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he materials that require storage in silos are hygroscopic and, in contact with moist air, cohesion builds up to such degree that the material can be loosened only by mechanical means. Raw sugar, salt, and some granular materials fall in this category. Flour is also hygroscopic, but in relatively dry storage the cohesion is weak, and the material can be easily loosened by compressed air so that it ows out of the cells by gravity. Nonhygroscopic or less hygroscopic materials are stored most economically in Table 1 Unit Weight and Angle of Internal Friction of Some Commonly Used Materials Unit weight Angle of internal friction Materials (kN/m( ) in degrees Wheat 8.5 2 Paddy 5.7 3 Rice 9.0 3 Maize 8.0 3 Barley 6.0 2 Corn 8.0 2 Sugar 8.2 3 Wheat our 7.0 3 Structural Considerations: Warehouse and Silo 2 relatively high cellular silos the individual cells of which are emptied by gravity ow. Hygroscopic materials are stored in large-spaced silos in which the material can be brokenloose mechanically before being taken out by appropriate conveyor systems. Also someof the highly hygroscopic materials (sugar, salt, etc.) may attack concrete, so that protec-tive coating for the inside surfaces of walls is required unless special cement and dense, closed surfaces are used. If hygroscopic sticky materials are not dried before storage, considerable cohesion and arching can be expected; consequently, the cell walls are sub-jected to unsymmetrical loading during emptying and have to resist considerably higherhorizontal pressure than in the fully-lled condition. Sometimes silos are required to beprovided with driers and equipment for infected grain-treatment facilities. 3.3 Assessment of Forces: Pressure Theories The stored materials in the storage bins exert horizontal pressure to the side walls of the bins in addition to the vertical forces from weight of the material. The horizontal pressurevaries under different conditions and for different congurations of the bins. P