422 IOWA DEPARTMENT OF AGRICULTURE 



material. We recommend that only blocks which have a low absorption 

 be used for silo construction. Blocks of this kind are more durable, and 

 a silo built of them will preserve silage better. 



The mortar used in laying the blocks should be a rich, water-tight mix- 

 ture. This will resist the passage of moisture or air through the joints. 

 To secure a more perfect wall, it is recommended that the inside be 

 washed with cement. This will seal any minute openings or imperfec- 

 tions. 



A careful personal inspection has been made of the silage in the thir- 

 teen Iowa silos which have been built up to the time of writing and, with 

 one unimportant exception, the silage was found in first-class condition 

 throughout the entire feeding season. There is no reason why the walls 

 of the Iowa silo should not be entirely satisfactory for the preservation 

 of silage. That they are satisfactory has been demonstrated by actual 

 test. 



RIGIDITY, STRENGTH AXD SMOOTHNESS OF WALLS. 



The outward or bursting pressure of silage has been found by previous 

 experiment to amount to about 11 pounds per cubic foot. To be well 

 designed, any silo must be constructed of material of suflBcient strength 

 to resist this pressure. Square silos are not well adapted to resist this 

 bursting pressure on account of their flat sides. They are almost sure to 

 bulge. For this reason the round silo is the most desirable form. 



The silo, owing to its diameter and height, offers a rather large amount 

 of surface upon which the force of the wind may act. For this reason 

 the silo walls should be of rigid construction and not readily distorted 

 or damaged by high winds. Furthermore, it is best that the structure 

 should be heavy enough when empty to resist being moved readily from 

 its foundation by strong winds. Some tjT)es are so light that they must 

 be thoroughly tied to adjacent buildings and held to place by guy wires. 

 The importance of this feature is emphasized through the fact that certain 

 insurance companies do not care to insure such types of silos and either 

 will not insure at all or only when extra charge is made for exposure. 

 The Iowa silo is rigid enough and heavy enough so that it is not affected 

 by wind. 



Enough steel reinforcement is laid in the mortar joints of the Iowa 

 silo, to carry the entire bursting pressure of the silage with a reasonable 

 factor of safety. Originally, the safe working load of the steel was 

 taken at 20,000 pounds per square inch, which gives a factor of safety 

 of three for steel rolled in relatively large bars. As it has been found 

 that steel wire is the most desirable reinforcement, the safe working 

 strength has been raised to 30,000 pounds per square inch. The drawing 

 process through which wire must pass in its manufacturing increases its 

 tensile strength and this change on the part of the designers is entirely 

 justified as there is no reduction in the strength of the reinforcement. 

 As originally dsigned, practically as much steel was included in the walls 

 of the Iowa silo as was to be found in the hoops of the average stave 

 silo which not only must resist the bursting pressure of the silage, but 



