prevalent in those 

 parts of the country where the largest number of 

 silos is likely to be built; and as climatic disturb- 

 ances of this kind are more frequent in the spring 

 and early summer, when the silo is likely to be empty, 

 it is quite necessary that the silo itself have sufficient 

 stability to meet these storms without any dependence 

 upon the contents to weight it down. 



(7) Durability. By this term is meant the ability 

 of the silo to continue in service without -appreciable 

 impairment during 'a long term of years. The absolute 

 cost of any structure is not the initial expenditure, but 

 the cost per year through its period of usefulness. If 

 for instance, a silo costing $240 has a life of 40 years, 

 the capital investment is $6.00 per year. On the other 

 hand, if a silo costing $60'.00 has a life of but 4 years, 

 the capital investment is $15 per year. 



(8) Maintenance. This is a question which must 

 be considered in close connection with that of dura- 

 bility. The structure in order to be efficient must not 

 only entail a reasonable capital investment per year 

 of life, but must also be as free as possible from main- 

 tenance cost. A silo which requires considerable time 



