DESCRIPTION AND CLASSIFICATION OF SILOS

4.1 Description of Actions in Silos

(1) The actions on silos are to be estimated with regard to the silo structure, the properties of the stored bulk material and the flow profiles that arise during emptying of the silo.

(2) Ambiguities related to the flow profiles, the influence of the fill and discharge eccentricities on the fill and discharge processes, the influence of the silo shape and size on the type of the flow profile and those that are related to the time-dependant discharge and fill pressures are all to be taken into consideration

NOTE 1 The magnitude and the distribution of the rated loads depend upon the silo structure, the material parameters of the bulk materials and the flow profiles which build up during emptying. The inherent differences in the properties of the different bulk materials that are stored and the simplifications in the load models lead to variations between the silo loads that actually appear and the design loads (calculated loads) according to sections 6 and 7. Thus, to quote an example, the distribution of discharge pressures along the silo wall changes with time. An exact prediction of the prevailing mean pressure, its divergence and its temporal variability is not possible, given the present level of knowledge.

(3) Allowance should be made for loads on the vertical walls of the silo when it is filled and while it is emptying, with fill- and discharge- eccentricities being marginal; this is to be done using a symmetric load component and an unsymmetric reference surface load. In case of large eccentricities the loads are to be described using a pressure distribution curve.

(4) Should the chosen form of the silo structure show a sensitive reaction to changes of the estimated load-guidelines, allowance has to be made for this through appropriate investigations

(5) The symmetric loads on the silo walls are to be estimated as follows: a) by means of horizontal load components ph upon the inner surface of the vertical silo wall; b) by means of loads pn that act perpendicular to inclined walls; c) by means of frictional loads pw and pt that act in the tangential direction of the wall; and d) by means of vertical load components pv in the stored bulk material (see figure 1c)

(6) The unsymmetric loads on the vertical silo walls in case of marginal eccentricities during fill and discharge have to be taken into account by using a reference surface load. These reference surface loads consist of horizontal pressures ph that act upon the inner surface of the silo wall locally.

(7) The unsymmetric loads on the vertical silo walls in case of large eccentricities during fill and discharge are to be additionally registered using a unsymmetric distribution of horizontal pressures ph and friction loads pw

(8) Unplanned and unaccounted load influences are to be registered using the load augmentation factor C.

(9) The load augmentation factors C for silo cells in categories 2 and 3 (see 4.5) register unaccounted additional load influences alone, which arise due to the bulk material flow during emptying of the silo.

(10) The load augmentation factors C for silo bins in category 1 (see 4.5) register additional influences during emptying that are caused by the bulk material movement as well as the influences due to the deviation of the bulk material parameters.

NOTE 2 The load augmentation factors C are intended to cover the ambiguities related to the flow profile, the influences of eccentricities during filling and emptying, the influence of the shape of the silo on the manner of the flow profile and proximity influences which arise when allowance is not made for the presence of fill and discharge pressures that are time dependant. For category 1 silos (see 4.5) the load augmentation factor also takes into account the deviation of the material properties of the bulk material. In silos of categories 2 and 3, allowance for the deviation of the material parameters influenced by the loads is not made by a load augmentation factor C but by the formulation of the appropriate characteristic calculation values for the bulk material parameters γ, µ, K and φi.

(11) In silos of category 1 (see 4.5) the allowance for unsymmetric loads is made by means of an increase of the symmetric loads by applying a load augmentation factor for the discharge loads C.

(12) In silos of categories 2 and 3 (see 4.5) allowance for the unsymmetric reference surface loads can be made alternatively by a substitute augmentation of the symmetric loads.

4.2 Description of Action on Tanks

(1) Allowance for loads on tanks as a consequence of filling them up is made by hydrostatic load formulations

4.3 Classification of actions on silo bins

(1) Loads due to bulk materials stored in the silo bins are to be classified as variable actions in accordance with DIN 1055-100.

(2) Symmetric loads on silos are to be classified as variable stationary actions in accordance with DIN 1055-100.

(3) Reference surface loads for making allowances for the filling and discharge processes in silo bins are to be classified as variable free actions in accordance with DIN 1055-100.

(4) Eccentric loads for making allowances for the eccentric filling and discharge processes in silo bins are to be classified as variable stationary actions.

(5) Loads arising from air or gas pressures in connection with pneumatic conveyor systems are to be regarded as variable stationary actions.

(6) Loads due to dust explosions are to be classified as extraordinary actions as defined by DIN 1055-100.

4.4 CLASSIFICATION OF THE INFLUENCES ON TANKS

Loads on tanks that arise due to the filling up of the tanks can be classified as variable stationary influences acc. to DIN 1055-100.

4.5 STANDARDISED CATEGORIES

(1) Based upon the design of the silo structure and its susceptibility to different types of malfunctions, various accuracy standards are used in the process of determining the influences on silo structures.

(2) The silo influences should be determined in accordance with one of the following standardized categories specified in this standard (see Table 1).