Bulk Material Parameters

6.2.1 General

(1) The material properties of the bulk material stored in the silos, which are to be quantified for calculating the loads, are to be derived or obtained either as test results or as data in any other suitable form.

(2) While using values from test results and other sources of data, the same are to be evaluated in a suitable manner keeping in mind the type of load in question in each case.

(3) It should be kept in mind that there may be significant differences between the material parameters measured in tests and the parameters that are determined by the actual behaviour of the bulk material in the silo.

(4) While evaluating the differences in bulk material parameters mentioned in (3), the following are some of the factors that must be kept in mind:

a lot of parameters are not constant, and may be dependant upon the stress level and the background of load application

Influences on account of particle shape, sizes and distribution of grain size can have a strong impact on the test and the silo in a variety of ways.

temporal influences

fluctuations of the moisture content

influences of dynamic actions

brittleness or ductility of the tested bulk material

the manner of putting-in the bulk material in the silo and in the testing apparatus

(5) While evaluating the differences in bulk material parameters mentioned in (3) with ref. to the coefficients of wall friction, the following factors must be kept in mind:

corrosion and chemical reaction of the bulk material particles, dampness and the wall

abrasion and wear which can roughen or smoothen the wall of the silo

polishing of the wall surface

accumulation of fat deposits on the wall

particles which get impressed in the wall surface (usually an influence which leads to the roughening of the wall surface)

(6) While determining the values for the material parameters the following is to be kept in mind:

the facts regarding the application of the relevant tests should be well-publicised and common knowledge

a comparison of the values of the individual parameters which have been measured in the tests with the corresponding published parameters, taking into account the experimental values

the deviation of the parameters relevant to the calculations

the results obtained from the large scale measurements on silos of similar styles

correlation of results from different types of tests

perceptible changes in the material parameters during the period when the silo is in use

(7) The choice of the characteristic material parameters has to be made on the basis of values the have been determined through laboratory tests, with due regard for know-how acquired through experience.

(8) The characteristic value of a material is to be chosen after a careful evaluation of the value which has influenced the occurrence of the load.

CATEGORY DESCRIPTION OF

WALL-SURFACE TYPES OF MATERIAL

D1 Polished

Carbon steel with slight surface corrosion Coated carbon steel

Cast high-density polyethylene Smooth ceramic plates

Concrete surface manufactured with steel shell

D3 Rough

Rough shell concrete Scarred carbon steel

Steel silos with bolts on the inside surface of the wall

Roughly polished ceramic plates

D4 Corrugated

Horizontal corrugated wall

Contoured sheet metal with horizontal notches Non-standardised walls with large deviations The effect of wrinkling in these surfaces has to be very carefully examined by means of the

particles embedded in the wall surface.

NOTE The classification and description given in Table 3 refers to the friction rather than the roughness. The main reason for this is that there is only a small correlation between the degree of roughness and the measured amount of wall friction caused by the bulk material that slides along the wall surface.

6.2.2 Determination of the Bulk Material Parameter

(1) The material parameters to be used for the design calculation may have deviations due to the changes in the structure, the production procedure, the grain size distribution, moisture content, age and electrical charging during handling; these need to be taken into account.

(2) The bulk material parameters are to be determined either according to the simplified procedure laid down in 6.2.3 or by means of test measurements in accordance with 6.3.

(3) Bulk materials parameters which are not contained in Table E.1 are to be obtained by means of test measurements in accordance with 6.3.

(4) The calculated correction values for the coefficient of wall friction µ of the bulk materials should take into account the roughness of the wall surface along which they glide. In Table 3 the different classes of wall surfaces are defined for use in this standard.

(5) For silos with wall surfaces belonging to the class (category) D4 according to Table 3, the effective wall friction coefficients should be determined according to the procedure described in D.2.

(6) The bulk material correction value Cop for the reference surface loads is to be taken from Table E.1 or calculated according to the equation (8).

6.2.3 Simplified Procedure

(1) The parameters of commonly known bulk materials are to be taken from the Table E.1. The values given there for the specific gravity γ correspond to the upper

characteristic value, while the parameters for the wall friction µm, for the horizontal load ratio Km and for the angle of the internal friction φim represent mean values of these characteristic quantities.

(2) If individual bulk materials cannot be clearly classified under the bulk material categories listed in Table E.1, then their parameters are to be determined experimentally in accordance with the procedure described under 6.3

(3) For determining the characteristic parameters of µ, K and φi, the listed values of µm, Km and φim are to be multiplied or divided by the so called conversion factor.

The conversion factors ax are given in the table E.1 for the bulk materials listed therein. For calculating the maximum loads, the following combinations are to be used:

Upper characteristic value of K =a_kK_m (1) Lower characteristic value of

k m

a

K =K (2)

Upper characteristic value of µ =a_µµ_m (3) Lower characteristic value of

µ

µ µ

a

= m (4)

Upper characteristic value of ϕ_i =a_ϕϕ_im (5) Lower characteristic value of

ϕ

ϕ ϕ

a

im

i = (6)

(4) For determining the effect of action on silos of the requirement category 1, the mean values µm, Km and φim may be used instead of the upper and lower characteristic values.